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Fabry‐Pérot (FP) photonic absorbers have demonstrated their subwavelength thickness and integration convenience. However, without introducing abnormal phase shifts, it is hard to further miniaturize the FP absorbers. Herein, based on perfect optical phase‐matching, planar FP infrared (IR) absorbers can be more significantly thinned via ultrahigh refractive index (RI) artificial films, breaking the conventional quarter‐wavelength cavity limitation. Specifically, an ultrahigh RI (≈6.0) Ag nanowire‐Si metamaterial dielectric‐cavity covered by a giant RI (≈11.0) bismuth‐based absorbing layer is proposed to induce exceptional propagation phase accumulation and reflection phase shift. The resultant 155 nm FP absorber (not including the mirror layer) can support a resonance absorption at ≈4.4 µm. After vertically stacking, double‐cavity absorbers (below 310 nm) with multi‐resonant features are achieved, yielding broadband absorption from 1.4 to 14.4 µm. In addition, the whole FP absorbers are fabricated only through a sputtering process without any other procedures. In terms of optical phase management and refractive index engineering, this work creates a novel miniaturization scheme for FP infrared absorbers, offering a lot of opportunities applied in advanced nanophotonics. [ABSTRACT FROM AUTHOR]

Structural colors, resulting from the interaction of light with nanostructured materials rather than pigments, present a promising avenue for diverse applications ranging from ink-free printing to optical anti-counterfeiting. Achieving structural colors with high purity and brightness over large areas and at low costs is beneficial for many practical applications, but still remains a challenge for current designs. Here, we introduce a novel approach to realizing large-scale structural colors in layered thin film structures that are characterized by both high brightness and purity. Unlike conventional designs relying on single Fabry–Pérot cavity resonance, our method leverages coupled resonance between adjacent cavities to achieve sharp and intense transmission peaks with significantly suppressed sideband intensity. We demonstrate this approach by designing and experimentally validating transmission-type red, green, and blue colors using an Ag/SiO2/Ag/SiO2/Ag configuration on fused silica substrate. The measured spectra exhibit narrow resonant linewidths (full width at half maximum ∼60 nm), high peak efficiencies (>40 %), and well-suppressed sideband intensities (∼0 %). In addition, the generated color can be easily tuned by adjusting the thickness of SiO2 layer, and the associated color gamut coverage shows a wider range than many existing standards. Moreover, the proposed design method is versatile and compatible with various choices of dielectric and metallic layers. For instance, we demonstrate the production of angle-robust structural colors by utilizing high-index Ta2O5 as the dielectric layer. Finally, we showcase a series of printed color images based on the proposed structures. The coupled-cavity-resonance architecture presented here successfully mitigates the trade-off between color brightness and purity in conventional layered thin film structures and provides a novel and cost-effective route towards the realization of large-scale and high-performance structural colors. [ABSTRACT FROM AUTHOR]

Recently, the Fabry–Perot (F–P) cavity electrochromic devices, which are constructed by combining nano-size inorganic electrochromic material WO 3 and reflective metal layers, have attracted considerable attention due to their potentials in the fields of dynamic multicolor displays and active camouflage. However, in the few studies that have been reported, the peak reflectance of flexible F–P cavity multicolor electrochromic devices in the visible wavelength band is generally lower than 30 %, which limits their reflective display and camouflage effects in high-brightness environments. Thus, we have combined multicolor reflective electrochromic electrodes in WO 3 /Au asymmetric F–P nanocavity constructed by magnetron sputtering on a Nylon 66 porous substrate with a highly transparent UV-cured gel electrolyte and a PET plastic sealing procedure. As a result, a flexible and dynamic multicolor electrochromic skin with high reflectance (R max ＞50 %) and excellent flexibility (bending radius of 4 mm) has been fabricated. The WO 3 electrochromic layer at the top works as the dynamic dielectric layer in the broadband absorption Fabry–Perot cavity, and the inert metal Au layer at the bottom works as the reflective layer. The WO 3 /Au asymmetric F–P cavity can acutely capture the variation in optical constants of WO 3 due to the thickness change and electric-driven effects. This cavity facilitates the selective reflection and absorption of the energy distribution of the spectrum after the induced interferometric resonance. As a result, the cavity exhibits a rich array of structural colors, including ocher, taupe, yellow, orange, green, and violet. The electrochromic skin exhibits a fast switching time and maintains 99.43 % of the optical modulation amplitude (ΔR) after 110 coloring/bleaching cycles. It provides a potential option in the field of flexible high-brightness reflective displays and dynamic camouflage in the future. [ABSTRACT FROM AUTHOR]

Polaritons are generated by strong interaction between photons and matter, with the hybridization fundamentally changing the energy landscape of the system. Future exploitation of polaritons will benefit from implementing low‐cost, flexible, and easily tuneable configurations, as electronics have before. Here, coherent coupling of the carbonyl stretch vibrations of polyester (poly(ethylene terephthalate), PET) films to the optical modes of a Fabry–Perot (FP) cavity is presented, in which the FP cavity is directly formed on flexible, free‐standing, and commercially available PET films. For 2‐µm thick PET films, the carbonyl stretch vibration of the PET overlaps with the half‐wavelength FP cavity mode, leading to the coupling of the two modes, a Rabi splitting above 160 cm−1, and associated anticrossing in angular dispersion measurements. The study demonstrates dynamic control of light‐matter interaction strength by modulating film thickness under tension, finding that a 25% tensile stretch can tune the light‐matter interaction from 20:80 mixing to near‐perfect 50:50 hybridization at normal incidence. These findings are discussed in the context of future foldable/twistable/wearable polaritonic devices. [ABSTRACT FROM AUTHOR]

The long-wave infrared band (8–14 µm) is essential for several applications, such as infrared detection, radiative cooling, and near-field heat transfer. However, according to Kirchhoff's law, the intrinsic balance between thermal absorption and emission limits the further improvement of photon energy conversion and thermal management. Thus, breaking Kirchhoff's balance and achieving nonreciprocal thermal radiation in the long-wave infrared band are necessary. Most existing designs for nonreciprocal thermal emitters rely on grating or photonic crystal structures to achieve nonreciprocal thermal radiation at narrow peaks, which are relatively complex and typically realize bands larger than 14 µm. Here, a sandwich structure consisting of an epsilon-near-zero (ENZ) magneto-optical layer (MOL), a dielectric layer (DL), and a metal layer is proposed to achieve a strong nonreciprocal effect in the long-wave infrared band, which is mainly attributed to the strengthening of the asymmetric Berreman mode by the Fabry–Perot cavity. In addition, the impact of the incident angle, DL thickness, and DL refractive index on the nonreciprocal thermal radiation has been investigated. Moreover, by replacing the ENZ MOL with the gradient ENZ MOL, the existence of the DL can further improve the nonreciprocity of the broadband nonreciprocal thermal radiation. The proposed work promotes the development and application of nonreciprocal energy devices. [ABSTRACT FROM AUTHOR]

Optical fiber sensor emerges as a highly promising technology for trace gas detection due to their high sensitivity, remote capability, and immunity to electromagnetic interference. However, the state‐or‐the‐art fiber‐optic gas sensors typically use lengthy optical fibers as gas absorption cells or coatings with functional materials to achieve more sensitive gas detection, which poses challenges such as slow response and/or poor selectivity, as well as limitations on their use in confined spaces. Here, an ultraminiature optical fiber‐tip photothermal gas sensor via direct 3D micro‐printing of a Fabry‐Pérot cavity on the end face of a standard single‐mode optical fiber is reported. It enables not only direct interaction between light and gas molecules at the fiber output but also remote interrogation through an interferometric read‐out scheme. With a low‐finesse microcavity of 66 µm in length, a noise equivalent concentration of 160 parts‐per‐billion acetylene gas is demonstrated with an ultra‐fast response time of 0.5 s. Such a small high‐performance photothermal gas sensor offers an approach to remotely detecting trace gases for a myriad of applications ranging from in‐reactor monitoring to medical diagnosis. [ABSTRACT FROM AUTHOR]

We have demonstrated a wide range of hydrogen gas detection from 200 ppm to 100% at room temperature by using optical cavity type of sensor elements with polarization interference. This explosion‐proof type of sensor element provides a hydrogen monitoring system in flammable environments for gaseous fuels. [ABSTRACT FROM AUTHOR]

Photoacoustic spectroscopy (PAS) gas sensors based on optomechanical resonators (OMRs) have garnered significant attention for ultrasensitive trace-gas detection. However, a major challenge lies in balancing small size with high performance when developing ultrasensitive miniaturized optomechanical resonant PAS (OMR-PAS) gas sensors for space-constrained applications. Here, we present a miniature optical fiber PAS gas sensor based on a planar-spiral spring OMR (PSS-OMR) that is in situ 3D micro-printed on the end-face of a fiber-optic ferrule. Experimental results demonstrate that mechanical vibrational resonance can enhance the sensor's acoustic sensitivity by over two orders of magnitude. Together with a 1.4 μL non-resonant photoacoustic cell, it can detect C2H2 gas concentration at the 45-ppb level, and its response is very fast approximating 0.2 seconds. This optical fiber OMR-PAS gas sensor holds great promise for the detection or monitoring of rapidly varying trace gas in many applications ranging from production process control to industrial environmental surveillance.

Abstract We present quad-layered reflective structural color filters generating vivid additive primary colors by controlling a mode number in a Fabry–Perot (FP) cavity and an anti-reflective (AR) coating layer, thus accomplishing high spectral contrast which is highly demanded in creating sharp colors. The reflection brightness of fabricated structural color filters is over 78% and a color gamut is comparable to the standard color gamut (sRGB). Higher-order resonant modes are exploited yielding a narrow passband with strong suppression of the reflection at shorter and longer wavelength ranges for a green color, while red and blue colors are produced by employing fundamental resonant modes. Besides, the structural color filters maintain both high brightness and high color purity at oblique incidence angles up to 40° due to a small angle of refraction by a cavity medium with high refractive index. Moreover, a large-scale fabrication is enabled owing to the simplicity of a device structure, where thin film deposition is used. The scheme presented in this work may open the door to a number of applications, such as reflective displays, imaging devices, colored photovoltaics, and decorations.

Abstract Fabry-Pérot (FP) microcavities have attracted tremendous attention in recent years due to their favorable optical characteristics of the high quality (Q) factor and small mode volume. In this work, we presented a novel approach that utilized the soft lithography and imprinting technology to incorporate the convex micro-lens array structure into the FP (FP-lens) cavity. A strong mode-profile restriction of the micro-lens simultaneously reduced the mode volume and enhanced the Q factor, exhibiting high tolerance to non-parallelism of mirrors compared with that of the plane-plane FP (PP-FP) microcavities. In the experiment, the Q factor of the FP-lens cavity was measured to be 8.145×104, which exhibited a 5.6-fold increase than that of the PP-FP cavity. Furthermore, we experimentally measured the refractive index sensing performance of the FP-lens cavity with the sensitivity of 594.7 nm/RIU and a detection limit of 4.26×10−7 RIU. On the basis of this superior sensing performance, the FP-lens cavity has the great potential for applications in biosensors.

We present quad-layered reflective structural color filters generating vivid additive primary colors by controlling a mode number in a Fabry–Perot (FP) cavity and an anti-reflective (AR) coating layer, thus accomplishing high spectral contrast which is highly demanded in creating sharp colors. The reflection brightness of fabricated structural color filters is over 78% and a color gamut is comparable to the standard color gamut (sRGB). Higher-order resonant modes are exploited yielding a narrow passband with strong suppression of the reflection at shorter and longer wavelength ranges for a green color, while red and blue colors are produced by employing fundamental resonant modes. Besides, the structural color filters maintain both high brightness and high color purity at oblique incidence angles up to 40° due to a small angle of refraction by a cavity medium with high refractive index. Moreover, a large-scale fabrication is enabled owing to the simplicity of a device structure, where thin film deposition is used. The scheme presented in this work may open the door to a number of applications, such as reflective displays, imaging devices, colored photovoltaics, and decorations. [ABSTRACT FROM AUTHOR]

Strain sensors that can rapidly and efficiently detect strain distribution and magnitude are crucial for structural health monitoring and human–computer interactions. However, traditional electrical and optical strain sensors make access to structural health information challenging because data conversion is required, and they have intricate, delicate designs. Drawing inspiration from the moisture-responsive coloration of beetle wing sheaths, we propose using Ecoflex as a flexible substrate. This substrate is coated with a Fabry–Perot (F–P) optical structure, comprising a "reflective layer/stretchable interference cavity/reflective layer", creating a dynamic color-changing visual strain sensor. Upon the application of external stress, the flexible interference chamber of the sensor stretches and contracts, prompting a blue-shift in the structural reflection curve and displaying varying colors that correlate with the applied strain. The innovative flexible sensor can be attached to complex-shaped components, enabling the visual detection of structural integrity. This biomimetic visual strain sensor holds significant promise for real-time structural health monitoring applications. [ABSTRACT FROM AUTHOR]

Mid-infrared (mid-IR) photodetection is important for various applications, including biomedical diagnostics, security, chemical identification, and free-spacing optical communications. However, conventional "photon" mid-IR photodetectors require liquid nitrogen cooling (i.e., MCT). Furthermore, acquiring mid-IR spectra usually involves a complex and expensive Fourier Transform Infrared spectrometer, a tabletop instrument consisting of a meter-long interferometer and MCT detectors, which is not suitable for mobile and compact device applications. In this work, we present tunable photoresponsivity in the mid-IR wavelength in palladium diselenide (PdSe2) – molybdenum disulfide (MoS2) heterostructure field-effect transistors (FETs), operating at room temperature. Furthermore, we applied a tunable membrane cavity to modulate the Fabry–Pérot resonance to modulate the absorption spectrum of the device layer. We used a robust polyetherimide (PEI) membrane with CVD-grown graphene to electrically tune the membrane structure. For the next step, we will integrate the PdSe2-based photodetector and tunable membrane to increase detection sensitivity and spectrum tunability to realize the 'learning'-based spectroscopy. [ABSTRACT FROM AUTHOR]

In this paper, we study the nonlinear optical bistability (OB) in a symmetrical multilayer structure. This multilayer structure is constructed by embedding a nonlinear three-dimensional Dirac semimetal (3D DSM) into a Fabry–Perot cavity composed of one-dimensional photonic crystals. The OB phenomenon stems from the third order nonlinear conductivity of 3D DSM. The local field of resonance mode could enhance the nonlinearity and reduce the thresholds of OB. This structure achieves the tunability of OB due to the fact that the transmittance could be modulated by the Fermi energy. It is found that the OB threshold and threshold width could be remarkably reduced by increasing the Fermi energy of the 3D DSM. Besides, we also found that the OB curve depends heavily on the angle of incidence of the incident light, the structural parameters of the Fabry–Perot cavity, and the position of the 3D DSM inside the cavity. After parameter optimization, we obtained OB with a threshold of 10 6   V / m . We believe this simple multilayer structure could provide a reference idea for realizing low-threshold and tunable all-optical switching devices. [ABSTRACT FROM AUTHOR]

Mid-infrared (mid-IR) photodetection is important for various applications, including biomedical diagnostics, security, chemical identification, and free-spacing optical communications. However, conventional "photon" mid-IR photodetectors require liquid nitrogen cooling (i.e., MCT). Furthermore, acquiring mid-IR spectra usually involves a complex and expensive Fourier Transform Infrared spectrometer, a tabletop instrument consisting of a meter-long interferometer and MCT detectors, which is not suitable for mobile and compact device applications. In this work, we present tunable photoresponsivity in the mid-IR wavelength in palladium diselenide (PdSe2) – molybdenum disulfide (MoS2) heterostructure field-effect transistors (FETs), operating at room temperature. Furthermore, we applied a tunable membrane cavity to modulate the Fabry–Pérot resonance to modulate the absorption spectrum of the device layer. We used a robust polyetherimide (PEI) membrane with CVD-grown graphene to electrically tune the membrane structure. For the next step, we will integrate the PdSe2-based photodetector and tunable membrane to increase detection sensitivity and spectrum tunability to realize the 'learning'-based spectroscopy. [ABSTRACT FROM AUTHOR]

In this paper, we construct a near-infrared Fabry–Perot cavity composed of two sodium (Na) layers and an antimony trisulfide (Sb2S3) layer. By cascading two Fabry–Perot cavities, the transmittance peak splits into two transmittance peaks due to the coupling between two Fabry–Perot modes. We utilize a coupled oscillator model to describe the mode coupling and obtain a Rabi splitting of 60.0 meV. By cascading four Fabry–Perot cavities, the transmittance peak splits into four transmittance peaks, leading to a near-infrared transparent band. The near-infrared transparent band can be flexibly tuned by the crystalline fraction of the Sb2S3 layers. In addition, the effects of the layer thickness and incident angle on the near-infrared transparent band and the mode coupling are investigated. As the thickness of the Na layer increases, the coupling strength between the Fabry–Perot modes becomes weaker, leading to a narrower transparent band. As the thickness of the Sb2S3 layer increases, the round-trip propagating of the Sb2S3 layer increases, leading to the redshift of the transparent band. As the incident angle increases, the round-trip propagating of the Sb2S3 layer decreases, leading to the blueshift of the transparent band. This work not only provides a viable route to achieving tunable near-infrared transparent bands, but also possesses potential applications in high-performance display, filtering, and sensing. [ABSTRACT FROM AUTHOR]

Fabry–Pérot (F–P) cavity and metal hole array are classic photonic devices. Integrating F–P cavity with holey metal typically enhances interfacial reflection and dampens wave transmission. In this work, a hybrid bound surface state is found within rectangular metal holes on a silicon substrate by merging an extraordinary optical transmission (EOT) mode and a high-order F–P cavity mode both spatially and spectrally. Transmission, Q-factor, and bandwidth can be enhanced significantly with respect to the classical EOT and F–P interference by simply sweeping the cavity length. This state can provide EOT properties and ten times broader EOT bandwidth well below the effective plasma frequency of the periodic metal holes, where the metal holes typically show evanescent properties and do not support EOT in theory. Furthermore, a large modulation range of 25 % and 39 % is demonstrated with various graphene patterns for the transmittance of this hybrid state at 500 and 582 GHz, respectively. [ABSTRACT FROM AUTHOR]

A dual-parameter sensor for simultaneous strain and temperature measurement is proposed and demonstrated based on the introduction of higher-order modes in a single-mode-multimode-single-mode (SMS) structure cascaded with a Fabry–Perot (FP) cavity as a compact and low-cost fiber optic sensor. In this structure, a multimode fiber is spliced between two single-mode fibers, and the single-mode fiber at the other end is fusion spliced to the HF-corroded single-mode fiber to form the SMS-FP cascade structure. Light transmitted from the multimode fiber to the FP cavity can couple light into the cladding, introducing higher-order modes into modal interference. Moreover, multiple-beam interference generates spectra. The experimental results indicate that the sensor performs effectively within the range of 150∘C–250∘C and 0–875 μ ε. The strain sensitivities of SMS and FP were found to be −2.45 and −2.34 pm/ μ ε , respectively, and their temperature sensitivities are 11.85 and 5.59 pm/∘C, respectively. The interesting features of the sensor include good operating linearity, compact size, high sensitivity, simple structure and low cost. [ABSTRACT FROM AUTHOR]

Smart windows that incorporate photothermal modulators (PMs) can independently regulate solar transmittance and infrared (IR) emissivity to improve building comfort and reduce energy consumption. Herein, a novel all‐solid‐state variable IR emissivity device (VED) is first designed and fabricated with a high visible irradiation transmittance (T′vis = 0.79) and solar irradiation transmittance (T′sol = 0.75) using an ITO/SiO2/ITO Fabry–Perot cavity structure. The VED exhibits different IR emissivity (ɛ) values at positive (ɛP = 0.80) and negative (ɛN = 0.38) bias, allowing for dynamic regulation of radiation by controlling the electrical conductivity of the indium tin oxide (ITO) layer. Furthermore, an all‐solid‐state PM with a structure of ITO/SiO2/ITO/Glass/ITO/NiO/ZrO2/Li/WO3/ITO, which is capable of independently regulating solar transmittance (ΔT'sol = 0.31) and IR emissivity (Δɛ2.5–25 µm = 0.42), is fabricated. The multi‐mode smart window incorporating PMs can achieve "bright," "dark," "warm," and "cool" modes, making them suitable for deployment in diverse climate zones. The innovative smart window holds a massive potential for use in reducing building energy consumption. [ABSTRACT FROM AUTHOR]

DECIGO is a future Japanese project for the detection of gravitational waves in space. To conduct various scientific missions, including the verification of cosmic inflation through the detection of primordial gravitational waves as the main objective, DECIGO is designed to have high sensitivity in the frequency band from 0.1 to 10 Hz, with arms of length 1000 km. Furthermore, the use of the Fabry-Perotcavity in these arms has been established for the DECIGO project. In this paper, we scrutinize the significance of the Fabry-Perot cavity for promoting this project, with a focus on the possibility of observing gravitational waves from cosmic inflation and binary compact star systems as indicators. The results show that using the Fabry-Perot cavity is extremely beneficial for detecting them, and it is anticipated to enable the opening of a new window in gravitational wave astronomy. [ABSTRACT FROM AUTHOR]

Industrial-grade optical semiconductor films have attracted considerable research interest because of their potential for wafer-scale mass deposition and direct integration with other optoelectronic wafers. The development of optical thin-film processes that are compatible with complementary metal-oxide-semiconductor (CMOS) processes will be beneficial for the improvement of chip integration. In this study, a multilayer periodically structured optical film containing Fabry–Perot cavity was designed, utilizing nine pairs of SiN/SiO2 dielectrics. Subsequently, the multilayer films were deposited on Si substrates through the inductively coupled plasma chemical vapor deposition (ICPCVD) technique, maintaining a low temperature of 80 °C. The prepared films exhibit narrow bandpass characteristics with a maximum peak transmittance of 76% at 690 nm. Scanning electron microscopy (SEM) shows that the film structure has good periodicity. In addition, when the optical films are exposed to p/s polarized light at different angles of incidence, the cavity mode of the film undergoes a blueshift, which greatly affects the color appearance of the film. As the temperature rises, the cavity mode undergoes a gradual redshift, while the full width at half maximum (FWHM) and quality factor remain relatively constant. [ABSTRACT FROM AUTHOR]

Low-frequency and 1/f noise are common measurement limitations that arise in a variety of physical processes. Mitigation methods for these noises are dependent on their source. Here, we present a method for removing 1/ f noise of optical origin using a micro-cavity Fabry–Pérot (FP) interferometer. A mechanical modulation of the FP cavity length was applied to a previously studied opto-mechanical sensor. It effectively mimics an up-conversion of the laser frequency, shifting signals to a region where lower white-noise sources dominate and 1/ f noise is not present. Demodulation of this signal shifts the results back to the desired frequency range of observation with the reduced noise floor of the higher frequencies. This method was found to improve sensitivities by nearly two orders of magnitude at 1 Hz and eliminated 1/f noise in the range from 1 Hz to 4 kHz. A mathematical model for low-finesse FP cavities is presented to support these results. This study suggests a relatively simple and efficient method for 1/ f noise suppression and improving the device sensitivity of systems with an FP interferometer readout. [ABSTRACT FROM AUTHOR]

A novel Fabry-Perot cavity (FPC) antenna with metasurface is presented, which can achieve broad bandwidth and low profile. Traditional FPC antennas, with rectangular microstrip antennas as feeds, have limited impedance bandwidth and struggle to make a compromise in the gain bandwidth and maximum gain value. To obtain wide bandwidth, the FPC antenna proposed in this paper utilizes a feed antenna loaded with parasitic patches. To widen impedance bandwidth and gain bandwidth and reduce the profile, a positive phase gradient partially reflective surface (PRS) and an artificial magnetic conductor (AMC) are located above and below the feed antenna, respectively. The phase property of the PRS and AMC also brings in a more smooth gain value curve. To further increase gain values, four metal reflector plates are located around the proposed antenna. The overall dimension of the antenna is 2.5λ0×2.5λ0×0.25λ0 (λ0 is the free space wavelength at 7.5 GHz). Simulated results show that the resonant cavity antenna proposed in this letter exhibits an impedance bandwidth of 13.3% (7-8 GHz) and a 3 dB gain bandwidth of 14.3% (7.02-8.10 GHz). The maximum gain in the whole operating band is 14.5 dBi. The measured results are in good agreement with the simulated ones. [ABSTRACT FROM AUTHOR]

In the field of in situ measurement of high-temperature pressure, fiber-optic Fabry–Perot pressure sensors have been extensively studied and applied in recent years thanks to their compact size and excellent anti-interference and anti-shock capabilities. However, such sensors have high technological difficulty, limited pressure measurement range, and low sensitivity. This paper proposes a fiber-optic Fabry–Perot pressure sensor based on a membrane-hole-base structure. The sensitive core was fabricated by laser cutting technology and direct bonding technology of three-layer sapphire and develops a supporting large-cavity-length demodulation algorithm for the sensor's Fabry–Perot cavity. The sensor exhibits enhanced sensitivity, a simplified structure, convenient preparation procedures, as well as improved pressure resistance and anti-harsh environment capabilities, and has large-range pressure sensing capability of 0–10 MPa in the temperature range of 20–370 °C. The sensor sensitivity is 918.9 nm/MPa, the temperature coefficient is 0.0695 nm/(MPa∙°C), and the error over the full temperature range is better than 2.312%. [ABSTRACT FROM AUTHOR]

Abstract We present a stabilization technique developed to lock and dynamically tune the resonant frequency of a moderate finesse Fabry-Pérot (FP) cavity used in precision atom-cavity quantum electrodynamics (QED) experiments. Most experimental setups with active stabilization either operate at one fixed resonant frequency or use transfer cavities to achieve the ability to tune the resonant frequency of the cavity. In this work, we present a simple and cost-effective solution to actively stabilize an optical cavity while achieving a dynamic tuning range of over 100 MHz with a precision under 1 MHz. Our unique scheme uses a reference laser locked to an electro-optic modulator (EOM) shifted saturation absorption spectroscopy (SAS) signal. The cavity is locked to the PDH error signal obtained from the dip in the reflected intensity of this reference laser. Our setup provides the feature to efficiently tune the resonant frequency of the cavity by only changing the EOM drive without unlocking and re-locking either the reference laser or the cavity. We present measurements of precision control of the resonant cavity frequency and vacuum Rabi splitting (VRS) to quantify the stability achieved and hence show that this technique is suitable for a variety of cavity QED experiments.

This paper reports a novel Fabry-Perot fiber optic microphone based on Au/PI composite film, with an optimal F-P cavity length of $500 \, \mu \text{m}$ , and the maximum extinction ratio (ER) obtained is about 12 dB. Besides, the sensor has flat response performance in the audio frequency range of 200–10 kHz, with a maximum signal-to-noise ratio (SNR) of 76 dB and a sensitivity of approximately 170 mV/Pa at the sound pressure level of 1000 Hz/94 dB, which is much higher than Brüel & Kjær’s electric microphone 4939. Within a certain range of sound pressure, there is a fine linear relationship between the output response of the sensor and the input sound pressure, with a fitting coefficient of 0.9987. The new diaphragm-type fiber optic microphone proposed in our work has superb economy, sensitivity, frequency response and linearity within the audible sound range, which is expected to achieve large-scale, low-cost fabrication and long-term stable application in audio detection and other fields.

To achieve omnidirectional sensitive detection of partial discharge (PD) in transformers and to avoid missing PD signals, a fiber optic omnidirectional sensing method for PD in transformers combined with the fiber Bragg grating (FBG) and Fabry-Perot (F-P) cavity is proposed. The fiber optic omnidirectional sensor for PD as a triangular prism was developed. The hollow structure of the probe was used to insert a single-mode fiber to form an F-P cavity. In addition, the three sides of the probe were used to form a diaphragm-type FBG sensing structure. The ultrasound sensitization diaphragm was designed based on the frequency characteristics of PD in the transformer and the vibration model of the diaphragm in the liquid environment. The fiber optic sensing system for PD was built and the performance test was conducted. The results show that the resonant frequency of the FBG acoustic diaphragm is around 20 kHz and that of the F-P cavity acoustic diaphragm is 94 kHz. The sensitivity of the developed fiber optic sensor is higher than that of the piezoelectric transducer (PZT). The lower limit of PD detection is 68.72 pC for the FBG sensing part and 47.97 pC for the F-P cavity sensing part. The directional testing of the sensor and its testing within a transformer simulation model indicate that the proposed sensor achieves higher detection sensitivity of PD in all directions. The omnidirectional partial discharge ultrasound sensing method proposed in this paper is expected to reduce the missed detection rate of PD. [ABSTRACT FROM AUTHOR]

Transport of excitons in organic materials can be enhanced through polariton formation when the interaction strength between these excitons and the confined light modes of an optical resonator exceeds their decay rates. While the polariton lifetime is determined by the Q(uality)‐factor of the optical resonator, the polariton group velocity is not. Instead, the latter is solely determined by the polariton dispersion. Yet, experiments suggest that the Q‐factor also controls the polariton propagation velocity. To understand this observation, the authors perform molecular dynamics simulations of Rhodamine chromophores strongly coupled to Fabry–Pérot cavities with various Q‐factors. The results suggest that propagation in the aforementioned experiments is initially dominated by ballistic motion of upper polariton states at their group velocities, which leads to a rapid expansion of the wavepacket. Cavity decay in combination with non‐adiabatic population transfer into dark states, rapidly depletes these bright states, causing the wavepacket to contract. However, because population transfer is reversible, propagation continues, but as a diffusion process, at lower velocity. By controlling the lifetime of bright states, the Q‐factor determines the duration of the ballistic phase and the diffusion coefficient in the diffusive regime. Thus, polariton propagation in organic microcavities can be effectively tuned through the Q‐factor. [ABSTRACT FROM AUTHOR]

Perfect absorbers have received significant attention due to their tunable and extraordinary absorptive behaviour of light at selected wavelengths. Such absorption can be achieved by means of nanostructured materials with various types of flexibility. Despite these advantages, nanofabrication is a challenging process. In this study, we manufacture and characterize a gold thin-film-based ultra narrow-band perfect absorber that does not require nanostructures for full absorption of the incident light. Perfect absorption is based on Fabry-Perot interference created using a metal-insulator-metal triple layer. The fabrication involves precise deposition of a gold film (35 nm) and silicon nitride dielectric film (110-170 nm) in a controlled fashion. Optical characterization is achieved using a portable, simple reflection probe. The fabricated multi-layer thin film demonstrates ultra-narrow bandwidth (25 nm) and 99.4% maximum absorption. The use of gold thin film ensures inertness and makes the proposed system robust under harsh environmental conditions, unlike other metals. Our findings underscore the potential of this engineered perfect absorber for applications in photovoltaics and as optical filters; but not in sensing. We show that the electric field is bound between the metallic thin films and not affected by the change of refractive index outside the system. [ABSTRACT FROM AUTHOR]

A dual-polarized filtering Fabry–Perot antenna (FPA) with high selectivity and high isolation is proposed for in-band full-duplex (IBFD) applications. The proposed antenna utilizes a square patch as the feeding element, which is fed by a double differential-fed scheme for dual-polarized radiation with high isolation. The patch is loaded with a symmetrical cross-slot and four shorting pins for a broad passband filtering feature. To enhance broadside gain across a wide frequency range, the patch is incorporated with a partially reflecting surface (PRS), which is composed of two complementary cross-slot and patch arrays. Moreover, the frequency selectivity of PRS is exploited to improve the filtering characteristic. The double differential feeds are realized based on out-of-phase power dividers, which are combined with simple low-pass filters to further improve the out-of-band suppression. The final design was fabricated and measured. The measurement results show excellent results with a 10-dB return loss bandwidth of 21.5% (4.91–6.09 GHz), isolation of greater than 40 dB, peak gain of 13.7 dBi, out-of-band suppression level of better than 27 dB, and a cross-polarization level of less than −27 dB.

Epsilon-near-zero (ENZ) materials have recently emerged as a promising platform for infrared nanophotonics. A significant challenge in the design of ENZ-based optics is to control the dispersion of ENZ modes that otherwise have a flat profile near the ENZ frequency. Strong coupling with an optical cavity is a promising approach to ENZ dispersion engineering, which however has limitations due to the lack of tunability or nanofabrication demands of the cavity employed. Here, we theoretically and numerically show that much of the limitations of previous approaches can be overcome by strongly coupling the ENZ mode to an unpatterned Fabry–Perot cavity. We demonstrate this unprecedented ENZ dispersion control in coupled cavities by designing tunable infrared polarizers that can absorb s and reflect p-polarized components, or vice versa, for almost any oblique angle of incidence, i.e. omnidirectional polarizers. The feasibility of active control is also demonstrated using a phase change material within the cavity, which predicts dynamic switchability of polariton dispersions across multiple resonant levels at mid-infrared wavelengths. These results are expected to advance the current understanding of strongly coupled ENZ interactions and demonstrate their potential in tailoring dispersions for active and passive control of light.

Recently, high-performance thin-film lithium niobate optical modulators have emerged that, together with advanced multiplexing technologies, are highly expected to satisfy the ever-growing demand for high-capacity optical interconnects utilizing multiple channels. Accordingly, in this study, a compact lithium-niobate-on-insulator (LNOI) photonic chip was adopted to establish four-channel wavelength-division-multiplexing (WDM) transmitters, comprising four optical modulators based on ultracompact 2 × 2 Fabry-Perot cavities and a four-channel WDM filter based on multimode waveguide gratings. The fabricated chip with four wavelength channels has a total footprint as compact as 0.3 × 2.8 mm2, and exhibits an excess loss of ~0.8 dB as well as low inter-channel crosstalk of < –22 dB. Using this LNOI photonic chip, high-capacity data transmissions of 320 Gbps (4 × 80 Gbps) on-off-keying signals and 400 Gbps (4 × 100 Gbps) four-level pulse amplitude signals were successfully realized with the ultra-low power consumption of 11.9 fJ/bit.

Strain sensors that can rapidly and efficiently detect strain distribution and magnitude are crucial for structural health monitoring and human–computer interactions. However, traditional electrical and optical strain sensors make access to structural health information challenging because data conversion is required, and they have intricate, delicate designs. Drawing inspiration from the moisture-responsive coloration of beetle wing sheaths, we propose using Ecoflex as a flexible substrate. This substrate is coated with a Fabry–Perot (F–P) optical structure, comprising a “reflective layer/stretchable interference cavity/reflective layer”, creating a dynamic color-changing visual strain sensor. Upon the application of external stress, the flexible interference chamber of the sensor stretches and contracts, prompting a blue-shift in the structural reflection curve and displaying varying colors that correlate with the applied strain. The innovative flexible sensor can be attached to complex-shaped components, enabling the visual detection of structural integrity. This biomimetic visual strain sensor holds significant promise for real-time structural health monitoring applications.

In this paper, we construct a near-infrared Fabry–Perot cavity composed of two sodium (Na) layers and an antimony trisulfide (Sb2S3) layer. By cascading two Fabry–Perot cavities, the transmittance peak splits into two transmittance peaks due to the coupling between two Fabry–Perot modes. We utilize a coupled oscillator model to describe the mode coupling and obtain a Rabi splitting of 60.0 meV. By cascading four Fabry–Perot cavities, the transmittance peak splits into four transmittance peaks, leading to a near-infrared transparent band. The near-infrared transparent band can be flexibly tuned by the crystalline fraction of the Sb2S3 layers. In addition, the effects of the layer thickness and incident angle on the near-infrared transparent band and the mode coupling are investigated. As the thickness of the Na layer increases, the coupling strength between the Fabry–Perot modes becomes weaker, leading to a narrower transparent band. As the thickness of the Sb2S3 layer increases, the round-trip propagating of the Sb2S3 layer increases, leading to the redshift of the transparent band. As the incident angle increases, the round-trip propagating of the Sb2S3 layer decreases, leading to the blueshift of the transparent band. This work not only provides a viable route to achieving tunable near-infrared transparent bands, but also possesses potential applications in high-performance display, filtering, and sensing.

Abstract Transport of excitons in organic materials can be enhanced through polariton formation when the interaction strength between these excitons and the confined light modes of an optical resonator exceeds their decay rates. While the polariton lifetime is determined by the Q(uality)‐factor of the optical resonator, the polariton group velocity is not. Instead, the latter is solely determined by the polariton dispersion. Yet, experiments suggest that the Q‐factor also controls the polariton propagation velocity. To understand this observation, the authors perform molecular dynamics simulations of Rhodamine chromophores strongly coupled to Fabry–Pérot cavities with various Q‐factors. The results suggest that propagation in the aforementioned experiments is initially dominated by ballistic motion of upper polariton states at their group velocities, which leads to a rapid expansion of the wavepacket. Cavity decay in combination with non‐adiabatic population transfer into dark states, rapidly depletes these bright states, causing the wavepacket to contract. However, because population transfer is reversible, propagation continues, but as a diffusion process, at lower velocity. By controlling the lifetime of bright states, the Q‐factor determines the duration of the ballistic phase and the diffusion coefficient in the diffusive regime. Thus, polariton propagation in organic microcavities can be effectively tuned through the Q‐factor.

Hydrogen (H2) sensors are critical to various applications such as the situation where H2 is used as the clean energy for industry or the indicator for human disease diagnosis. Palladium (Pd) is widely used as the hydrogen sensing material in different types of sensors. Optical fiber H2 sensors are particularly promising due to their compactness and spark-free operation. Here, we report a Fabry–Pérot (FP)-cavity-based H2 sensor that is formed with a freestanding Pd membrane and integrated on a conventional single-mode optical fiber end. The freestanding Pd membrane acts both as the active hydrogen sensing material and as one of the reflective mirrors of the cavity. When the Pd film absorbs H2 to form PdHx, it will be stretched, resulting in a change of the cavity length and thus a shift of the interference spectrum. The H2 concentration can be derived from the amplitude of the wavelength shift. Experimental results showed that H2 sensors based on suspended Pd membranes can achieve a detection sensitivity of about 3.6 pm/ppm and a detection limit of about 3.3 ppm. This highly sensitive detection scheme is expected to find applications for sensing low-concentration H2. [ABSTRACT FROM AUTHOR]

Ultrathin broadband absorbers with high efficiency, wide angular tolerance, and low fabrication cost are in demand for various applications. Here, we present an angle-insensitive ultrathin (<150 nm) broadband absorber with an average 96.88% (experiment) absorptivity in the whole visible range by utilizing a simple dielectric–semiconductor–lossy metal triple-layer film structure. The excellent broadband absorption performance of the device results from the combined action of the enhanced absorptions in the semiconductor and lossy metal layers exploiting strong interference effects and can be maintained over a wide viewing angle up to ±60°. Benefiting from the lossy metal providing additional absorption, our design reduces the requirement for the semiconductor's material dispersion and has great flexibility in the material selection of the metal layer. Additionally, the lithography-free nature of the proposed broadband visible absorber provides a high-throughput fabrication convenience, thus holding great potential for its large-area applications in various fields. [ABSTRACT FROM AUTHOR]

A refractive index (RI) sensor based on micro-nano fiber (MN-fiber) with chirped fiber Bragg grating (CFBG) Fabry--Perot cavity (FP-cavity) for a microfluidic chip has been proposed. A single -mode fiber is drawn by hydrogen flame heading come into MN-fiber. Two CFBGs are written into this MN-fiber by the ultraviolet (UV) laser mask exposure method. One is at the tapered region, another is at the micro-nano region. Then a micro-nano fiber with chirped fiber Bragg grating (MN-CFBGs) FP-cavity sensor is formed. The Bragg reflection wavelengths of two CFBGs are 1620 nm, 3-dB bandwidth are above 50 nm. The reflectance of two CFBGs are 70% and 99%, respectively. The effects of reflectivity and bandwidth of the CFBGs FP-cavity, diameter and length of MN-fiber with this sensor's optical properties are analysed is and discussed. This sensor is embedded in a microfluidic chip and the MN-fiber region is immersion microfluid in different channels. The experimental results show that refractive index sensitivity of the sensor is --986 nm/refractive index unit (RIU), and the signal of the sensor has little noise. The CFBG-FP sensor not only has high sensitivity and lager measurement range, but also high contrast resonance signal and stability. [ABSTRACT FROM AUTHOR]

The Fano resonance is realized in a Fabry–Perot (FP) cavity with W S 2 sheet. The Fano resonance originates from the interaction between the FP cavity mode and the excition mode supported by the tungsten disulfide ( W S 2 ) sheet. The proposed system is modeled theoretically by coupled-mode theory (CMT) and numerically studied by finite difference-time domain method (FDTD). Results show that the Fano resonance coupling strength is up to 14.1 meV due to the large dipole transition moment of W S 2 , and the line-shapes can be tuned by the height of the FP cavity, the position of the W S 2 sheet, and the refractive index of the dielectric in the FP cavity, respectively. Moreover, the Fano resonance profile can be changed to a symmetrical Lorentz line-shape by tuning the orientation of the W S 2 sheet. The designed structure can find potential applications in enhanced light absorption in two-dimensional materials and sensing nano-devices in applied fields. [ABSTRACT FROM AUTHOR]

No membranous optical sensors have excellent development prospects in aerospace and other industrial fields due to their small size and anti-electromagnetic interference. Here, we proposed a novel Fabry–Pérot (FP) cavity acoustic sensor based on silicon optical waveguide Bragg gratings. The FP cavity consists of two Bragg gratings written on the silicon-based optical waveguide and a miniature air groove. When the sound signal acts on the miniature air groove, the sound pressure changes the density of air molecules near the waveguide grating's evanescent field, causing variation in the air's refractive index. This results in a shift in the reflection spectrum of the FP cavity to detect the sound signal. The effects of the grating period, grating pitch quantity, and groove depth of the FP cavity on acoustic sensing were studied. The modelling predicts that the sensing sensitivity could be 0.4 nm/Pa. Theoretically, the compact self-designed acoustic sensor can withstand temperatures above 800 °C. Therefore, it has significant potential applications in precision measurement in high-temperature and high-pressure environments. [ABSTRACT FROM AUTHOR]

This study proposes a high-frequency pneumatic system and demodulation method based on polydimethylsiloxane (PDMS) film-embedded fiber-optic Fabry-Perot sensor to meet the conflicting requirements of small sizes and wide frequency-response range not achieved by conventional pneumatic probes. The high modulus of elasticity of PDMS films provides the potential for an upper limit of a pneumatic frequency response up to 20 kHz, and the new demodulation algorithm, based on the ability of the F-P cavity to return to its initial length from the maximum cavity length change, is utilized to analyze its variation. The process of recovering F-P cavity corresponds with the sparse part of the signal, which takes advantage of the unique characteristics of interference fringes in this part. Moreover, the relationship between the relative change of the F-P cavity length and pneumatic pressure, as well as the relationship between the duration of the relative change of the F-P cavity length and the frequency are analyzed comprehensively. The rationality of the proposed demodulation scheme is verified from the angle-error analyses. These analyses could be helpful for integrated and high-frequency response pneumatic detection.

Structural colors with tunable properties have extensive applications in surface decoration, arts, absorbers, and optical filters. Planar structures have more advantages over other forms studied to date due to their easy manufacturability. Metal-insulator-metal-based structures are one of the known methods to fabricate structural colors where colors can be tuned mainly by the thickness of the intermediate lossless insulator layer. However, generating colors by MIM structure requires a thin metallic layer on top, and the top metals’ abrasiveness and/or oxidation may degrade the colors quickly. Thus, we propose a lossy dielectric layer to replace the top metallic layer as a solution to ensure the structure’s durability by preventing scratches and oxidation. Herein, CrON/Si3N4/Metal structures have been studied where theoretical investigations suggest that highly saturated colors can be generated in the lossy-lossless dielectric structures. Experimental data validated such simulations by revealing a range of vivid colors. Furthermore, these structures can easily achieve strong light absorption (SLA) even for a thick top layer of ∼100 nm. The colors realized by these structures are appeared due to a combination of the interference effect of the asymmetric Fabry–Perot cavity structure and the absorption rate in the CrOxN1−x layer.

We present a novel terahertz (THz) Fabry–Perot (FP) microcavity biosensor that uses a porous polytetrafluoroethylene (PTFE) supporting film to improve microorganism detection. The THz FP microcavity confines and enhances fields in the middle of the cavity, where the target microbial film is placed with the aid of a PTFE film having a dielectric constant close to unity in the THz range. The resonant frequency shift increased linearly with increasing amount of yeasts, without showing saturation behavior under our experimental conditions. These results agree well with finite-difference time-domain (FDTD) simulations. The sensor's sensitivity was 11.7 GHz/μm, close to the optimal condition of 12.5 GHz/μm, when yeast was placed at the cavity's center, but no frequency shift was observed when the yeast was coated on the mirror side. We derived an explicit relation for the frequency shift as a function of the index, amount, and location of the substances that is consistent with the electric field distribution across the cavity. We also produced THz transmission images of yeast-coated PTFE, mapping the frequency shift of the FP resonance and revealing the spatial distribution of yeast. [ABSTRACT FROM AUTHOR]

Based on the ab initio theory, the physical parameters such as molar polarization, molar susceptibility and Virial coefficient of argon gas were calculated, and the theoretical parameter model of gas pressure about refractive index was established by combining the Virial equation of state and Lorentz-Lorenz equation. The optical vacuum measuring device based on the Fabry-Perot cavity was used to accurately measure the resonant frequency change of the laser in the Fabry-Perot cavity to obtain the gas refractive index and gas pressure. At 105Pa, the relative uncertainty of the refractive index is 1.8×10-11, and the relative uncertainty of the gas pressure is 4.4×10-6. The measured pressures obtained by the optical vacuum measuring device based on the Fabry-Perot cavity and the capacitance film vacuum gauge were compared and analyzed. The results showed that the Fabry-Perot cavity based optical vacuum measuring method has higher stability and accuracy. [ABSTRACT FROM AUTHOR]

Fiber sensors possess characteristics such as compact structure, simplicity, electromagnetic interference resistance, and reusability, making them widely applicable in various practical engineering applications. Traditional fiber sensors based on different microstructures solely rely on the thermal expansion effect of silica material itself, limiting their usage primarily to temperature or pressure sensing. By employing thin film technology to form Fabry–Perot (FP) cavities on the end-face or inside the fiber, sensitivity to different physical quantities can be achieved using different materials, and this greatly expands the application range of fiber sensing. This paper provides a systematic introduction to the principle of FP cavity fiber optic sensors based on thin film technology and reviews the applications and development trends of this sensor in various measurement fields. Currently, there is a growing need for precise measurements in both scientific research and industrial production. This has led to an increase in the variety of structures and sensing materials used in fiber sensors. The thin film discussed in this paper, suitable for various types of sensing, not only applies to fiber optic FP cavity sensors but also contributes to the research and advancement of other types of fiber sensors. [ABSTRACT FROM AUTHOR]

Tunable structural color has gained significant attention due to its dynamic characteristics. However, conventional devices are usually regulated only in their color capabilities by structural parameters, restricting real-time dynamic applications. In this study, we propose an ultra-thin asymmetric Fabry–Perot cavity patterned with phase-change materials (MPMP). The reversible phase transition of VO2 induces changes in the MPMP's optical performance, enabling color mode switching through temperature control and resulting in rapid color conversion and low-temperature regulation. By adjusting relevant structural parameters of the VO2 layer and nanodiscs, the color performance range can be tailored. Through numerical investigations, we demonstrate that MPMP can produce stable transformation of dynamic structural colors by harnessing the phase-change effect. Our research unveils new possibilities for applications such as anti-counterfeiting, bio/chemical sensing, and temperature sensing. [ABSTRACT FROM AUTHOR]