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In the field of physical oceanography, the temperature and pressure of seawater are important fundamental parameters. In order to realize the simultaneous measurement of the temperature and pressure in seawater, a reflective optical fiber sensor based on polydimethylsiloxane (PDMS)-sealed optical microfiber coupler combined with a sagnac loop (OMCSL) is proposed. Benefiting from the high thermo-optical coefficient and large elasticity of PDMS, and the large-scale swift field transmission characteristics of optical microfiber coupler (OMC), the sensitivity and structural stability of our sensor is largely improved. The response performance of the sensor is further analyzed by numerical simulation calculations and theoretical modeling. Experimental results show that the temperature and pressure sensitivity of the sensor could reach -2.133 nm/C and 3.416 nm/Mpa, respectively, about one order higher than that of bare fiber OMCSL. Furthermore, via inversely calculating the cross-sensitivity matrix, the temperature and pressure sensing can be demodulated simultaneously, and the average errors of the preliminary experimental settlements are 1.61% and 5.02%, respectively. Due to the merits of compact structure, easy fabrication, high sensitivity, fast response speed and high stability, the basic performance of this dual parametric sensor is comparable to the existing electrical temperature-depth (TD) sensor, which is expected to meet the practical application requirements of marine environmental monitoring and ocean dynamics research.

We propose and demonstrate an all-optical tunable polymer bottle micro-resonator fabricated by droplet dispensing method. A microfiber is used to excite whispering gallery mode (WGM) of the micro-resonator. Continuous wave (CW) laser is employed as pump light to tune the WGM. To illustrate tuning performance of the fabricated device, external and internal pump light methods are both employed. For external pump light method, the tuning efficiency is about 5.23 pm/mW. For internal pump light, tuning efficiency of 68.3 pm/mW is obtained. The experimental results reveal that the bottle micro-resonator (BMR) can achieve wavelength tuning more than one free spectral range (FSR), which indicates the WGM can be tuned within a transparent window covered the full wavelength range. In addition, the fabricated device has advantages of high tuning efficiency, small size, easy fabrication, and extremely low cost.

Nanophotonic devices with high densities are extremely attractive because they can potentially merge photonics and electronics at the nanoscale. However, traditional integrated photonic circuits are designed primarily by manually selecting parameters or employing semi-analytical models. Limited by the small parameter search space, the designed nanophotonic devices generally have a single function, and the footprints reach hundreds of microns. Recently, novel ultra-compact nanophotonic devices with digital structures were proposed. By applying inverse design algorithms, which can search the full parameter space, the proposed devices show extremely compact footprints of a few microns. The results from many groups imply that digital nanophotonics can achieve not only ultra-compact single-function devices but also miniaturized multi-function devices and complex functions such as artificial intelligence operations at the nanoscale. Furthermore, to balance the performance and fabrication tolerances of such devices, researchers have developed various solutions, such as adding regularization constraints to digital structures. We believe that with the rapid development of inverse design algorithms and continuous improvements to the nanofabrication process, digital nanophotonics will play a key role in promoting the performance of nanophotonic integration. In this review, we uncover the exciting developments and challenges in this field, analyse and explore potential solutions to these challenges and provide comments on future directions in this field.

A mid-infrared (mid-IR) dual-band filter with ultra-high resolving power is designed and numerically investigated. It is composed of an Al2O3 layer between a gold grating and a CaF2 substrate. Each unit cell of the grating consists of two gold strips with different heights. The filter provides two high-extinction ultra-narrow stop bands. One is at a wavelength of ~3.8392 μm with a full-width at half-maximum (FWHM) of ~0.8 nm, and the other is at a wavelength of ~3.8435 μm with a FWHM of ~1.8 nm. The resolving power of the filter is >2100. The two stop bands result from the (+1) and (-1) waveguide modes, respectively. An interaction between the two resonance modes exists and affects the stop bands. We can control the interaction by adjusting the grating parameters, thus designing the FWHMs and lineshapes of the two stop bands flexibly.

Mode division multiplexing (MDM) technology has been well known to researchers for its ability to increase the link capacity of photonic network. While various mode processing devices were demonstrated in recent years, the reconfigurability of multi-mode processing devices, which is vital for large-scale multi-functional networks, is rarely developed. In this paper, we first propose and experimentally demonstrate a scalable mode-selective converter using asymmetrical micro-racetrack resonators (MRRs) for optical network-on-chip. The proposed device, composed of cascaded MRRs, is able to convert the input monochromatic light to an arbitrary supported mode in the output waveguide as required. Thermo-optical effect of silicon waveguides is adopted to tune the working states of the device. To test the utility, a device for proof-of-concept is fabricated and experimentally demonstrated based on silicon-on-insulator substrate. The measured spectra of the device show that the extinction ratios of MRRs are larger than 18 dB, and modal crosstalk for selected modes are all less than −16.5 dB. The switching time of the fabricated device is in the level of about 40 μs. The proposed device is believed to have potential applications in multi-functional and intelligent network-on-chip, especially in reconfigurable MDM networks.

Plasmonic nanocircuits can deliver light in subwavelength scale, however, require state-of-the-art fabrication process due to the ultra-small footprints. Here, we introduce direct coupling strategy based on metal-insulator-metal (MIM) waveguide systems to reduce the system loss as well as the fabrication difficulty and increase the structural stability. Following this strategy, the coupling between the input waveguide and square ring resonator (SRR) can be realized via an aperture, and for the coupling between SRRs, the metal gap can be removed. The numerical results show that such direct coupling can produce similar effects with conventional indirect coupling in MIM waveguide systems, and the physics mechanism behind as well as influences of geometric parameters on transmission spectrum is also investigated. This work provides a simpler approach to realize on-chip plasmonic nanodevices, such as filters, sensors, and optical delay lines, in practice.

Silicon photonics is becoming the leading technology in photonics for a variety of new applications. However, due to the large volume and high cost of traditional optical devices, they are not suitable for high integration. It is challenging to further improve the integration and performance of silicon photonics. Here, we proposed a power splitter designed by inverse design algorithm has high transmission efficiency and compact structure size, which is helpful to the integration of photonic integrated circuit (PIC). The emergence of inverse design algorithm makes a great breakthrough in the problems existing in optical devices. In recent years, inverse design algorithms have attracted researchers' attention because of their ability to regulate light transmission by changing the refractive index distribution in the subwavelength structure. Direct-binary-search (DBS) algorithm, as the most commonly used inverse design algorithm, is applied to the design of on-chip photonic devices because of its simple working principle and high optimization efficiency. As one of the important components of photonic integrated circuits, on-chip power splitter plays an important role in optical communication system. Power splitters which can achieve any power ratio are widely used in optical interconnect devices. The traditional arbitrary power splitter can achieve different split ratios through different structures, but they can not achieve controllable split ratios in the same device, which is an obstacle to the integration of PIC. Phase change materials have been widely used in controllable photonic devices due to their unique optical properties. We combined the DBS algorithm to program and control the Ge2Sb2Se4Te1 (GSST), divided the whole device into multiple units, and optimized the design of each unit. Finally, the phase distribution in line with the target splitting ratio was obtained, and the high-efficiency and small-size power distributor was realized. A 3D finite-difference time-domain (FDTD) solution was used to simulate the device, and the TE0 mode light from the input waveguide was transmitted through the coupling region to the upper and lower output waveguides. Simulation results show that the device size is only 2.4 × 2.4 um2, and in the wavelength range of 1530 nm-1560 nm, the power split ratio of 1:1.5 and 1:2.5 is achieved. This method is helpful for the development of programmable integrated photonic interconnect devices.

In this paper, an optical fiber magnetic field and temperature sensor based on an optical microfiber coupler (OMC), Polydimethylsiloxane (PDMS), and magnetic fluid (MF) is proposed, and its magnetic field and temperature sensing characteristics are analyzed theoretically and verified experimentally. Based on the OMC and using MF as the sensing medium, the sensor can respond to the magnetic field and temperature respectively after encapsulated by PDMS. The experimental results show that the maximum magnetic field sensitivity is 96.8 pm/Oe, and the maximum temperature sensitivity is 919.1 pm/°C. To overcome the cross-sensitivity of the magnetic field and temperature of the sensor, the sensitivity matrix is established and demodulated. In addition, we discuss the optimization of the sensitivity demodulation matrix by the size design of the PDMS package and the OMC structure. The proposed two-parameter sensor in this article has the advantages of high sensitivity, low cost, small volume and high integration, which is of great significance for the multi-parameter sensing of basic physical parameters such as magnetic field and temperature.

Waveguide taper, a key component in the photonic integrated circuit (PIC), enables on-chip mode conversion, but large-footprint tapers are detrimental to the PIC, which desires compact and efficient devices. Polarization sensitivity also limits the tapers in the applications involving orthogonal modes. In this work, we design an efficient polarization-insensitive ultra-short MMI-based waveguide taper, through the mode spreading principle and the self-image principle. The proposed taper is 26.3 µm long, one order of magnitude shorter than the standard linear taper. We fabricate the taper, and experimentally demonstrate that it exhibits a high transmission efficiency of ∼ 70 % and a wide 1 dB bandwidth of > 54 n m , for both TE and TM polarizations.

In this paper, a novel and compact magnetic field sensor based on the combination of an optical microfiber coupler interferometer (OMCI) and magnetic fluid (MF) is proposed. The sensor is made up of an OMCI cover with polydimethylsiloxane (PDMS) and MF, and it uses MF as a material for adjusting the magnetic refractive index and magnetic field response. The sensing characteristics of the sensor are analyzed, and the experimental test is carried out. Under the condition of the same OMC waist length, the sensor sensitivity increases with the decrease of the OMC waist radius. The sensitivity of 54.71 and 48.21 pm/Oe was obtained when the OMC waist radius was set at 3.5 and 4 μm, respectively. In addition, we also tested the sensing response time and vector response characteristics of the sensor. At the same time, we discuss the demodulation idea about the cross-sensitivity of the magnetic field and temperature. The sensor has the advantages of high sensitivity, low cost, small size, optimized performance, and convenient integration. It has huge application potential in the fields of navigation and industrial intelligent manufacturing.

In order to meet the needs of phase generated carrier (PGC) demodulation technology for interferometric fiber optic hydrophones, we proposed an optical microfiber all-optical phase modulator (OMAOPM) based on the photo-induced thermal phase shift effect, which can be used as a phase carrier generation component, so as to make the modulation efficiency and working bandwidth of this type of modulator satisfy the requirements of underwater acoustic signal demodulation applications. We analyzed the modulation principle of this modulator and optimized the structural parameters of the optical microfiber (OM) when the waist length and waist diameter of OM are 15 mm and 1.4 μm, respectively. The modulation amplitude of the modulator can reach 1 rad, which can meet the requirements of sensing applications. On this basis, the fiber optical hydrophone PGC-Atan demodulation system was constructed, and the simulated underwater acoustic signal test demodulation research was carried out. Experimental results showed that the system can demodulate underwater acoustic signals below 1 kHz.

An ultra-compact multimode wavelength demultiplexer (MWD) with a footprint of 3.6 × 2.4 μm2 was designed. Its size is approximately two orders of magnitude smaller than the size of the traditional waveguide-based device. Moreover, the designed demultiplexer is multifunctional, and can realize wavelength demultiplexing and mode conversion at the same time. This MWD splits light at the wavelengths of 1450 nm and 1550 nm and simultaneously converts the input light from the fundamental transverse electric (TE0) mode to the first-order transverse electric (TE1) mode. The direct binary search (DBS) algorithm was employed to simulate the design. The results show that the insertion loss of the demultiplexer is −1.26 dB and −0.82 dB for the upper channel (wavelength 1450 nm) and lower channel (wavelength 1550 nm), while the crosstalk values are maintained as low as −18.10 dB and −23.35 dB, respectively. Additionally, a similar structure can be expanded to several modes. Based on these advantages, this multifunctional photonics device is expected to be used extensively in advanced photonic integration circuits.

We demonstrated a mode-locked fiber laser based on a novel photonic device that combined optical microfiber coupler (OMC) and saturable absorption materials. The stable ultrafast laser was formed based on the interaction between the deposited Indium Antimonide (InSb) and the evanescent field on OMC. Different from optical microfiber (OM), OMC can directly output the mode-locked laser without additional beam splitting devices, which further improves the integrated characteristics of the fiber laser. The pulse duration of the output pulse is 405 fs at the central wavelength of 1560 nm. To the best of our knowledge, this is the first time that optical microfiber coupler based saturable absorber (OMC-SA) for mode-locked fiber laser is demonstrated.

In this paper, we use the inverse design method to design an optical interconnection system composed of wavelength demultiplexer and the same direction waveguide crossing on silicon-on-insulator (SOI) platform. A 2.4 μm × 3.6 μm wavelength demultiplexer with an input wavelength of 1.3–1.6 μm is designed. When the target wavelength of the device is 1.4 μm, the insertion loss of the output port is − 0.93 dB, and there is − 18.4 dB crosstalk, in TE0 mode. The insertion loss of the target wavelength of 1.6 μm in TE0 mode is − 0.88 dB, and the crosstalk is − 19.1 dB. Then, we designed a same direction waveguide crossing, the footprint is only 2.4 μm × 3.6 μm, the insertion loss of the wavelength 1.4 μm and 1.6 μm in TE0 mode is − 0.99 dB and − 1 dB, and the crosstalk is − 12.14 dB and − 14.34 dB, respectively. Finally, an optical interconnect structure composed of two devices is used, which can become the most basic component of the optical interconnect network. In TE0 mode, the insertion loss of the output wavelength of 1.4 μm at the output port is − 1.3 dB, and the crosstalk is − 29.36 dB. The insertion loss of the output wavelength of 1.6 μm is − 1.39 dB, and the crosstalk is − 38.99 dB.

In recent years, metasurfaces have demonstrated a tremendous development potential and applicability in various fields as they have overcome the problems associated with traditional optical devices, including bulkiness and difficulties related to integration. The vortex beam can carry an unlimited amount of orbital angular momentum which can greatly increase the capacity of the communication system and thus attain a tremendous applicability potential in communication and information transmission. In this study, a vortex-focused beam metalens array is designed based on computer-generated holography technology for use in the visible light range. The antenna’s unit structure is arranged according to the phase distribution designed using computer-generated holography technology. The metalens array can convert circularly polarized light into a vortex beam and can focus the vortex beam. At the wavelengths of 420 nm, 550 nm, and 660 nm, the average conversion efficiency exceeds 80%, and the phase distribution is in the range of 0–2π. The metalens array can simultaneously generate and focus the vortex beams. We also studied the focusing effect of different vortex beams, and we analyzed and compared the focal length and full width at half maximum of metalens at different wavelengths and different topological charges. In addition, the focusing of the primary color light by the 3 × 3 metalens array is simulated. The design presented in this study is envisaged to be applied extensively in the fields of optical photography and optical communication systems.

A dielectric guided-mode resonance (GMR) notch filter with multiple channels was designed to operate in the mid-wave infrared (MWIR, 3 ∼ 4 μ m ). The transmission characteristics of filters were analyzed using a finite difference time domain (FDTD) method. In the structure of notch filter, two different periodic ( p 1 = 1 . 6 μ m and p 2 = 2 . 0 μ m) silicon (Si) grating waveguide layers (GWLs) are constructed on the CaF2 substrate. Si waveguide layer thickness and CaF2 cladding layer thickness were designed based on optical interference principle to give maximum transmission across the MWIR. The transmission spectrum presents two narrow band located at 3. 410 μ m and 3. 833 μ m with not only a high bandstop coefficient (>99%), but also a broadband high transmission (>90%) off-resonance. The resonant wavelengths can be flexible modulated in wide range by changing grating periods or filling factors, and it is proved that filter is stable and useful in actual atmospheric conditions. Because of the good performance in the bandstop features, the multiband GMR notch filter designed in this method may have a potential for a variety of imaging or detection applications.

An all optical fiber CTD sensor used in seawater based on an OMC is demonstrated. And the OMC is fabricated by fusing and tapering two twisted conventional communication fibers based on the improved flame-brushing method. By monitoring the sensing dips, in situ measurement of salinity, temperature, and depth (pressure) in seawater can be realized, respectively. Especially, the water depth (pressure) sensing function of the OMC is demonstrated for the first time, and the pressure sensitivity is 50 times higher than a bare fiber Bragg grating (with a wide pressure measurement range from 0 to 25 MPa). The highest sensitivities of salinity, temperature, and depth are 1596 pm/‰, 2326 pm/°C, and 169 pm/Mpa, respectively. These are high enough for the requirements of marine dynamic environmental monitoring and underwater target detection of fiber-optic hydrophone system. The all optical fiber CTD sensor demonstrated here show advantages of easy fabrication, compact structure, high sensitivity, low cost, and compatibility with the optical fiber system, which may find applications in multifunction optical sensors in ocean detections and other applications.

As one kind of organic semiconductor materials, Chlorophyll has been attracted great attention with remarkable optical properties. Here, we demonstrate photoresponse and thermoelectric current in the biological Chlorophyll A field effect phototransistors (FEpTs). Meanwhile, it exhibits excellent performances including high responsivity (5 × 103 A/W) and relatively fast response time (rise time ∼ 20.3 s, fall time ∼ 28.2 s) under illumination by 405 nm. Utilizing thermionic emission theory and typical Arrehenius plot, Schottky barrier Φ B is found to be 76.6 meV. The biological phototransistor via low-cost, simple, and scalable fabrication processes opens up new opportunities to realize flexible, transparent organic electronic, and biologically optoelectronic devices.

In this paper, we theoretically and experimentally investigate the all-optical modulation characteristics of a microfiber coupler combined Sagnac loop (MCSL). We inject pump lights with different intensities into the MCSL and utilize the thermo-optical effect from the microfiber coupler to control the phase relationship between the counter-propagating beams inside the Sagnac loop. The spectral characteristics from the Sagnac loop can be controlled accordingly, and the rate of the frequency comb (interference characteristic wavelength) shift can be 1 pm/mW. In addition, at a specific wavelength (e.g., 1550 nm), the MCSL can be used as an intensity modulator as well as a reflectivity-tunable Sagnac reflector. The reflectivity variation of the Sagnac reflector can reach about 10% when the pump light is at milliwatt level. The intensity modulation depth can be 9.3% at 100 Hz when the intensity-modulated 980 nm pump light imposed on the MCSL is 5.5 mW. Based on the thermo-optical effect, the MCSL can achieve the all-optical intensity modulation and all optically tunable filtering functions. The device is expected to play an important role in the development of tunable, multi-wavelength, and compact fiber lasers.

A transmission notch filter based on surface-plasmon resonance is proposed and numerically studied in the mid-infrared region. The metal structure is a dual-period grating constructed by using smaller-period gold arrays as the duty portions of a larger-period grating. The plasmonic filter shows high transmission of >85% over broad mid-infrared range, and a single notch at ∼ 4. 073 μ m with 24 dB extinction ratio. The stop bandwidth of the filter is ∼ 0.75 nm ( ∼ 0.45 cm−1), corresponding to the quality factor as high as 5400, which is improved by at least one order of magnitude. The rejection wavelength and bandwidth of the filter can be designed by adjusting the structure parameters. Additionally, the angle and polarization of the incidence and refractive index of the surrounding medium affect the performance of the filter.

We numerically and theoretically investigate a metal-insulator-metal (MIM) system based on aperture-coupled cascade resonators without gap to achieve plasmon-induced transparency (PIT). Such structure not only can be fabricated easily, but also possesses low intrinsic loss. Consequently, a high transmitted PIT peak more than 90% can be achieved, and this device also has good performance on sensing and slow light. Finally, multi-channel PIT and slow light can be achieved via cascade multiple resonators. This work provides a valuable approach to realize on-chip PIT in practical experiments.

Borophene, a new member of the two-dimensional material family, has been found to support surface plasmon polaritons in visible and infrared regimes, which can be integrated into various optoelectronic and nanophotonic devices. To further explore the potential plasmonic applications of borophene, we propose an infrared plasmonic sensor based on the borophene ribbon array. The nanostructured borophene can support localized surface plasmon resonances, which can sense the local refractive index of the environment via spectral response. By analytical and numerical calculation, we investigate the influences of geometric as well as material parameters on the sensing performance of the proposed sensor in detail. The results show how to tune and optimize the sensitivity and figure of merit of the proposed structure and reveal that the borophene sensor possesses comparable sensing performance with conventional plasmonic sensors. This work provides the route to design a borophene plasmonic sensor with high performance and can be applied in next-generation point-of-care diagnostic devices.

A novel and compact pressure sensor for marine application based on optical microfiber coupler interferometer(OMCI) is proposed theoretically. We theoretically analyzed the characteristics of OMCI's dual sensing unit, The highest pressure sensitivity of the waist sensing unit is 654pm/Mpa, and it increases with longer wavelength. The pressure sensitivity of the interference arm sensing unit is 7pm/Mpa, the sensitivity increases with the length of the embedded elastomer. Its excellent performance can provide strong theoretical guidance for the practical application of optical ocean monitoring equipment

In this paper, a metasurface Terahertz absorber based on the fractal technology of a graphene geometry resonator to realize ultra-wideband, ultrathin, adjustable double-layer cross-fractal formation is introduced. This paper proposes a dynamically tuned graphene absorbing material. The structure is composed of one- to four-level-fractal graphene pattern layers, MgF2 layers and metal reflective layers to form a two-sided mirror of an asymmetric Fabry&ndash, Perot cavity. To confine the terahertz electromagnetic wave, four different fractals are integrated into a supercell, and the coupling and superposition of adjacent resonant cavities form a broadband high-absorption absorber. Using finite element-based full-wave electromagnetic simulation software to simulate the response frequency of 0.4&ndash, 2.0 THz, we found that the absorber achieves a broadband 1.26 THz range (absorption &gt, 80%) and a relative bandwidth of 106.8%. By adjusting the Fermi energy, it can realize free switching and expand to wider broadband terahertz absorption, by adjusting the polarization angle (&Phi, ) from 0 to 90&deg, to prove that the structure is not sensitive to polarization, the absorber provides a 60&deg, large angle of incidence, polarization for TE and TM the absorption pattern remains basically the same. Compared with the previous work, our proposed structure uses fractal technology to expand the bandwidth and provide dynamic adjustable characteristics with great degrees of freedom. The appearance of the fractal structure reduces the difficulty of actual processing.

Inverse design is one of the most important design methods of nanophotonic devices. In recent years, with the rapid development of deep learning technique and applications, deep learning assisted inverse design method has been introduced into the field of nanophotonic device design. In this work, by combining the direct binary search method with multilayer convolutional neural networks, we present the inverse design of a wavelength demultiplexer which has 1352 design variables. The dropout strategy has been employed to avoid overfitting in training the inverse design model. The simulation results indicate that the trained CNN can both efficiently forward predict the spectrum and inverse design the structure.

The traditional optical system is composed of multiple optical elements, which will lead to the huge volume and high cost of the optical system, which can not meet the needs of people in terms of performance. The emergence of metasurface has made a great breakthrough in the problems existing in optical systems. In recent years, the metasurface has attracted the attention of researchers because it can breakthrough the limitation of traditional electromagnetic law and realize the modulation of light phase, amplitude and other parameters. In this paper, a kind of multiwavelength confocal plane multifocal metalens in visible wavelengths is designed. The design of metalens is realized by computer generation holography(CGH), and the phase distribution of the metalens is analyzed and calculated. Transmission metalens designed with all-dielectric materials is used and the imaging effect of confocal plane four-focal metalens at different wavelengths of visible wavelengths is analyzed. The surface phase characteristics of the multifocal metalens at different wavelengths were analyzed, and the Pancharatnam-Berry (P-B) phase was used to match the rotational phase of the unit cell structure with the phase of the surface of the metalens, and the confocal plane focusing at multiple wavelengths was successfully realized, with the focusing efficiency reaching 43%.

Magnetic field sensors have important application values in the fields of geophysical surveying and navigation, military equipment applications, biomedical sensing, and aerospace field. This paper proposes a magnetic field sensor based on side-polished fiber and microsphere. The sensor uses optical fiber microspheres as the light sensing platform and magnetic fluid as the magnetic field response and sensitizing material. The magnetic field sensor is made of a multi-mode side-polished optical fiber and SiO2 microspheres, which are put into a capillary package, and Fe3O4 magnetic fluid is injected into the capillary.We used COMSOL Multiphysics 5.4 based on finite element analysis to simulate the proposed sensor structure. Simulation results show that the magnetic field sensor has a maximum intensity sensitivity of 0.75dB / Oe, a refractive index sensitivity of 125nm / RIU, a Q-factor of 2.5x104, and the FOM of 2.5x103. Compared with the traditional magnetic fluid magnetic field sensor, the proposed sensor has a simple structure, low production cost, high Q-factor and high FOM. It has potential application prospects in developing magnetic field sensors compatible with high-sensitivity sensing and high-precision detection.

Metasurfaces, the two-dimensional artificial metamaterials, have attracted intensive attention due to their abnormal ability to manipulate the electromagnetic wave. Although there have been considerable efforts to design and fabricate beam steering devices, continuously tunable devices with a uniform bias-voltage have not been achieved. Finding new ways to realize more convenient and simpler wavefront modulation of light still requires research efforts. In this article, a series of novel reflective metasurfaces are proposed to continuously modulate the wavefront of terahertz light by uniformly adjusting the bias-voltage. By introducing the innovation of nonuniform periodic structures, we realize the gradient distribution of the reflected light phase-changing-rate which is the velocity of phase changing with Fermi energy. Based on strict phase distribution design scheme, a beam scanner and a variable-focus reflective metalens are both demonstrated successfully. Furthermore, dynamic and continuous control of either the beam azimuth of beam scanner or the focal length of metalens can be achieved by uniformly tuning the Fermi energy of graphene. Our work provides a potentially efficient method for the development and simplification of the adjustable wavefront controlling devices.

A method of coding patterns is proposed to achieve flexible control of absorption response at terahertz frequencies. The designed absorber consists of an Au-graphene pattern layer, a SiO2 layer and a metal reflective layer. Among them, we use concentrical circle structure to achieve broadband absorption, and adjust graphene&rsquo, s Fermi level to achieve tunable absorption. In addition, we propose an encoding method that can achieve flexible control of the absorption response at the terahertz frequency based on the external voltage applied on the graphene membrane, thereby having a programmable function. We also use COMSOL to simulate the electric field distribution diagram to explain the underlying physical mechanism. The programmable broadband adjustable absorber proposed in this paper has potential application prospects in the fields of optical equipment, information transmission, digital coding and artificial intelligence (AI).

A novel magnetic field sensor comprising a photonic crystal fiber (PCF) is designed and investigated based on surface plasmon resonance (SPR). We use finite element analysis in order to analyze the sensing characteristics of the magnetic field sensor. The simulation results show that the sensor is very sensitive to the change of refractive index and has good linearity in the refractive index range from 1.43&ndash, 1.45. The thickness of the metal film and the metal material has great influence on the resonance wavelength and the peak of the loss spectrum, the diameter of the central air hole will affect SPP excitation. When the thickness of gold layer is 50 nm, the refractive index sensitivity is 4125 nm/RIU in the refractive index range from 1.43&ndash, 1.45. Using the designed sensor for magnetic field sensing, the loss spectrum is red-shifted with the increase of the magnetic field, the highest magnetic field sensitivity can reach 61.25 pm/Oe in the range from 50 Oe to 130 Oe. The sensor not only has high sensitivity of refractive index, but it can also realize accurate measurement of magnetic field. It has huge application potential in complex environment, remote sensing, real-time monitoring, and other fields.

In this paper, we report a novel and compact sensor based on an optic microfiber coupler interferometer (OMCI) for seawater salinity application. The OMCI device is fabricated by connecting Faraday rotating mirrors to the two out-ports of the microfiber coupler, respectively. The sensor signal processing is based on a wavelength demodulation technique. We theoretically analyze the sensing characteristics with different device structure parameters. Besides, the results show that the date reading error decreases with the thinner waist region and longer arm difference. Through the experiment, the reflection spectra red-shifted as the sea water salinity increased; the highest response sensitivity of the OMCI salinity sensor reached 303.7 pm/‰ for a range of 16.6–23.8‰, and the resolution was less than 0.03‰. This study provides a new technical solution for the development of practical optical fiber seawater salinity sensors.

In this paper, a novel high-sensitivity temperature sensor with two sensing regions based on optical microfiber coupler interferometer (OMCI) for ocean application is proposed. The OMCI sensor is constructed by connecting Faraday mirrors to the two ports of the microfiber coupler respectively. Its sensing characteristics analysis and experimental test are conducted. Using a broad-spectrum light source as input light, temperature sensor demodulation can be implemented by tracking the drift of the characteristic wavelength (dip wavelength or peak wavelength) of the reflection spectrum. Experimental results show that the temperature sensitivity of the OMCI sensor can reach 1007.4 pm/&deg, C and the detection dynamic range up to 17.6 &deg, C. Besides, due to the two sensing regions in OMCI, self-calibration of seawater temperature sensing and optimization of multi-parameter cross-sensitive demodulation are performed by affecting the non-equal-arm interferometer through a specific package design of the external environment. The sensor has the advantages of high sensitivity, large dynamic range, small size, easy to manufacture, which will play an important role in the practical application of marine environment monitoring.

The subwavelength structure based on metamaterial has been widely used in the application of structure color due to its unique characteristics. Here, a composite structure consisting of a circular hole and different number of metal strips is proposed. By changing the polarization direction of the incident light, the resonance peak in the visible light range shifts, and the material presents different colors. At the same time, take advantage of the shift of the resonance peak, a plasmonic optical switch, of which on/off states can be indicated by the structural color, is constructed, and the maximum contrast ratio is more than 20 dB. In addition, by means of changing the geometric parameters and materials of the metal strip, the colors presented basically cover the whole visible light range. This method theoretically proves the feasibility of using subwavelength structure to construct visual plasmonic optical switch using structural color in the visible light range, which provides a broad prospect for the application of multiple physical mechanism in nanostructure design.

Optical measurement techniques are commonly used in order to inspect the thermal warpage behavior and the deformation of packages during reflow. JEDEC standard has offered four types of methods to measure the warpage: Confocal, Digital Image Correlation (DIC), Shadow Moire, and Digital fringe projection. In this paper, the recently emerged new confocal imaging system is introduced and compared with DIC method in different aspects.The objection of the comparison is not to decide which tool is better yet to provide a general guide for both advantages and disadvantages of two kinds of methods under different situations of applications. First, the same component is measured by confocal as well as DIC method in order to verify the accuracy of the new confocal inspection system. For this measurement, surface treatment is needed for DIC method yet the confocal method does not have such a requirement. Another application for confocal method is that except for the substrate warpage, it can also measure the bump top behavior like coplanarity during cycling, which DIC is not able to measure. However, the DIC is capable of inspecting the in-plane and out-of-plane deformation so that it has the CTE measurement function that confocal method does not possess.

The optical response of a metamaterial can be engineered by manipulating the size, pattern, and composition of its cells. Here, we present a coloring device, which increases resolution while retaining adjustability. By adding different nanoparticles in the nanohole, the shift of the transmission peak in the visible regions is realizable and manageable, which means a series of different colors are revealed in this device. At the same time, it is also possible to fill the holes with dielectric materials of different refractive indices to achieve the purpose of color diversity. This method theoretically confirms the feasibility of designing a coloring device via surface plasmons-based metamaterial nanostructure, which holds great promise for future versatile utilization of multiple physical mechanisms to render multiple colors in a simple nanostructure.

Using the principle of Surface Plasmons Polaritons, we propose a graphene broadband terahertz absorption structure that achieves the tunable absorption of electromagnetic waves. In our absorption structure, the method of patterning graphene is used to realize continuous broadband absorption from 0.5THz to 2.1THz. The absorption which is more than 50% reaches 1.1THZ, especially the structure designed here has three plasmonic resonance peaks which above 98% at 0.79THz, 1.18THz and 1.35THz, respectively. In addition, the symmetry in the pattern design consider that our absorption structure is not sensitive to the polarization and incident angle. Due to a series of excellent characteristics of the absorption structure, it may play an important role in the field of aircraft stealth, absorber, and light wave modulation.

In this paper, we combine inverse design concept and direct binary search algorithm to demonstrate three ultra-compact high efficiency and low crosstalk on-chip integrated optical interconnection basic devices in the entire wavelength range of 1,400–1600 nm based on silicon-on-insulator platform. A 90-degree waveguide bend with a footprint of only 2.4 × 2.4 μm2 is designed, whose transmission efficiency up to 0.18 dB. A waveguide crossing with a footprint of only 2.4 × 2.4 μm2 is designed, which can provide insertion loss of less than 0.5 dB and crosstalk (CL) of lower than − 19 dB. A same direction waveguide crossing with footprint of only 2.4 × 3.6 μm2 is designed, which can provide the insertion loss of less than 0.56 dB and the crosstalk of lower than − 21 dB. Then, we use them to form several ultra-compact optical interconnect basic structures and performed the simulation calculation. They overall achieve high performance. This will significantly improve the integration density.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Recently, phase-change materials (PCMs) have drawn more attention due to the dynamically tunable optical properties. Here, we investigate the active control of electromagnetically induced transparency (EIT) analogue based on terahertz (THz) metamaterials integrated with vanadium oxide (VO2). Utilizing the insulator-to-metal transition of VO2, the amplitude of EIT peak can be actively modulated with a significant modulation depth. Meanwhile the group delay within the transparent window can also be dynamically tuned, achieving the active control of slow light effect. Furthermore, we also introduce independently tunable transparent peaks as well as group delay based on a double-peak EIT with good tuning performance. Finally, based on broadband EIT, the active tuning of quality factor of the EIT peak is also realized. This work introduces active EIT control with more degree of freedom by employing VO2, and can find potential applications in future wireless and ultrafast THz communication systems as multi-channel filters, switches, spacers, logic gates and modulators.

In this paper, we design a stacked round hole disk absorber, the simulation and calculation are done by FDTD Solutions. We demonstrate absorber with ultra-wideband absorption in full spectrum (0.25µm-4µm). Due to plasmon resonance, the absorption rate of the absorber in the 3450 nm spectral range (0.25µm-3.7 µm) is more than 90% with an average absorption rate of 97.5%, and the average absorption rate is more than 99% in the visible light band. By comparing with the standard solar spectrum (0.28µm-4.0 µm) under AM 1.5, the absorber can effectively absorb sunlight and coincide with the standard solar spectrum. Subsequently, we study the effects of different structures and materials on the performance of absorber. The absorber is insensitive to incident light angle and polarization angle, and can be used under natural environment. Finally, we further study the influence of the hole shape on the absorber. The relevant conclusions of the research exhibits better performance in the field of solar energy collection and conversion.

As a plasmonic analogue of electromagnetically induced transparency (EIT), plasmon-induced transparency (PIT) has been widely studied duo to the excellent performance on sensing, slow light and enhancing nonlinear effects, which has great potential to be applied on compact optical communication system. Here, we introduce an all-optical PIT multi-channel switching based on the metal–insulator–metal (MIM) plasmonic waveguide device with small footprint. Single/double PIT is investigated both theoretically and numerically by multi-oscillator theory (MOT) and finite-difference time-domain (FDTD), and utilizing the nonlinear medium filled in the cavity, the PIT can be dynamically tunable to achieve various kinds of single/multi-channel switching for telecommunication wavelengths. Besides, utilizing the unprecedented high Kerr coefficient of graphene, the low-power and ultrafast all-optical switching can be made possible. This work provides a novel scheme to realize all-optical switching based on PIT, which can promote on-chip ultrafast optical communication and signal processing.

A nanoslit array is introduced on the silicon waveguide, and the phase difference is controlled by the slit width to satisfy the function of the focusing lens. If keep the designed width and depth of the slit in accordance with the focusing effect unchanged, when the incident wavelength changes, the focal position must change accordingly, and the dispersion effect is significant. In order to achieve the achromatic effect, the refractive index of the surrounding medium is changed while changing the wavelength. Finally, the refractive index of the surrounding medium which can keep the focal length constant at the wavelength of 1550-1950nm is obtained, and the equation that the change of the refractive index and the wavelength of the medium makes the focal length constant is obtained. The achromatic effect can be effectively achieved, and applications range of achromatic metalens from imaging in optical communications to telescopes in the astronomical field.

In this paper, we investigate the characteristics of all-optical modulation via thermo-optic effects of an optical microfiber coupler (OMC) and the theoretical analysis and experimental results are shown. The intensity modulation and the phase modulation efficiency are proportional to the length of the OMC' waist region, the pump light intensity, etc. We experimentally characterize the OMC, presenting that stable intensity and phase modulation can be realized by pumps at microwatts level. For OMC' intensity modulation, the modulation bandwidth is measured to be 50 kHz under 1.7 mW pump light intensity. We also demonstrate OMC's phase modulation function, and the frequency response characteristics of the OMC phase modulator shows similar properties with the OMC's intensity modulation, which mainly depends on the waveguide structures of the OMC' waist region and the heat conduction property of the waveguide materials. These results are valuable for the development of all-optical modulators and all-optical tunable filters based on subwavelength scales optical fiber, and hold great potential in laser and optical sensing applications.

A subwavelength fine-structured silicon&ndash, gold metagrating was designed for realizing mid-infrared (mid-IR) narrowband absorbers. The metagrating consisted of a silicon grating on the stack of a gold film and a quartz substrate. The silicon grating consisted of two periodically arranged silicon strips in each unit cell. The numerical results reveal that perfect absorption of the traverse-magnetic (TM) polarized light at a wavelength of 4.071 &mu, m can be achieved, with an absorption rate of ~99.2% and an absorption full-width at half-maximum (FWHM) bandwidth of ~31 nm. Thus, the proposed structure is useful for the spectral control of mid-IR signals. When used as a refractive index sensor, the structure has a measuring range of 1.0&ndash, 2.0 with a quasi linear response, with a figure of merit (FOM) of ~103.

In this paper, an on-chip polarizer passing the transverse-magnetic mode and a polarization beam splitter are proposed. The polarizer achieves an extinction ratio exceeding 20 dB within a spectral range from 1500 nm to 1610 nm, and has a value of 24dB at 1550 nm with a coupling efficiency of 84%. By employing a specific wavelength of 1570 nm, the polarizer behaves as the polarization beam splitter with extinction ratio for the transverse-electric mode and the transverse-magnetic mode of 29dB and 24 dB, respectively. In addition, the coupling efficiency of 95.98% and 78.18% for above two polarizations, demonstrate that our approach has a potential to efficiently control coupling splitting ratio via silicon subwavelength grating.

A hybrid metal–graphene metamaterial (MM) is reported to achieve active control of broadband plasmon-induced transparency (PIT) in the THz region. The unit cell consists of one cut wire (CW), four U-shaped resonators (USRs) and monolayer graphene sheets under the USRs. Via near-field coupling, broadband PIT can be produced through the interference between different modes. Based on different arrangements of graphene positions, not only can we achieve electrical switching of the amplitude of broadband PIT, but can also realize modulation of the bandwidth of the transparent window. Simultaneously, both the capability and region of slow light can be dynamically tunable. This work provides schemes to manipulate PIT with more degrees of freedom, which will find significant applications in multifunctional THz modulation.

The power splitter is a device that splits the energy from an input signal into multiple outputs with equal or uneven energy. Recently, the use of algorithms to intelligently design silicon-based photonic devices has attracted widespread attention. Thus, many optimization algorithms, which are called inverse design algorithms, have been proposed. In this paper, we use the Direct Binary Search (DBS) algorithm designed with three 1 × 3 power splitters with arbitrary directions theoretically. They have any direction and can be connected to other devices in any direction, which greatly reduces the space occupied by the optical integrated circuit. Through the simulation that comes about, we are able to get the insertion loss (IL) of the device we designed to be less than 5.55 dB, 5.49 dB, and 5.32 dB, separately. Then, the wavelength is 1530–1560 nm, so it can be used in the optical communication system. To discuss the impact of the footprint on device performance, we also designed another device with the same function as the second one from the above three devices. Its IL is less than 5.40 dB. Although it occupies a larger area, it has an advantage in IL. Through the design results, three 1 × 3 power splitters can be freely combined to realize any direction, multi-channel, ultra-compact power splitters, and can be better connected with different devices to achieve different functions. At the same time, we also show an example of a combination. The IL of each port of the combined 1 × 6 power splitter is less than 8.82 dB.