Search

Herein a new class of optoelectronic devices beneficial for infrared light absorption and high‐frequency application in the terahertz frequency domain are designed and fabricated. The devices are formed by coating a highly transparent thin layer of Nb2O5 onto a selenium‐thin film to form Se/Nb2O5 (SNO) optical interfaces. Although coating of Nb2O5 nanosheets decreased the crystallite sizes and increased the strain and defect concentration in the hexagonal structured Se films, they successfully increased the light absorption by ≈148% in the infrared range of light. A blueshift in the energy band gap of Se from 2.02 to 2.30 eV is observed. The coating of the Nb2O5 onto Se suppressed the free carrier absorption in Se and Nb2O5. As dielectric active layers, SNO interfaces showed a major resonance dielectric peak centered at 1.67 eV. The optical conductivity and terahertz cutoff frequency analyses which are handled using the Drude‐Lorentz approach revealed the highest drift mobility and free carrier concentration of 17.17 cm2 Vs−1 and 5.0 ×1017$ \times \ {{10}^{17}}$ cm−3 when an oscillator of energy of 1.75 eV is activated. In addition, the terahertz cutoff frequency spectra which varied in the range of 4.0–131 THz showed the suitability of the SNO devices for terahertz technology and other optoelectronics. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS2) possesses excellent potential for applications in the field of optoelectronic detection. However, the atomic-level thickness of the monolayer MoS2 leads to weak light absorption and a restricted absorption spectrum. The performance of monolayer MoS2 devices has reached a bottleneck. Fortunately, the above issues can be effectively solved by coupling with various types of photosensitivity nanostructures. In this work, we integrated MoS2 quantum dots (QDs) with high efficient light absorption with monolayer MoS2 to fabricate 0D/2D MoS2 QDs/MoS2 hybrid dimensional homojunction photodetectors. In this structure, MoS2 is used as an efficient carrier transport channel, while MoS2 QDs act as effective light absorbers to enhance the local electric field around MoS2. The synergistic effect of MoS2 QDs and MoS2 is utilized to accelerate the migration rate of photogenerated carriers in the structure, and in particular, the highest responsivity of the MoS2 QDs/MoS2 hybrid device is 27.6 A W−1 with the detectivity as high as 2.13 × 1011 Jones under 532 nm laser, which is an order of magnitude higher than that of the pristine MoS2 devices. The synergistic effect of MoS2 QDs with monolayer MoS2 is verified by finite-difference time-domain simulation. The results will pave the way for the future development of high-performance MoS2-based photodetectors. [ABSTRACT FROM AUTHOR]

In this study, semitransparent lead films serve as substrates for depositing niobium pentoxide thin films, forming versatile electro‐optical devices. Using vacuum evaporation and ion sputtering techniques at ≈10−5 mbar, stacked layers of crystalline Pb and amorphous Nb2O5 are created. This process reduces free carrier absorption in Nb2O5 and forms Urbach tail states with a width of 0.91 eV. Pb/Nb2O5 thin films exhibit remarkable broadband absorption, exceeding 440% in the visible and 98% in the infrared. Moreover, Pb substrates induce a redshift in Nb2O5's energy bandgap. Electrical analysis using impedance spectroscopy on Pb/Nb2O5/Ag structures reveals their series/parallel resonance and bandstop filter properties. Notably, the bandstop filters exhibit reflection coefficient minima at a notch frequency of 1.66 GHz, with a bandwidth of 280 MHz, return loss of 26 dB, and voltage standing wave ratio of 1.13. These findings underscore the device's potential for wide‐ranging electro‐optical applications across the electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

Cadmium sulfide thin films are deposited using the thermal evaporation technique and coated with 50 nm thick Sb nanosheets. Both the coated and uncoated films undergo structural, morphological, and optical investigations to explore potential modifications resulting from the Sb coating. The antimony nanosheets successfully increase the crystallite sizes from 28 to 34 nm and decrease the defect concentration from 4.36 × 1011 lines/cm2 to 3.02 × 1011 lines/cm2. The deposition of Sb nanosheets on CdS films induces the formation of nanowires with a length of 3.0 μm. Sb nanosheet coatings improve visible light absorption by more than nine times, redshift the energy band gap, suppress free carrier absorption in the infrared (IR) range, and increase the optical conductivity of CdS films. Additionally, as optical filters and waveguides, Sb-coated CdS films exhibit a terahertz cutoff frequency range of 0.47–32.06 THz as light energy increases from the IR to ultraviolet ranges. It is also observed that Sb coating alters the third-order nonlinear susceptibility of CdS, making it tunable, more polarizable, and suitable for nonlinear optical applications. Photocurrent measurements showed that Sb nanosheets improved the light responsivity by 330% and significantly enhanced the response time. The enhanced features of CdS achieved through Sb nanosheet coatings position CdS in the preferred group for nonlinear optical waveguides applicable in terahertz and nonlinear optical applications. [ABSTRACT FROM AUTHOR]

Background: Cetirizine is a second-generation antihistamine with anti-allergy and anti-itching properties. The topical formulation of this medicine is used in androgenic alopecia treatment. Due to the hydrophilic nature of cetirizine, its skin absorption is negligible, so to increase its absorption, various enhancers were examined to see which can be used in the design of a topical formulation. Methods: First, the skin was exposed to enhancers, including eucalyptus, menthol, Tween 80, propylene glycol, and oleic acid, for 1 or 2 hours. Then, the permeability parameters of the cetirizine solution and the structural changes of the skin after exposure to enhancers were analyzed by differential scanning calorimetry (DSC) and Fourier-transform infrared spectroscopy (FTIR) techniques. Results: The obtained results show that all used enhancers increased the permeability of the drug cetirizine compared to water. Various mechanisms, such as liquefaction of lipids, destruction of lipid structure, and irreversible denaturation of intracellular keratin, are involved in the increase in drug penetration caused by eucalyptus, mint, Tween 80, propylene glycol, and oleic acid. Conclusions: The results showed that among the studied absorption enhancers, eucalyptus and Tween 80 had the strongest, and propylene glycol had the weakest absorption enhancement effect after 2- and 1-hour pre-contact, respectively. [ABSTRACT FROM AUTHOR]

The quantum efficiency of an InAs/InAsSb type-II superlattice (T2SL) high operating temperature (HOT) long-wavelength infrared (LWIR) photodetector may be significantly improved by integrating a two-dimensional subwavelength hole array in a metallic film (2DSHA) with the detector heterostructure. The role of the metallic grating is to couple incident radiation into surface plasmon polariton (SPP) modes whose field overlaps the absorber region. Plasmon-enhanced infrared photodetectors have been recently demonstrated and are the subject of intensive research. Optical modelling of the three-dimensional detector structure with subwavelength metallic components is challenging, especially since its operation depends on evanescent wave coupling. Our modelling approach combines the 3D finite-difference time-domain method (FDTD) and the rigorous coupled wave analysis (RCWA) with a proposed adaptive data-point selection for calculation time reduction. We demonstrate that the 2DSHA-based detector supports SPPs in the Sommerfeld-Zenneck regime and waveguide modes that both enhance absorption in the active layer. [ABSTRACT FROM AUTHOR]

Tailoring critical light‐matter coupling is a fundamental challenge of nanophotonics, impacting fields from higher harmonic generation and energy conversion to surface‐enhanced spectroscopy. Plasmonic perfect absorbers (PAs), where resonant antennas couple to their mirror images in adjacent metal films, excel at obtaining different coupling regimes by tuning the antenna‐film gap size. However, practical PA applications require constant gap size, making it impossible to maintain critical coupling beyond singular wavelengths. Here, a new approach for plasmonic PAs is introduced by combining mirror‐coupled resonances with the unique loss engineering capabilities of plasmonic quasi‐bound states in the continuum. This novel combination allows to tailor the light–matter interaction within the under‐coupling, over‐coupling, and critical coupling regimes using flexible tuning knobs including asymmetry parameter, dielectric gap, and geometrical scaling factor. The study demonstrates a pixelated PA metasurface with optimal absorption over a broad range of mid‐infrared wavenumbers (950–2000 cm−1) using only a single gap size and applies it for multispectral surface‐enhanced molecular spectroscopy. Moreover, the asymmetry parameter enables convenient adjustment of the quality factor and resonance amplitude. This concept expands the capabilities and flexibility of traditional gap‐tuned PAs, opening new perspectives for miniaturized sensing platforms towards on‐chip and in situ detection. [ABSTRACT FROM AUTHOR]

The enhancement of light absorption in an ultrathin two-dimensional material is critical for its optoelectronic and photonic applications. In this work, we investigated the enhanced light absorption of layered PtSe2 by engineering the optical impedance and the attenuation of the PtSe2-based metal–insulator–metal (MIM) absorber. For a monolayer PtSe2-based MIM absorber, the undesirable impedance mismatch can be compensated for using the top patterned metal array in the MIM structure to achieve near-perfect absorption (99.95%), and the absorption of monolayer PtSe2 is enhanced by 8.6 times in the visible spectra. For a few-layer PtSe2 MIM absorber, the self-impedance of the PtSe2 layer becomes an important factor in modulating the optical absorption and the PtSe2-based absorbers show excellent features with broadband absorption, insensitive to the incident angle and polarization. Our results improve the viability of the PtSe2-based optoelectronic and photonic devices and shed light on the design of absorbers with hybrid 2D materials. [ABSTRACT FROM AUTHOR]

Herein amorphous WO3 thin films prepared by the thermal evaporation technique under a vacuum pressure of 10−5 mbar are treated via pulsed laser welding technique (PLW) in an argon atmosphere. An induced crystallization process within one second for a spot of diameter of 2 mm is achieved via this technique. The crystalline films exhibited hexagonal structure and showed cuboid grains of dimensions of (length × width) 4.6 × 1.1 (μm)2. In addition, the PLW treated tungsten oxide thin films exhibited enhanced light absorbability reaching 90 times at 3.09 eV. Moreover in addition to the existing indirect energy band gap (3.50 eV) of WO3 films, the PLW treatment resulted in formation of another direct energy band gap of value of 1.92 eV. The pulsed laser welding of the WO3 films increased the optical conductivity of the films by 66 times and shifted the plasmon frequency from 0.19 to 6.99 GHz. The free charge carrier density also increased by two orders of magnitude. On the other hand, the impedance spectroscopy studies have shown domination of negative capacitance effect in samples treated with PLW technique. The conversion of the structure of the films from amorphous to polycrystalline that is associated with enhanced light absorption and improved optical conduction which is achieved within 41 s for a large area of 1.25 cm2 of the films is accounted as a smart new approach for improving the physical properties of thin films. [ABSTRACT FROM AUTHOR]

Highly sensitive detection of enhanced terahertz (THz) fingerprint absorption spectrum of trace-amount tiny molecules is essential for biosensing. THz surface plasmon resonance (SPR) sensors based on Otto prism-coupled attenuated total reflection (OPC-ATR) configuration have been recognized as a promising technology in biomedical detection applications. However, THz-SPR sensors based on the traditional OPC-ATR configuration have long been associated with low sensitivity, poor tunability, low refractive index resolution, large sample consumption, and lack of fingerprint analysis. Here, we propose an enhanced tunable high-sensitivity and trace-amount THz-SPR biosensor based on a composite periodic groove structure (CPGS). The elaborate geometric design of the spoof surface plasmon polaritons (SSPPs) metasurface increases the number of electromagnetic hot spots on the surface of the CPGS, improves the near-field enhancement effect of SSPPs, and enhances the interaction between THz wave and the sample. The results show that the sensitivity (S), figure of merit (FOM) and Q-factor (Q) can be increased to 6.55 THz/RIU, 4234.06 1/RIU and 629.28, respectively, when the refractive index range of the sample to measure is between 1 and 1.05 with the resolution 1.54 × 10 − 5 RIU. Moreover, by making use of the high structural tunability of CPGS, the best sensitivity (SPR frequency shift) can be obtained when the resonant frequency of the metamaterial approaches the biological molecule oscillation. These advantages make CPGS a strong candidate for the high-sensitivity detection of trace-amount biochemical samples. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide (MoS 2 ) possesses excellent potential for applications in the field of optoelectronic detection. However, the atomic-level thickness of the monolayer MoS 2 leads to weak light absorption and a restricted absorption spectrum. The performance of monolayer MoS 2 devices has reached a bottleneck. Fortunately, the above issues can be effectively solved by coupling with various types of photosensitivity nanostructures. In this work, we integrated MoS 2 quantum dots (QDs) with high efficient light absorption with monolayer MoS 2 to fabricate 0D/2D MoS 2 QDs/MoS 2 hybrid dimensional homojunction photodetectors. In this structure, MoS 2 is used as an efficient carrier transport channel, while MoS 2 QDs act as effective light absorbers to enhance the local electric field around MoS 2 . The synergistic effect of MoS 2 QDs and MoS 2 is utilized to accelerate the migration rate of photogenerated carriers in the structure, and in particular, the highest responsivity of the MoS 2 QDs/MoS 2 hybrid device is 27.6 A W -1 with the detectivity as high as 2.13 × 10 11 Jones under 532 nm laser, which is an order of magnitude higher than that of the pristine MoS 2 devices. The synergistic effect of MoS 2 QDs with monolayer MoS 2 is verified by finite-difference time-domain simulation. The results will pave the way for the future development of high-performance MoS 2 -based photodetectors., (© 2024 IOP Publishing Ltd. All rights, including for text and data mining, AI training, and similar technologies, are reserved.)

Absorbers such as photonic crystals, metasurfaces, and nanowires have been demonstrated to have important applications in solar cells, detectors, etc. The application of thin-layer-based absorbers to photodetectors is beneficial to simultaneously improve responsivity and 3 dB bandwidth. In this work, we propose a high-contrast-gratings-like absorber to achieve ultra-high absorption based on a thin Germanium layer. High absorption approximately 95% at wavelength 1310 nm is reached for TE-polarized incidence due to the interference of two guided modes, which is 4 times compared to the planar film with the same thickness. The high absorption of the absorber is also experimentally demonstrated. This work shows a new method for enhanced absorption with thin Germanium layer, indicating a promising prospect for high-efficiency photodetection.

Absorbers such as photonic crystals, metasurfaces, and nanowires have been demonstrated to have important applications in solar cells, detectors, etc. The application of thin-layer-based absorbers to photodetectors is beneficial to simultaneously improve responsivity and 3 dB bandwidth. In this work, we propose a high-contrast-gratings-like absorber to achieve ultra-high absorption based on a thin Germanium layer. High absorption approximately 95% at wavelength 1310 nm is reached for TE-polarized incidence due to the interference of two guided modes, which is 4 times compared to the planar film with the same thickness. The high absorption of the absorber is also experimentally demonstrated. This work shows a new method for enhanced absorption with thin Germanium layer, indicating a promising prospect for high-efficiency photodetection. [ABSTRACT FROM AUTHOR]

Plasmonic photodetection based on the hot-electron generation in nanostructures is a promising strategy for sub-band detection due to the high conversion efficiencies; however, it is plagued with the high dark current. In this paper, we have demonstrated the plasmonic photodetection with dark current suppression to create a Si-based broadband photodetector with enhanced performance in the short-wavelength infrared (SWIR) region. By hybridizing a 3 nm Au layer with the spherical Au nanoparticles (NPs) formed by rapid thermal annealing (RTA) on Si substrate, a well-behaved ITO/Au/Au NPs/n-Si Schottky photodetector with suppressed dark current and enhanced absorption in the SWIR region is obtained. This optimized detector shows a broad detection beyond 1200 nm and a high responsivity of 22.82 mA/W at 1310 nm at −1 V, as well as a low dark current density on the order of 10−5 A/cm2. Such a Si-based plasmon-enhanced detector with desirable performance in dark current will be a promising strategy for realization of the high SNR detector while keeping fabrication costs low. [ABSTRACT FROM AUTHOR]

Monolayer (1L) transition metal dichalcogenides (TMDCs) have been attracting tremendous interest in recent years as promising candidate materials in atomic-scale optoelectronic devices due to their direct band gaps (1.5–2.2 eV) and strong light—matter interactions. Unfortunately, their practical applications are limited by low visible light absorption stemming from atomic thickness and negligible infrared response. Here, we report the triangular Sb2O3 microresonators in wide thickness and lateral size distributions grown on 1L TMDCs and their created significant broadband enhancement of light adsorption and photoresponse in 1L WSe2 crystal via coexisting Fabry—Perot and whispering gallery type resonances. As an example of demonstration, 1L WSe2 crystal coupled to Sb2O3 microresonators with widely distributed sizes exhibits the enhanced visible light absorption by up to 5 folds and the simultaneously extended near infrared (NIR) one of more than 50%. For application of 1L WSe2 in photodetection, incorporation of Sb2O3 microresonators leads to significantly enhanced visible light responsivity by ∼ 104 order and expanded NIR one of more than 400 mA·W−1. Similar results have been observed in the other 1L W(Mo) dichalcogenides coupled to Sb2O3 microresonators. This work provides a new route for development of the high-performance monolayer TMDCs-based optoelectronic devices. [ABSTRACT FROM AUTHOR]

Sensitive detection of terahertz fingerprint absorption spectrum for tiny molecules is essential for bioanalysis. However, it is extremely challenging for traditional terahertz spectroscopy measurement because of the weak spectral response caused by the large mismatch between terahertz wavelengths and biomolecular dimensions. Here, we proposed a wideband-tunable metal plasmonic terahertz biosensor to detect tiny biomolecules, employing attenuated total reflection in an Otto prism configuration and tightly confined spoof surface plasmons on the grooved metal surface. Benefitting from the plasmonic electric field enhancement, such a biosensor is able to identify the molecular terahertz fingerprints. As a proof of concept, a hypothetical molecule modeled by the Lorentz model with two vibrational modes is used as the sensing analytes. Simulation results show that the absorption of two vibrational modes of analytes can be selectively enhanced up to ten times by plasmonic resonance, and their fingerprints can be resolved by sweeping incident angle in a wide waveband. Our work provides an effective approach for the highly sensitive identification of molecular fingerprints in fields of biochemical sensing for tiny analytes. [ABSTRACT FROM AUTHOR]

Highly sensitive detection of enhanced terahertz (THz) fingerprint absorption spectrum of trace-amount tiny molecules is essential for biosensing. THz surface plasmon resonance (SPR) sensors based on Otto prism-coupled attenuated total reflection (OPC-ATR) configuration have been recognized as a promising technology in biomedical detection applications. However, THz-SPR sensors based on the traditional OPC-ATR configuration have long been associated with low sensitivity, poor tunability, low refractive index resolution, large sample consumption, and lack of fingerprint analysis. Here, we propose an enhanced tunable high-sensitivity and trace-amount THz-SPR biosensor based on a composite periodic groove structure (CPGS). The elaborate geometric design of the spoof surface plasmon polaritons (SSPPs) metasurface increases the number of electromagnetic hot spots on the surface of the CPGS, improves the near-field enhancement effect of SSPPs, and enhances the interaction between THz wave and the sample. The results show that the sensitivity (S), figure of merit (FOM) and Q-factor (Q) can be increased to 6.55 THz/RIU, 4234.06 1/RIU and 629.28, respectively, when the refractive index range of the sample to measure is between 1 and 1.05 with the resolution 1.54×10−5 RIU. Moreover, by making use of the high structural tunability of CPGS, the best sensitivity (SPR frequency shift) can be obtained when the resonant frequency of the metamaterial approaches the biological molecule oscillation. These advantages make CPGS a strong candidate for the high-sensitivity detection of trace-amount biochemical samples.

Terahertz (THz) detecting technology has attracted many attentions. In this work, a novel THz detector based on a metal gratings/Si-based blocked-impurity-band (BIB) hybrid structure was designed and theoretically calculated. The highly integrated hybrid structure was found useful for modulating the response wavelength and improving the responsivity of Si-based BIB detector. The enhanced THz absorption of the hybrid device structure of metal gratings and Si-based BIB absorption layer was carefully investigated by finite difference time domain. The peak wavelength can be moved from 26.9 to 36.53 μm when metal gratings (period: 32 μm) were added on the Si-based BIB. The peak absorptivity of the hybrid detector was increased by 87.3 % with the metal gratings, compared with that of conventional Si-based BIB detector. Our results indicate that a highly integrated hybrid device structure can replace the divided band filters, and realize selective resonant enhanced absorption and tunable THz spectral response. [ABSTRACT FROM AUTHOR]

Herein, the effects of insertion of Ag2O nanosheets of thicknesses 25–75 nm between stacked layers of GeO2 on the structural, morphological, and optical properties of germanium dioxide are explored. While the stacking of GeO2/Ag2O/GeO2 layers does not alter the amorphous nature of the structure, significant effects on the transmittance, reflectance, absorption coefficient, Urbach's tails, and dielectric constant are observed. Silver oxide nanosheets successfully enhance the light absorbability of germanium dioxide in the visible and the infrared (IR) range of light. The light absorbability is increased by more than 12 times after the insertion of 75 nm‐thick Ag2O nanosheets. In addition, a widening in Urbach's tails and narrowing in the indirect bandgap of GeO2 are detected as a result of increasing Ag2O layer thickness. The dielectric constant and optical conductivity are also increased by ≈59.6% and 191.4% in the IR range of light. The enhancement in the optical properties of GeO2 that results from the insertion of Ag2O nanosheets makes germanium dioxide more appropriate for optoelectronic applications. [ABSTRACT FROM AUTHOR]

Plasmonic photodetection based on the hot-electron generation in nanostructures is a promising strategy for sub-band detection due to the high conversion efficiencies; however, it is plagued with the high dark current. In this paper, we have demonstrated the plasmonic photodetection with dark current suppression to create a Si-based broadband photodetector with enhanced performance in the short-wavelength infrared (SWIR) region. By hybridizing a 3 nm Au layer with the spherical Au nanoparticles (NPs) formed by rapid thermal annealing (RTA) on Si substrate, a well-behaved ITO/Au/Au NPs/n-Si Schottky photodetector with suppressed dark current and enhanced absorption in the SWIR region is obtained. This optimized detector shows a broad detection beyond 1200 nm and a high responsivity of 22.82 mA/W at 1310 nm at −1 V, as well as a low dark current density on the order of 10−5 A/cm2. Such a Si-based plasmon-enhanced detector with desirable performance in dark current will be a promising strategy for realization of the high SNR detector while keeping fabrication costs low.

In the field of modern optical communication systems and photoelectric detection, new components with complex functions and excellent performance are urgently needed. In this paper, a graphene-based parity–time (PT) symmetry structure is proposed, which is achieved by preparing the graphene layer on the top of a PT-symmetry photonic crystal. The transfer matrix method was used to calculate the absorptance of graphene, and a unique amplified absorption effect was found. Meanwhile, the peak value and wavelength position of the absorption can be modulated via the applied electric field. The results show that by adjusting the negative square-wave electric field from −3.5 × 10−5 to −13.5 × 10−5 V/nm (or the positive square-wave electric field from 2 × 10−5 to 11 × 10−5 V/nm), the proposed structure can achieve in-phase (or out-phase) enhanced absorption for the communication wavelength 1550 nm, with the absorption of graphene from 17 to 28 dB (or 30 to 15 dB) corresponding to the square-wave modulation electric field change. The modulable absorption properties of graphene in the structure have potential in optoelectronic devices and optical communication systems.

Tellurium (Te)-doped black silicon (Si) with enhanced absorption and photoelectric performance over a broad wavelength range of 0.2-2.5 μm was obtained using femtosecond (fs) laser irradiation in liquid water. Prior to laser irradiation, the Si sample was covered with a Te thin film (thickness 200 nm) over an adhesion layer of Cr (thickness 5 nm). Surface analyses by scanning electron microscopy and three-dimensional confocal microscopy evidence the presence of hierarchical surface structures combining quasi-periodic stripes with a spatial period of about 5 μm and subwavelength laser-induced periodic surface structures directed in directions parallel and perpendicular to the direction of the laser polarization, respectively. Moreover, the incorporation of Te generates intermediate levels within the Si bandgap. The Te-doped black Si shows a significant enhancement of the absorption, which reaches values of about 48% in the UV and visible (0.2-1.1 μm) and 70% in the near-infrared (1.1-2.5 μm) spectral ranges, respectively, due to the synergistic effects of multiscale surface structures and Te incorporation. Moreover, the surface reflectance is reduced to almost zero across the entire spectrum. The Te-doped black Si sample is used to realize a photodetector which displays an impressive photoelectric capability, being characterized by a responsivity of 328 mA/W, and an external quantum efficiency of 49.27% at a voltage bias of -10 V for 1064 nm light illumination, with rising and falling times of 55 and 67 ms, respectively. These figures remarkably outperform the response of unprocessed Si under the same experimental conditions.

The terahertz (THz) absorption properties of a gated graphene monolayer placed on top of a one-dimensional photonic crystal is investigated in the presence of a perpendicular magnetostatic bias. The response of electrons to the magnetic field is inspected in the quantum regime, due to the low doping level of graphene grown on the C-terminated surface of silicon-carbide. It has been shown that there is the possibility of achieving enhanced absorption at low magnetic fields for certain states of circular polarization of light. Furthermore, adjusting the gate voltage of the graphene provides another method of tuning absorption in the proposed structure. Therefore, one can obtain enhanced absorption with the appropriate choices of magnetic and electric biases. We believe that these properties make our structure suitable for designing tunable graphene-based THz absorbers. [ABSTRACT FROM AUTHOR]

The wavevector of electromagnetic wave propagation in a hexagonal boron nitride (hBN) slab can be controlled by tilting its optical axis. This property can be used to manipulate the absorption in a hBN slab. By carefully analyzing the dependence of the absorptivity of a thin hBN slab on the tilted angle of its optical axis, we propose a structure that can realize great absorptivity enhancement in a band by stacking hBN slabs of different tilted angles. Our numerical results show that the absorptivity of a structure made of 91 stacked hBN slabs can be achieved higher than 0.94 in the wavenumber range from 1367 to 1580 cm −1 when the tilted angles of the slabs are properly arranged. The strong absorption is attributed to the combination of impedance matching at the slab interfaces and enlarged wavevectors in the slabs. This work reveals a novel way to realize strong absorption with anisotropic materials for applications in areas such as thermal radiative energy harvesting and conversion. [ABSTRACT FROM AUTHOR]

We present the design and realization of a graphene-based absorber with nearly 60% absorption efficiency using the encapsulated cascaded gratings (ECGs). The absorption locations can be estimated by using the dispersion relation of the ECGs, and it is confirmed that the absorption of the graphene-based ECGs is resulted from the excitation of the guided mode resonance (GMR). The absorption location is almost immune to the variation of the thickness of the top grating, and the absorption location is red-shifted with the increase of the fill factor. The shift of the absorption peak can be adjusted to the design value by tuning the incident angle as the grating period is altered. Multiple absorption bands can be achieved by increasing the thickness of the bottom grating. The absorption properties can be tailored based on the dispersion diagram of the ECGs structure. [ABSTRACT FROM AUTHOR]

In this paper, we predict unexpected optical absorption in an optical thick free-standing transmission metallic grating by introducing periodical back grooves into it. The absorption peak could be very high up to 90%. In the structure, the introduced periodical back grooves bring about different orders of transmission minima into the transmission spectrum, which are named as transmission minima introduced by back grooves (TMIBBGs) here. When TMIBBGs approach the Fabry-Pérot (FP)-like resonant transmission peaks supported in the slits, the latter will be strongly suppressed. However, the reflection dips are almost unaffected. As a result, absorption is greatly enhanced. More interesting, the positions of TMIBBGs can be tuned by modulating the structural parameters of back grooves to selectively inhibit different orders of FP-like resonant transmission peaks and realize corresponding enhanced absorption, meanwhile, without influence to other orders of FP-like resonant transmissions. Moreover, due to the asymmetry of the structure, the enhanced absorption is unidirectional. The physical mechanisms are qualitatively and quantitatively analyzed in this paper.

This paper theoretically shows that nearly perfect absorption can be achieved by adjusting the period numbers of the two graphene based photonic crystals surrounding a lithium niobate (LiNbO 3 ) layer. Since LiNbO 3 is an electro-optical (EO) material with voltage dependent refractive index and high EO coefficient, peak wavelength tunability of absorption has been revealed. Our investigations show that the relation between the peak wavelength and applied external voltage is linear, so that the peak wavelength moves toward shorter wavelengths by increasing the applied external voltage. Moreover, it is possible to achieve multi-peak by varying the thickness of LiNbO 3 defect layer. Finally, we discuss the possibility of control of the absorption via controllable parameter (chemical potential) of the graphene. The results indicate that for a given structure, i.e, definite period numbers surrounding the LNO layer, the enhanced absorption can be achieved, by adjusting the chemical potential. [ABSTRACT FROM AUTHOR]

We demonstrate a broadband absorber using random structures on refractory plasmonic material. The random microstructure is fabricated by femtosecond laser on tungsten and characterized with surface roughness which described by root mean square (RMS) and correlation length. Results show that the absorption efficiency of random microstructure with RMS of 0.8 μm and correlation length of 0.55 μm is over 90 % in the wavelength range from 200 to 1100 nm. However, the sample with surface structure RMS of 0.08 μm has much lower absorption (less than 70 % for λ > 600 nm). Numerical simulations agree well with the experimental results and illustrate that the structure with 0.8-μm RMS and 0.55-μm correlation length has the cut-off wavelength of 2400 nm which prevents mid-infrared emission. The possibility of realizing broadband absorption by using random structures presents a flexible and efficient way for solar cell, thermophotovoltaics, and energy harvesting. [ABSTRACT FROM AUTHOR]

In this work, a rapid, replicable method for imprinting concave nanostructures to be used as functional light-trapping nanostructures in organic thin-films is presented. Porous anodic alumina templates were fabricated both by anodization of thick Al foils and by anodization of submicrometer thin Al films evaporated via e-beam evaporation on Si substrates. The template formation leads to natural patterning of the underlying Al layers that are used as rigid masters for stamp fabrication, after selective etching of the porous anodic alumina. PDMS stamps were made after replicating the Al concave patterns and used for imprinting of spin coated photoresist on glass substrates. We have investigated semiperiodic and aperiodic imprinted large concave patterns fabricated from rigid masters after anodization of Al in H3PO4. We show that metal covered imprinted concaves show enhancement in absorption that is attributed to field enhancement and diffuse scattering, leading to efficient light trapping for a selected active layer material (P3HT:PCBM). [ABSTRACT FROM AUTHOR]

Constructing semiconductor photocatalysts with enhanced absorption and rapid carrier dynamics is essential for applying semiconductor photocatalysis in environmental remediation and energy conversion. In this study, we modified carbon quantum dots (CQDs) onto the surface of Bi 4 O 5 Br 2 using a one-step hydrothermal method. The modification enabled the excellent degradation of bisphenol A (BPA) due to the up-conversion luminescence of CQDs and the Z-scheme charge transfer path formed by the unique energy level of the two materials. The CQDs/Bi 4 O 5 Br 2 composite added with 4 mg of CQDs completely degraded BPA (20 mg/L) within 100 min when the radiation wavelength exceeded 400 nm, and Bi 4 O 5 Br 2 itself only degraded BPA by 1.44%. In sharp contrast, if a xenon lamp was directly irradiated without a 400 nm filter to sift the ultraviolet components, the degradation rate of the Bi 4 O 5 Br 2 monomer could reach 40% under the same conditions. This indicates that introducing CQDs plays a vital role in visible-light-driven degradation. Furthermore, on the basis of the structural characterisation, the successful introduction of CQDs was confirmed in the CQDs/Bi 4 O 5 Br 2 hybrid system. On this basis, the possibility of Z-scheme charge transfer was proved by X-ray photoelectron spectroscopy and ultraviolet electron spectroscopy measurements. The degradation mechanism of BPA under Z-type charge transfer was proposed through electron paramagnetic resonance, free radical capture and total organic carbon tests. This work is expected to provide a new idea for designing an energy band that matches semiconductor heterojunction photocatalysts, enhancing the unique narrow band gap structure and up-conversion properties of CQDs. In addition, it facilitates the construction of photocatalysts with better performance. [Display omitted] • CQDs/Bi 4 O 5 Br 2 composites with enhanced solar absorption and efficient carrier separation were successfully prepared. • The present CQDs/Bi 4 O 5 Br 2 photocatalyst can efficiently degrade BPA and TC under visible light. • The up-conversion luminescence of CQDs and Z-scheme charge transfer lead to the enhancement of photocatalysis. [ABSTRACT FROM AUTHOR]