Broadband Response Research Articles

High-performance Self-powered Perovskite Photodetector Based On Cesium Iodide Doped Spiro-OMeTAD Hole Transport Material

Hole transport materials play an essential role in the performance of perovskite detectors (PDs). Therefore, hole transport materials with excellent performance can significantly promote photoinduced holes' extraction rate and mobility. Spiro-OMeTAD, a small organic molecular material, has high solubility and excellent film-forming properties and is ideal for the hole transport layer (HTL) of photodetectors. In this work, a small amount of CsI is introduced into Spiro-OMeTAD together with Li-TFSI and TBP as additives. It is found that CsI and TBP formed a complex, which inhibited the agglomeration of Li-TFSI in Spiro-OMeTAD, thus preventing the rapid evaporation of TBP from leaving some cracks Spiro-OMeTAD. The photodetectors exhibit broadband response from near-ultraviolet to near-infrared (300–800 nm), achieving fast response (rise/fall time is 54.1/10.7 μs) and low dark current density (9.72 ×10−10 A cm−2). The detector can be self-powered at zero bias voltage, the external quantum efficiency (EQE) is 88%, the Responsivity (R) is 0.48 A W−1, and the detectivity (D*) is close to 2.7 × 1013 Jones. When the photovoltaic detector is placed in a nitrogen-filled glove box without special packaging for more than two months, the EQE remains 98.1% of the original. This study provides a simple method for fabricating a large area of high-performance planar PDs.

Bipolar dual-broadband photodetectors based on perovskite heterojunctions

The development of efficient photodetectors for color recognition is of great importance for many applications. In this paper, we report a novel bipolar dual-broadband photodetector equipped with a perovskite heterojunction, with bidirectional broadband responses in the short-wavelength and long-wavelength regions at zero bias voltage, enabled by a charge separation reversion mechanism. The unique aerosol–liquid–solid technique allowed the perovskite heterojunction to be fabricated by successively depositing wide-bandgap perovskite (WBP) and narrow-bandgap perovskite (NBP) layers directly on the transparent substrate. For photodetectors based on the perovskite heterojunctions, the short-wavelength photons were depleted by the bottom WBP layer and generated negative responses, while the long-wavelength photons were absorbed by the top NBP layer and generated positive responses. Moreover, the demarcation wavelength between the bipolar responses and the cut-off wavelength can be easily tuned by adjusting the bandgaps (or compositions) of the bottom and top perovskite layers.

Broadband Graphene Field-Effect Coupled Detectors: From Soft X-Ray to Near-Infrared

Here, we report a broadband graphene-silicon field-effect coupled detector (FCD) sensitive to photons spanning from soft X-ray to Near-Infrared region. Unlike traditional charge-coupled devices (CCD) relying on serial charge transfer between potential wells, the FCD integrates photo-sensing, charge integration, field-effect amplification, and random readout in one pixel. The charge integration in the potential well of the semiconductor substrate and field-effect coupled amplification in graphene transistor result in a highly sensitive broadband response. In the X-ray region, the device displays a high sensitivity of 1088 CGy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">air</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> and a fast response time < ~5 ms. Linear array imaging reveals the potential of our devices for array-based broadband imaging. Our FCD offers a viable strategy to monolithically integrate 2D materials into conventional solid-state imaging technology, exploring the next-generation broadband photodetectors for high-quality imaging and super vision technology.

Plasmonic Polycrystals within Microbowl Arrays

AbstractPlasmonic polycrystals composed of loosely packed plasmonic nanoparticles formed microscale plasmonic grains with random orientations may exhibit interesting optical properties. Their fabrication includes placing nanoparticles in order to form plasmonic grains and forming the random orientation of the plasmonic grains. The seemingly mutually exclusive nanoparticle ordering and random grain orientation processes challenge existing technologies. Here a self‐assembled bowl‐in‐bowl template‐confined dewetting process to prepare plasmonic polycrystals is developed. The size and composition of the plasmonic nanoparticles can be conveniently designed before experiments, which are inaccessible to conventional fabrication methods. Complex plasmonic coupling between the Au nanoparticles and the plasmonic grains within the plasmonic polycrystals is observed. The microbowls further endow plasmonic nanostructures with more interesting optical properties arising from multiple scattering processes within the concave bowl surfaces. The broadband response of the plasmonic polycrystals makes them active under 532, 633, and 785 nm laser excitation for surface‐enhanced Raman scattering (SERS) sensing applications. Finite‐difference time‐domain simulations are performed to explain the optical properties of the plasmonic polycrystals and the SERS enhancement. The simple template‐confined dewetting process provides a powerful approach to design plasmonic polycrystals with application potentials in plasmonic, sensing, and photonic fields.

High-operating temperature far-infrared Si:Ga blocked-impurity-band detectors

Silicon-based blocked impurity band (BIB) detectors have become the preferred candidate for the astronomical observation field because of their excellent ability for far-infrared detection, easy integration with the readout circuit, and potential for large-scale preparation. We fabricate Si:Ga BIB far-infrared detectors by a molecular beam epitaxy technique with an impressive blackbody specific detectivity of 4.21 × 1011 cm Hz1/2 W−1 at 10 K and nearly uniform broadband response between 2.5 and 20 μm. A response mechanism with variable temperature is described minutely by the varying temperature optoelectronic characterization and theoretical calculation as well as energy band diagram. The substantial results indicate that the responsivity of the detector can steadily maintain up to 26 K for far-infrared. This paper not only increases the accessibility of BIB detectors' fabrication tools but also provides an approach of high-operating temperature far-infrared detectors for astronomy explorations.

A novel broadband l inear‐cross and l inear‐circular reflective polarizer based on interdigital capacitance loaded dipole for radar cross‐section applications

A novel interdigital capacitance (IDC) frequency selective surface (FSS) based linear-cross reflective polarizer for K- and Ka-band applications is demonstrated in this work. The suggested device includes an IDC-loaded dipole printed on a thin FR-4 grounded dielectric substrate. From 17.02 to 37.30 GHz, it shows linear-cross conversion with a minimum 90% polarization conversion ratio (PCR) of 20.28 GHz bandwidth (74.7% fractional bandwidth [FBW]) for a y/x polarized incident electromagnetic (EM) wave. Along with linear-cross conversion, this polarizer also performs linear-circular conversions with axial ratio ≤ 3 dB in two frequency bands (Ku band-right circular, 15.13–16.15 GHz; Ka-band-left circular, 38.52–39.78 GHz) with 6.52%, and 3.14% FBW, respectively. The design performance is stable up to 45° for both transverse electric (TE) and transverse magnetic (TM) modes. The polarizer unitcell has structural dimensions 0.177 × 0.177 × 0.061 λ L 3 , $$ {\boldsymbol{\lambda}}_{\boldsymbol{L}}^{\mathbf{3}}, $$ where λL corresponds to the free-space wavelength of the lowest working frequency. The polariser's transmission line equivalent circuit model is extracted, and a good match is observed with the EM solver response. The broadband behavior of FSS and the ground plane is better understood by looking at surface current patterns at resonant frequencies. The proposed design's radar cross-section (RCS) analysis is compared to perfect electric conductor for the bistatic condition. The design novelty lies in its compact architectural volume of the unitcell 1.911 ( λ L 3 / 1000 $$ \Big({\boldsymbol{\lambda}}_{\boldsymbol{L}}^{\mathbf{3}}/\mathbf{1000} $$ ), low-profile, broadband PCR response with superior angle stability, dual-polarization conversion with circular rotation switching makes it a good option for real-time applications.

Simultaneous Characterization of Two Ultrashort Optical Pulses at Different Frequencies Using a WS2 Monolayer.

The precise characterization of ultrashort laser pulses has been of interest to the scientific community for many years. Frequency-resolved optical gating (FROG) has been extensively used to retrieve the temporal and spectral field distributions of ultrashort laser pulses. In this work, we exploit the high, broad-band nonlinear optical response of a WS2 monolayer to simultaneously characterize two ultrashort laser pulses with different frequencies. The relaxed phase-matching conditions in a WS2 monolayer enable the simultaneous acquisition of the spectra resulting from both four-wave mixing (FWM) and sum-frequency generation (SFG) nonlinear processes while varying the time delay between the two ultrashort pulses. Next, we introduce an adjusted double-blind FROG algorithm, based on iterative fast Fourier transforms between two FROG traces, to extract the intensity distribution and phase of two ultrashort pulses from the combination of their FWM and SFG FROG traces. Using this algorithm, we find an agreement between the computed and observed FROG traces for both the FWM and SFG processes. Exploiting the broad-band nonlinear response of a WS2 monolayer, we additionally characterize one of the pulses using a second-harmonic generation (SHG) FROG trace to validate the pulse shapes extracted from the combination of the FWM and SFG FROG traces. The retrieved pulse shape from the SHG FROG agrees well with the pulse shape retrieved from our nondegenerate cross-correlation FROG measurement. In addition to the nonlinear parametric processes, we also observe a nonlinearly generated photoluminescence (PL) signal emitted from the WS2 monolayer. Because of its nonlinear origin, the PL signal can also be used to obtain complementary autocorrelation and cross-correlation traces.

Mid- and long-wave infrared point spectrometer (MLPS): a miniature space-borne science instrument.

The mid- and long-wave infrared point spectrometer (MLPS) is an infrared point spectrometer that utilizes unique technologies to meet the spectral coverage, spectral sampling, and field-of-view (FOV) requirements of many future space-borne missions in a small volume with modest power consumption. MLPS simultaneously acquires high resolution mid-wave infrared (∼2-4 µm) and long-wave infrared (∼5.5-11 µm) measurements from a single, integrated instrument. The broadband response of MLPS can measure spectroscopically resolved reflected and thermally emitted radiation from a wide range of targets and return compositional, mineralogic, and thermophysical science from a single data set. We have built a prototype MLPS and performed end-to-end testing under vacuum showing that the measured spectral response and the signal-to-noise ratio (SNR) for both the mid-wave infrared (MIR) and long-wave infrared (LIR) channels of MLPS agree with established instrument models.

Na+-doped lead-free double perovskite Cs2AgInCl6 for broadband solar-blind UV detection

As a new generation of photonic materials, lead-free halide double perovskite materials demonstrate strong UV-light excited broadband emission that are promising for solar-blind ultraviolet (UV) light detectors. However, their practical applications are strongly limited by the low photoluminescence quantum efficiency because of the parity-forbidden nature of electronic transitions. Herein, the idea of cation alloying into lead-free halide double perovskite is proposed to mitigate the weak electronic transition and to improve the luminescence yield. By using the hydrothermal method, a series of Cs2NaxAg1-xInCl6 microcrystals were fabricated and the influences of Na-doping on the structural evolution and optical properties are investigated. Notably, the luminescence properties can be greatly improved by introducing an alloying element. By leveraging the UV-to-visible spectral conversion capability of the developed halide crystals, we fabricated a luminescent composite film by incorporating Cs2NaxAg1-xInCl6 perovskite nanocrystals into a polymethyl methacrylate matrix. Using this composite film as a spectral converter, we successfully demonstrated a solar-blind ultraviolet light detector with broadband and efficient response. Our work provides a novel solution for broadband solar-blind UV detection that could have practical implications for commercialization of metal halide based luminescent materials.

Cascaded nanobeam spectrometer with high resolution and scalability

An on-chip reconstruction spectrometer provides a powerful approach for miniaturized and portable applications. Owing to the cross-correlation and line shape of broadband responses, the reconstruction spectrometer exhibits limited resolution and a fixed bandwidth. Here we demonstrate a cascaded nanobeam spectrometer with assistance of a reconstruction algorithm. The transmissions of tunable Fano-enhanced nanobeams (FENs) are of good orthogonality and have improved quality factors. We retrieved a narrowband signal of 0.16 nm linewidth and a dual peak at 0.32 nm distance for resolution characterization. This result breaks the limit of the full width at half maximum for narrowband filter spectrometers. An expanded bandwidth is realized by a three-channel cascaded device, and signal spectra in the 16 nm range are reconstructed successfully. Each FEN unit has an ultra-compact footprint of 18 × 18 µ m 2 . In view of the improved performance and flexibility, our concept is an ideal candidate for precise and miniaturized applications.

Generation of hollow Gaussian beams by restoring structured light with meta-optics

Hollow Gaussian beams (HGBs) with annular intensity distributions but uniform phase and polarization distributions have aroused enormous interest due to their advantages in optical trapping, optical limiting, etc. Although extensive HGB generation methods have been performed previously, there are still existing limitations in polarization independence and broadband response, which hinder the application of HGBs. Here, we propose and experimentally demonstrate an HGB generation method by restoring structured light with a pair of cascaded dielectric metasurface q-plates. The metasurface q-plates are based on Pancharatnam-Berry (PB) phase and work as elements of optical spin-to-orbital angular momentum conversion. The first metasurface q-plate is used to introduce orbital angular momentum to an incident circularly polarized beam, and the output light field possesses both annular intensity distribution and spiral phase with reversed circular polarization. Since the phase retardation of the metasurface q-plates isπ, the reversed circularly polarized beam output from the first metasurface q-plate will obtain opposite spiral phase after the second metasurface q-plate. Then, the spiral phase of the output light field is counteracted, but its annular intensity distribution is retained as a new HGB. Arbitrarily polarized Gaussian beams are available for incident beams because they can be decomposed into orthogonal circularly polarized components. Furthermore, this method is applicable to a broad spectrum of visible light owing to the dispersion-free characteristic of the PB phase. This work suggests a polarization-independent and broadband HGB generation method, which may benefit HGB-based fundamental research.

Si2Te3 Photodetectors for Optoelectronic Integration at Telecommunication Wavelengths

High-performance two-dimensional (2D) -photodetectors with the potential for on-chip integration are desired for telecommunication applications. This work presents the photoresponse of planar silicon telluride photodetector (Si2Te3) with under wavelengths of 1310 and 1550 nm light illuminations. We utilized mechanically exfoliated multilayered Si2Te3 to fabricate back-gated phototransistor. The device with 50 nm Si2Te3 flack thickness demonstrated a hole mobility of 0.36 cm2 V1s1 and photo-responsivities of 170.5 AW-1 (0.35 W), and 12.61 AW-1 (0.25 W) at 1310 nm and 1550 nm excitations, respectively. Furthermore, the frequency response of the device with two different metal contacts (Au/Cr and Al/Ti) was tested. The device exhibited moderate broadband response with Au/Cr metal contact of 3dB bandwidth of 1.6 MHz, while 3.8 MHz bandwidth is realized with Al/Ti metal contacts. We also demonstrated a prototype of a heterogeneously integrated Si2Te3 photodetector onto a Si waveguide. The transmission losses in the waveguide were measured before and after the integration. Results demonstrated an attenuation of the optical signal by 24.3 dB and 18 dB for 1310 nm and 1550 nm wavelengths, respectively, that can attribute to the material-induced losses. These findings suggest that Si2Te3 is a promising 2D semiconductor material for optical communication photodetection.

Ultraviolet Narrowband Photomultiplication Type Organic Photodetectors with Fabry−Pérot Resonator Architecture

AbstractUltraviolet (UV) narrowband photodetectors play a critical role in missile detection, flame monitoring, optical communication, etc. It is a great challenge to realize UV narrowband organic photodetectors due to wide photo‐harvesting property of organic materials, especially for photomultiplication type organic photodetectors (PM‐OPDs). In this work, a smart strategy is proposed to achieve UV narrowband response by coupling Fabry−Pérot microcavity with PM‐OPDs. PM‐OPDs are realized by using poly(3‐hexylthiophene‐2,5‐diyl):[6,6]‐phenyl‐C71‐butyric acid methyl ester (100:1, w/w) as active layers, exhibiting broadband response range covering from UV–vis region. Series of optical microcavities consisting of Ag/LiF/Ag with UV spectral selectivity are prepared, which are employed to couple with the PM‐OPDs for achieving UV narrowband response. The UV spectral selectivity of optical microcavity can be optimized by tuning the thickness of spacer layer and mirror layers, which can further regulate the photogenerated electron distribution near Al electrode to optimize the external quantum efficiency (EQE) spectra of the PM‐OPDs coupled with optical microcavity. The optimized PM‐OPDs coupled with optical microcavity exhibit EQE of 9300% at 350 nm and narrowband response with 33 nm full‐width at half‐maximum under −15 V bias. This work indicates that PM‐OPDs coupled with optical microcavity should be an efficient strategy for achieving UV narrowband response.

Tunable terahertz absorption modulation in graphene nanoribbon-assisted dielectric metamaterial

Designing metamaterial absorbers with tunable and broadband response with high modulation depth is crucial for application in switches, polarization sensitive devices, and optical filters. In this article, we report a tunable broadband metamaterial absorber in terahertz regime. The design comprises frustum shaped dielectric (SiO2) metamaterial structures on top of a one-dimensional period array of graphene nanoribbons (GNRs) deposited over the ultrathin metal-backed dielectric. For TM polarization, our design offers a near-perfect absorption in the range of 0.72 THz to 0.9 THz with 72.7 fractional bandwidth. On the contrary, we get a negligible absorption for the TE polarization of the incident light, thereby giving an absorption contrast ratio up to 20.5 dB. The excitation of plasmons in GNRs combined with resonance induced by the graphene-dielectric-metal cavity leads to nearly perfect broadband absorption for the TM polarized radiation. Further, modulation of the absorption spectrum has been achieved by changing graphene conductivity with the help of its Fermi energy. A theoretical model based on effective medium theory and transfer matrix method has been presented to validate absorption response for various Fermi energy values of the graphene. The figure of merit of our design can outperform some of the recently reported tunable metamaterial absorbers.

Broadband and Dual-Polarized Terahertz Wave Anomalous Refraction Based on a Huygens’ Metasurface

Terahertz wavefront manipulation is one of the key terahertz technologies. While few of the research works on terahertz wavefront manipulation has broadband and dual-polarized responses. Here a broadband dual-polarized Huygens’ metasurface is proposed to realize high efficient terahertz wave anomalous refraction. By constructing simultaneous electric and magnetic responses in a bi-layer metasurface to produce Huygens’ resonance, broadband and large phase changes for dual-polarized terahertz wave are achieved. A phase change over 300° with transmission magnitude beyond 0.75 is realized between 0.4 THz and 1 THz. An array made of the metasurface with phase gradient is designed to achieve a 14.0° anomalous refraction for two orthogonal linear polarized waves at 0.93 THz. The structure consists of only two metal layers, providing a simple and high-efficiency design scheme for achieving efficient dual-polarized terahertz wavefront manipulation.

Hollow semiconductor photocatalysts for solar energy conversion

The development of high-efficient photocatalysts plays an important role in the sustainable utilization of solar energy. Hollow nanostructured photocatalysts are vital for solar light utilization and charge carrier separation in photocatalytic processes. Therefore, the construction of hollow semiconductor photocatalysts is a promising strategy for preparing novel high-efficient photocatalysts. This paper reviews common hollow semiconductor nanomaterials, such as oxides, sulfides, nitrides, C 3 N 4 , MOFs, and their composite photocatalysts. The characteristics of hollow-structure photocatalysts, the application of solar energy conversion, and their understanding of the photocatalytic mechanism are also reviewed. In addition, future challenges will be focused on designing and majorizing broadband response hollow-structure photocatalysts to further enhance solar energy conversion. Hollow semiconductor photocatalysts will have potential applications in the natural environment, and these synthesized strategies can also provide new possibilities for synthesizing other high-performance semiconductor photocatalysts. Hollow semiconductor photocatalytic nanomaterials including oxides, sulfides, nitrides, g-C 3 N 4 , MOFs and their composites are reviewed. The characteristics, formation, applications for solar energy conversion and the deep understanding of photocatalytic mechanism for hollow semiconductor photocatalysts are also reviewed, which may provide new insights for fabricating high-performance hollow photocatalysts. • This review summarizes hollow photocatalysts including oxides, sulfides, nitrides, C 3 N 4 , MOF. • The effects of different modification methods of hollow photocatalysts are reviewed. • The recent development for preparing hollow semiconductor photocatalysts is summarized. • The application of hollow photocatalysts for solar energy conversion is reviewed. • The potential directions for hollow photocatalysts are proposed.

Acoustic waveguide demultiplexer based on Fano resonance: Experiment and simulation

A compact acoustic waveguide demultiplexer configuration is studied via finite-element numerical modeling and audio frequency experiments. The demultiplexer consists of a Y-shaped waveguide with a single input and two outputs. The narrow transmission bands created by stubs side-loaded on each output arm lead to selective transmission of certain frequencies. The experimental work characterizes the broadband response along each output arm by using an impulse response method. Finite-element numerical simulations are conducted using COMSOL. The results of the experiment and the simulation are compared to an existing analytic theory.

Subwavelength Bayer RGB color routers with perfect optical efficiency

Abstract We introduce subwavelength color routers with perfect optical efficiency in a red-green-green-blue (RGGB) Bayer layout for solid state image sensors. This is the first demonstration of a subwavelength device concept that shows the full potential of color routing, i.e., perfect routing without loss of photons, with a broadband, polarization-independent, and angular robust response. As an example, we show a color router for 320 nm wide image sensor pixels, which are two times smaller than the smallest state-of-the-art pixels, that features perfect optical efficiency, i.e., no crosstalk between color channels, no reflections, and no leakage into non-photodetector regions, even though the pixel photodetectors are 2–3 times smaller than the wavelength of the incident light. Our color router outperforms all other color separation approaches and can replace the entire optical stack in solid state image sensors.

Underwater Multispectral Computational Imaging Based on a Broadband Water-Resistant Sb2Se3 Heterojunction Photodetector

Exploration, utilization, and protection of marine resources are of great significance to the survival and development of mankind. However, currently classical optical cameras suffer information loss, low contrast, and color distortion due to the absorption and scattering nature for the underwater environment. Here, we demonstrate an underwater multispectral computational imaging system combined with single-photodetector imaging algorithm technology and a CdS/Sb2Se3 heterojunction photodetector. The computational imaging technology coupled with an advanced Fourier algorithm can capture a scene by a single photodetector without spatial resolution that avoids the need to rely on high-density detectors array and bulky optical components in traditional imaging systems. This convenient computational imaging method provides more flexible possibilities for underwater imaging and promises to give more imaging capabilities (such as multispectral imaging, antiscattering imaging capability) to meet ever-changing demand of underwater imaging. In addition, the water-resistant CdS/Sb2Se3 heterojunction photodetector fabricated by the close spaced sublimation (Sb2Se3) and chemical bath deposition (CdS) shows excellent self-powered photodetection performance at zero bias with high LDR of 128 dB, broadband response spectrum range of 300-1050 nm, high responsivity up to 0.47 A/W, and high specific detectivity over 5 × 1012 jones. Compared with the traditional optical imaging system, our designed computational imaging system that combines the advanced Fourier algorithm and a high-performance CdS/Sb2Se3 heterojunction photodetector exhibits outstanding antiscattering imaging capability (shielded by frosted glass), weak light imaging capability (∼0.2 μW/cm2, corresponding to moonlight intensity), and multispectral imaging capability. Therefore, we believe that this work will boost the progress of marine science.

A solution-processed octahedral nano-blocks structure CaIn2S4 film for UV/vis photodetection

A novel CaIn2S4 with three-dimensional octahedral nano-blocks (ONBs) are successfully synthesized on fluorine-doped tin oxide (FTO) substrate by a simple hydrothermal method. The CaIn2S4 ONBs are uniform grown and scattered on the whole FTO substrate with high regular and symmetric morphology as well as average diagonal length of about 600 nm. Based on the as-synthesized CaIn2S4 ONBs, a photodetector (PD) is fabricated. Satisfyingly, it is found that the CaIn2S4 ONBs PD achieves a broad-band response ranging from ultraviolet (UV) to visible ( vis) light at zero bias voltage. It is also significant that the CaIn2S4 ONBs PD enables a fast response, in which the rise time and decay time are less than 0.15 and 0.2 s, respectively. Furthermore, the morphological evolution of the CaIn2S4 ONBs and plausible UV/vis detection mechanism of the CaIn2S4 ONBs PD are discussed.