Graphene-based photodetectors have attracted much attention due to their unique properties, such as high-speed and wide-band detection capability. However, they suffer from very low external quantum efficiency in the infrared (IR) region and lack spectral selectivity. Here, we construct a plasmon-enhanced macro-assembled graphene nanofilm (nMAG) based dual-band infrared silicon photodetector. The Au plasmonic nanostructures improve the absorption of long-wavelength photons with energy levels below the Schottky barrier (between metal and Si) and enhance the interface transport of electrons. Combined with the strong photo-thermionic emission (PTI) effect of nMAG, the nMAG–Au–Si heterojunctions show strong dual-band detection capability with responsivities of 52.9 mA/W at 1342 nm and 10.72 mA/W at 1850 nm, outperforming IR detectors without plasmonic nanostructures by 58–4562 times. The synergy between plasmon–exciton resonance enhancement and the PTI effect opens a new avenue for invisible light detection.
2023, 11(10): 1657
Metasurfaces provide an effective technology platform for manipulating electromagnetic waves, and the existing design methods all highlight the importance of creating a gradient in the output phase across light scattering units. However, in the emerging research subfield of meta-waveguides where a metasurface is driven by guided modes, this phase gradient-oriented approach can only provide a very limited emission aperture, significantly affecting the application potential of such meta-waveguides. In this work, we propose a new design approach that exploits the difference between meta-atoms in their light scattering amplitude. By balancing this amplitude gradient in the meta-atoms against the intensity decay in the energy-feeding waveguide, a large effective aperture can be obtained. Based on this new design approach, three different wavefront shaping functionalities are numerically demonstrated here on multiple devices in the terahertz regime. They include beam expanders that radiate a plane wave, where the beam width can increase by more than 900 times as compared to the guided wave. They also include a metalens that generates a Bessel-beam focus with a width 0.59 times the wavelength, and vortex beam generators that emit light with a tunable topological charge that can reach . This amplitude gradient design approach could benefit a variety of off-chip light shaping applications such as remote sensing and 6G wireless communications.
2023, 11(9): 1542
Temperature sensing is essential for human health monitoring. High-sensitivity () fiber sensors always require long interference paths and temperature-sensitive materials, leading to a long sensor and thus slow response (6–14 s). To date, it is still challenging for a fiber optic temperature sensor to have an ultrafast () response simultaneously with high sensitivity. Here, a side-polished single-mode/hollow/single-mode fiber (SP-SHSF) structure is proposed to meet the challenge by using the length-independent sensitivity of an anti-resonant reflecting optical waveguide mechanism. With a polydimethylsiloxane filled sub-nanoliter volume cavity in the SP-SHSF, the SP-SHSF exhibits a high temperature sensitivity of 4.223 nm/°C with a compact length of 1.6 mm, allowing an ultrafast response (16 ms) and fast recovery time (176 ms). The figure of merit (FOM), defined as the absolute ratio of sensitivity to response time, is proposed to assess the comprehensive performance of the sensor. The FOM of the proposed sensor reaches up to , which is more than two to three orders of magnitude higher than those of other temperature fiber optic sensors reported previously. Additionally, a three-month cycle test shows that the sensor is highly robust, with excellent reversibility and accuracy, allowing it to be incorporated with a wearable face mask for detecting temperature changes during human breathing. The high FOM and high stability of the proposed sensing fiber structure provide an excellent opportunity to develop both ultrafast and highly sensitive fiber optic sensors for wearable respiratory monitoring and contactless in vitro detection.
2023, 11(8): 1397
Transparent absorbers, with a functional integration of broadband electromagnetic shielding, microwave camouflage, and optical transparency, have attracted increasing attention in the past decades. Metal mesh, an artificial, optically transparent, conducting material composed of periodic metallic gratings, is the optimal choice for the microwave shielding layer of transparent absorbers because of its excellent compatibility between high transparency and low resistance. However, the micrometer-level periodicity of metallic grating concentrates the diffraction of light, which degrades the imaging quality of cameras and sensors in common. In this study, we report on a generalized Thiessen-polygon-randomization method that prevents the concentration of the diffraction of light in periodic metallic grating and demonstrate an ultrawide-band optically transparent diffraction-immune metamaterial absorber. The absorber is constructed with a multilayer indium-tin-oxide-based metasurface and a Thiessen-polygon-randomized metal-mesh reflector. The lossy metasurface provides multimode absorption, whereas the Thiessen-polygon randomization prevents the concentration of the diffraction of light. The practical sample achieves a 10 dB absorptivity and shielding effectiveness over a range of 8–26.5 GHz, and the optical transparency is also preserved over the entire visible and near-infrared regions. The point spread function and field of view are both improved by using the antidiffraction absorber. Our study paves the way for the application of optically transparent electromagnetic devices, display, and optoelectronic integration in a more practical stage.
2023, 11(7): 1354
Whispering gallery mode (WGM) microcavities have been widely used for high-sensitivity ultrasound detection, owing to their optical and mechanical dual-resonance enhanced sensitivity. The ultrasound sensitivity of the cavity optomechanical system is fundamentally limited by thermal noise. In this work, we theoretically and experimentally investigate the thermal-noise-limited sensitivity of a WGM microdisk ultrasound sensor and optimize the sensitivity by varying the radius and a thickness of the microdisk, as well as using a trench structure around the disk. Utilizing a microdisk with a radius of 300 μm and thickness of 2 μm, we achieve a peak sensitivity of at 82.6 kHz. To the best of our knowledge, this represents the record sensitivity among cavity optomechanical ultrasound sensors. Such high sensitivity has the potential to improve the detection range of air-coupled ultrasound sensing technology.
2023, 11(7): 1139
Integrating novel materials is critical for the ultrasensitive, multi-dimensional detection of biomolecules in the terahertz (THz) range. Few studies on THz biosensors have used semiconductive active layers with tunable energy band structures. In this study, we demonstrate three THz biosensors for detecting casein molecules based on the hybridization of the metasurface with graphitic carbon nitride, graphene, and heterojunction. We achieved low-concentration detection of casein molecules with a 3.54 ng/mL limit and multi-dimensional sensing by observing three degrees of variations (frequency shift, transmission difference, and phase difference). The favorable effect of casein on the conductivity of the semiconductive active layer can be used to explain the internal sensing mechanism. The incorporation of protein molecules changes the carrier concentration on the surface of the semiconductor active layer via the electrostatic doping effect as the concentration of positively charged casein grows, which alters the energy band structure and the conductivity of the active layer. The measured results indicate that any casein concentration can be distinguished directly by observing variations in resonance frequency, transmission value, and phase difference. With the heterojunction, the biosensor showed the highest response to the protein among the three biosensors. The Silvaco Atlas package was used to simulate the three samples’ energy band structure and carrier transport to demonstrate the benefits of the heterojunction for the sensor. The simulation results validated our proposed theoretical mechanism model. Our proposed biosensors could provide a novel approach for THz metasurface-based ultrasensitive biosensing technologies.
2023, 11(5): 858
Optical fiber microresonators have attracted considerable interest for acoustic detection because of their compact size and high optical quality. Here, we have proposed, designed, and fabricated a spring-based Fabry–Pérot cavity microresonator for highly sensitive acoustic detection. We observed two resonator vibration modes: one relating to the spring vibration state and the other determined by the point-clamped circular plate vibration mode. We found that the vibration modes can be coupled and optimized by changing the structure size. The proposed resonator is directly 3D printed on an optical fiber tip through two-photon polymerization and is used for acoustic detection and imaging. The experiments show that the device exhibits a high sensitivity and low noise equivalent acoustic signal level of at 75 kHz that can detect weak acoustic waves, which can be used for underwater object imaging. The results demonstrate that the proposed work has great potential in acoustic detection and biomedical imaging applications.
2023, 11(5): 780
Highly accurate biosensors for few or single molecule detection play a central role in numerous key fields, such as healthcare and environmental monitoring. In the last decade, laser biosensors have been investigated as proofs of concept, and several technologies have been proposed. We here propose a demonstration of polymeric whispering gallery microlasers as biosensors for detecting small amounts of proteins, down to 400 pg. They have the advantage of working in free space without any need for waveguiding for input excitation or output signal detection. The photonic microsensors can be easily patterned on microscope slides and operate in air and solution. We estimate the limit of detection up to 148 nm/RIU for three different protein dispersions. In addition, the sensing ability of passive spherical resonators in the presence of dielectric nanoparticles that mimic proteins is described by massive ab initio numerical simulations.
2023, 11(5): 732
Valley Hall topological photonic crystals, inspired by topological insulators in condensed matter physics, have provided a promising solution to control the flow of light. Recently, the dynamic manipulation property of topological photonic crystals has been widely studied. Here, we propose a novel solution for programmable valley photonic crystals, called field programmable topological edge array (FPTEA), based on the field reorientation property of nematic liquid crystals and robust valley-protected edge modes. FPTEA is composed of an array of graphene-like lattices with symmetry, in which the birefringence of liquid crystal is larger than 0.5105. Due to the dielectric anisotropy of liquid crystals being sensitive to external fields such as light, heat, electric, and magnetic fields, each lattice is tunable, and the topological propagation routes and even the lattice parameters can be dynamically changed while changing the distribution of external fields. We numerically demonstrate three methods of composing an FPTEA device to design arbitrary passive optical devices by electric driving, thermal inducing, or UV writing. These results show the great application potential of liquid crystals in topological photonic crystals, and enrich the design of programmable integrated topological devices with broad working bandwidth ranging from microwave to visible light.
2023, 11(3): 476
Dynamical control of perfect absorption plays an indispensable role in optical switch and modulators. However, it always suffers from the limited modulation range, small depth, and susceptible absorption efficiencies. Here, we propose a new strategy based on Friedrich–Wintgen bound states in the continuum (F–W BICs) to realize a tunable perfect absorber with large dynamic modulation range. For proof of concept, we demonstrate a pentaband ultrahigh absorption system consisting of graphene gratings and graphene sheets through elaborately tuning F–W BIC. The nature of the F–W BIC arises from the destructive interference between Fabry–Perot resonance and guided mode resonance modes in the coherent phase-matching condition. The radiation channels are avoided from crossing. The BIC can be dynamically modulated by engineering the Fermi level of graphene gratings, which breaks the traditional modulation methods with an incidence angle. Remarkably, the perfect absorber with this F–W BIC approach achieves the largest modulation range of up to 3.5 THz. We believe that this work provides a new way to dynamically engineer perfect absorption and stimulates the development of multiband ultracompact devices.
2023, 11(3): 456