2021, 9(9) Column
Photonics Research 第9卷 第9期
The emergence of metasurfaces provides a novel strategy to tailor the electromagnetic response of electromagnetic waves in a controlled manner by judicious design of the constitutive meta-atom. However, passive metasurfaces tend to perform a specific or limited number of functionalities and suffer from narrow-frequency-band operation. Reported reconfigurable metasurfaces can generally be controlled only in a 1D configuration or use p-i-n diodes to show binary phase states. Here, a 2D reconfigurable reflective metasurface with individually addressable meta-atoms enabling a continuous phase control is proposed in the microwave regime. The response of the meta-atom is flexibly controlled by changing the bias voltage applied to the embedded varactor diode through an elaborated power supply system. By assigning appropriate phase profiles to the metasurface through voltage modulation, complex beam generation, including Bessel beams, vortex beams, and Airy beams, is fulfilled to demonstrate the accurate phase-control capability of the reconfigurable metasurface. Both simulations and measurements are performed as a proof of concept and show good agreement. The proposed design paves the way toward the achievement of real-time and programmable multifunctional meta-devices, with enormous potential for microwave applications such as wireless communication, electromagnetic imaging, and smart antennas.
On-chip ultrafast mode-locking lasers are basic building blocks for the realization of a chip-based optical frequency comb. In this paper, an ultrafast saturable absorber made up of a graphene pad on top of a silicon waveguide is applied to implement an ultrafast pulse laser. Benefiting from the small mode area of the graphene/silicon hybrid waveguide, the saturable pulse energy is reduced by two orders of magnitude compared with the fiber. A mode-locked pulse with a duration of 542 fs and a repetition rate of 54.37 MHz is realized. Pump–probe measurement shows that the carrier relaxation process of free carrier recombination with atomic-thin graphene/silicon junctions is three orders of magnitude faster than silicon, which plays a fundamental role in pulse narrowing. The chip-scale silicon ultrafast laser lays a foundation for a new class of nonlinear devices, in which a combination with multiple functional silicon photonic circuits enables efficient nonlinear interaction at the micrometer scale and less than 1 W of power consumption.
Chiro-optical effects offer a wide range of potential applications in nanophotonics, such as advanced imaging and molecular sensing and separation. Flat single-layer metasurfaces composed of subwavelength meta-atoms have gained significant attention due to their exceptional characteristics in light–matter interactions. Although metasurface-based devices have manipulated electromagnetic waves, the compact on-chip realization of giant chiro-optical effects remains a challenge at optical frequencies. In this work, we experimentally and numerically demonstrate an all-dielectric metasurface to realize large chiro-optical effects in the visible regime. Notably, the proposed strategy of utilizing achiral nanofins instead of conventional chiral structures provides an extra degree of design freedom. The mutual coupling between carefully engineered nanofins produces constructive and destructive interference, leading to the asymmetric transmission of 70% and average circular dichroism exceeding 60%. We investigate the underlying mechanism behind the chiro-optical effects using the theory of multipolar decomposition. The proposed design mechanism maximizes the chiro-optical response through a single-layer metasurface with potential applications in high-efficiency integrated ultrathin polarization rotators and shapers, chiral polarizers for optical displays, chiral beam splitters, and chiral sensors.
An optical funnel, which performs as a passive electromagnetic compressor, can guide electromagnetic waves from a wide inlet to a narrow outlet without reflectance/scattering and squeeze electromagnetic fields uniformly to an air neck. In this study, an optical funnel is designed by precisely filling subwavelength ceramic blocks with a gradient refractive index inside a tapered waveguide. The gradient refractive index is designed by transformation optics, which is isotropic and all above unit, thus exhibiting a broadband feature. Due to the mechanism of impedance matching over the whole funnel, extremely low reflectance/scattering and stable enhancement of fields can be achieved. The field enhancement factor in different regions of the funnel (e.g., in the air neck) can be flexibly designed just by modifying the funnel-width ratios.
We fabricated p-i-n tunnel junction (TJ) contacts for hole injection on c-plane green micro-light-emitting diodes (micro-LEDs) by a hybrid growth approach using plasma-assisted molecular beam epitaxy (PA-MBE) and metal–organic chemical vapor deposition (MOCVD). The TJ was formed by an MBE-grown ultra-thin unintentionally doped InGaN polarization layer and an
layer on the activated layer prepared by MOCVD. This hybrid growth approach allowed for the realization of a steep doping interface and ultrathin depletion width for efficient inter-band tunneling. Compared to standard micro-LEDs, the TJ micro-LEDs showed a reduced device resistance, enhanced electroluminescence intensity, and a reduced efficiency droop. The size-independent J-V characteristics indicate that TJ could serve as an excellent current spreading layer. All these results demonstrated that hybrid TJ contacts contributed to the realization of high-performance micro-LEDs with long emission wavelengths.
Metasurface provides miniaturized devices for integrated optics. Here, we design and realize a meta-converter to transform a plane-wave beam into multiple Laguerre-Gaussian (LG) modes of different orders at various diffraction angles. The metasurface is fabricated with Au nano-antennas, which vary in length and orientation angle for modulation of both the phase and the amplitude of a scattered wave, on a silica substrate. Our error analysis suggests that the metasurface design is robust over a 400 nm wavelength range. This work presents the manipulation of LG beams through controlling both radial and azimuthal orders, which paves the way in expanding the communication channels by one more dimension (i.e., radial order) and demultiplexing different modes.
Surface plasmon resonance (SPR) sensors are a prominent means to detect biological and chemical analytes and to investigate biomolecular interactions in various fields. However, the performance of SPR sensors is ultimately limited by ohmic loss, which substantially weakens the resonance signal and broadens the response linewidth. Recent studies have shown that ohmic loss can be fully compensated in plasmonic nanolasers, which leads to a novel class of lasing-enhanced surface plasmon resonance (LESPR) sensors with improved sensing performance. In this paper, we detail the underlying physical mechanisms of LESPR sensors and present their implementation in various sensing devices. We review recent progress on their applications, particularly for refractive index sensing, gas detection and biological imaging, labeling, tracking, and diagnosis. We then summarize the review and highlight remaining challenges of LESPR sensing technology.
Widely tunable, high-power, femtosecond noncollinear optical parametric oscillator in the visible spectral range
Ultrafast visible radiation is of great importance for many applications ranging from spectroscopy to metrology. Because some regions in the visible range are not covered by laser gain media, optical parametric oscillators offer an added value. Besides a high-power broadband laser source, the ability to rapidly tune the frequency of pulses with high-power spectral density offers an extra benefit for experiments such as multicolor spectroscopy or imaging. Here, we demonstrate a broadband, high-power, rapidly tunable femtosecond noncollinear optical parametric oscillator with a signal tuning range of 440–720 nm in the visible range. The oscillator is pumped by the third harmonic of an Yb-fiber laser at 345 nm with a repetition rate of 50.2 MHz. Moreover, the signal wavelength is tuned by changing the cavity length only, and output powers up to 452 mW and pulse durations down to 268 fs are achieved. This is, to the best of our knowledge, the first demonstration of a quickly tunable femtosecond optical parametric oscillator that covers nearly the entire visible spectral range with high output power.
Aging degradation and seismic damage of civil infrastructures have become a serious issue for society, and one promising technology for monitoring their conditions is optical fiber sensing. Glass optical fibers have been predominantly used for the past several decades to develop fiber sensors, but currently polymer or plastic optical fibers (POFs) have also been used extensively to develop advanced fiber sensors because of their unique features, such as high flexibility, large breakage strain, and impact resistance. This review focuses on recently developed distributed and quasi-distributed POF-based sensing techniques based on Rayleigh scattering, Brillouin scattering, and fiber Bragg gratings.
Hyperspectral imaging (HSI) with rich spectral and spatial information holds potential for applications ranging from remote sensing to biomedicine. However, charge-coupled device (CCD) detectors used in conventional HSI systems suffer from inferior and unbalanced responsivity in the visible region, which is not a perfect choice for high-performance visible HSI. That is, conventional Si-based CCDs exhibit poor responsivity at short wavelengths (e.g., 400–600 nm) compared with that at longer wavelengths due to the nature of the indirect bandgap in silicon of around 1.1 eV. To solve this challenge, we introduce a
perovskite layer to shape the spectrum of a Si/PEDOT:PSS heterojunction photodetector (PD), resulting in a fabricated Si- hybrid PD with enhanced responsivity at 400–600 nm. This results in an approximately flat spectral responsivity curve in the visible region (400–800 nm). Therefore, the stable hybrid PD with a flat spectrum overcomes the shortcomings of traditional Si-based PDs and makes it more suitable for HSI. Further, we set up a first perovskite HSI system with high spectrum resolution and demonstrate potential applications for tumor detection and tissue identification. We believe that this perovskite optimization can be integrated into modern CCD, thus becoming a step in future CCD fabrication processes, which is a milestone for high-performance HSI systems.
The class quantum Merlin–Arthur (QMA), as the quantum analog of nondeterministic polynomial time, contains the decision problems whose YES instance can be verified efficiently with a quantum computer. The problem of deciding the group non-membership (GNM) of a group element is conjectured to be a member of QMA. Previous works on the verification of GNM, which still lacks experimental demonstration, required a quantum circuit with
group oracle calls. Here, we provide an efficient way to verify GNM problems, in which each quantum circuit only contains group of oracle calls, and the number of qubits in each circuit is reduced by half. Based on this protocol, we then experimentally demonstrate the new verification process with a four-element group in an all-optical circuit. The new protocol is validated experimentally by observing a significant completeness-soundness gap between the probabilities of accepting elements in and outside the subgroup. This work efficiently simplifies the verification of GNM and is helpful in constructing more quantum protocols based on the near-term quantum devices.
Spin defects in silicon carbide (SiC) have attracted much attentions in various quantum technologies. In this work, we study the optical manipulation of charge state and coherent control of multifold divacancy spins ensemble in SiC under resonant excitation. The results reveal that the resonantly excited divacancy ensemble counts have dozens of enhancements by repumping a higher-energy laser. Moreover, it has a similar optimal repump laser wavelength of around 670 nm for multiple divacancies. On the basis of this, the optically detected magnetic resonance (ODMR) experiment shows that repump lasers with different wavelengths do not affect the ODMR contrast and line width. In addition, the repump lasers also do not change the divacancy spins’ coherence times. The experiments pave the way for using the optimal repump excitation method for SiC-based quantum information processing and quantum sensing.
Laplace operation, the isotropic second-order differentiation, on spatial functions is an essential mathematical calculation in most physical equations and signal processing. Realizing the Laplace operation in a manner of optical analog computing has recently attracted attention, but a compact device with a high spatial resolution is still elusive. Here, we introduce a Laplace metasurface that can perform the Laplace operation for incident light-field patterns. By exciting the quasi-bound state in the continuum, an optical transfer function for nearly perfect isotropic second-order differentiation has been obtained with a spatial resolution of wavelength scale. Such a Laplace metasurface has been numerically validated with both 1D and 2D spatial functions, and the results agree well with that of the ideal Laplace operation. In addition, the edge detection of a concerned object in an image has been demonstrated with the Laplace metasurface. Our results pave the way to the applications of metasurfaces in optical analog computing and image processing.
All-inorganic perovskite has attracted significant attention due to its excellent nonlinear optical characteristics. Stable and low-toxic perovskite materials have great application prospects in optoelectronic devices. Here, we study the nonlinear optical properties of
( , 1.5, 2) nanocrystals (NCs) glass by open-aperture Z-scan. It is found that the two- (2PA) and three-photon absorption (3PA) intensity can be adjusted by the treatment temperature and the ratio of halide anions. The perovskite NCs glass treated at a high temperature has better crystallinity, resulting in stronger nonlinear absorption performance. In addition, the value of the 2PA parameter of NCs glasses decreases when the incident pump intensity increases, which is ascribed to the saturation of 2PA and population inversion. Finally, the research results show that the 2PA coefficient ( ) and 3PA coefficient ( ) of NCs glass with high Br anion content are larger than those of and NCs glasses. This is mainly due to the greater influence of Br anions on the symmetry of the perovskite structure, which leads to the redistribution of delocalized electrons. The revealed adjustable nonlinear optical properties of perovskite NCs glass are essential for developing stable and high-performance nonlinear optical devices.
It is still very challenging to determine a freeform lens for converting a given input beam into a prescribed output beam where not only the irradiance distribution but also the phase distribution hardly can be expressed analytically. Difficulties arise because the ray mapping from the input beam to the output beam is not only intertwined with the required double freeform surfaces but also intertwined with the output phase distribution, whose gradient represents the directions of the output rays. Direct determination of such a problem is very difficult. Here, we develop a special iterative wavefront tailoring (IWT) method to tackle this problem. In a certain iteration, the current calculation data of the double freeform surfaces and the output phase gradient are used to update the coefficients of a Monge–Ampère equation describing an intermediate wavefront next to the entrance freeform surface. The solution to the wavefront equation could lead to an improved ray mapping to be used to update the corresponding phase gradient data and reconstruct the double freeform surfaces. In a demonstrative example that deviates much from the paraxial or small-angle approximation, the new IWT method can generate a freeform lens that performs much better than that designed by a conventional ray mapping method for producing two irradiance distributions in the forms of numerals “1” and “2” on two successive targets, respectively.
Suppressing surface plasmon losses to improve the efficiency of blue organic light-emitting diodes using the plasmonic quasi-bandgap phenomenonDownload：520次
It is a persistent problem in organic light-emitting diode (OLED) display devices that the efficiency of blue-light source materials is lower than that of green- or red-light source materials. To address this problem, numerous studies have investigated blue-light-emitting materials. However, ensuring the reliability of the blue-light-emitting materials has been difficult in most studies. In this study, electrodes using asymmetric dielectric/metal/dielectric structures with plasmonic quasi-bandgap characteristics were developed to achieve a highly efficient blue fluorescent OLED. The electrodes were applied in a microcavity OLED and a transparent OLED. Using the developed electrode in fabricated OLED devices not only maximized the cavity resonance effect and transparency, but also preserved the advantage of the work function of the metal in terms of electrical properties, with high device stability. The approach also minimized losses caused by surface plasmon polaritons, which is a blind spot in the optical aspect of metal electrodes, resulting in improved light extraction efficiency.
630-nm red InGaN micro-light-emitting diodes (<20 μm × 20 μm) exceeding 1 mW/mm2 for full-color micro-displays
arrays of InGaN micro-light-emitting diodes (μLEDs) with a peak wavelength from 662 to 630 nm at . The on-wafer external quantum efficiency reached 0.18% at . The output power density of the red μLEDs was obtained as , which was estimated to be higher than that of AlInGaP red μLEDs ( ). Finally, we demonstrate that InGaN red/green/blue μLEDs could exhibit a wide color gamut covering 81.3% and 79.1% of the Rec. 2020 color space in CIE 1931 and 1976 diagrams, respectively.
An integrated polymer microtoroid resonator system fabricated using single-step, two-photon polymerization laser lithography is reported. The integrated microtoroid-waveguide system includes an integrated coupling waveguide for ease of optical coupling and has a quality factor of 16,000. We further demonstrate a nested double-spiral waveguide that allows higher spatial density of data, akin to a macroscopic form of space-division multiplexing. Experimental characterization reveals good transmission properties in the double-spiral waveguide device. In addition, the waveguides are demonstrated to support 30 Gb/s nonreturn-to-zero and 28 Gb/s pulse amplitude modulation 4 high-speed data.
Indium arsenide phosphide (InAsP) nanowires (NWs), a member of the III–V semiconductor family, have been used in various photonic and optoelectronic applications thanks to their unique electrical and optical properties such as high carrier mobility and adjustable band gap. In this work, we synthesize InAsP NWs and further explore their nonlinear optical properties. The ultrafast carrier dynamics and nonlinear optical response are thoroughly studied based on the nondegenerate pump probe and Z-scan experimental measurements. Two different characteristic carrier lifetimes (
and ) from InAsP NWs are observed during the excited-carrier relaxation process. Based on the physical model analysis, the relaxation process can be ascribed to the carrier cooling process via carrier-phonon scattering and Auger recombination. In addition, based on the measured excited-carrier lifetime and Pauli-blocking principle, we discover that InAsP NWs show strong saturable absorption properties at the wavelengths of 532 and 1064 nm. Last, we demonstrate for the first time a femtosecond ( ) solid-state laser based on an InAsP NWs saturable absorber at 1.04 μm. We believe that our work provides a better understanding of the InAsP NWs optical properties and will further advance their photonic applications in the near-infrared range.
Space-charge effect on photogenerated-current and -voltage in III-nitride optoelectronic semiconductors
In this study, we attempted to elucidate the carrier dynamics behind the abnormal characteristics of photogenerated current and voltage (
and ) at cryogenic temperature in III-nitride optoelectronic semiconductors by employing space-charge theory. To this end, we carefully investigated and analyzed excitation-power-dependent (PDIV) curves operated by quasiresonant excitation of an AlGaInN-based junction semiconductor at 300 K and 15 K. At 300 K, the curves exhibited typical characteristics and were well described by the conventional theory. However, the PDIV curves at 15 K could no longer be described by the conventional theory. To elucidate the mechanism behind this phenomenon, we proposed a model in which the space-charge effect (SCE) plays a key role. Based on this model, we proposed the modified Shockley diode equation, which can explain the PDIV characteristic at 15 K, including the SCE. We also discussed the SCE on the efficiency of devices.
500 μm field-of-view probe-based confocal microendoscope for large-area visualization in the gastrointestinal tract
Rapid large-area tissue imaging at the cellular resolution is important for clinical diagnosis. We present a probe-based confocal microendoscope (pCM) with a field-of-view (FOV) diameter over 500 μm, lateral resolution of 1.95 μm, and outer diameter of 2.6 mm, compatible with the biopsy channel of conventional gastroscopes. Compared to a conventional pCM, our system’s FOV increased by a factor of 4 while maintaining the probe size and cellular resolution—comparable to the FOV of Zeiss Axio Observer’s
numerical aperture objective lens. Ex vivo imaging of wide areas in healthy and diseased rat gastrointestinal tract tissues demonstrates the system’s practicality.
Time-varying materials in the presence of dispersion: plane-wave propagation in a Lorentzian medium with temporal discontinuity
We study the problem of a temporal discontinuity in the permittivity of an unbounded medium with Lorentzian dispersion. More specifically, we tackle the situation in which a monochromatic plane wave forward-traveling in a (generally lossy) Lorentzian-like medium scatters from the temporal interface that results from an instantaneous and homogeneous abrupt temporal change in its plasma frequency (while keeping its resonance frequency constant). In order to achieve momentum preservation across the temporal discontinuity, we show how, unlike in the well-known problem of a nondispersive discontinuity, the second-order nature of the dielectric function now gives rise to two shifted frequencies. As a consequence, whereas in the nondispersive scenario the continuity of the electric displacement
and the magnetic induction suffices to find the amplitude of the new forward and backward wave, we now need two extra temporal boundary conditions. That is, two forward and two backward plane waves are now instantaneously generated in response to a forward-only plane wave. We also include a transmission-line equivalent with lumped circuit elements that describes the dispersive time-discontinuous scenario under consideration.
We demonstrate that multiple higher-order topological transitions can be triggered via the continuous change of the geometry in kagome photonic crystals composed of three dielectric rods. By tuning a single geometry parameter, the photonic corner and edge states emerge or disappear with higher-order topological transitions. Two distinct higher-order topological insulator phases and a normal insulator phase are revealed. Their topological indices are obtained from symmetry representations. A photonic analog of the fractional corner charge is introduced to distinguish the two higher-order topological insulator phases. Our predictions can be readily realized and verified in configurable dielectric photonic crystals.
The orbital angular momentum (OAM) carried by photons defines an infinitely dimensional discrete Hilbert space. With OAM modes, high-dimensional quantum states can be achieved for quantum communication and cryptography. Here we demonstrate a heralded single-photon source with a switchable OAM mode, which consists of a heralded single-photon source and an integrated OAM emitter as the mode converter. As the first step, the heralded single-photon source is based on the dispersion-shifted fiber. In this work, the OAM mode (quantized by topological charge
) carried by the heralded single photon (at fixed wavelength of 1555.75 nm) can be switched within the range of while the mode purity is more than 80%.
We demonstrate a blind zone-suppressed and flash-emitting solid-state Lidar based on lens-assisted beam-steering technology. As a proof-of-concept demonstration, with the design of a subwavelength-gap 1D long-emitter array and multiwavelength flash beam emitting, the device was measured to have 5% blind zone suppression, 0.06°/point-deflection step, and 4.2 μs scanning speed. In time-of-flight ranging experiments, Lidar systems have a field of view of
(normal device) or (blind-zone suppressed device), far-field number of resolved points of 192, and a detection distance of 10 m. This work demonstrates the possibility that a new integrated beam-steering technology can be implemented in a Lidar without sacrificing other performance.
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