Extreme single-excitation subradiance from two-band Bloch oscillations in atomic arrays

Atomic arrays provide an important quantum optical platform with photon-mediated dipole–dipole interactions that can be engineered to realize key applications in quantum information processing.A major obstacle for such applications is the fast decay of the excited states.By controlling two-band Bloch oscillations of single excitation in an atomic array under an external magnetic field,here we show that exotic subradiance can be realized and maintained with orders of magnitude longer than the spontaneous decay time in atomic arrays with the finite size.The key finding is to show a way for preventing the wavepacket of excited states scattering into the dissipative zone inside the free space light cone,which therefore leads to the excitation staying at a subradiant state for an extremely long decay time.We show that such operation can be achieved by introducing a spatially linear potential from the external magnetic field in the atomic arrays and then manipulating interconnected two-band Bloch oscillations along opposite directions.Our results also point out the possibility of controllable switching between superradiant and subradiant states,which leads to potential applications in quantum storage.

Frequency stabilization of C-band semiconductor lasers through a SiN photonic integrated circuit

Integrated semiconductor lasers represent essential building blocks for integrated optical components and circuits and their stability in frequency is fundamental for the development of numerous frontier applications and engineering tasks.When dense optical circuits are considered,the stability of integrated laser sources can be impaired by the thermal cross-talk generated by the action of neighboring components,leading to a deterioration of the long-term system performance(on the scale of seconds).In this work we show the design and the experimental characterization of a silicon nitride photonic integrated circuit(PIC)that is able to frequency stabilize 16semiconductor lasers,simultaneously.A stabilized 50 GHz-spaced two-channel system is demonstrated through the detection of the related beating note and the stability of the resulting waveform is characterized via the use of artificially induced thermal cross-talk stimuli.

Round-trip multi-band quantum access network

The quantum network makes use of quantum states to transmit data,which will revolutionize classical communication and allow for some breakthrough applications.Quantum key distribution(QKD)is one prominent application of quantum networks,and can protect data transmission through quantum mechanics.In this work,we propose an expandable and cost-effective quantum access network,in which the round-trip structure makes quantum states travel in a circle to carry information,and the multi-band technique is proposed to support multiuser access.Based on the round-trip multi-band quantum access network,we realize multi-user secure key sharing through the continuous-variable QKD(CV-QKD)protocol.Due to the encoding characteristics of CV-QKD,the quadrature components in different frequency bands can be used to transmit key information for different users.The feasibility of this scheme is confirmed by comprehensive noise analysis,and is verified by a proof-of-principle experiment.The results show that each user can achieve excess noise suppression and 600 bit/s level secure key generation under 30 km standard fiber transmission.Such networks have the ability of multi-user access theoretically and could be expanded by plugging in simple modules.Therefore,it paves the way for near-term largescale quantum secure networks.

High-efficiency reflector-less dual-level silicon photonic grating coupler

We present the design and experimentally demonstrate a dual-level grating coupler with subdecibel efficiency for a 220 nm thick silicon photonics waveguide which was fabricated starting from a 340 nm silicon-on-insulator wafer.The proposed device consists of two grating levels designed with two different linear apodizations,with opposite chirping signs,and whose period is varied for each scattering unit.A coupling efficiency of-0.8 d B at1550 nm is experimentally demonstrated,which represents the highest efficiency ever reported in the telecommunications C-band in a single-layer silicon grating structure without the use of any backreflector or indexmatching material between the fiber and the grating.

Spectrally programmable Raman fiber laser with adaptive wavefront shaping

Raman fiber lasers(RFLs)have broadband tunability due to cascaded stimulated Raman scattering,providing extensive degrees of freedom for spectral manipulation.However,the spectral diversity of RFLs depends mainly on the wavelength flexibility of the pump,which limits the application of RFLs.Here,a spectrally programmable RFL is developed based on two-dimensional spatial-to-spectral mapping of light in multimode fibers(MMFs).Using an intracavity wavefront shaping method combined with genetic algorithm optimization,we launch light with a selected wavelength(s)at MMF output into the active part of the laser for amplification.In contrast,the light of undesired wavelengths is blocked.We demonstrate spectral shaping of the high-order RFL,including a continuously tunable single wavelength and multiple wavelengths with a designed spectral shape.Due to the simultaneous control of different wavelength regions,each order of Raman Stokes light allows flexible and independent spectral manipulation.Our research exploits light manipulation in a fiber platform with multieigenmodes and nonlinear gain,mapping spatial control to the spectral domain and extending linear light control in MMFs to active light emission,which is of great significance for applications of RFLs in optical imaging,sensing,and spectroscopy.

Integrated photonic RF self-interference cancellation on a silicon platform for full-duplex communication

In-band full-duplex(IBFD) technology can double the spectrum utilization efficiency for wireless communications,and increase the data transmission rate of B5G and 6G networks and satellite communications. RF self-interference is the major challenge for the application of IBFD technology, which must be resolved. Compared with the conventional electronic method, the photonic self-interference cancellation(PSIC) technique has the advantages of wide bandwidth, high amplitude and time delay tuning precision, and immunity to electromagnetic interference.Integrating the PSIC system on chip can effectively reduce the size, weight, and power consumption and meet the application requirement, especially for mobile terminals and small satellite payloads. In this paper, the silicon integrated PSIC chip is presented first and demonstrated for IBFD communication. The integrated PSIC chip comprises function units including phase modulation, time delay and amplitude tuning, sideband filtering, and photodetection, which complete the matching conditions for RF self-interference cancellation. Over the wide frequency range of C, X, Ku, and K bands, from 5 GHz to 25 GHz, a cancellation depth of more than 20 dB is achieved with the narrowest bandwidth of 140 MHz. A maximum bandwidth of 630 MHz is obtained at a center frequency of10 GHz. The full-duplex communication experiment at Ku-band by using the PSIC chip is carried out. Cancellation depths of 24.9 dB and 26.6 dB are measured for a bandwidth of 100 MHz at central frequencies of 12.4 GHz and14.2 GHz, respectively, and the signal of interest(SOI) with 16-quadrature amplitude modulation is recovered successfully. The factors affecting the cancellation depth and maximum interference to the SOI ratio are investigated in detail. The performances of the integrated PSIC system including link gain, noise figure, receiving sensitivity, and spurious free dynamic range are characterized.

Broadband silicon-based tunable metamaterial microfluidic sensor

Tunable metamaterial absorbers play an important role in terahertz imaging and detection.We propose a multifunctional metamaterial absorber based on doped silicon.By introducing resonance and impedance matching into the absorber,a broadband absorption greater than 90%in the range of 0.8–10 THz is achieved.At the same time,the light regulation characteristics of the doped semiconductor are introduced into the absorber,and the precise amplitude control can be achieved in the range of 0.1–1.2 THz by changing the pump luminous flux.In addition,based on the principle of light-regulating the concentration of doped silicon carriers,the medium-doped silicon material is replaced by a highly doped silicon material,and a sensor with a sensitivity of up to 500 GHz/RIU is realized by combining the wave absorber with the microfluidic control.Finally,the broadband absorption characteristics and sensing performance of alcohol and water on the prepared device are verified by experiments,indicating that the absorber may have great potential in the field of sensor detection.

Dynamic full-field refractive index distribution measurements using total internal reflection terahertz digital holography

Massive usage scenarios prompt the prosperity of terahertz refractive index(THz RI) measurement methods.However, they are very difficult in measuring the full-field dynamical RI distributions of either solid samples without a priori thickness or liquid samples. In this study, we propose total internal reflection THz digital holography and apply it for measuring RI distributions for both solid and liquid samples dynamically. An RI measurement model is established based on an attenuated total reflection prism with a pitching angle. The pitching angle and the field of view can be numerically calculated from the spectrogram of the off-axis Fresnel hologram,which solves the adjustment of the visually opaque prism irradiated by the invisible THz beam. Full-field RI distributions of the droplets of solid-state soy wax and distilled water are obtained and compared with THz time-domain spectroscopy. The evaporation of an ethanol solution droplet is recorded, and the variation of the RI distribution at the sample–prism interface is quantitatively visualized with a temporal resolution of 10 Hz. The proposed method greatly expands the sample range for THz RI measurements and provides unprecedented insight into investigating spontaneous and dynamic THz phenomena.

Single-shot 3D tracking based on polarization multiplexed Fourier-phase camera:erratum

Figures 5(b)and 5(c)in the original article[1]are not consistent with their captions.Correct images are shown as follows.The article[1]was corrected online on 29 March 2022.

Strong coupling between colloidal quantum dots and a microcavity with hybrid structure at room temperature

The interaction between light and matter has always been the focus of quantum science, and the realization of truly strong coupling between an exciton and the optical cavity is a basis of quantum information systems. As a special semiconductor material, colloidal quantum dots have fascinating optical properties. In this study, the photoluminescence spectra of colloidal quantum dots are measured at different collection angles in microcavities based on hybrid refractive-index waveguides. The photon bound states in the continuum are found in the low–high–low refractive-index hybrid waveguides in the appropriate waveguide width region, where the photoluminescence spectra of colloidal quantum dots split into two or more peaks. The upper polaritons and lower polaritons avoid resonance crossings in the systems. The Rabi splitting energy of 96.0 meV can be obtained. The observed phenomenon of vacuum Rabi splitting at room temperature is attributed to the strong coupling between quantum dots and the bound states in the continuum.

Continuous-wave operation of 405 nm distributed Bragg reflector laser diodes based on GaN using 10th-order surface gratings

Single longitudinal mode continuous-wave operation of GaN-based distributed Bragg reflector(DBR) laser diodes with 10 th-order surface gratings is demonstrated. The DBR consists of periodic V-shaped grooves on a 2 μm wide ridge waveguide fabricated by using electron-beam lithography and plasma etching. The effect of different lengths of the DBR section and the gain section on the device performance has been studied. Periodic mode hops to the adjacent longitudinal Fabry–Perot resonator mode at shorter wavelength have been observed when increasing the operation current. Between the mode hops, single longitudinal mode emission at around 405 nm is achieved with a full width at half-maximum of 0.03 nm. A linear redshift of the emission wavelength with increasing temperature of 0.019 nm/K was derived.

All-optical sampling of few-cycle infrared pulsesusing tunneling in a solid

Recent developments in ultrafast laser technology have resulted in novel few-cycle sources in the mid-infrared.Accurately characterizing the time-dependent intensities and electric field waveforms of such laser pulses is essential to their applications in strong-field physics and attosecond pulse generation,but this remains a challenge.Recently,it was shown that tunnel ionization can provide an ultrafast temporal“gate”for characterizing highenergy few-cycle laser waveforms capable of ionizing air.Here,we show that tunneling and multiphoton excitation in a dielectric solid can provide a means to measure lower-energy and longer-wavelength pulses,and we apply the technique to characterize microjoule-level near-and mid-infrared pulses.The method lends itself to both all-optical and on-chip detection of laser waveforms,as well as single-shot detection geometries.

Si-substrate LEDs with multiple superlattice interlayers for beyond 24 Gbps visible light communication

High-speed visible light communication(VLC)using light-emitting diodes(LEDs)is a potential complementary technology for beyond-5 G wireless communication networks.The speed of VLC systems significantly depends on the quality of LEDs,and thus various novel LEDs with enhanced VLC performance increasingly emerge.Among them,In Ga N/Ga N-based LEDs on a Si-substrate are a promising LED transmitter that has enabled VLC data rates beyond 10 Gbps.The optimization on the period number of superlattice interlayer(SL),which is a stressrelief epitaxial layer in a Si-substrate LED,has been demonstrated to be an effective method to improve Si-substrate LED’s luminescence properties.However,this method to improve LED’s VLC properties is barely investigated.Hence,we for the first time experimentally studied the impact of SL period number on VLC performance.Accordingly,we designed and fabricated an integrated 4×4 multichromatic Si-substrate wavelength-divisionmultiplexing LED array chip with optimal SL period number.This chip allows up to 24.25 Gbps/1.2 m VLC transmission using eight wavelengths,which is the highest VLC data rate for an In Ga N/Ga N LED-based VLC system to the best of our knowledge.Additionally,a record-breaking data rate of 2.02 Gbps over a 20-m VLC link is achieved using a blue Si-substrate LED with the optimal SL period number.These results validate the effectiveness of Si-substrate LEDs for both high-speed and long-distance VLC and pave the way for Si-substrate LED design specially for high-speed VLC applications.

Flexible,video-rate,and aberration-compensated axial dual-line scanning imaging with field-of-view jointing and stepped remote focusing

Parallel dual-plane imaging with a large axial interval enables the simultaneous observation of biological structures and activities in different views of interest.However,the inflexibility in adjusting the field-of-view(FOV)positions in three dimensions and optical sectioning effects,as well as the relatively small effective axial range limited by spherical aberration,have hindered the application of parallel dual-plane imaging.Herein,we propose a flexible,video-rate,and defocus-aberration-compensated axial dual-line scanning imaging method.We used a stepped mirror to remotely generate and detect dual axial lines with compensation for spherical aberration and FOV-jointing to rearrange into a head-to-head line for high-speed optical sectioning acquisition.The lateral and axial positions of the two FOVs could be flexibly adjusted before and during imaging,respectively.The method also allows the adjustment of optical sectioning effects according to specific experimental requirements.We experimentally verified the consistent imaging performance over an axial range of 300μm.We demonstrated high throughput by simultaneously imaging Brownian motions in two 250μm×250μm FOVs with axial and lateral intervals of 150μm and 240μm,respectively,at 24.5 Hz.We also showed potential application in functional imaging by simultaneously acquiring neural activities in the optic tectum and hindbrain of a zebrafish brain.The proposed method is,thus,advantageous compared to existing parallel dual-plane imaging and potentially facilitates intravital biological study in large axial range.

Ultrastable Gd^(3+)doped CsPbCl_(1.5)Br_(1.5) nanocrystals blue glass for regulated and low thresholds amplified spontaneous emission

Here,Gd-doped CsPbCl_(1.5)Br_(1.5) nanocrystals(NCs)in borosilicate glass matrix(B_(2)O_(3)−SiO_(2)−ZnO)were prepared by melting quenching and in-situ crystallization.The optical performance of CsPbCl_(1.5)Br_(1.5) NCs glasses under different heat-treatment temperatures and the content of Gd^(3+) were analyzed in detail.After CsPbCl_(1.5)Br_(1.5) NCs glass is doped with Gd^(3+)ions,the photoluminescence intensity increases and the synthesized Gd-doped CsPbCl_(1.5)Br_(1.5) NCs glasses have excellent water stability and thermal cycling performance.In addition,the influence of Gd-doped concentrations and heat-treatment temperatures on the amplified spontaneous emission(ASE)thresholds of CsPbCl_(1.5)Br_(1.5) NCs glasses was studied,and the Gd-doped CsPbCl_(1.5)Br_(1.5) NCs glasses achieve controllable ASE thresholds at room temperature.The ASE threshold can be as low as 0.39 mJ∕cm^(2).This work offers a neoteric reference for the research in the application of metal ion-doped perovskite NCs and a new idea for the realization of controllable and low ASE thresholds on perovskite NCs.

Photonic extreme learning machine by free-space optical propagation

Photonic brain-inspired platforms are emerging as novel analog computing devices,enabling fast and energyefficient operations for machine learning.These artificial neural networks generally require tailored optical elements,such as integrated photonic circuits,engineered diffractive layers,nanophotonic materials,or time-delay schemes,which are challenging to train or stabilize.Here,we present a neuromorphic photonic scheme,i.e.,the photonic extreme learning machine,which can be implemented simply by using an optical encoder and coherent wave propagation in free space.We realize the concept through spatial light modulation of a laser beam,with the far field acting as a feature mapping space.We experimentally demonstrate learning from data on various classification and regression tasks,achieving accuracies comparable with digital kernel machines and deep photonic networks.Our findings point out an optical machine learning device that is easy to train,energetically efficient,scalable,and fabrication-constraint free.The scheme can be generalized to a plethora of photonic systems,opening the route to real-time neuromorphic processing of optical data.

Circular dichroism-like response of terahertz wave caused by phase manipulation via all-silicon metasurface

Chiral metasurfaces based on asymmetric meta-atoms have achieved artificial circular dichroism(CD),spin-dependent wavefront control,near-field imaging,and other spin-related electromagnetic control.In this paper,we propose and experimentally verify a scheme for achieving high-efficiency chiral response similar to CD of terahertz(THz)wave via phase manipulation.By introducing the geometric phase and dynamic phase in an all-silicon metasurface,the spin-decoupled terahertz transmission is obtained.The giant circular dichroism-like effect in the transmission spectrum is observed by using a random phase distribution for one of the circular polarization components.More importantly,the effect can be adjusted when we change the area of the metasurface illuminated by an incident terahertz beam.In addition,we also demonstrate the spin-dependent arbitrary wavefront control of the transmitted terahertz wave,in which one of the circularly polarized components is scattered,while the other forms a focused vortex beam.Simulated and experimental results show that this method provides a new idea for spin selective control of THz waves.

Individually resolved luminescence from closely stacked GaN/AlN quantum wells

Investigating closely stacked GaN/AlN multiple quantum wells(MQWs)by means of cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope,we have reached an ultimate spatial resolution ofσCL=1.8 nm.The pseudomorphically grown MQWs with high interface quality emit in the deep ultraviolet spectral range.Demonstrating the capability of resolving the 10.8 nm separated,ultra-thin quantum wells,a cathodoluminescence profile was taken across individual ones.Applying a diffusion model of excitons generated by a Gaussian-broadened electron probe,the spatial resolution of cathodoluminescence down to the free exciton Bohr radius scale has been determined.

Ultraviolet-to-microwave room-temperature photodetectors based on three-dimensional graphene foams

Highly sensitive broadband photodetection is of critical importance for many applications.However,it is a great challenge to realize broadband photodetection by using a single device.Here we report photodetectors(PDs)based on three-dimensional(3 D)graphene foam(GF)photodiodes with asymmetric electrodes,which show an ultra-broadband photoresponse from ultraviolet to microwave for wavelengths ranging from 10~2 to 10~6 nm.Moreover,the devices exhibit a high photoresponsivity of 10~3 A·W^-1,short response time of 43 ms,and3 d B bandwidth of 80 Hz.The high performance of the devices can be attributed to the photothermoelectric(PTE,also known as the Seebeck)effect in 3 D GF photodiodes.The excellent optical,thermal,and electrical properties of 3 D GFs offer a superior basis for the fabrication of PTE-based PDs.This work paves the way to realize ultra-broadband and high-sensitivity PDs operated at room temperature.

Laser fabrication of graphene-based supercapacitors

Supercapacitors(SCs)have broad applications in wearable electronics(e.g.,e-skin,robots).Recently,graphenebased supercapacitors(G-SCs)have attracted extensive attention for their excellent flexibility and electrochemical performance.Laser fabrication of G-SCs exhibits obvious superiority because of the simple procedures and integration compatibility with future electronics.Here,we comprehensively summarize the state-of-the-art advancements in laser-assisted preparation of G-SCs,including working mechanisms,fabrication procedures,and unique characteristics.In the working mechanism section,electric double-layer capacitors and pseudocapacitors are introduced.The latest advancements in this field are comprehensively summarized,including laser reduction of graphene oxides,laser treatment of graphene prepared from chemical vapor deposition,and laserinduced graphene.In addition,the unique characteristics of laser-enabled G-SCs,such as structured graphene,graphene hybrids,and heteroatom doping graphene-related electrodes,are presented.Subsequently,laser-enabled miniaturized,stretchable,and integrated G-SCs are also discussed.It is anticipated that laser fabrication of G-SCs holds great promise for developing future energy storage devices.