Integrated photonics provides a promising platform for quantum key distribution (QKD) system in terms of miniaturization, robustness, and scalability. Tremendous QKD works based on integrated photonics have been reported. Nonetheless, most current chip-based QKD implementations require additional off-chip hardware to demodulate quantum states or perform auxiliary tasks such as time synchronization and polarization basis tracking. Here, we report a demonstration of resource-efficient chip-based BB84 QKD with a silicon-based encoder and a decoder. In our scheme, the time synchronization and polarization compensation are implemented relying on the preparation and measurement of the quantum states generated by on-chip devices; thus, we need no additional hardware. The experimental tests show that our scheme is highly stable with a low intrinsic quantum bit error rate of 0.50%±0.02% in a 6 h continuous run. Furthermore, over a commercial fiber channel up to 150 km, the system enables the realization of secure key distribution at a rate of 866 bit/s. Our demonstration paves the way for a low-cost, wafer-scale manufactured QKD system.

Dual-comb spectroscopy (DCS) has revolutionized numerous spectroscopic applications due to its high spectral resolution and fast measurement speed. Substantial efforts have been made to obtain a coherent dual-comb source at various spectral regions through nonlinear frequency conversion, where the preservation of coherence has become a problem of great importance. In this study, we report the generation of coherent dual-comb sources covering from the ultraviolet to mid-infrared region based on high-order harmonic generation. Driven by high-repetition-rate femtosecond mid-infrared dual-comb pump pulses, up to ninth-order harmonic was generated from the ultraviolet to mid-infrared region using an aperiodically poled lithium niobate waveguide. To investigate the coherence property of the high-order harmonic generation, DCS was performed at every generated spectral region from 450 to 3600 nm. The measured dual-comb spectra with distinctive tooth-resolved structures show the well-preserved coherence without apparent degradation after the cascaded quadratic nonlinear processes. The subsequent methane absorption spectroscopy at multiple spectral regions of different harmonics was carried out to characterize the spectroscopic capability of the system. These results demonstrate the potential of our scheme to generate compact and coherent broadband optical frequency combs for simultaneous multi-target detections.

We proposed and experimentally demonstrated a simple and novel photonic spiking neuron based on a distributed feedback (DFB) laser chip with an intracavity saturable absorber (SA). The DFB laser with an intracavity SA (DFB-SA) contains a gain region and an SA region. The gain region is designed and fabricated by the asymmetric equivalent π-phase shift based on the reconstruction-equivalent-chirp technique. Under properly injected current in the gain region and reversely biased voltage in the SA region, periodic self-pulsation was experimentally observed due to the Q-switching effect. The self-pulsation frequency increases with the increase of the bias current and is within the range of several gigahertz. When the bias current is below the self-pulsation threshold, neuronlike spiking responses appear when external optical stimulus pulses are injected. Experimental results show that the spike threshold, temporal integration, and refractory period can all be observed in the fabricated DFB-SA chip. To numerically verify the experimental findings, a time-dependent coupled-wave equation model was developed, which described the physics processes inside the gain and SA regions. The numerical results agree well with the experimental measurements. We further experimentally demonstrated that the weighted sum output can readily be encoded into the self-pulsation frequency of the DFB-SA neuron. We also benchmarked the handwritten digit classification task with a simple single-layer fully connected neural network. By using the experimentally measured dependence of the self-pulsation frequency on the bias current in the gain region as an activation function, we can achieve a recognition accuracy of 92.2%, which bridges the gap between the continuous valued artificial neural networks and spike-based neuromorphic networks. To the best of our knowledge, this is the first experimental demonstration of a photonic integrated spiking neuron based on a DFB-SA, which shows great potential to realizing large-scale multiwavelength photonic spiking neural network chips.

We report a high-power single-mode InP-based 2 μm distributed feedback (DFB) laser with a second-order buried grating and corrugated sidewalls. A second-order semiconductor grating is used for in-plane feedback and vertical out-coupling. The corrugated sidewalls are used to eliminate higher-order transverse modes. For the DFB laser with a 2 mm long cavity and 15 μm wide ridge, the maximum continuous-wave edge-emitting and surface-emitting single-mode powers at 300 K are up to 81 and 42 mW, respectively. A single-lobed far-field radiation pattern with a low divergence angle of approximately 8.6° is achieved by a device with a ridge width of 15 μm. The single-longitudinal-mode emission wavelength of the fabricated laser can be adjusted from 2003.8 nm at 288 K to 2006.9 nm at 313 K without any mode hopping. Robust single-mode emission with a side-mode suppression ratio of 30 dB is achieved under all injection currents and temperature conditions.

Temperature sensing is essential for human health monitoring. High-sensitivity (>1nm/°C) 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 (∼ms) 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 263.94(nm/°C)/s, 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.

Using freeform optical surfaces in lens design can lead to much higher system specifications and performance while significantly reducing volume and weight. However, because of the complexity of freeform surfaces, freeform optical design using traditional methods requires extensive human effort and sufficient design experience, while other design methods have limitations in design efficiency, simplicity, and versatility. Deep learning can solve these issues by summarizing design knowledge and applying it to design tasks with different system and structure parameters. We propose a deep-learning framework for designing freeform imaging systems. We generate the data set automatically using a combined sequential and random system evolution method. We combine supervised learning and unsupervised learning to train the network so that it has good generalization ability for a wide range of system and structure parameter values. The generated network FreeformNet enables fast generation (less than 0.003 s per system) of multiple-solution systems after we input the design requirements, including the system and structure parameters. We can filter and sort solutions based on a given criterion and use them as good starting points for quick final optimization (several seconds for systems with small or moderate field-of-view in general). The proposed framework presents a revolutionary approach to the lens design of freeform or generalized imaging systems, thus significantly reducing the time and effort expended on optical design.

Structured light carrying orbital angular momentum (OAM) opens up a new physical dimension for studying light–matter interactions. Despite this, the complex fields created by OAM beams still remain largely unexplored in terms of their effects on surface plasmons. This paper presents a revelation of anomalous plasmon excitations in single particles and plasmon couplings of neighboring nanorods under OAM beams, which are forbidden using non-OAM sources. The plasmon excitation of single nanoparticles is determined both by photon spin angular momentum (SAM) and OAM and influenced by the locations of the nanoparticles. Specifically, when SAM and OAM are equal in magnitude and opposite in direction, a pure plasmon excitation along light propagation direction is achieved. Two plasmon dipoles show end-to-end antibonding coupling and side-by-side bounding coupling, which are the opposite of the typical couplings. Furthermore, we observe Fano resonance with a nanorod dimer: one aligned along light propagation direction acting as the bright mode and the other aligned along the global polarization direction of light acting as the dark mode, which is the opposite of the usual plasmonic Fano resonance. By taking advantage of the unique property of the OAM source, this investigation presents a novel way to control and study surface plasmons, and the research of plasmon behavior with OAM would open new avenues for controlling electromagnetic waves and enriching the spectroscopies with more degrees of freedom.

We propose an integrated W-band transmitter enabled by an integrated dual-mode distributed feedback (DFB) laser and cascaded silicon photonic microring modulators for next-generation wireless communication. 10 Gb/s error-free intensity modulation and direct detection W-band transmission are achieved in experiments by using the dual-mode DFB laser and two free-running lasers. Moreover, we conduct an experiment of dual-carrier modulation based on cascaded microring modulators, achieving 3 dB signal-to-noise ratio improvement and better signaling integrity for wireless communication. The proposed photonic integrated W-band transmitter will be a viable solution for a high-speed and low-power wireless communication system.

Epsilon-near-zero (ENZ) media were demonstrated to exhibit unprecedented strong nonlinear optical properties including giant second-harmonic generation (SHG) due to their field-enhancement effect. Here, on the contrary, we report the quenching of SHG by the ENZ media. We find that when a tiny nonlinear particle is placed very close to a subwavelength ENZ particle, the SHG from the nonlinear particle can be greatly suppressed. The SHG quenching effect originates from the extraordinary prohibition of electric fields occurring near the ENZ particle due to evanescent scattering waves, which is found to be universal in both isotropic and anisotropic ENZ particles, irrespective of their shapes. Based on this principle, we propose a kind of dynamically controllable optical metasurface exhibiting switchable SHG quenching effect. Our work enriches the understanding of optical nonlinearity with ENZ media and could find applications in optical switches and modulators.

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 multi-user 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 large-scale quantum secure networks.

Low-intensity light detection necessitates high-responsivity photodetectors. To achieve this, we report In0.53Ga0.47As/InAs/In0.53Ga0.47As quantum well (InAs QW) photo-field-effect-transistors (photo-FETs) integrated on a Si substrate using direct wafer bonding. Structure of the InAs QW channel was carefully designed to achieve higher effective mobility and a narrower bandgap compared with a bulk In0.53Ga0.47As, while suppressing the generation of defects due to lattice relaxations. High-performance 2.6 nm InAs QW photo-FETs were successfully demonstrated with a high on/off ratio of 105 and a high effective mobility of 2370cm2/(V·s). The outstanding transport characteristics in the InAs QW channel result in an optical responsivity 1.8 times greater than InGaAs photo-FETs and the fast rising/falling times. Further, we experimentally confirmed that the InAs QW photo-FET can detect light in the short-wavelength infrared (SWIR; 1.0–2.5 μm) near 2 μm thanks to bandgap engineering through InAs QW structures. Our result suggests that the InAs QW photo-FET is promising for high-responsivity and extended-range SWIR photodetector applications.

We demonstrate a GeSi electro-absorption modulator with on-chip thermal tuning for the first time, to the best of our knowledge. Theoretical simulation proves that the device temperature can be tuned and the effective operating wavelength range can be broadened. When the heater power is 4.63 mW, the temperature of the waveguide increases by about 27 K and the theoretical operating wavelength range is broadened by 23.7 nm. The experimental results show that the optical transmission line shifted to the longer wavelength by 4.8 nm by every 1 mW heater power. The effective static operating wavelength range of the device is increased from 34.4 nm to 60.1 nm, which means it is broadened by 25.7 nm. The band edge shift coefficient of 0.76 nm/K is obtained by temperature simulation and linear fitting of the measured data. The device has a 3 dB EO bandwidth of 89 GHz at 3 V reverse bias, and the eye diagram measurement shows a data rate of 80 Gbit/s for non-return-to-zero on–off keying modulation and 100 Gbit/s for 4 pulse amplitude modulation in the 1526.8 nm to 1613.2 nm wavelength range as the heater power increases from 0 mW to 10.1 mW.

Optical microcavities offer a promising platform for highly efficient light–matter interactions. Recently, the combination of microresonators and 2D materials in the nanoscale has further enriched the optoelectronics of microcavity geometries, spurring broad advances including lasers, nonlinear converters, modulators, and sensors. Here, we report the concept of compact dual-laser cogeneration in a graphene-microcavity fiber, which offers a way to cancel the optical common mode noises. Driven by a single 980 nm pump, orthogonally polarized laser lines are generated in a pair of degeneracy breaking modes. The two laser lines produce a heterodyne beat note at 118.96 MHz, with frequency noise down to 200Hz2/Hz at 1 MHz offset, demonstrating a linewidth of 930 Hz in vacuum. This compact device enables on-line and label-free NH3 gas detection with high resolution, realizing a detection limit on a single pmol/L level, and a capability to quantitatively trace gas–graphene interactions. Such a combination of graphene optoelectronics and microcavity photonics demonstrates a novel physical paradigm for microlaser control and offers a new scheme for in situ chemical sensing.

Transient thermal instability represents a significant challenge in generating soliton microcombs. Fast laser sweep can be an efficient method to mitigate thermal instability, but it requires an ultrahigh laser sweep rate for crystalline microresonators with fast thermal relaxation. Here, we engineer a laser sweep waveform to generate AlN-on-sapphire soliton microcombs with an intermediate sweep speed (<30GHz/μs). Two laser sweep methods with backward plus forward tuning or two-step backward tuning added after the fast forward laser sweep were demonstrated to stabilize solitons. Reducing the soliton number is found to be useful to stabilize solitons in fast laser sweep. The effectiveness of the methods was numerically verified. Our measurements and simulations also reveal the impacts of different thermal relaxation processes occurring at quite different time scales on thermal instability. The requirement of the laser sweep protocols is discussed.

Whispering gallery mode (WGM) microbubble cavities are a versatile optofluidic sensing platform owing to their hollow core geometry. To increase the light–matter interaction and, thereby, achieve higher sensitivity, thin-walled microbubbles are desirable. However, a lack of knowledge about the precise geometry of hollow microbubbles prevents us from having an accurate theoretical model to describe the WGMs and their response to external stimuli. In this work, we provide a complete characterization of the wall structure of a microbubble and propose a theoretical model for the WGMs in this thin-walled microcavity based on the optical waveguide approach. Structural characterization of the wavelength-scale wall is enabled by focused ion beam milling and scanning electron microscopy imaging. The proposed theoretical model is verified by finite element method simulations. Our approach can readily be extended to other low-dimensional micro-/nanophotonic structures.