The Hong–Ou–Mandel interference plays a vital role in many quantum optical applications where indistinguishability of two photons is important. Such photon pairs are commonly generated as the signal and idler in the polarization-degenerate spontaneous parametric downconversion (SPDC). To scale this approach to a larger number of photons, we demonstrate how two independent signal photons radiated into different spatial modes can be rendered conditionally indistinguishable by a heralding measurement performed on their respective idlers. We use the SPDC in a whispering gallery resonator, which is already proven to be versatile sources of quantum states. Its extreme conversion efficiency allowed us to perform our measurements with only 50 nW of in-coupled pump power in each propagation direction. The Hong–Ou–Mandel interference of two counterpropagating signal photons manifested itself in the fourfold coincidence rate, where the detection of two idler photons heralds a pair of signal photons with a desired temporal overlap. We achieved the Hong–Ou–Mandel dip contrast of 74% ± 5%. Importantly, the optical bandwidth of all involved photons is of the order of a MHz and is continuously tunable. This, on the one hand, makes it possible to achieve the necessary temporal measurement resolution with standard electronics and, on the other hand, creates a quantum state source compatible with other candidates for qubit implementation, such as optical transitions in solid-state or vaporous systems. We also discuss the possibility of generating photon pairs with similar temporal modes from two different whispering gallery resonators.

Computational temporal ghost imaging (CTGI) allows for retrieving the fast temporal object using a slow photodetector and can naturally encrypt the transmission data to enhance communication security. Here, we first demonstrate free-space data encryption based on frequency down-conversion CTGI in the mid-infrared (MIR) atmospheric window, where commercial fast photodetectors are lacking. The target temporal signal is digitally encrypted by a secret key and modulated at 1.5 μm light by an acousto-optic intensity modulator (AOM). The encrypted signal is then optically transferred to MIR light through nonlinear frequency down-conversion. The data decryption can be realized from the correlation operation between the intensity recorded by a 1 MHz bandwidth MIR detector and the secret key. Moreover, when the data rate is significantly higher than the bandwidth of the AOM, the modulated optical signal will be severely distorted, which adds extra physical encryption. As a result, we experimentally demonstrate MIR data encryption and transmission over a 3 m free-space link at a speed more than 60 times higher than the MIR detector bandwidth in a 3.3–3.5 μm spectral region. We anticipate the frequency down-conversion CTGI could provide a new platform for secure free-space communication in the MIR region.

Phase measuring deflectometry has been proven as a competitive technique for measuring complex optical surfaces. However, the commonly used imaging model simply treats the light beam associated with each camera pixel as a single ray, which in turn leads to significant measurement errors for complex surfaces with high curvedness, low reflectivity, or nonideal specularity. To broaden the measurable scope of deflectometry, beam phase measuring deflectometry is proposed. A beam imaging model is developed by defining a discrete light field function and assigning a beam composed of a set of rays to each camera pixel. The conventional framework of the one-to-one mapping between the points on the screen, camera, and surface under test is broken through for the first time, enabling a direct physics-compliant description of the imaging process in deflectometric measurement. The flexibility of the imaging model is guaranteed by a set of tunable parameters, and the difficulty of intensity quantification of images is bypassed through phase rendering. The proposed method shows superior accuracy over traditional approaches for complex surfaces, which reduces the measurement errors from an order of micrometers to hundreds of nanometers, while maintaining numerical reliability and measurement efficiency.

Perfect vortices have garnered significant attention due to their role as orbital angular momentum (OAM) beams with tunable ring-like intensity distributions. More recently, the non-separable combination of perfect vortices with opposite OAM and spin, known as perfect vector vortices, has extended their applicability in optical manipulation, high-resolution lithography, imaging, and telecommunications. The generation of perfect vector vortices with a size that remains constant regardless of the encoded topological charge typically requires both phase and amplitude modulation. In this study, we propose an efficient and innovative solution that leverages a multifocal approach encoded on phase-only silicon metalenses. A simultaneous control over dynamic and geometric phases was achieved through the careful design and fabrication of the individual meta-atoms comprising the metalens. This meta-atom engineering approach, modulating both phase components, enabled the generation and characterization of orbital angular momentum-independent perfect vector beams with an improved depth of focus. These devices represent a crucial step toward developing integrated optical architectures for information and communication technologies, with potential applications in both classical and quantum domains.

The extraction of singly scattered photons is crucial for achieving high-quality imaging. Singular value decomposition of the optical transmission matrix (TM) can effectively separate singly and multiply scattered photons. However, traditional TM construction methods usually require capturing a large number of light field images, a process that is both time-consuming and complex, limiting its widespread use in practical applications. This paper introduces the light field contribution matrix (LFCM) model, which separates singly and multiply scattered photons using a single light field collection under low scattering conditions, enhancing measurement efficiency and enabling imaging in dynamic environments. Experimental results show that under low scattering conditions with a high proportion of singly scattered photons, the technique doubles the signal-to-noise ratio and increases the contrast by 1.5 times. In fly brain structure imaging experiments, LFCM successfully revealed hidden features and details, demonstrating its advantages in complex structural analysis. Additionally, since the technique is based on light field intensity information imaging, it has potential applications in digital image processing, particularly for reducing image blur caused by scattering, as demonstrated in the case of scatter blur correction of digital images of cataracts in the human eye. Therefore, this method provides a practical and efficient solution for high-quality imaging under low scattering conditions.

We propose and demonstrate a broadband two-dimensional multi-beam optical phased array (MOPA) with four beams based on a Butler matrix on a silicon nitride (Si3N4) platform. A beamforming network utilizing cascaded Butler matrix splitting arrays has been proposed to scale up the MOPA for the first time, to the best of our knowledge, significantly reducing the number of waveguide crossings. A 32-channel MOPA with four beams has been designed and fabricated with a uniform antenna pitch of 3.5 μm. Experimental results indicate that the field of view is 25.6° × 8.1°, with a beam divergence of 0.655° × 0.085°. Simultaneous multi-beam 2D high-power operation and independently scanning along the wavelength steering axis have been demonstrated. Furthermore, parallel and high-speed free-space optical communication based on the fabricated MOPA has been performed. Data transmission of 40 Gbps non-return-to-zero on–off keying has been achieved, revealing the potential of low system complexity and power-efficient multi-access data transmission.

Exceptional points (EPs), representing unique non-Hermitian degeneracies in open systems, have been the focus of significant research due to their remarkable phenomena and applications across various fields. While second-order EPs are extensively studied, higher-order EPs—requiring the degeneracy of multiple eigenvalues and corresponding eigenvectors—remain less explored. In this work, we propose a generic approach for realizing, 2Nth-order EP in an N-element photonic molecule (PM). Differing from a common PM consisting of identical microresonators, here a spiral-shaped microresonator is incorporated to form an asymmetric PM, offering unidirectional coupling between eigenmodes and transferring the chirality throughout the whole PM. Moreover, the hierarchical construction of 2Nth-order EPs can be achieved in gain–loss asymmetric PMs, as well as in all-passive ones. In the prototype demonstration based on numerical simulations, the obtained fourth-order EP reveals its superior sensitivity with a three-fold enhancement compared with that of a second-order EP to external perturbations at the single-molecule level. We envisage that the proposed asymmetric PM, circumventing the need for external components or precise control of gain–loss profiles, will pave the way for constructing higher-order EPs in integrated photonic devices and enabling applications in both linear and nonlinear regimes.

Light propagation through artificially patterned anisotropic materials, such as dielectric metasurfaces, enables precise control of the spatio-vectorial properties of optical fields using highly transparent, thin, and flat optical elements. Liquid-crystal cells are a common realization of such devices. Optical losses are typically assumed to be polarization-independent and are therefore often overlooked in modeling these systems. In this work, we introduce electrically tunable liquid-crystal metasurfaces with patterned birefringence and dichroism, achieved by incorporating dichroic dye molecules into the liquid-crystal mixture. These dye molecules align with the liquid crystal, effectively coupling birefringence and dichroism effects. The behavior of these metasurfaces is described using non-unitary Jones matrices, validated through polarimetric measurements. In the case of devices that are patterned to form polarization gratings, we also characterize the diffraction efficiency as a function of the dichroism and birefringence parameters, which can be tuned jointly by applying an electric field across the cell. This study not only introduces a new class of optical components but also deepens our understanding of light propagation through anisotropic materials, where dichroism can naturally arise from bulk material properties or from reflection and transmission laws at their interfaces.

Kerr-microresonator frequency combs in integrated photonics waveguides are promising technologies for next-generation positioning, navigation, and timing applications, with advantages that include platforms that are mass-producible and CMOS-compatible and spectra that are phase-coherent and octave-spanning. Fundamental thermal noise in the resonator material typically limits the timing and frequency stability of a microcomb. The small optical mode volume of the microresonators exaggerates this effect, as it both increases the magnitude and shortens the timescale of thermodynamic fluctuations. In this work, we investigate thermal instability in silicon nitride microring resonators as well as techniques for reducing their effects on the microcomb light. We characterize the time-dependent thermal response in silicon nitride microring resonators through experimental measurements and finite element method simulations. Through fast control of the pump laser frequency, we reduce thermal recoil due to heating. Finally, we demonstrate the utility of a coupled microresonator system with tunable mode interactions to increase the stability of a soliton against thermal shifts.

Label-free quantitative phase imaging is vital for optical microscopy and metrology applications. A multimode fiber stands out as a desirable platform for imaging. Here, we propose and experimentally demonstrate a non-interferometric non-iterative approach for high-speed high-resolution label-free quantitative phase imaging via a random light scattering in a multimode fiber.