We introduce a reflection-mode diffraction tomography technique that enables the simultaneous recovery of forward- and backward-scattering information for high-resolution 3D refractive index reconstruction. Our technique works by imaging a sample on a highly reflective substrate and employing a multiple-scattering model and a reconstruction algorithm. It combines the modified Born series as the forward model, Bloch and perfect electric conductor boundary conditions to handle oblique incidence and substrate reflections, and the adjoint method for efficient gradient computation in solving the inverse-scattering problem. We validate the technique through simulations and experiments, achieving accurate reconstructions in samples with high refractive index contrasts and complex geometries. Forward scattering captures smooth axial features, while backward scattering reveals complementary interfacial details. Experimental results on dual-layer resolution targets, 3D randomly distributed beads, phase structures obscured by highly scattering fibers, fixed breast cancer cells, and fixed C. elegans demonstrate its robustness and versatility. This technique holds promise for applications in semiconductor metrology and biomedical imaging.

Coherent detection can provide enhanced receiver sensitivity and spectral efficiency in free-space optical (FSO) communications. However, turbulence can cause modal power coupling effects on a Gaussian data beam and significantly degrade the mixing efficiency between the data beam and a Gaussian local oscillator (LO) in the coherent detector. Specifically, for widely used single-mode-fiber (SMF)-coupled coherent detectors, such degradation is mainly caused by the significantly reduced efficiency when coupling the multi-mode data beam into the SMF. Optical phase conjugation (OPC) in a photorefractive crystal can “automatically” mitigate turbulence by (a) recording a back-propagated turbulence-distorted probe beam, and (b) creating a phase-conjugate beam that has the inverse phase distortion of the medium as the transmitted data beam. However, previously reported crystal-based OPC approaches for FSO links have demonstrated either: (1) a relatively fast response time of 35 ms but at a relatively low data rate (e.g., <1Mbit/s), or (2) a relatively high data rate of 2-Gbit/s but at a slow response time (e.g., >60s). Here, we report an OPC approach for the automatic mitigation of dynamic turbulence that enables both a high data rate (8 Gbit/s) data beam and a rapid (<5ms) response time. For a similar data rate, this represents a 10,000-fold faster response time than previous reports, thereby enabling mitigation for dynamic effects. In our approach, the transmitted pre-distorted phase-conjugate data beam is generated by four-wave mixing in a GaAs crystal of three input beams: a turbulence-distorted probe beam, a Gaussian reference beam regenerated from the probe beam, and a Gaussian data beam carrying a high-speed data channel. We experimentally demonstrate our approach in an 8-Gbit/s quadrature-phase-shift-keying coherent FSO link through emulated dynamic turbulence. Our results show an up to ∼10-dB improvement in the free-space-to-SMF coupling efficiency for the data beam under dynamic turbulence with a bandwidth of up to ∼260Hz (Greenwood frequency). Our approach has the potential to significantly increase the resilience of high-performance coherent FSO links to turbulence.

Raman spectroscopy is an essential optical tool for tracing gases that exhibit weak or no infrared absorption. Existing Raman spectroscopic methods are deficient for precision sensing applications due to the extremely low Raman cross-section of gas. Herein, we report an approach, named stimulated Raman photothermal spectroscopy (SRPTS), to indirectly probe stimulated Raman scattering (SRS) via detecting the induced photothermal phase modulation in a gas-filled hollow-core fiber (HCF). Photothermal interferometry enables a linear increase of the SRPTS signal with the product of the pump and Stokes power while maintaining low background noise at the idle probe wavelength. The HCF tightly confines the light beams and the gas sample to significantly enhance both the SRS gain and the photothermal phase modulation efficiency. Preliminary experimentation with a 3.9-m-long HCF demonstrates hydrogen detection with a noise equivalent concentration of 6.8 ppm (parts-per-million) under 100 s averaging time and 6 bar gas pressure, indicating the potential for high-precision gas detection in chemical, medical, and energy industries.

Light, being massless, casts no shadow; under ordinary circumstances, photons pass right through each other unimpeded. Here, we demonstrate a laser beam acting like an object — the beam casts a shadow upon a surface when the beam is illuminated by another light source. We observe a regular shadow in the sense it can be seen by the naked eye, it follows the contours of the surface it falls on, and it follows the position and shape of the object (the laser beam). Specifically, we use a nonlinear optical process involving four atomic levels of ruby. We are able to control the intensity of a transmitted laser beam by applying another perpendicular laser beam. We experimentally measure the dependence of the contrast of the shadow on the power of the laser beam, finding a maximum of approximately 22%, similar to that of a shadow of a tree on a sunny day. We provide a theoretical model that predicts the contrast of the shadow. This work opens new possibilities for fabrication, imaging, and illumination.

Short-reach optical interconnects, employing an 850 nm wavelength multimode vertical cavity surface emitting laser (VCSEL) and multimode fiber (MMF), are confronted with transmission distance limitation as the transmission speed increases. Achieving higher speeds over the same transmission distance necessitates a significant enhancement in the fiber’s effective bandwidth, resulting in substantial costs. Employing single-mode VCSEL with single-mode fiber transmission presents as a more economically viable solution. Here we report, to our knowledge, the first demonstration of an 850 nm single-mode VCSEL using a chirped high-contrast metastructure (HCM) as the top mirror with spatially graded reflectivity to suppress higher-order modes. The chirped HCM top reflector is designed to favor lasing of the fundamental mode. We show stable single-mode lasing with a >40dB side mode suppression ratio (SMSR) up to 15 times threshold current and an open eye through a short single-mode fiber SM800 transmission at 106 Gbps PAM4 modulation. This study highlights the potential for scalable single-mode VCSEL in advanced optical interconnects, providing a cost-effective pathway for high-speed applications.

The technologies to examine the neuronal microenvironment label free remain critically underexplored. There is a gap in our knowledge of underlying metabolic, biochemical, and electrophysiological mechanisms behind several neurological processes at a cellular level, which can be traced to the lack of versatile and high-throughput tools to investigate neural networks. In this paper, four label-free contrasts were explored as mechanisms to study neuronal activity, namely, scattering, birefringence, autofluorescence from metabolic cofactors and molecules, and local biochemistry. To overcome challenges of observing neuronal activity spanning three orders of magnitude in space and time, microscopes had to be developed to simultaneously capture these contrasts quickly, with high resolution, and over a large FOV. We developed versatile autofluorescence lifetime, multiharmonic generation, polarization-sensitive interferometry, and Raman imaging in epi-detection (VAMPIRE) microscopy to simultaneously observe multiple facets of neuronal structure and dynamics. The accelerated computational-imaging-driven acquisition speeds, the utilization of a single light source to evoke all contrasts, the simultaneous acquisition that provides an otherwise impossible multimodal dynamic imaging capability, and the real-time processing of the data enable VAMPIRE microscopy as a powerful imaging platform for neurophotonics and beyond.

The large-scale clock network is the key ingredient to obtain high precision in many scenarios, from fundamental research to cutting-edge applications. The advantage of the time synchronization among microwave clocks is their cost, size, and accessibility. Here, we demonstrate a femtosecond-level time synchronization of microwave clocks through a commercial link of 205.86 km via dual-comb-enhanced optical two-way time transfer, which achieves a 6.23-fs residual time deviation between synchronized timescales at 1 s and an instability below 6×10−18 at 10,000 s. Further, the high-precision time synchronization of microwave clocks significantly enhances the probe ability of subtle reciprocity changes of fiber to the sub-picosecond level. This work provides a path toward secure fiber time-frequency networks to support future microwave-clock-based precise timing and sensing systems.

Frequency-modulated (FM) combs are produced by mode-locked lasers in which the electric field has a linearly chirped frequency and nearly constant amplitude. This regime of operation occurs naturally in certain laser systems and constitutes a valuable alternative to generate spectra with equidistant modes. Here, we use a low-noise fs-pulse comb as the local oscillator and combine dual comb heterodyne detection with time domain analysis of the multi-heterodyne signal to reveal the temporal trace of both amplitude and phase quadratures of FM comb lasers’ electric field. This technique is applied to both a dense and a harmonic mid-infrared free-running quantum cascade laser frequency comb and shows direct evidence of the FM behavior together with the high degree of coherence of these sources. Our results furnish a deeper insight on the origin of the FM combs and pave the way to further improvement and optimization of these devices.

The use of vortex laser beams has increased in various areas of optics and photonics, necessitating enhanced beam quality for wide usage. In the field of high-power lasers, a range of methods to implement vortex laser beams has been proposed. Nonetheless, enhancing beam quality becomes more challenging as the laser power increases. This study presents the successful implementation of a high-quality, high-intensity vortex laser beam exceeding 1020W/cm2—an essential advancement for enhancing the performance and applicability of high-power lasers. We achieved this by integrating a spiral phase mirror (SPM) with an orbital angular momentum of l=1, specially designed for the vortex beam, along with an adaptive optical system, into a 150-TW femtosecond Ti:sapphire laser. Importantly, SPM, featuring a modulation structure to correct for a 45° incidence angle, was utilized to produce the vortex beam. The adaptive optical system, comprising a Shack–Hartmann wavefront sensor and a deformable mirror, employed a feedback loop to correct wavefront aberrations distorting the vortex beam. Notably, the negligible difference between the Hartmanngrams of the Gaussian and vortex beams (l=1) allowed the Gaussian beam’s Hartmanngram to serve effectively as the input reference for wavefront correction, resulting in a successfully corrected vortex beam wavefront. Experimental results, both pre- and post-wavefront correction, were compared with simulation results obtained via field-tracing. The peak intensity of the focused high-quality vortex laser beam ultimately reached 1.8×1020W/cm2, exceeding previously reported experimental results. The results of this study could significantly contribute to the exploration of high-intensity angular momentum transfer in relativistic laser-plasma interactions.

Spatial optical analog differentiation allows ultrahigh-speed and low-power-consumption of image processing, as well as label-free imaging of transparent biological objects. Optical analog differentiation with broadband and incoherent sources is appealing for its multi-channels and multi-task information processing, as well as the high-quality differentiation imaging. Currently, broadband and incoherent optical differentiation is still challenging. Here, a compact and broadband achromatic optical spatial differentiator is demonstrated based on the intrinsic spin–orbit coupling in a natural thin crystal. By inserting a uniaxial crystal just before the camera of a conventional microscope, the spin to orbit conversion will embed an optical vortex to the image field and make a second-order topological spatial differentiation to the field, thus an isotropic differential image will be captured by the camera. The wavelength-independent property of the intrinsic spin–orbit coupling effect allows us to achieve broadband analog computing and achromatic spatial differentiation imaging. With this differentiation imaging method, both amplitude and pure phase objects are detected with high contrast. Transparent living cells and biological tissues are imaged with their edge contours and intracellular details protruded in the edge detection mode and edge enhancement mode, respectively. These findings pave the way for optical analog computing with broadband incoherent light sources and concurrently drive the advancement of high-performance and cost-effective phase contrast imaging.