Yongchun Zhong

Structured-light manipulation has advanced rapidly, but most existing approaches rely on linear modulation. The nonlinear geometric phase offers a route to combine frequency conversion with wavefront control during harmonic generation. However, a platform that simultaneously provides solid-state stability and multifunctional structured-light generation remains needed. Here, we demonstrate a nonlinear structured-light platform based on the nonlinear Berry phase in lithium tantalate (LiTaO ₃ ). An eight-segment device with stepwise-rotated optical axes imprints a discrete 2 π phase winding on the second-harmonic (SH) field. Under circularly polarized excitation, it directly generates spin-dependent SH vortex beams with a central phase singularity. Under linear excitation, the coherent superposition of opposite-spin components produces vector vortex beams with spatially varying polarization. Coherent interference with a Gaussian SH reference further yields skyrmion-like polarization textures with a phase-tunable orientation. These results establish the nonlinear Berry phase as a versatile approach for frequency conversion, phase and polarization control, and topological-field generation.

Optical spatial differentiation utilizing spiral vortex phases provides an efficient framework for edge detection, pure-phase object imaging, and pattern recognition. However, the intrinsic dispersion of spiral phase elements leads to non-integer vortex orders under broadband illumination, which may limit their performance in white-light applications. Here, we systematically investigate the spatial differentiation imaging capability of a spiral phase element under white-light illumination. Using a spiral phase element designed for 532 nm, high-quality differentiation images are obtained for both pure amplitude and pure-phase objects under red, green, and blue LED source illuminations, generating integer and fractional optical vortices, respectively. The white-light imaging capability is further demonstrated through full-color static samples and living biological specimens, where sharp edge contrast is consistently preserved across the visible spectrum. These findings not only broaden the understanding of vortex-based imaging but also establish a practical, dispersion-tolerant strategy for broadband optical spatial differentiation.

The detection of low humidity levels remains a great challenge in relative humidity (RH) sensing technologies. In this work, methacryloxyethyl trimethyl ammonium chloride (DMC) was coated around SiO₂ microspheres to form DMC/SiO₂ composite microspheres, which were self-assembled into a three-dimensional (3D) network structure for low humidity detection. The hydrophilic nature of the DMC component enhances the adsorption capacity for water molecules even at ultra-low humidity levels (1–18.6% RH), while the 3D network structure provides abundant channels for fast water molecule transport, facilitating rapid response and recovery processes. The optimized sensor shows high response (13,544%) in 1–18.6% RH, with short response/recovery time (6 s/10 s) and a small humidity hysteresis (1.4% RH). Such high performance shows that this type of sensor has great potential for application in widespread fields, such as electricity, semiconductor manufacturing, pure gas supply, aerospace, and pharmaceutical formulations.

We report a deep learning-assisted quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for trace water vapor detection in air. A 1392 nm butterfly-packaged DFB laser is wavelength-modulated at f₀/2, and the QEPAS signal is retrieved by second-harmonic (2f) lock-in demodulation using a commercial quartz tuning fork gas cell. After optimizing the modulation depth to 400 mV, a 1D U-Net denoising network trained with pseudo-clean supervision is applied to the measured 2f traces, yielding an SNR improvement of 2.05× (3.11 dB). Allan deviation analysis indicates a minimum detection limit (MDL) of ~2.21 ppm at an optimum averaging time of ~619 s, corresponding to an ~2.1× improvement compared with the raw output. These results demonstrate that neural-network-based post-processing can improve QEPAS water vapor sensing performance without modifying the optical hardware.

A compact and high-sensitivity carbon monoxide (CO) detection system based on multi-pass cell enhanced light-induced thermoelastic spectroscopy (MPC-LITES) was developed for real-time monitoring. A 2.3 μm distributed feedback (DFB) diode laser targeting the CO absorption line at 4300.699 cm⁻¹ was employed, offering strong line intensity and minimal interference from H₂O, CO₂, NO₂, and SO₂. The optimal modulation depth of 0.76 cm⁻¹ produced the maximum second harmonic (2f) signal. Experimental results demonstrated excellent linearity (R² = 0.998) and a minimum detection limit of 230 ppb at 1 s, further reduced to 47 ppb at 367 s by Allan deviation analysis. Application tests were carried out for real-time monitoring of cigarette smoke in a 20 m² indoor environment. Under closed conditions, the CO concentration rapidly increased to approximately 165 ppm, while in ventilated conditions, it peaked at 45 ppm and decayed quickly due to air exchange. The results confirm that the proposed MPC-LITES sensor enables accurate, real-time detection of transient CO variations, demonstrating strong potential for indoor air quality evaluation, environmental safety, and public health protection.

As a promising sensing material, Molybdenum disulfide (MoS₂) nanosheets is being increasingly studied for Nitrogen dioxide (NO₂) gas sensing. However, the MoS₂ nanosheets is prone to the stacking effect that compromises the sensing performances. Here, the stacking effect is mitigated by engineering MoS₂ nanosheets into a three dimensional (3D) network microstructure, which was fabricated by method of electrostatically self-assembling of MoS₂/SiO₂ microspheres. The fabricated sensor based on 3D MoS₂ network observed a significantly improved response of 15% to 12.3 ppm NO₂, which is a 75-fold increase compared to the control sensor with pure MoS₂ nanosheets. In addition, the sensitivity of the sensor with 3D MoS₂ network was 6.15 times larger than that of the control sensor with pure MoS₂ nanosheets. The detection limit of our sensor was 0.297 ppm, lower than most of reported MoS₂-based NO₂ sensors. The enhanced sensitivity and dynamic response stem from the improved interaction between NO₂ molecules and MoS₂ network, thanks to its increased surface area per footprint of MoS₂ nanosheets compared to pure 2D MoS₂ film (single- or few-layer). This work presents a new approach to enhancing the performance of gas sensors based on 2D materials.

In this work, we developed off-plane quartz-enhanced photoacoustic spectroscopy (OP-QEPAS). In the OP-QEPAS the light beam went neither through the prong spacing of the quartz tuning fork (QTF) nor in the QTF plane. The light beam is in parallel with the QTF with an optimal distance, resulting in low background noise. A radial-cavity (RC) resonator was coupled with the QTF to enhance the photoacoustic signal by the radial resonance mode. By offsetting both the QTF and the laser position from the central axis, we enhance the effect of the acoustic radial resonance and prevent the noise generated by direct laser irradiation of the QTF. Compared to IP-QEPAS based on a bare QTF, the developed OP-QEPAS with a RC resonator showed a >10× signal-to-noise ratio (SNR) enhancement. The OP-QEPAS system has great advantages in the use of light emitting devices (LEDs), long-wavelength laser sources such as mid-infrared quantum cascade lasers, and terahertz sources. When employing a LED as the excitation source, the noise level was suppressed by ∼2 orders of magnitude. Furthermore, the radial and longitudinal resonance modes can be combined to further improve the sensor performance.

A tunable near-infrared surface plasmon resonance (SPR) biosensor based on gate-controlled graphene plasmons is numerically investigated by using the finite element method (FEM) and the transfer matrix method (TMM). The novel properties of chemical potential sensing make the proposed sensor promising in the application of ultra-sensitive and highly specific biosensing technology. The sensitivity of chemical potential sensing in wavelength interrogation mode can be calculated to be 1.5, 1.89, 2.29, 3.21, 3.73 and 4.68 nm/meV, respectively, at the resonance wavelengths of 1100, 1200, 1310, 1550, 1700 and 1900 nm. The figure of merit (FOM) achieves 129.3, 101.1, 84.5, 67.7, 69.5 and 59.7 eV⁻¹, respectively, at these resonance wavelengths. The sensitivity of chemical potential sensing in gate voltage interrogation mode also can be calculated to be 156.9822, 143.6147, 131.0779, 111.0351, 101.3415 and 90.6038 mV/meV, respectively, at the incident wavelengths of 1100, 1200, 1310, 1550, 1700 and 1900 nm. The FOM achieves 135.6, 103.0, 88.9, 62.2, 66.6 and 61.5 eV⁻¹, respectively, at these incident wavelengths. Theoretical estimates suggest that the limit of detection (LOD) of the sensor’s DNA sensing can reach the level of femtomolar or even attomolar, comparable to and even lower than that of 2D nanomaterial-enhanced metal SPR sensors with AuNPs as a sensitivity enhancement strategy. The feasibility of preparation and operation of this new concept SPR biosensor is also analyzed and discussed.

A tunable near-infrared surface plasmon resonance (SPR) biosensor based on gate-controlled graphene plasmons is investigated theoretically. The novel characteristics of chemical potential sensing make the proposed sensor promising in the application of ultra-sensitive and highly specific biosensing technology. The sensitivity of chemical potential sensing in wavelength interrogation mode can be calculated to be 1.5, 1.89, 2.29, 3.21, 3.73 and 4.68 nm/meV respectively at the central wavelength of 1100, 1200, 1310, 1550, 1700 and 1900 nm. The much smaller full width half maximum (FWHMs) comparing with that of 2D nanomaterial-enhanced metal SPR sensors indicates higher figure of merit. The sensitivity of chemical potential sensing in gate voltage interrogation mode also can be calculated to be 156.9822, 143.6147, 131.0779, 111.0351, 101.3415 and 90.6038 mV/meV respectively at the incident wavelength of 1100, 1200, 1310, 1550, 1700 and 1900 nm. It can be estimated theoretically that the limit of detection (LOD) in DNA sensing of the proposed sensor can reach femtomolar level and even attomolar level, comparable to and even lower than that of 2D nanomaterial-enhanced metal SPR sensors with AuNPs as a sensitivity enhancement strategy. The feasibility of preparation and operation of this new concept SPR biosensor is also analyzed and discussed.

When a paraxial light beam is reflected from/transmitted through an optical element with rotation symmetry, a tiny small incident angle can arise wave-vector-varying Pancharatnam-Berry (PB) phases, resulting in high-performance photonic spin Hall effect (PSHE). Differently, here we investigate the PSHE in photonic crystal slabs of square lattices constructed with air holes, and demonstrate that the C _4v symmetry of the slabs enables giant PSHE in both isotropic and anisotropic band structures with a small incident angle. The guided-mode resonances of the subwavelength-thickness slab enhance the PSHE, allowing the spin separation to exceed the incident beam waist. Via changing the incident angle, wavelength and even the beam waist, the spin separation can be adjusted in a wide range, which makes our proposal more flexible in manipulation of spin photons.