Optics letters最新文献

The latest breakthroughs in time-varying photonics are fueling novel, to the best of our knowledge, thermal emission phenomena, e.g., showing that the dynamic amplification of quantum vacuum fluctuations, induced by the time modulation of material properties, enables a mechanism to surpass the blackbody spectrum. So far, this issue has only been investigated under the assumption of non-dispersive time modulations. In this work, we identify the existence of a non-physical diverging behavior in the time-modulated emission spectra at high frequencies and prove that it is actually attributed to the simplistic assumption of a non-dispersive (temporally local) response of the time modulation associated with memory-less systems. Accordingly, we upgrade the theoretical formalism by introducing a dispersive response function, showing that it leads to a high-frequency cutoff, thereby eliminating the divergence and hence allowing for the proper computation of the emission spectra of time-modulated materials.

We study the optical modulation of the three-dimensional (3D) quantum Hall effect in Weyl semimetals by using the four-terminal Hall-bar probing geometry. By utilizing the Floquet theorem, we calculate the Hall conductance under the combined effect of the circularly polarized light (CPL) and the perpendicular magnetic field. The Hall conductance plateaus change from negative values to positive values induced by the left-handed CPL. Moreover, the Hall conductance plateaus keep negative values under the action of the right-handed CPL. Accordingly, the chiral dependence of the Hall conductance can be elucidated in terms of the optical modulation of the spatial distribution of the probability density and the Landau levels induced by the CPL. The Hall conductance depends on the thickness of the Weyl semimetals and is robust against the on-site strong disorders. This work will be instructive for realizing an effective optical modulation of the Weyl orbitals in Weyl semimetals.

Efficient and compact grating coupler designs are crucial for enhancing the performance of photonic integrated chips. In this work, we propose a design approach that combines metalens technology and topology optimization. As a proof of concept, we first designed an on-chip metalens to preliminarily improve the transmission efficiency of a compact mode converter. Building upon this, we applied a topology optimization algorithm to further optimize the tapered conversion region, compensating for the insertion loss caused by local periodic approximations in the metalens design. This process ultimately improved the transmission efficiency to 93.69%. By integrating this with the previously designed grating coupler, we experimentally demonstrated that the mode converter, with a size of 20 × 12 μm and a minimum feature size greater than 180 nm, achieves an insertion loss of only -0.4 dB. The compact grating coupler, designed within a 35 × 12 μm footprint, shows an insertion loss of -3.6 dB at a wavelength of 1550 nm.

Ablative lasers such as erbium-doped laser and carbon dioxide laser are currently primary tools for skin rejuvenation and treating dermatological disorders. However, during treatment, as the thermal effect exerts on both target and normal tissues simultaneously, significant effectiveness is often accompanied by a high risk of adverse reactions. To attain an appropriate thermal diffusion and thus favorable therapeutic outcome and fewer side effects, collagen-resonant femtosecond (fs) lasers hold promise as innovative tools for laser cosmetic treatments. In this study, we report, for the first time to the best of our knowledge, an in vivo experiment of fs laser resurfacing with collagen-resonant wavelengths of 6.1 and 7.5 μm, via an optical parametric amplifier. Our results demonstrate that long-wavelength infrared (LWIR) lasers effectively enhance the components of the dermal matrix without causing dermal ablation. The structure of collagen fiber is significantly improved with a substantial amount of new collagen formation. The increased expression of various collagen types in immunofluorescence image further demonstrates the efficacy of the LWIR fs laser in skin rejuvenation. In addition, improvement in the epidermis is more pronounced at a wavelength of 6.1 μm, with a more suitable depth of action. We anticipate that LWIR fs laser could become widely applicable in clinical settings for skin regeneration and rejuvenation.

Under linear conditions, power injected from a single waveguide into a multi-core fiber array results in multimode propagation, progressively diminishing the spatial coherence of light. In this work, we introduce a comprehensive approach to mitigate this coherence loss by means of a nonlinear thermodynamic Joule-Thomson expansion. By leveraging the tools of optical thermodynamics, we demonstrate that as light undergoes a sudden transition from a small to a larger nonlinear optical array, it can abruptly drop its optical temperature to near-zero values. During this cooling process, light irreversibly flows into the system's fundamental mode with very high efficiency, synchronizing all elements of the lattice with the input port. We show that this nonlinear effect is highly predictable even in systems of arbitrary geometry and shape and can be controlled precisely by the initial conditions at the input of the array. In particular, for a single injection point, the reduction in optical temperature can be directly determined by the total power, irrespective of the input location.

In this Letter, we present a novel distributed pumping scheme (DPS) for effective stimulated Brillouin scattering (SBS) suppression in a kilowatt-level, single-frequency-amplified Tm-fiber master oscillator power amplifier (MOPA). Theoretical analysis of laser gain distribution and temperature variation along the cascaded hybrid active fiber in the DPS indicates enhanced power scalability. By incorporating distributed pumping and pseudo-random binary sequence (PRBS) phase modulation, an output power of 1025 W at 1998 nm was successfully achieved with a spectral linewidth of 480 MHz and signal-to-noise ratio of >46 dB. Further power scaling is primarily limited by high-intensity self-pulses originating from the spectral spikes. Over an ∼60 min time scale, the power stability (RMS) was measured to be approximately 0.48%. A diffraction-limited beam quality factor M² is estimated to be ∼1.20. Additionally, the dynamic process of backward Stokes light generation under varying phase modulation conditions was revealed. The concept presented herein paves a new pathway for the generation of kilowatt-level single-frequency laser sources across a broad spectral region.

Microsphere nano-imaging is a promising technique for label-free and real-time imaging, making optical sub-diffraction resolution possible. Due to the limited size and high surface curvature of microspheres, the magnified imaging suffers from the limited depth of field and low contrast. The performance of this technique depends not only on the geometric parameters of microspheres but also on the illumination conditions of an optical system. In this work, a specially designed filter is added to the microscope to adjust the illumination angle and area on the microsphere. Experimental results demonstrate that with the filter, the imaging contrast is increased by 2.77 times, and the resolution is improved from 125 nm to 100 nm. It also increases the depth of field, extending it from 519 nm to 900 nm coupled with a 20× objective lens. This effective light manipulation strategy establishes suitable illumination conditions to enhance the imaging contrast and resolution. It is also applicable to improve the performance of microspheres in other optical applications.

We characterized the intra-cavity mode patterns due to the concurrence of dual-optical parametric oscillations (OPOs) followed by second-harmonic generation (SHG) and sum-frequency generation (SFG) within a gain-modulated quasi-phase-matching nonlinear photonic crystal (QPM-NPC). The proposed device contains a bi-grating QPM period for downconversion, followed by mono- or tri-grating QPM periods for upconversion on periodically poled lithium tantalate. The pairs of infrared dual-OPO beams are found to spatially reside at the opposite sidelobes of the cavity mode, each exhibiting distinct spectral contents corresponding to the pair of (signal, idler) waves oscillating at (979, 1167) nm and (964, 1189) nm, respectively. The QPM-SHG waves at 582 and 593 nm are found to overlap with their respective OPO counterparts in the sidelobes, whereas the QPM-SFG at 588 nm wavelength is located at the center. Such spatial-spectral configurability reveals a subtle spatial overlap in the dual-OPO idlers due to wave continuity in the gain-modulated NPC, which agrees with our model calculation.

Optical neural networks (ONNs) offer advantages in parallel processing, low power consumption, and high-speed operation. However, existing ONN designs face challenges in miniaturization, stability, tunability, and integration. This study proposes a graphene surface plasmon polariton (GSPP) waveguide switch array for all-optical neural networks. The design features a compact structure with a lateral area of only 0.045 $boldsymbol{mathrm{mu}}{{mathbf m}^2}$. Numerical simulations show that within the 30.2 to 49.4 THz range, the transmission rate is tunable from 0 to 0.875, accurately simulating synaptic weights in ONNs. The compact switch array achieves a recognition accuracy of 93.83% on the CIFAR-10 dataset, demonstrating its potential for high-speed, low-power, and highly integrated neural network computing platforms.

Conventional lensless imaging systems require complex phase diversity measurements and sequential processing steps, limiting their practical application despite their compact design. We present a differentiable end-to-end pixel-super-resolution (dPSR) technique that unifies PSR hologram synthesis, autofocusing, and complex-field reconstruction within a single optimization framework. By jointly optimizing these traditionally separate processes, our method eliminates both phase diversity requirements and error accumulation from sequential processing. Our method achieves superior position estimation accuracy (mean error 0.0282 pixels versus 0.1172 pixels with conventional methods), delivering precise autofocusing with accuracy better than 0.3 µm, and enabling a twofold resolution enhancement beyond the sensor's native pixel size. This robust performance is validated through both simulated and experimental results, including challenging phase objects and label-free cell imaging, establishing dPSR as a practical solution for high-resolution microscopy applications.