Nonlinear optical heating of all-dielectric super-cavity: efficient light-to-heat conversion through giant thermorefractive bistability

Low-Power Threshold Optical Bistability Enabled by Hydrodynamic Kerr Nonlinearity of Free Carriers in Heavily Doped Semiconductors

We develop an efficient numerical model based on the semiclassical hydrodynamic theory for studying Kerr nonlinearity in degenerate electron systems such as heavily doped semiconductors. This model provides direct access to the electromagnetic responses of the quantum nature of the plasmons in heavily doped semiconductors with complex geometries, which is nontrivial for conventional frameworks. Using this model, we demonstrate nanoscale optical bistability at an exceptionally low-power threshold of 1 mW by leveraging Kerr-type hydrodynamic nonlinearities supported by the heavily doped semiconductor's free carriers. This high nonlinearity is enabled by a strong coupling between metallic gap plasmons and longitudinal bulk plasmons in the semiconductor due to quantum pressure. These findings offer a viable approach to studying Kerr-type nonlinearity and lay the groundwork for developing efficient and ultrafast all-optical nonlinear devices.

Second harmonic generation and broad-band photoluminescence in mesoporous Si/SiO2 nanoparticles

Efficient second harmonic generation and broad-band photoluminescence from deeply subwavelength and nontoxic nanoparticles is essential for nanophotonic applications. Here, we explore nonlinear optical response from mesoporous Si/SiO2, SiO2, and Si nanoparticles, considering various fabrication and treatment procedures. We show that thermal annealing (including femtosecond laser treatment) of mesoporous Si/SiO2 nanoparticles provides the transformation of Si phase from amorphous to crystalline, enhancing the second harmonic and nonlinear photoluminescent response. Notably, the SiO2 mesoporous frame of the considered Si/SiO2 nanoparticles plays a dual positive role for the nonlinear process: it stabilizes the Si material, and SiO2:OH- material has a second-order nonlinearity itself and impacts to the observed second harmonic signal.

Photo-thermo-optical modulation of Raman scattering from Mie-resonant silicon nanostructures

Raman scattering is sensitive to local temperature and thus offers a convenient tool for non-contact and non-destructive optical thermometry at the nanoscale. In turn, all-dielectric nanostructures, such as silicon particles, exhibit strongly enhanced photothermal heating due to Mie resonances, which leads to the strong modulation of elastic Rayleigh scattering intensity through subsequent thermo-optical effects. However, the influence of the complex photo-thermo-optical effect on inelastic Raman scattering has yet to be explored for resonant dielectric nanostructures. In this work, we experimentally demonstrate that the strong photo-thermo-optical interaction results in the nonlinear dependence of the Raman scattering signal intensity from a crystalline silicon nanoparticle via the thermal reconfiguration of the resonant response. Our results reveal a crucial role of the Mie resonance spectral sensitivity to temperature, which modifies not only the conversion of the incident light into heat but also Raman scattering efficiency. The developed comprehensive model provides the mechanism for thermal modulation of Raman scattering, shedding light on the photon-phonon interaction physics of resonant material, which is essential for the validation of Raman nanothermometry in resonant silicon structures under a strong laser field.

Multistability manipulation by reinforcement learning algorithm inside mode-locked fiber laser

Fiber mode-locked lasers are nonlinear optical systems that provide ultrashort pulses at high repetition rates. However, adjusting the cavity parameters is often a challenging task due to the intrinsic multistability of a laser system. Depending on the adjustment of the cavity parameters, the optical output may vary significantly, including Q-switching, single and multipulse, and harmonic mode-locked regimes. In this study, we demonstrate an experimental implementation of the Soft Actor-Critic algorithm for generating a harmonic mode-locked regime inside a state-of-the-art fiber laser with an ion-gated nanotube saturable absorber. The algorithm employs nontrivial strategies to achieve a guaranteed harmonic mode-locked regime with the highest order by effectively managing the pumping power of a laser system and the nonlinear transmission of a nanotube absorber. Our results demonstrate a robust and feasible machine-learning-based approach toward an automatic system for adjusting nonlinear optical systems with the presence of multistability phenomena.

Ultrasensitive Photothermal Switching with Resonant Silicon Metasurfaces at Visible Bands

Dynamic access to quasi-bound states in the continuum (q-BICs) offers a highly desired platform for silicon-based active nanophotonic applications, while the prevailing tuning approaches by free carrier injections via an all-optical stimulus are yet limited to THz and infrared ranges and are less effective in visible bands. In this work, we present the realization of active manipulations on q-BICs for nanoscale optical switching in the visible by introducing a local index perturbation through a photothermal mechanism. The sharp q-BIC resonance exhibits an ultrasensitive susceptibility to the complex index perturbation, which can be flexibly fulfilled by optical heating of silicon. Consequently, a mild pump intensity of 1 MW/cm2 can yield a modification of the imaginary part of the refractive index of less than 0.05, which effectively suppresses the sharp q-BIC resonances and renders an active modulation depth of reflectance exceeding 80%. Our research might open up an enabling platform for ultrasensitive dynamic nanophotonic devices.

Optical Bistability in Nanosilicon with Record Low Q-Factor

We demonstrated optical bistability in an amorphous silicon Mie resonator with a size of ∼100 nm and Q-factor as low as ∼4 by utilizing photothermal and thermo-optical effects. We not only experimentally confirmed the steep intensity transition and the hysteresis in the scattering response from silicon nanocuboids but also established a physical model to numerically explain the underlying mechanism based on temperature-dependent competition between photothermal heating and heat dissipation. The transition between the bistable states offered particularly steep superlinearity of scattering intensity, reaching an effective nonlinearity order of ∼100th power over excitation intensity, leading to the potential of advanced optical switching devices and super-resolution microscopy.

Broadband generation of quasi bound-state-in-continuum modes using subwavelength truncated cone resonators

To allow a high quality factor (Q-factor) to a sub-wavelength dielectric resonator, quasi-bound states in the continuum (Q-BICs) have gained much interest. However, the Q-BIC resonance condition is too sensitive to the geometry of the resonator, and its practical broadband generation on a single-wafer platform has been limited. Here we present that, employing the base angle as a structural degree of freedom, the truncated nano-cone resonator supports the Q-BIC resonance with a high Q-factor of >150 over a wide wavelength range of >100 nm. We expect our approach will boost the utilization of the Q-BIC resonance for various applications requiring broadband spectral tuning.