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Scientific Reports

Enhanced optical bistability in slotted photonic crystal structure for microwave frequency generation

Akash Kumar Pradhan, Chandra Prakash, Sambit Satpathy, Jibitesh Kumar Panda

Published Nov 11, 2025• 2 citations

Abstract

This study proposes a novel silicon nanocrystal SiNC/SiO[Formula: see text] embedded slotted photonic crystal nanocavity employing precise width modulation to explore optical bistability and self-pulsing behavior for microwave signal generation. The designed cavity achieves an ultra-high quality factor (Q) of [Formula: see text] and a low modal volume of [Formula: see text]. Theoretical modeling incorporating Kerr nonlinearity, two-photon absorption, and free carrier effects is used to analyze the bistability response. Simulation results reveal a low threshold power of [Formula: see text] for optical bistability under a - 20 pm detuning condition. Additionally, the cavity demonstrates microwave frequency generation through self-pulsing oscillations, with a fundamental mode and observable second harmonic at 21.34 GHz. Fabrication tolerance is also evaluated, showing that the design sustains performance with up to 9% randomness in hole radii, 12% in x-position, and 20% in z-position of air holes. These findings confirm the feasibility of the proposed structure for low-power, high-frequency integrated photonic applications.

DOI: 10.1038/s41598-025-20992-w

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This study proposes a novel silicon nanocrystal SiNC/SiO[Formula: see text] embedded slotted photonic crystal nanocavity employing precise width modulation to explore optical bistability and self-pulsing behavior for microwave signal generation. The designed cavity achieves an ultra-high quality factor (Q) of [Formula: see text] and a low modal volume of [Formula: see text]. Theoretical modeling incorporating Kerr nonlinearity, two-photon absorption, and free carrier effects is used to analyze the bistability response. Simulation results reveal a low threshold power of [Formula: see text] for optical bistability under a - 20 pm detuning condition. Additionally, the cavity demonstrates microwave frequency generation through self-pulsing oscillations, with a fundamental mode and observable second harmonic at 21.34 GHz. Fabrication tolerance is also evaluated, showing that the design sustains performance with up to 9% randomness in hole radii, 12% in x-position, and 20% in z-position of air holes. These findings confirm the feasibility of the proposed structure for low-power, high-frequency integrated photonic applications.

We demonstrate extremely low-power all-optical bistability by utilizing silicon photonic crystal nanocavities, based on the plasma effect of carriers generated by two-photon absorption. Owing to the high quality factor and the small volume of the nanocavities, the photon density inside the cavity becomes extremely high, which leads to a large reduction in operation power. Optical bistable operation in a single nanocavity permits optical read-write memory operation, which opens the possibility of an integrated optical logic circuit on a single chip, based on photonic crystals. The demonstrated bistable threshold power is 0.4 mW with a set pulse energy of 74 fJ, at a switching speed of <100 ps.

The authors observe experimentally optical bistability enhanced through Fano interferences in high-Q localized silicon photonic crystal resonances (Q∼30000 and modal volume ∼0.98 cubic wavelengths). This phenomenon is analyzed through nonlinear coupled-mode formalism, including the interplay of χ(3) effects such as two-photon absorption and related free-carrier dynamics, and optical Kerr as well as thermal effects and linear losses. Experimental and theoretical results demonstrate Fano resonance based bistable states with switching thresholds of 185μW and 4.5fJ internally stored cavity energy (∼540fJ consumed energy) in silicon for scalable optical buffering and logic.

We report on bistable operations in a microcavity formed on an AlGaAs-based photonic crystal slab at telecommunication wavelengths. We designed the cavity to achieve a high quality (

Q

)-factor, while maintaining a small mode volume; this was accomplished by removal of a single air hole. The fabricated microcavity exhibited a resonant peak with a

Q

-factor of 1900 at 1548 nm. In nonlinear transmission measurement, when the input power was increased, a bistable characteristic which was dependent on the detuned wavelength from the resonance was observed. The bistability was considered to originate from thermal nonlinearity due to two-photon absorption.

Optical bistability with low power and large modulation depth is observed in the silicon 2D photonic crystal 3 defect-long (L3) cavity coupling with a photonic crystal waveguide experimentally. The cavity-waveguide resonator system operates under the critical coupling condition. The triangular line shape of the cavity resonance and the hysteresis loop of the system is observed. The switching contrast is 7.7 dB and the modulation depth is 0.70 at the switching power of -12.1 dBm. A nonlinear coupled mode model is established to analyze the behavior of the cavity, and numerical simulation results are in good agreement with the experiment results.

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