[Paper Review] Photo-induced cascaded harmonic and comb generation in silicon nitride microresonators

Jianqi Hu, Edgars Nitišs+6 more|arXiv (Cornell University)|Mar 29, 2022

Advanced Fiber Laser Technologies4 citations

TL;DR

This paper demonstrates photo-induced cascaded harmonic and comb generation in silicon nitride microresonators by optically writing quasi-phase-matched χ² gratings via the photogalvanic effect. The resulting χ² grating enables sum-frequency generation and phase-matching for primary comb initiation, while doubly resonant pump and second-harmonic fields drive cascaded third-harmonic generation, yielding a low-noise, broadband microcomb state in a normal dispersion platform.

ABSTRACT

Silicon nitride (Si$_3$N$_4$) is an ever-maturing integrated platform for nonlinear optics. Yet, due to the absence of second-order ($χ^{(2)}$) nonlinearity, Si$_3$N$_4$ is mostly considered for third-order ($χ^{(3)}$) nonlinear interactions. Recently, this limitation was overcome by optical poling in both Si$_3$N$_4$ waveguides and microresonators via the photogalvanic effect, resulting in the inscription of quasi-phase-matched $χ^{(2)}$ gratings. Here, we report cascaded nonlinear effects in a normal dispersion Si$_3$N$_4$ microresonator with combined $χ^{(2)}$ and $χ^{(3)}$ nonlinearities. We demonstrate that the photo-induced $χ^{(2)}$ grating also provides phase-matching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs at pump wavelength. Additionally, the doubly resonant pump and second-harmonic fields allow for cascaded third-harmonic generation, where a secondary optically written $χ^{(2)}$ grating is identified. Finally, we reach a low-noise, broadband microcomb state evolved from the sum-frequency coupled primary comb. These results expand the scope of cascaded effects in $χ^{(2)}$ and $χ^{(3)}$ microresonators.

Motivation & Objective

• To overcome the intrinsic lack of second-order nonlinearity (χ²) in silicon nitride (Si3N4) for integrated nonlinear optics.
• To demonstrate that optically induced χ² nonlinearity via the photogalvanic effect enables cascaded nonlinear processes in Si3N4 microresonators.
• To achieve phase-matched sum-frequency generation and third-harmonic generation using self-organized, photo-written χ² gratings.
• To generate a low-noise, broadband microcomb state in a normal dispersion Si3N4 microresonator through cascaded χ² and χ³ nonlinearities.
• To validate the optical inscription of multiple quasi-phase-matching gratings and their role in enabling complex nonlinear dynamics.

Proposed method

• Optical poling via the photogalvanic effect induces periodic space-charge distributions in Si3N4 microresonators, creating effective χ² gratings.
• The interference of fundamental and second-harmonic (SH) waves at the resonator circumference forms a periodic χ² grating with a period matching the beat length of the pump and SH fields.
• Time-resolved pump wavelength sweeps (tens of seconds) are used to probe slow photogalvanic dynamics and enable reproducible observation of cascaded nonlinearities.
• Transverse phase-mapping (TPM) imaging is employed to visualize the spatial structure of the inscribed χ² grating by scanning a femtosecond laser and detecting the SH signal.
• The microresonator is designed with a 924 µm radius, ultralow propagation loss, and supports multiple spatial modes at harmonic wavelengths.
• Phase-matching conditions for sum-frequency generation and third-harmonic generation are confirmed by analyzing the spectral and spatial overlap of resonant modes.

Experimental results

Research questions

• RQ1 Can photo-induced χ² nonlinearity in Si3N4 microresonators enable sum-frequency generation and comb initiation in the normal dispersion regime?
• RQ2 How do self-organized, optically written χ² gratings support cascaded third-harmonic generation in the presence of doubly resonant pump and SH fields?
• RQ3 What is the role of the photogalvanic effect in creating phase-matched χ² gratings that simultaneously support multiple nonlinear processes?
• RQ4 Can the interplay between χ² and χ³ nonlinearities in Si3N4 microresonators lead to the formation of low-noise, broadband microcomb states?
• RQ5 How do the spatial and spectral characteristics of the inscribed χ² grating influence the efficiency and stability of cascaded harmonic and comb generation?

Key findings

• A photo-induced χ² grating is unambiguously confirmed via transverse phase-mapping imaging, showing a periodic structure with a period matching the beat length of the pump and second-harmonic waves.
• The inscribed χ² grating provides phase-matching for sum-frequency generation, enabling the initiation of a primary microcomb at the pump wavelength.
• Cascaded third-harmonic generation is observed due to the doubly resonant interaction of pump and second-harmonic fields, with a secondary optically written χ² grating identified.
• A low-noise, broadband microcomb state is achieved, evolving from the sum-frequency-coupled primary comb, indicating stable nonlinear dynamics.
• The method enables reliable, hand-tunable reproduction of cascaded nonlinear effects due to the slow time dynamics of the photogalvanic effect (on the order of tens of seconds).
• The platform supports efficient harmonic and comb generation in a normal dispersion Si3N4 microresonator, expanding the scope of integrated nonlinear optics beyond conventional Kerr comb generation.