Athermal third harmonic generation in micro-ring resonators

The third-harmonic generation (THG) process is a class of high-harmonic generation processes in nonlinear optics where the nonlinear medium combines three photons having identical frequency to generate a new photon with triple the original frequency. For THG to happen, the nonlinear medium must allow the photons to meet the strict phase matching condition between the input pump signal and the generated third-harmonic signal. Besides, the medium must also be able to amplify the generated third-harmonic signal. The integrated optical micro-ring resonator is an ideal structure for the THG process as it naturally preserves the precise phase of the signal in the resonator thus allowing the generated new photons to multiply in the resonator as they circulate, as long as the resonance frequencies of the resonator match with both the pump and third harmonics signals. However, as the physical properties of the nonlinear optical waveguide are influenced by temperature, the thermal optic (TO) effect, the optical intensity, and the Kerr effect, maintaining the strict phase matching condition for THG is a great challenge. In this study, the authors recognized that the overall TO effect in the nonlinear ring resonator is composed of two separate components, namely a linear component corresponds to the overall device temperature and a nonlinear component that is due to the optical intensity in the resonator and is a function of the detuning of the signal from the resonance. They developed a dynamic model and presented a comprehensive study of the thermal behavior of the generated visible third-harmonic (TH) modes in mirco-resonators when pumped with telecom wavelengths. Due to the overall temperature dependent wavelength shift of the visible TH modes is a function of the difference between the compensation wavelength shift rates from the linear and nonlinear TO effects which can be negative or positive, they predicted by using the model and demonstrated that there are third-harmonic mode families with positive thermal, negative thermal and athermal coefficients which the emission wavelength increases, decreases and nearly keep independent with temperature, respectively. They have also identified athermal orthogonally pumped visible TH modes with a temperature dependent wavelength shift of 0.05 pm/°C over a temperature range of 12 °C. This corresponds to the actual temperature variation of only 0.0026 °C when the device is controlled by an external thermal controller.

Fig. 1 (left) The four port MRR schematic for visible emission via THG. (right) As long as the visible mode is thermally matched with the pump mode, their resonances can be kept correlated while maintaining the necessary phase matching condition while the temperature changes without the need for any external compensation. The inset shows the captured green side-emissions via THG in MRRs.

Fuzhou University, Xi’an institute of optics and precision mechanics of the Chinese Academy of Science, and the City University of Hong Kong report a study on the thermal behavior of micro-ring resonators in the visible band. It examined third harmonic generation emission under the pump of a continuous laser at telecom wavelengths and experimentally observed athermal third harmonic generation. Besides the ability to generate visible emission in the highly-doped silica glass platform, the study provides a new method to control the temperature drift of the emitted visible signal. In CMOS compatible integrated optical micro-ring resonators, the linear TO effect can impart temperature dependent resonance wavelength shift at a rate of tens of pm/°C that needs to be mitigated in applications that requires precise control of the wavelength location. The conventional approach to reduce the thermal dependent wavelength shift is to use an external temperature controller and on-chip temperature sensor to actively control the chip temperature in this type of devices. Besides the external controller, the thermal dependency can be further reduced by applying a layer of cladding material that has a negative thermal optic coefficient on the resonator to compensate the shift. This study shows a new approach to compensate the visible emission mode without any coating materials with negative thermal optic coefficient on the resonator. The interesting and solid results should benefit the fields of nonlinear optics, micro/nano photonics and visible photonics due to the realization of stable third harmonic signal generation and the extended the wavelength range. In particular for applications in bio-sensing, confocal microscopy, flow cytometry and spectroscopy system, where it can serve as 2f-3f self-references in the visible region. The article is entitled “Athermal third harmonic generation in micro-ring resonator” and published in Opto-Electronic Advances Issue 12 2020.

About The Group

Dr Wang’s research group at Fuzhou University focuses on the nonlinear optics in optical fiber and integrated photonics circuit. The group is now under the FZU-Jinjiang Joint Institute of Microelectronics. Our research group at the City University of Hong Kong along with our collaborators design and fabricate novel high-index-contrast planar lightwave circuits (PLC) and structures that provide unique photonic signal processing function onto a single robust solid-state chip. These devices find applications in a broad range of photonics research areas that include linear and nonlinear optics, sensing and metrology, RF and microwave photonics and integrated quantum photonics. Our group has active research collaborations with groups all over the world, including Australia, Canada, China, Japan and the UK.

Article

Wang S H, Li Y H, Little B E, Wang L R, Wang X et al. Athermal third harmonic generation in micro-ring resona-tors. Opto-Electron Adv 3, 200028 (2020).

DOI: 10.29026/oea.2020.200028