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Nonlinear Optics in Nanostructures Fabricated on Lithium-Niobate-on-Insulator
Published in Ya Cheng, Lithium Niobate Nanophotonics, 2021
A true third-order nonlinear optical process is FWM, which originates from the interplays between four interacting waves. In a high-Q-factor LN WGM resonator, FWM can occur by generating two new frequencies equally separated from the pump wave, as determined by the cavity resonance. The required phase-matching condition for such process can be expressed as Δk=2k3−k1−k2, where k1, k2, and k3 are the wave vectors of the two newly generated signal waves and the wave vector of the pump, respectively. Recently, an efficiency FWM process has been observed around the second harmonic wavelength of the pump laser in a photonic molecule (PM) fabricated on the LNOI platform, as shown in Fig. 3.12a [99]. Figure 3.12a shows that when the pump laser is tuned to a cavity resonance wavelength of 1565.46 nm, complex nonlinear signals near the second harmonic wavelength can be generated, which evolve with the increasing pump power. At a pump power of 10.4 mW, only the second harmonic signal at 782.73 nm is detected with a quasi-TM mode. When the pump power is increased to 14 mW, two equally spaced FWM peaks emerged symmetrically around the second harmonic mode, of which the wavelengths are determined to be 778.09 nm (F1) and 787.37 nm (F2), respectively. A further increase of the pump power results in cascaded FWM process, giving rise to the generation of the waves peaked at 773.44 nm (F3) and 792.01 nm (F4). Remarkably, an ultrahigh FWM conversion efficiency PF1/PSH (~14%) is revealed at only the μW level of the second harmonic signal power, as shown in Fig. 3.12b. Meanwhile, cascaded Raman scattering is generated with Raman shifts of 580 cm−1, 581 cm−1, and 252 cm−1, as indicated by the Raman peaks R1, R2, and R3 in Fig. 3.12b, respectively. The high Stokes wave intensities indicate the occurrence of stimulated Raman scattering. The fact that the cascaded FWM and Raman scattering processes are directly pumped by the second harmonic wave has been confirmed by checking the spectrum around the fundamental wavelength. As shown in the inset of Fig. 3.12b, no pronounced FWM or Raman peaks are visible in the vicinity of the fundamental wavelength.
Progress in wafer bonding technology towards MEMS, high-power electronics, optoelectronics, and optofluidics
Published in International Journal of Optomechatronics, 2020
Jikai Xu, Yu Du, Yanhong Tian, Chenxi Wang
Optical frequency combs are important building blocks for optical communication and precision spectroscopy. The current combs are produced by mode-locked lasers or dispersion-engineered resonators. Alternatively, the comb can also be generated by the electro-optic (EO) modulation. Previous EO combs have been limited by narrow widths because of the weak tuning. To overcome this problem, Mian Zhang et al. from Marko Loncar’s group realize an integrated EO comb generator on the thin-film LiNbO3 platform with advantages of ultralow loss and highly co-localized microwave engineering.[103] The schematic diagram of the enhanced EO comb generator is presented in Figure 5(f). The measured results show that the EO comb can cover the frequencies more than the entire telecommunications L-band, as shown in Figure 5(g). Moreover, this comb generator has a high tolerance in the frequency detuning. This phenomenon can be utilized to generate dual-frequency combs in a single resonator. The results demonstrate that the integrated EO comb generators can provide wide and stable comb spectra. Additionally, Cheng Wang et al. also show a frequency comb on the single monolithic nanophotonic LiNbO3 substrate. It simultaneously achieves large electro-optic and Kerr nonlinearities with low optical losses, as shown in Figure 5(h).[104] This device configuration is a powerful complement to integrated Kerr combs, enabling a wide range of applications from spectroscopy to optical communications. In addition to the aforementioned breakthrough in the LiNbO3-based thin-film modulator, it can also be used for the electronically programmable photonic molecule.[105] The coupled LiNbO3 microring resonators are used and controlled by the external microwave excitation, as displayed in Figure 5(i). Both the frequency and phase conversion of light can be precisely controlled by programmed microwave signals. This kind of dynamic control for light opens the door in the field of microwave signal processing, quantum photonic gates, optical computing, and topological physics.