Whispering gallery mode (WGM) microresonators with high quality (
Photonics Research, Volume. 5, Issue 6, 623(2017)
Sum-frequency generation in on-chip lithium niobate microdisk resonators
We report the first observation, to the best of our knowledge, of sum-frequency generation in on-chip lithium niobate microdisk resonators. The sum-frequency signal in the 780 nm band, distinct in wavelength from second-harmonic signals, was obtained in lithium niobate microresonators under the pump of two individual 1550 nm band lasers. The sum-frequency conversion efficiency was measured to be 1.4×10?7 mW?1. The dependence of the intensities of the nonlinear signals on the total pump power and the wavelength of one pump laser was investigated while fixing the wavelength of the other. This work paves the way for applications of on-chip lithium niobate microdisk resonators ranging from infrared single-photon detection to infrared spectroscopy.
1. INTRODUCTION
Whispering gallery mode (WGM) microresonators with high quality (
In comparison with Fabry–Perot and photonic crystal microcavities, WGM microresonators allow simultaneous resonances at very different wavelengths due to light confinement via total internal reflection. WGM microresonators can obtain high-
Nonlinear optical effects associated with multiple wavelengths with low power pumps can be realized in multiple-resonance WGM resonators with high
Sign up for Photonics Research TOC Get the latest issue of Advanced Photonics delivered right to you!Sign up now
Second-order nonlinear optical effects were mainly realized in microcavities made from materials such as lithium niobate (LN) [
Benefiting from progress in micro-fabrication techniques, on-chip LN WGM microresonators were fabricated by several groups [
Compared with SHG [
In this paper, we report the first observation of SFG in LN microdisk cavities on a chip. The 780 nm band SFG signal, usually together with SHG signals, was generated by pumping the LN resonator with two individual lasers working in communication band. The influences of various experimental conditions on the intensity and wavelength of nonlinear optical signals were studied systematically.
2. SAMPLES AND EXPERIMENTAL SETUPS
SFG experiments were performed in an LN microdisk resonator of 40 μm radius and 500 nm thickness. The resonator was fabricated from an LN on insulator (LNOI) chip produced by NANOLN by using UV lithograph, reactive ion etching, and hydrogen fluoride wet etching in turn [
The experimental setup for the SFG experiment is illustrated in Fig.
Figure 1.Illustration of the experimental setup for investigation of nonlinear effects in LN microdisks. AFG, arbitrary function generator; PD, photodetector.
In order to efficiently and simultaneously couple the pump beams in the 1550 nm band and nonlinear signals in the 780 nm band into and out of the LN resonator, a tapered fiber with relatively thinner diameter was employed. The waist diameter of the tapered fiber was less than 2 μm. The position of the tapered fiber with respect to the LN microdisk was optimized to maximize the SFG signal. We found that, when the SFG signal reaches its maximum value, plenty of modes with quality factors extending for several orders are efficiently or even over-coupled. Additionally, the resonance dips in the transmission spectrum were broadened and shifted to the longer wavelengths due to the thermal effect and the increase in effective refractive index induced by the introduction of the coupling tapered fiber. The distortions in transmission spectrum bring us difficulties in finding the quantum numbers of the WGMs regarding the nonlinear optical process. The WGM quantum numbers are usually obtained by comparing the theoretical resonance wavelengths of the eigenmodes of the resonator in the absence of the tapered fiber to the measured spectrum in a weak coupling condition rather than in the strong coupling regime.
3. EXPERIMENTAL RESULTS AND DISCUSSIONS
We first incident one light beam from either the ps laser or the cw laser into the LN microdisk while slowly scanning the laser wavelength. When the input wavelength matches the resonance of the resonator and thus its second harmonics, we observed a second-harmonic signal from the grating spectrometer. Figures
Figure 2.Spectra of nonlinear optical signals and the corresponding transmission of the pump. SHG signals of (a) pulsed and (b) cw pumps, respectively. (c) SFG (red bold line) and SHG signals (blue and black lines) obtained when both cw and pulsed pump lasers were coupled into the LN resonator. (d)–(f) Typical transmission of the pump correspond to (a)–(c), respectively. The wavelengths (input power) of the cw and pulsed lasers were 1547.0 and 1554.6 nm (8.02 and 0.52 mW), respectively.
We found that a series of the SFG signals show up, when the wavelength of the cw pump is tuned to a broad spectral range with the ps laser wavelength fixed. In experiments, we notice that, within a spectral range from 769.3 to 777.4 nm, the SFG signal arises more than 30 times. The average distance in wavelength between two adjacent SFG frequencies is about 0.28 nm, which is much smaller than the free spectrum range
Figure 3.Spectra showing nonlinear optical signals for various cw laser wavelengths. The red, blue, and black peaks represent the SFG, SHG of cw laser, and SHG of pulsed laser, respectively. For demonstration, the vertical coordinate for each spectrum is shifted by 10 pW with respect to its preceding one. The wavelength of the pulsed pump was fixed at 1549.7 nm. The input power of the cw and pulsed pumps were set to be 7.81 and 0.51 mW, respectively.
When the wavelength of the ps laser is fixed at a particular value with a maximum SHG, and the cw laser wavelength is scanned around a resonance wavelength of the LN microdisk, the change in intensities of the SHG and SFG signals with respect to the frequency detuning of the cw laser is shown in Figs.
Figure 4.Dependence of the SFG and SHG intensities on the wavelength of the cw pump and transmission spectra of the cw pump. (a)–(c) Nonlinear signals measured by scanning the wavelength of the cw laser near 1547.0 nm. The wavelength of the pulsed pump was fixed at 1554.6 nm. The input power of the cw and pulsed pumps were set to be 10.92 and 0.71 mW, respectively. (d) Sawtooth voltage for cw laser wavelength scan. Transmission spectra of the cw laser around 1543.0 nm under (e) strong and (f) weak pump, respectively.
The dependence of the SFG on the pump power was also investigated. The input power of both the cw and pulsed pumps was tuned simultaneously by using a tunable attenuator after the 50:50 combiner (not shown in Fig.
Figure 5.Dependence of nonlinear signals on pump power and the deduced SFG conversion efficiency. (a)–(c) Power of SHG signals of the cw and pulsed lasers and SFG signal with respect to the total pump power. (d) SFG conversion efficiency deduced from (c). The central wavelengths of the cw and ps lasers were set as 1547.0 and 1554.6 nm, respectively. The power ratio of the continuous laser and the pulsed laser was kept as 15.4:1.
The SFG conversion efficiency is defined as
We also investigated the impact of the pump power contributed only by the cw laser on the intensities of the nonlinear signals, while fixing the wavelength and pump power of the ps laser. In this case, the variation of the SFG and SHG of the cw laser increases with the growth of the pump power of the cw laser; however, the SHG of the ps laser shows opposite variation tendency due to energy transportation from the pulse pump to the SFG.
4. CONCLUSIONS
In conclusion, we demonstrated SFG in on-chip LN microdisk resonators for the first time, to the best of our knowledge. The SFG signal commonly accompanied by SHG signals was generated by simultaneously pumping the LN resonator with a cw laser and a ps laser operating at different wavelengths. We investigated the influences of the experimental conditions, such as wavelength of the cw laser and the pump power, on the intensities of the SFG and SHG signals. This work not only expands the research field of nonlinear effects in micro- or nano-photonics devices but also paves the way for applications such as infrared single-photon detection and infrared spectroscopy based on on-chip LN microresonators.
Acknowledgment
Acknowledgment. Z. Hao and J. Wang contributed equally to this work.
[24] R. Wang, S. A. Bhave. Free-standing high quality factor thin-film lithium niobate micro-photonic disk resonators(2014).
[28] J. Moore, J. K. Douglas, I. W. Frank, T. A. Friedmann, R. Camacho, M. Eichenfield. Efficient second harmonic generation in lithium niobate on insulator. Conference on Lasers and Electro-Optics (CLEO), STh3P.1(2016).
[29] I. W. Frank, J. Moore, J. K. Douglas, R. Camacho, M. Eichenfield. Entangled photon generation in lithium niobate microdisk resonators through spontaneous parametric down conversion. Conference on Lasers and Electro-Optics (CLEO), SM2E.6(2016).
[31] H. Liang, W. C. Jiang, X. B. Sun, X. C. Zhang, Q. Lin. Thermo-optic oscillation dynamics in a high-Q lithium niobate microresonator. Conference on Lasers and Electro-Optics (CLEO), STu1E.4(2016).
Get Citation
Copy Citation Text
Zhenzhong Hao, Jie Wang, Shuqiong Ma, Wenbo Mao, Fang Bo, Feng Gao, Guoquan Zhang, Jingjun Xu, "Sum-frequency generation in on-chip lithium niobate microdisk resonators," Photonics Res. 5, 623 (2017)
Category: Nonlinear Optics
Received: Jul. 27, 2017
Accepted: Sep. 13, 2017
Published Online: Dec. 7, 2017
The Author Email: Fang Bo (bofang@nankai.edu.cn)