Whispering gallery mode (WGM) microcavities have attracted much attention in recent years for their broad applications ranging from ultra-sensitive sensing[
It is known that putting a nonlinear optical medium into a high-Q optical cavitiy can improve the efficiencies of the nonlinear optical effects and release the requirement of high intensity pump. Therefore, a pulse laser with high peak power can be replaced by a continuous pump with power even lower than milliwatts. Compared with bulk optical cavities containing nonlinear optical materials, WGM microcavities usually have not only a much smaller size but also a broader resonance window in which high Q can be achieved[
Lithium niobate (LN) is a noncentrosymmetrical material with outstanding nonlinear optical properties ([
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However, all of the nonlinear optical effects reported in LN WGM microcavities are associated with only laser sources. In this paper, we report the observation of a series of nonlinear optical signals, besides second harmonic signals, in an on-chip LN WGM microcavity. These nonlinear optical signals were attributed to the SFGs between the pump laser and its background. This work elucidates the origin of unknown nonlinear optical signals always observed in high-Q WGM resonators that cannot be attributed to high-order harmonic generation.
2. Samples and Experimental Setup
LN WGM microcavities with a size of 200 nm in thickness and 40 µm in radius were employed to conduct nonlinear optical experiments. The Q factors of these WGM microcavities are on the order of , which were fabricated from Z-cut LN film produced by NANOLN by using the microfabrication technique including UV lithography, argon ion etching, and hydrogen fluoride etching. For a Z-cut LN WGM microcavity, the optical axis of LN is parallel to the rotational axis of the microdisk cavity, in other words, perpendicular to the plane in which the microdisk lies.
Figure 1 indicates the experimental setup to observe the sum-frequency signal in the LN WGM microcavities. A tapered fiber was used to couple the 1550 nm pump into the LN microdisk and extract nonlinear optical signals from the same cavity. The 1550 nm pump is generated by a tunable laser with a linewidth less than 200 kHz and a tuning range covering 1520–1570 nm. The nonlinear optical signals were detected by a spectrometer allowing for the detection of weak light with power down to several picowatts (pW). The laser wavelength can be finely tuned by applying a triangular wave voltage on the piezo mirror of the external reference cavity of the laser. The transmission of the pump light was monitored by using a photodetector and an oscilloscope. An optical spectrum analyzer (not shown in Fig. 1) working from 600 nm to 1700 nm that covers the output wavelength of the pump laser was used to measure the transmission spectrum of the laser and its background and to calibrate the wavelengths measured by the spectrometer and shown by the laser controller. Therefore, we can approximately assign the WGMs related to the sum-frequency processes.
Figure 1.Schematic of the experimental setup to measure sum-frequency signals. An arbitrary function generator (AFG) is used to precisely control the output wavelength of the pump laser and to trigger the oscilloscope. The pump light passes through a fiber polarization controller (PC) and a beam splitter, and then couples into the LN WGM microcavity via a tapered fiber. The transmission of the pump is monitored by a photodetector connected to an oscilloscope. The tapered fiber that is used to couple the pump collects the nonlinear optical signals as well. The nonlinear optical signals are detected by a spectrometer.
3. Experimental Results and Discussions
It is known that second-order nonlinear optical signals can be detected in WGM microcavities made from material lacking of central symmetry with pump down to less than milliwatts[
We observed nonlinear optical signals with about 10 peaks, as shown in Fig. 2, when only one pump laser at 1521.36 nm was launched into an LN WGM microcavity. Peak 1, which is highlighted in red, has a wavelength of 760.68 nm. It is considered the second harmonic signal of the pump according to the principle of energy conservation, which indicated that the wavelength of the second harmonic signal was half of the pump wavelength. Besides the second harmonic signal, a series of signals ranging from 770 nm to 780 nm, marked in blue in Fig. 2, were unexpectedly observed as well. The nonlinear optical signals in blue have wavelengths between the pump at 1521.36 nm and its second harmonic signal of 760.68 nm, which is different from the aforementioned four kinds of nonlinear optical signals in the wavelength. Such nonlinear signals have not been discussed in detail in LN WGM cavities.
Figure 2.Nonlinear optical signals. Peak 1, marked in red, corresponding to the second harmonic generation (SHG) of the pump laser at 1521.36 nm. Peaks 2–9 in blue are the sum-frequency generation (SFG) signals of the pump laser and its background.
To find the origin of the unexpected nonlinear optical signals, we first suppose they are the sum-frequency signals of the pump and unknown light sources. In this situation, the wavelengths of the undiscovered light sources can be calculated according to energy conversation for sum-frequency processes, i.e., , where is the Planck constant; is the light speed in vacuum; and are the wavelengths of the nonlinear signal and the pump, respectively; stands for the wavelength of the unknown light that is needed to mix with the pump to produce the SFG signal. The calculated wavelengths are shown in the third column of Table 1. On the other hand, the transmission spectrum of the pump laser was measured by an optical spectrum analyzer, which is shown in Fig. 3(a). Figure 3(b) is the enlarged version showing the detailed information of the yellow-background highlighted part of Fig. 3(a). We found that the wavelengths of the blue peaks in Fig. 3(b) coincide with with deviations from 0 nm to 0.220 nm. Accordingly, we attribute the unexpected nonlinear optical signals to the sum-frequency of the pump laser and its background.
Figure 3.Transmission spectra of the pump laser and its background. (a) A typical broad transmission spectrum of an LN WGM microcavity coupled to a tapered fiber. (b) The enlarged view of the yellow highlighted part of (a) showing the pump laser background in detail. The peaks marked in blue represent the WGMs associated with the sum-frequency processes.
Figure 4.Dependence of the conversion efficiency of the typical nonlinear optical signals on that of the pump laser. (a) and (b) show the data for the second harmonic signal and that for the sum-frequency signal marked as Peak 7, respectively.
Table 1. Wavelengths of Light Associated with the Sum-Frequency Processes
Initially, we suspected these nonlinear optical signals may due to the sum-frequency of the pump laser and its third-order nonlinear optical outputs, such as stimulated Raman or four-wave mixing signal. We measured the transmission spectrum around the pump, and neither the Raman nor four-wave mixing signal was observed in the 1550 nm band, especially at the wavelengths that may generate observed nonlinear signals via the sum-frequency process under a 1521.36 nm pump. The absence of the Raman and four-wave mixing signals further verify that the nonlinear optical signals extending from 770 nm to 780 nm are due to the sum-frequency of the laser and its background that was coupled into the LN WGM microcavity. The high power of the background from 1560 nm to 1600 nm makes it easier to generate sum-frequency signals ranging from 770 nm to 780 nm.
The normalized conversion efficiencies of the strongest sum-frequency signal (Peak 7 in Fig. 2) and the associated second harmonic signal were derived by measuring the slope of the curve of the conversion efficiency () with respect to the pump power; see Fig. 4. The measured conversion efficiencies of the sum-frequency and SHGs are and , respectively. The efficiency of the SHG is lower than the highest efficiency of second-order nonlinear optical processes related to birefringent phase matching conditions in LN WGM microcavities, which is the order of [
It is noted that the SFG process related to the phenomenon itself has meaningful application and has been reported in both millimeter-sized and micrometer-sized LN WGM microcavities[
To conclude, we observed a series of multi-peak signals in an LN WGM microcavity under the pump of only a continuous laser in the 1550 nm band. These signals were ascribed to the wave mixing of the pump laser and its background, which is usually considered natural light. The influences of SFG between the laser and the background on the nonlinear process were analyzed. The method to suppress the phenomenon was proposed. This work provides a reasonable interpretation for the unknown nonlinear optical signal in WGM microcavities and the suggestions to use and avoid similar phenomena.
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