Visualizing Whispering-Gallery Modes through Second-Harmonic and Sum-Frequency Generation in Al0.4Ga0.6As-on-Insulator Microdisk Resonators

We report second-harmonic and sum-frequency generation in Al0.4Ga0.6As-on-insulator microdisk resonators at ~775 nm wavelengths. Our experiments reveal clear polygonal-shaped whispering-gallery mode profiles.


Abstract:
We report secondharmonic and sumfrequency generation in Al0.4Ga0.6Asoninsulatormicrodisk resonators at ~775 nm wavelengths.Our experiments reveal clear polygonalshaped whisperinggallery mode profiles.OCIS code: (130.0130)Integrated optics, (190.0190)Nonlinear Optics, (140.3948)Microcavity devices AlxGa1xAs is emerging as a promising semiconductor material platform for integrated nonlinear optical frequency conversions for applications towards alloptical signal processing [1] and quantum photonic circuits [2].The high secondorder nonlinear susceptibility pm/V (GaAs at 1553 nm) [3], large refractive index and direct bandgap (x < 0.45) enables a compact monolithic photonic platform for optical signal processing and quantum photonic circuits.Researchers exploited various phasematching schemes, including modal phasematching [4], periodic domain inversions [5] and quasiphasematching [6,7] to demonstrate efficient secondorder nonlinear optical frequency conversions in AlGaAs despite its lack of birefringent phasematching.Whisperinggallery mode (WGM) microresonators are commonly adopted to enable cavityresonance enhancement of the nonlinear optical frequency conversions.Under weak perturbations, such as cavity shape deformations from a symmetric shape or surface roughnessinduced scattering, orthogonal WGMs can coherently couple to form polygonalshaped WGMs [8].In this paper, we demonstrate secondharmonic generation (SHG) and sumfrequency generation (SFG) in multiple polygonalshaped WGMs that can be clearly observed in AlGaAsoninsulator (AOI) circular microdisk resonators without designed cavity deformations.We adopted commercially available Al0.4Ga0.6Assubstrates with an aluminum content of x = 0.4 for our AOI photonic chips to tailor the bandgap energy (Eg ~ 1.92 eV) for near transparency at the ~775 nm wavelengths (far detuned from the bandedge transition) and normal dispersion spanning the nearinfrared (NIR) wavelengths to the telecommunications 1550nm wavelengths.We design the waveguide geometries to tailor the modal dispersion for a typeI modal phasematching (using ) between the transversemagnetic (TM)polarized mode at ~775 nm and the transverseelectric (TE)polarized at ~1550 nm. Figure 1(a) shows the schematic of various WGMs illustrating the various ray trajectories that form a regular WGM, periodic orbital modes and irregular WGMs.Figures 1(b) and (c) show the topview and crosssectional schematics of our waveguidecoupled microdisk resonator.Our microdisk resonators have a circumference of 400 m and are cladded with silica films above and beneath the AlGaAs film for optical confinement.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
We measure the linear transmission spectra of our device at the ~1550 nm wavelengths to characterize the waveguidecoupled WGMs. Figure 2(a) shows the measured TEpolarized transmission spectrum of a waveguide coupled microdisk resonator.The inset shows the zoomin view of the resonance at ~1554 nm, indicating a loaded quality (Q) factor of ~78,000.
We input two NIR laser light at two resonances and marked by the red and orange arrows, as shown in Fig. 2(a).We collect the offplane scattered light from the SHG and SFG signals using a tapered fiber.Figure 2(b) shows the measured SHG and SFG spectra.The inset shows the imaged topview modeintensity pattern of the total generated SHG and SFG light scattered from the microdisk resonator using a longworkingdistance objective lens (NA 0.42) and a CMOS camera.The open circle indicates where we positioned the taperedfiber tip for measuring the SHG and SFG spectra.The secondharmonic of and the sumfrequency of and are evident from the measured spectra.The secondharmonic of is below the noise floor (possibly due to a weaker inputwaveguidecoupling at ).
Figure 2(c) shows another measured transmission spectrum.We observe clear scattering of the SHG light from the microdisk resonator when the input wavelengths are tuned to onresonance wavelengths labeled in Fig. 2(c).We discern at least four distinct WGMs of different polygonal shapes within one freespectral range.Figure 2(d) shows the imaged modeintensity distributions of four different WGMs (denoted the colored markers).We notice the presence of periodicorbital WGMs, such as the period6 hexagons and period7 heptagons, suggesting 6bounce and 7bounce trajectories.Irregular WGM distributions with more complex finegrained intensity distributions and a wide radial spread are also observed experimentally.Further analysis and experiments are needed to investigate the perturbations needed to excite various periodicorbital WGMs and their effects on the SHG and SFG efficiencies.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.
Figure 1(d) shows the scanningelectron micrograph of the curved sidewall of one device, indicating a smooth and vertical sidewall.

Figure 1 .
Figure 1.(a) .(b) Topview and (b) crosssectionalview schematic of our designed AOI waveguidecoupled microdisk resonator.The colored arrows indicate the input of two nearinfrared frequencies and generating the second harmonic and sumfrequencies of the input frequencies.(d) SEM image of our device sidewall.

Figure 2 .
Figure 2. (a) Measured TEpolarized transmission spectrum of the waveguidecoupled microdisk resonator in the ~1550 nm wavelengths using an erbiumdoped fiber amplifier as a broadband light source and measured by an optical spectrum analyzer.(b) Measured SHG and SFG signals integrated for 10 s by the spectrometer detector array.(b)i.The measured modeintensity profile of the combined SHG and SFG light scattered out ofplane from the microdisk resonator.The open red circle indicates the position of the collection tapered fiber tip.The dashed white lines indicate the tapered fiber.(c) Measured TEpolarized transmission spectrum of the waveguidecoupled microdisk resonator in the ~1550 nm wavelengths using a wavelengthtunable laser.(d) Imaged mode profiles of the SHG light scattered outofplane from the microdisk resonator.The white bars represent 100 m.