Fig. 3: Q-factor spectra and measurement set-up.
From: A mid-infrared Brillouin laser using ultra-high-Q on-chip resonators
a Q-factor spectrum of two different resonators, with and without an upper cladding, measured at wavelengths of 3.2-3.9 μm. The grey-shaded region at the 3.33-3.55 μm wavelength indicates the hydrocarbon (C-H) absorption band, with their characteristic peaks indicated with black arrows. The grey dashed line is given as a guideline for the effect of the H2O absorption tail. b Cross-section drawing of the resonator with an upper cladding used for the Q-factor measurement represented by the red line in Fig. 3a, which has geometric parameters of \(w=6.1\ {{{\mu }}}{{{\rm{m}}}},\theta=28^\circ,{t}_{1}=1.1\ {{{\mu }}}{{{\rm{m}}}},{t}_{2}=2.4\ {{{\mu }}}{{{\rm{m}}}},{t}_{3}=0.8\ {{{\mu }}}{{{\rm{m}}}}\). The parameters for the ‘without upper cladding’ case, represented by the blue line in Fig. 3a, are \({t}_{1}=1.0\ {{{\mu }}}{{{\rm{m}}}},{t}_{2}=2.0\ {{{\mu }}}{{{\rm{m}}}}\) and \({t}_{3}=0\ {{{\mu }}}{{{\rm{m}}}}\), with more details provided in Supplement 2.c Lorentzian fit of the record Q-factor measured at 3.86 μm wavelength on the same resonator. The magenta line indicates the resonator transmission spectrum, while the navy line is the interferogram of the calibrated Michelson interferometer (MI, 75.25 MHz), depicted in Figure d, used as a frequency reference. d A drawing of the mid-IR cavity coupling setup. A continuously tunable optical parametric oscillator (OPO) laser was used as the pump source. Details about the setup and interferometer are given in the Methods. BS: beam splitter, VA: variable attenuator, PC: polarization controller, L: lens, Res: resonator, WG: bus waveguide, PD: photodetector, OSC: oscilloscope, OSA: optical spectrum analyzer.
