Rayleigh scattering, mode coupling, and optical loss in silicon microdisks

Abstract

High refractive index contrast optical microdisk resonators fabricated from silicon-on-insulator wafers are studied using an external silica fiber taper waveguide as a wafer-scale optical probe. Measurements performed in the 1500 nm wavelength band show that these silicon microdisks can support whispering-gallery modes with quality factors as high as 5.2ϫ 10 5 , limited by Rayleigh scattering from fabrication induced surface roughness. Microdisks with radii as small as 2.5 m are studied, with measured quality factors as high as 4. 7ϫ 10 6 Applications for such devices include quantum networking, 7 low threshold nonlinear optical sources, 8 and compact micro-optical circuits. 4 The ability to create similar high quality factor ͑Q͒ WGM resonators in III-V or silicon ͑Si͒ semiconductors has thus far been hampered by the large refractive index of most semiconductors and the resulting sensitivity to surface roughness. 9,10 In this letter we describe measurements of micron-sized Si microdisk resonators supporting TM WGMs with significantly reduced sensitivity to disk-edge roughness. These modes have measured Q values as high as 5.2ϫ 10 5 and effective modal volumes ͑V eff ͒ as small as 5.3 cubic wavelengths in the material. The largest Q / V eff ratio is measured to be 8.8ϫ 10 4 , greater than the values measured in ultrasmall volume photonic crystals 11 and comparable to the values measured in ultrahigh-Q microspheres and microtoroids.

Unidirectional light emission from high-Qmodes in optical microcavities

“…Whispering-gallery modes (WGMs) in microdisks [2], microspheres [3], and microtori [4] have ultra-high Q-factors. For state-of-the-art semiconductor microdisks, the record Qfactor is > 3.5 × 10 5 [5,6]. The applicability of those cavities as microlasers and single-photon sources is, however, limited by isotropic light emission.…”

“…Hence, a light emitter couples easily to both WGMs which possibly leads to less directed emission. This problem is resolved in the following cases: (i) In realistic microdisks, the degeneracy is lifted due to Rayleigh scattering from the boundary [6]. (ii) For a singlephoton source it is desirable to place a single emitter (e.g.…”

Optical loss and lasing characteristics of high-quality-factor AlGaAs microdisk resonators with embedded quantum dots

“…The depth of coupling, however, has also considerably increased from 10% to 60%, corresponding to a photon collection efficiency ͑the ratio of "good" coupling to all other cavity losses including parasitic and intrinsic modal loss͒ of approximately 20%. It is believed that the high-Q values achieved in these measurements are due to a combination of the resist reflow process that reduces radial variations and subsequent Rayleigh scattering in the disk, 10 and the optimized dry etching processes that create very smooth disk-edge sidewalls.…”

“…2͑b͒, we show a "doublet" resonance of a microdisk ͑D = 4.5 m, one-DWELL structure͒ in the 1400 nm wavelength band when the taper is ϳ800 nm to the side of the disk; the separation is kept large in order to reduce taper loading effects. 9,10 The double resonance peaks correspond to standing wave modes formed from mixtures of the degenerate clockwise and counterclockwise whispering gallery modes that couple and split due to the disk-edge surface roughness. 10,15,16 The linewidth ͑⌬ ͒ of the shorter wavelength resonance corresponds to Q ϳ 3.6 ϫ 10 5 .…”

“…9,10,15 An optical fiber taper is formed by heating and adiabatically stretching a standard single-mode fiber until it reaches a minimum diameter ϳ1 m. A fiber-coupled scanning tunable laser ͑Ͻ5 MHz linewidth͒ is spliced to the taper's input, and when the taper is brought within a few hundred nanometers ͑nm͒ of the cavity, their evanescent fields interact, and power transfer can result. A schematic illustrating the coupling geometry for this system is shown in Fig.…”