Ar/Cl₂ etching of GaAs optomechanical microdisks fabricated with positive electroresist

Abstract

A method to fabricate GaAs microcavities using only a soft mask with an electrolithographic pattern in an inductively coupled plasma etching is presented. A careful characterization of the fabrication process pinpointing the main routes for a smooth device sidewall is discussed. Using the final recipe, optomechanical microdisk resonators are fabricated. The results show a very high optical quality factors of Q opt > 2 × 10 5 , among the largest already reported for dry-etching devices. The final devices are also shown to present high mechanical quality factors and an optomechanical vacuum coupling constant of g 0 = 2π × 13.6 kHz enabling self-sustainable mechanical oscillations for an optical input power above 1 mW. OCIS codes: (130.3990) Micro-optical devices; (230.4000) Microstructure fabrication; (220.4880) Optomechanics 1. Introduction Harnessing the confinement of light with wavelength-scale waveguides and cavities has enabled the realization of table-top scale nonlinear optical phenomena in silicon-compatible photonic chips. Recent examples show the versatility of this integrated photonics approach, such as frequency comb generation [1, 2], quantum computation [3-8], low voltage eletro-optical modulation [9], and optical to microwave coherent conversion [10]. One key ingredient that may also boost this revolution is the interaction between light and mechanical degrees of freedom, enabling both read-out and actuation of mesoscale mechanical modes in waveguides [11] and cavities [12][13][14][15], paving the road towards the sound circuits revolution [16,17]. Throughout the quest for better suited cavity geometries that can host optical and mechanical waves, microdisk cavities have proven to be a simple and effective choice. Their tight radial confinement of whispering gallery optical waves and rather strong interaction with radial breathing mechanical modes lead to high optomechanical coupling rates [18][19][20], which are necessary for efficient device-level functionalities [7,[21][22][23]. Other ingredients in the optomechanical enhancement are the cavity or waveguide material properties. Although silicon is widespread in many optomechanical devices [14,[24][25][26][27] due to its mature fabrication, the interest in III-V materials for optomechanical devices has increased [28-32] due to their unique optical, electronic and mechanical properties. Gallium Arsenide (GaAs), for instance, is advantageous because of its very high photoelastic coefficient [33], which leads to large electrostrictive forces and optomechanical coupling [18,34]. Moreover, optically active layers can be easily grown on GaAs wafers, allowing the fabrication of light-emitting optomechanical devices [35][36][37].Despite the tougher challenges in fabricating high optical quality factor in GaAs cavities, in comparison to the more mature fabrication of silicon, both wet and dry chemistry etching have been successfully developed. Wet etching, followed by surface passivation, resulted in record high intrinsic optical quality factors of 6 × 10 ...

Electroabsorption in gated GaAs nanophotonic waveguides

“…11,26,27 By removing mid-gap states, it is expected that the surface Fermi level unpins, thereby removing any source of electroabsorption due to bandbending. Finally, improving the fabrication process using intermediate hard masks 28 or resist reflow 29 is also expected to reduce the waveguide RMS roughness to < 1 nm, which can potentially reduce the intrinsic waveguide loss down to α INT < 1 dB/mm. Achieving such a target would enable integrating several hundreds of devices such as single-photon sources, filters, and switches, while taking advantage of the small size of such devices in the GaAs platform.…”

Integrated cavity opto-electromechanics: electrical tuning and control of optomechanical systems

“…B.2, reflowing resist causes a curved upper surface, with angled rather than vertical sidewalls (e.g. see also [337]). The particular advantage of performing resist reflow lies in smoothing out sub-micron roughness along the perimeter of the device, allowing for higher Q optical cavities.…”

“…Further, changing the sidewalls of exposed resist, for example through reflow [337], can consequently change the etched sidewall slope. Increasing the pressure of the etch increases the undercut (and Photoresist masks are not always ideal for HF etching due to delamination and their lack of sufficient etch resistance during long etch processes, though thick photoresists (> 2 µm) can be used to circumvent the latter problem.…”