Demonstration of critical coupling in an active III-nitride microdisk photonic circuit on silicon

Abstract

On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

Perspectives for III-nitride photonic platforms

“…Mode locking in monolithic two-section InGaN blue-violet semiconductor lasers was reported in 2013 and prefigures what can be achieved with blue lasers for the generation of optical frequency combs [138]. Microdisk resonators were embedded in air-suspended waveguiding layers on silicon [83][84][85].…”

“…Numerous demonstrations of individual microdisk lasers operating at room temperature under optical pumping from the UV-C to the green spectral range were reported [73][74][75][76][77][78][79][80][81][82]. Microdisk lasers operating under optical pumping and integrated into photonic circuits have also been demonstrated on silicon substrates [83][84][85]. Microdisk lasers under electrical injection were further demonstrated on Si (111) substrate [86] or on Si(100) substrate [87].…”

III-nitride photonic cavities

“…Such large air gaps also open access to the NIR spectral range, which remains presently unaccessible to epilayers grown on c-plane sapphire, and makes the realization of PICs that could be integrated to the Si photonic platform a tangible perspective. In this respect, regarding III-N microdisks that are part of PICs, the optical coupling of those resonators to the circuits is best ensured via side coupling to a suspended bus WG [25,[38][39][40]. In this latter case, the main challenge to overcome in terms of fabrication deals with the fine tuning of the distance between the microdisk and the WG, which will depend on the targeted applications and the desired figures of merit.…”

“…Lasing was also linked to second-order radial modes, whose presence was made possible thanks to a small post diameter, which are less sensitive to sidewall roughness and ion damage occurring during the RIE step. Beyond those studies, Tabataba-Vakili et al pointed out that the use of a bus WG is amenable with single-mode lasing operation, which would have to do with WGM selection taking place at the level of the whole PIC structure that involves a microdisk, a bus WG, and tethers [25], although those authors recently reported on two-mode lasing under pulsed excitation with such a PIC geometry [40].…”

Thermal characterization of electrically injected GaN-based microdisk lasers on Si

“…© 2020 The Japan Society of Applied Physics Supplementary material for this article is available online M icrocavity lasers featured with low-loss whispering gallery modes have been drawing increasing attention in communication, biological detection, temperature sensors and some other fields, due to their particular advantages of a small footprint, low power consumption and high quality factor compared with conventional laser diodes with Fabry-Pérot cavity. [1][2][3][4] GaN-based microdisk laser grown on Si is one of the promising candidates for the onchip light source of the optical interconnection, [5][6][7] because of the compatibility with Si-based microelectronics and lowcost large-scale manufacturing foundry. [8][9][10][11] In our previous work, we adopted a "sandwich-like" architecture by using AlGaN cladding layers on both sides of quantum wells for better electrical conductivity and reduced thermal resistance (R th ) than the conventional "mushroom-like" structure, and realized the first electrically pumped GaN-on-Si microdisk laser, of which the lasing wavelength and threshold current were 412 nm (violet) and 250 mA, respectively.…”