###### Abstract  
The authors present results on the performance of a hybrid external cavity photonic crystal laser-comprising semiconductor optical amplifier, and a 2D photonic crystal cavity fabricated in low-temperature amorphous silicon. The authors demonstrate that lithographic control over amorphous silicon photonic crystal cavity-resonant wavelengths is possible, and that single-mode lasing at optical telecommunications wavelengths is possible on an amorphous silicon platform. nanophotonics; silicon photonics; photonic crystals; lasers; amorphous silicon; CMOS processing; telecommunications +
Footnote †: journal: Applied Physics Letters  
services [7]. Much work has gone into the development of low-temperature fabrication processes to ensure that a-Si is compatible with CMOS process technology. Previously a-Si had to be deposited at \(550\,\mathrm{\SIUnitSymbolCelsius}\)[8], but recent work has shown that a-Si can be processed at temperatures as low as \(230\,\mathrm{\SIUnitSymbolCelsius}\)[9]. The potential of a-Si for use in photonic applications has been demonstrated with some success. In [10] an 8-channel optical add-drop multiplexer based on hydrogenated amorphous silicon (a-Si:H), micro-ring resonators with a radius of \(10\,\mathrm{\SIUnitSymbolMicro m}\) were shown to operate with \(100\,\mathrm{GHz}\) bandwidth. In [11] fiber-to-chip grating couplers, low-loss single-mode photonic wire waveguides, power splitters, Mach-Zehnder interferometers and micro-Ring resonators with Q-factors of 20,000 that were reported for \(200\,\mathrm{nm}\) a-Si:H platforms, device performance was comparable to devices with similar dimensions in the c-Si platform. The electro-optic effect, used to enable optical modulation, has been demonstrated in a-Si:H with a modulation efficiency of \(V_{\pi}\,L\,=\,5.9\,\mathrm{Vcm}\)[12]. Numerical schemes exist to optimize the design of a-Si:H Mach-Zehnder type modulators [13]. To fully realize the potential of a-Si for integrated photonic devices an a-Si-based light source must be developed. Recent work in this area has shown that it is possible to heterogeneously integrate a III-V semiconductor-distributed feedback laser onto standard c-Si SOI via a wafer bonding process where low temperature a-Si was used as the coupling medium between the laser and the SOI [14]. In this work we will demonstrate, for the first time, the operation of an external cavity photonic crystal laser-comprising semiconductor optical amplifier (SOA) gain medium, a wavelength selective mirror formed by a photonic crystal (PhC) cavity fabricated in low-temperature a-Si, and an SU8 polymer waveguide to enable low-loss optical propagation. Results obtained as part of this work will show, for the first time, that it is possible to obtain lithographic wavelength control of PhC cavity in a-Si and also that single-mode lasing can be achieved in a laser in which a-Si forms an integral part of the laser cavity. The hybrid external cavity photonic crystal laser has been previously demonstrated with c-Si as the medium in which the 2D PhC cavities were fabricated [15]. The laser uses a polymer bus waveguide concept to optimize coupling between the SOA and the PhC cavity [16]; this approach has the advantage of minimizing the propagation losses associated with a-Si. The PhC cavity is of the dispersion-adapted (DA) type and has been optimized for a-Si [17]. Other advantages associated with the hybrid external cavity photonic crystal laser are its reduced footprint, the fact that SOA gain chips can be fabricated with lengths on the order of \(400\,\mathrm{\SIUnitSymbolMicro m}\) while PhC cavity lengths are on the order of \(20\,\mathrm{\SIUnitSymbolMicro m}\) and increased electrical efficiency, and that hybrid external cavity laser configurations with wall plug efficiency of 12% have been reported [18].