Supercontinuum Generation in Hydrogenated Amorphous Silicon Waveguides in the Femtosecond Regime

Abstract

Supercontinuum generation in CMOS compatible hydrogenated amorphous silicon waveguides with femtosecond pulses at telecommunication wavelengths is experimentally studied. It is shown that stable 540 nm broad supercontinua can be obtained in 1 cm-long waveguides. Extreme spectral broadening that occurs when narrowband input pulses propagate in nonlinear optical media is known as supercontinuum (SC) generation [1]. In nonlinear bulk media, ultrahigh pulse energy delivered by cumbersome sources was required to observe this spectral widening. However, with the advent of guiding structures such as optical fibers, SC generation is possible with much lower pulse energy (and even with continuous waves) thanks to dispersion engineering and effective nonlinearity enhancement. Nevertheless, the integration of such sources still remains an important topic in photonic research owing to their possible applications. Hydrogenated amorphous silicon (a-Si:H) waveguides, thanks to their CMOS compatibility and high nonlinearity, are one of the most promising ultra-compact nonlinear devices for SC generation.In this work we focus on the generation of SC with femtosecond pulses in a-Si:H waveguides at telecommunication wavelengths around 1.55 µm. The advantage of this material over crystalline silicon is its larger bandgap of about 1.6 eV. This larger value results in lower two-photon absorption (TPA) and TPA induced free-carrier absorption, which are known to strongly limit the efficiency of nonlinear interactions [2]. SC generation has been recently observed in a-Si:H waveguide with picosecond pulses [3]. However in this latter work material degradation has been reported and we expect this degradation to be reduced in the femtosecond regime.Experiments were performed in 1 cm-long waveguides (cross-section 220⇥500 nm 2 ) fabricated using deep UV optical lithography and dry etching [4]. At telecommunication wavelengths, these waveguides are characterized by a linear loss of a = 3 dB/cm and a nonlinear parameter g = (740 + i 31) W 1 m 1 . At 1550 nm, the dispersion is anomalous and the GVD value has been estimated to be b 2 =-2 ps 2 /m [5]. The source is an OPO laser delivering pulses of about 150 fs around 1550 nm with a repetition rate of 82 MHz. The laser beam was coupled into the waveguide by means of a ⇥60 microscope objective to excite the quasi-TE mode of the waveguide. At the waveguide output, the light was collected with a lensed fiber (NA=0.4).Typical SC spectra at the waveguide output are reported in Fig.1(a). At an on chip peak power of 18 W, i.e. a pulse energy of 2.7 pJ, the spectrum extends from 1380 nm to 1920 nm at -20 dB from the peak maximum [see red curve in Fig.1(a)]. This spectrum is likely the result of self-phase modulation, soliton fission and dispersive wave generation. This latter phenomenon is an energy transfer from a soliton to a narrow frequency band in the normal GVD regime, and the peak around 1840 nm at 2 W input power is probably the signature of such a dispersive wave. The detailed physica...