Four-Wave Mixing in Highly Nonlinear Chalcogenide Glass-in-Silica Waveguides

Chalcogenide glasses (ChGs) are a favorable material platform for on-chip, all-optical signal processing applications, due to their pronounced nonlinearities, broad transparency windows and photo-sensitivity [1]. Examples include time-domain de-multiplexing of high-rate data based on four-wave mixing (FWM) [2], microwave-photonic filtering of radio-frequency waveforms using stimulated Brillouin scattering (SBS) [3], and many more. The most widely employed ChG composition is arsenic tri-sulfide (As₂S₃). Nonlinear propagation coefficients as high as 10 [W´m]-1 have been demonstrated in planar As₂S₃ waveguides [4]. The devices were fabricated based on dry etching of the ChG core layer [5]. Although low propagation losses could be achieved [5], the etching process remains difficult to implement due to the relative instability of ChGs. In addition, an individual protocol must be developed for each ChG composition, and the etching of the ChG layer following its deposition is not fully compatible with the fabrication of CMOS electronics.

In this work we propose and demonstrate an alternative method for the fabrication of highly-nonlinear ChG waveguides with small cross-sectional areas. The waveguide patterns are first defined as narrow, isolated and comparatively deep etched trenches in a silica lower cladding layer. A ChG layer is deposited to form a core region that partially fills the etched trenches, as a final process step. The ChG core is surrounded by silica on three sides. The fabrication procedure relies on well-established etching of silica, supports the deposition of any ChG composition, and requires no processing of the ChG core itself. Lastly, Four-wave mixing is demonstrated with a nonlinear coefficient of 6.5±1 [W´m]^-1. The platform holds much promise for future demonstrations and applications of nonlinear optics and opto-mechanics on-chip.