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A range of unique capabilities in optical and microwave signal processing have been demonstrated using stimulated Brillouin scattering. The desire to harness Brillouin scattering in mass manufacturable integrated circuits has led to a focus on silicon-based material platforms. Remarkable progress in silicon-based Brillouin waveguides has been made, but results have been hindered by nonlinear losses present at telecommunications wavelengths. Here, we report a new approach to surpass this issue through the integration of a high Brillouin gain material, As2S3, onto a silicon chip. We fabricated a compact spiral device, within a silicon circuit, achieving an order of magnitude improvement in Brillouin amplification. To establish the flexibility of this approach, we fabricated a ring resonator with free spectral range precisely matched to the Brillouin shift, enabling the first demonstration of Brillouin lasing in a silicon integrated circuit. Combining active photonic components with the SBS devices shown here will enable the creation of compact, mass manufacturable optical circuits with enhanced functionality.Stimulated Brillouin Scattering (SBS) has recently emerged as an impressive tool for optical processing and radio-frequency (RF) photonics. SBS is one of the strongest nonlinearities known to optics, and is capable of providing exponential gain over narrow bandwidths of the order of tens of megahertz. This narrowband amplitude response is accompanied with a strong dispersive response, capable of tailoring the phase or group delay of a counter propagating optical signal. In light of these effects a rich body of applications have been explored such as slow light 1 , stored light 2 , narrowband RF photonic filters [3][4][5] , dynamic optical gratings 6,7 , narrowband spectrometers 8 , optical amplifiers 9,10 and RF sources 11 among others. When pumped in a resonator configuration, a narrow linewidth spectrally pure SBS laser can be generated [12][13][14] . Highly coherent lasers are used in optical communication, LIDAR and producing pure microwave sources 15 among other applications. While the majority of previous works have traditionally utilised SBS in optical fiber, a number of these applications have been demonstrated in integrated form factors [16][17][18][19][20] . Most recently, the demonstration of 52 dB Brillouin gain 21 in centimeter length scale As 2 S 3 rib waveguides proves that performance equivalent to kilometers of optical fiber is achievable in integrated devices.The capability to embed SBS as a functional component in active photonic circuits will enable the creation of a new class of opto-electronic devices, in particular for integrated microwave photonics 22 . The desire to harness SBS optical processing in CMOS (Complementary metaloxide-semiconductor) compatible platforms has recently culminated in demonstrations of SBS in various silicon on insulator (SOI) device architectures [23][24][25][26] In this work we introduce a hybrid integration approach to generate large Brillouin gain in a sili...

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