Mid-wave infrared transmittance filters in suspended GaAs subwavelength gratings

The high-Q resonances observed in subwavelength gratings support many applications, including optical phase arrays, refractive index sensing, enhancement of nonlinear effects, and spectral filtering. Integrating these functions with active devices opens new possibilities for microscale optical systems. GaAs high-contrast gratings (HCGs) simplify the integration of resonant structures with active III–V semiconductor devices. These structures are particularly useful in the mid-wave infrared (5–7 μm), overcoming several limitations of the material systems presented in previous works. By selectively removing an underlying AlGaAs layer, a high index contrast is introduced between suspended GaAs gratings and air without using an exotic low-index substrate. Furthermore, epitaxial growth enables precise device thickness. In this work, we demonstrate GaAs/air HCG transmittance filters for operation in the mid-wave infrared. A numerical study investigates the dependence of filter performance on substrate proximity. We fabricate experimental filters using an HBr-based plasma etch and a selective wet etch. Optical characterization demonstrates quality factors >50 and transmittance peak efficiency >50%. The observed filter performance is comparable to other mid-wave infrared subwavelength grating filters. This work concludes with discussion of experimental nonidealities, emphasizing the considerable influence of grating size and optical measurement conditions. The suspended GaAs HCG filters show potential for direct integration with active devices such as photodetectors and lasers.