Electro-optics

Our research is focused on the use of compound semiconductor heterostructures in high-index-contrast waveguides to achieve ultra-low modulation voltages. State-of-the-art heterostructures such as coupled quantum wells should enable large optical phase shifts at low bias voltages without introducing significant loss. The integration of these materials into suspended high-index-contrast waveguides (in which the core is surrounded by air) reduces the necessary bias voltage further by dramatically reducing the size of the optical mode while maintaining excellent overlap with the semiconductor heterostructure. Our most recent devices have achieved VπL coefficients as low as 110 mV-cm in suspended waveguides with MQW cores only 215 nm thick.

A schematic (a) and SEM (b) of a suspended rib semiconductor quantum well waveguide. By suspending the waveguide (using a selective etch to remove the sacrificial layer) the electro-optic phase shift is enhanced by as much as 40%.
A schematic (a) and SEM (b) of a suspended rib semiconductor quantum well waveguide. By suspending the waveguide (using a selective etch to remove the sacrificial layer) the electro-optic phase shift is enhanced by as much as 40%.

Nonlinear Optics

High-index-contrast horizontal slots for difference frequency generation in suspended AlGaAsP waveguides. Such waveguides can be fabricated using selective etching to remove a sacrificial layer between the two waveguide layers, and provide birefringences large enough to enable phase matching for sum and difference frequency generation.
High-index-contrast horizontal slots for difference frequency generation in suspended AlGaAsP waveguides. Such waveguides can be fabricated using selective etching to remove a sacrificial layer between the two waveguide layers, and provide birefringences large enough to enable phase matching for sum and difference frequency generation.

High-index contrast semiconductor waveguides provide exciting new opportunities to both study basic nonlinear optics in these materials as well as investigate new photonic devices such as Raman lasers and chemical sensors. Waveguides fabricated from zinc-blende semiconductors (such as III-V compound semiconductors) also have large values of χ(2), enabling sum- and difference-frequency generation. However, efficient nonlinear frequency generation requires phase matching, an issue that has so far limited the use of III-V waveguides for this purpose. Our research is focused on the use of nanomachined high-index-contrast III-V waveguides to overcome problems associated with phase matching. Not only do nanomachining techniques enable sophisticated geometries that enable birefringent phase matching or quasi-phase matching, but they also insure a large waveguide intensity due to the highly confined optical mode. In particular, high-index-contrast horizontal slots provide birefringences large enough to enable phase matching for sum and difference frequency generation.

Polaritonics

The measured Raman scattering in a suspended InP waveguide (green) compared to the predicted peaks shown by the intersection of the guided-mode polariton dispersion curve (blue) and the polaritonic phase-matching curve (red dashed).
The measured Raman scattering in a suspended InP waveguide (green) compared to the predicted peaks shown by the intersection of the guided-mode polariton dispersion curve (blue) and the polaritonic phase-matching curve (red dashed).

Unlike a standard optical fiber (characterized by a core-cladding index difference of 0.01) in which most laser-induced fluorescence or Raman scattering will be lost out of the fundamental waveguiding mode, in a high-index-contrast waveguide (characterized by a core-cladding index difference as high as 2.5), much of the emitted Stokes-shifted light will be captured into a propagating mode. This property, coupled with the long interaction lengths provided by waveguides, make integrated high-index contrast semiconductor waveguides extremely efficient sources for optical experiments in which a pump laser induces signal light at a Stokes-shifted wavelength.