Mohammad Soltani

AlGaN/AlN integrated photonics platform for the ultraviolet and visible spectral range.

We analyze a photonic integrated circuit (PIC) platform comprised of a crystalline AlxGa1-xN optical guiding layer on an AlN substrate for the ultraviolet to visible (UV-vis) wavelength range. An Al composition of x~0.65 provides a refractive index difference of ~0.1 between AlxGa1-xN and AlN, and a small lattice mismatch (< 1%) that minimizes crystal dislocations at the AlxGa1-xN/AlN interface. This small refractive index difference is beneficial at shorter wavelengths to avoid extra-small waveguide dimensions. The platform enables compact waveguides and bends with high field confinement in the wavelength range from 700 nm down to 300 nm (and potentially lower) with waveguide cross-section dimensions comparable to those used for telecom PICs such as silicon and silicon nitride waveguides, allowing for well-established optical lithography. This platform can potentially enable cost-effective, manufacturable, monolithic UV-vis photonic integrated circuits.

Ultrahigh Q whispering gallery mode electro-optic resonators on a silicon photonic chip.

Crystalline whispering gallery mode (WGM) electro-optic resonators made of LiNbO3 and LiTaO3 are critical for a wide range of applications in nonlinear and quantum optics, as well as RF photonics, due to their remarkably ultrahigh Q(>108) and large electro-optic coefficient. Achieving efficient coupling of these resonators to planar on-chip optical waveguides is essential for any high-yield and robust practical applications. However, it has been very challenging to demonstrate such coupling while preserving the ultrahigh Q properties of the resonators. Here, we show how the silicon photonic platform can overcome this long-standing challenge. Silicon waveguides with appropriate designs enable efficient and strong coupling to these WGM electro-optic resonators. We discuss various integration architectures of these resonators onto a silicon chip and experimentally demonstrate critical coupling of a planar Si waveguide and an ultrahigh QLiTaO3 resonator (Q∼108). Our results show a promising path for widespread and practical applications of these resonators on a silicon photonic platform.

Broadband biphoton generation and statistics of quantum light in the UV-visible range in an AlGaN microring resonator

We present a physical investigation on the generation of correlated photon pairs that are broadly spaced in the ultraviolet (UV) and visible spectrum on a AlGaN/AlN integrated photonic platform which is optically transparent at these wavelengths. Using spontaneous four wave mixing (SFWM) in an AlGaN microring resonator, we show design techniques to satisfy the phase matching condition between the optical pump, the signal, and idler photon pairs, a condition which is essential and is a key hurdle when operating at short wavelength due to the strong normal dispersion of the material. Such UV-visible photon pairs are quite beneficial for interaction with qubit ions that are mostly in this wavelength range, and will enable heralding the photon-ion interaction. As a target application example, we present the systematic AlGaN microresonator design for generating signal and idler photon pairs using a blue wavelength pump, while the signal appears at the transition of ytterbium ion (171Yb+, 369.5 nm) and the idler appears in the far blue or green range. The photon pairs have minimal crosstalk to the pump power due to their broad spacing in spectral wavelength, thereby relaxing the design of on-chip integrated filters for separating pump, signal and idler.

Free-carrier electrorefraction and electroabsorption in wurtzite GaN.

We present a theoretical analysis of the change in refractive index and absorption of Wurtzite GaN due to free carriers for a wavelength range of 1 to 5 μm using the Drude model. The separate role of holes and electrons as well as their combined effect are discussed and the results are compared to those of Si and GaAs. The results show that promising modulators based on electrorefraction and electroabsorption can be implemented in a GaN integrated photonic platform. We also discuss the validity of the Drude model as well as the interaction of longitudinal optical (LO) phonons in GaN with the electron-hole plasmons, especially the impact on the absorption of GaN at high carrier concentrations. While LO phonon-plasmon (LOPP) interaction is shown to be stronger for electrons than for holes, the overall effect is negligible at the wavelength range of discussion (1-5 µm) for the moderate injection levels examined here for carrier-induced electro-optic modulation.

Stimulated Brillouin Scattering in an AlGaN Photonics Platform Operating in the Visible Spectral Range

We present Stimulated Brillouin Scattering (SBS) process in AlGaN integrated photonic waveguides. The wide bandgap of this III-Nitride material platform allows operating at visible wavelengths enabling large Stokes shifts. For this study, we employ a multiphysics approach that includes electric-photoelastic, magnetic-photoelastic, material interface displacement effects, and for optimal waveguide dimensions to find the Brillouin-active acoustic modes involved in the SBS process. The SBS power gain and the Stokes frequency shift are investigated for both backward and forward scattering processes, and it is shown that stokes shift larger than 50 GHz with high gain are achievable. Moreover, a parametric analysis is presented in order to demonstrate the possibility of realizing Brillouin lasers operating at blue wavelengths.

Emerging III-Nitride technology for ultraviolet and visible integrated photonics

We review our effort on developing a universal integrated photonic platform functional from ultraviolet (UV) through the visible range. In this context, III-Nitride materials, with their wide bandgap and unique UV-visible optical and electronic properties have reached a level of maturity for on-chip electronic-photonic integration.