Enhanced Interband Optical Nonlinearities from Coupled Quantum Wells

Abstract

The recent, rapid advances in nonlinear chipscale nanophotonics in the visible and near-infrared have been largely driven by manipulating the local dielectric environment proximate to decades-old workhorse bulk nonlinear optical materials, rather than increasing the inherent strength of their nonlinear response. While proposed decades ago, we demonstrate the first experimental realization of a new class of designer nonlinear materials that leverage the interband optical transition in asymmetric structures to provide strong second order susceptibility, $χ^{(2)}$. Using simple AlGaAs/GaAs coupled quantum wells operating in the near-infrared as a prototype, we observed strong second harmonic generation enhancement of 1550 nm to 775 nm over bulk controls. Extracted $χ^{(2)}$ values were as high as 2750 pm/V, which is $>$7x that of bulk GaAs. Furthermore, measured susceptibilities agreed well with quantum mechanical calculations of $χ^{(2)}$ using layer profiles extracted from electron microscopy. Growth interruptions were employed to improve interfacial abruptness in response to electron microscopy characterization, resulting in increased $χ^{(2)}$ toward the simulation predictions for ideal heterointerfaces. More complex layer designs showed predicted $χ^{(2)}$ up to 7 nm/V. Such materials are anticipated to find myriad applications, including entangled photon generation at telecommunications wavelengths for chipscale quantum information processing.

Figures (4)

• Figure 1: Asymmetric coupled quantum wells for enhanced second order optical nonlinearity.a) Diagram of the epitaxially-grown multi-QW samples with substrate transfer to sapphire. Each 30 nm period of coupled QW's is comprised of two GaAs QWs separated by a 1.8 nm AlGaAs tunneling barrier and an 18.2 nm AlGaAs period barrier between sets of coupled QWs. b) Interband second order nonlinearity utilizes transitions across the bandgap to access higher photon energies and shorter wavelengths compared to intersubband nonlinearities, which are limited by the achievable conduction band offset for a given material system. c) This band-edge diagram shows the coupled GaAs/Al$_{0.55}$Ga$_{0.45}$As QWs and the ground state (red) and first excited state (blue) energy levels and wavefunctions. The wavefunctions and energy levels are used to calculate $\chi^{(2)}$ as a function of QW asymmetry, which predicts the strongest $\chi^{(2)}$ for asymmetry s = 0.42. d) This graphic shows the overall scope of the work. Quantum mechanical simulations guided the coupled QW design and sample growth. The average enhanced $\chi^{(2)} \approx$ 1400 pm/V from measured second harmonic generation was in good agreement with $\chi^{(2)}$ simulations of measured composition profiles from electron microscopy. Two sets of coupled QWs, as measured by STEM, are shown with GaAs layers in light gray and AlGaAs layers in dark gray.
• Figure 2: Rotation-angle SHG measurements and signatures.a) The sample is rotated through 360$^o$ of azimuthal angle (around the marked x on the sample face in the figure) while under constant illumination at the fundamental wavelength. The sample is tilted such that the polar angle is 45$^o$, ensuring a component of the P-in polarized electric field to be perpendicular to the QW plane. The input and output are p- or s- polarized, and low-pass filters block the fundamental wavelength while passing the SH wavelength. b) The rotational dependence of the SH signal shows a four-lobed periodicity, and the SH signal is strong for p-polarized input but weak for s-polarized input. c) The SHG power scaled with the square of the fundamental power, shown by a linear fit to the data plotted against the squared power axis, and the coupled QW sample (red) shows enhanced SHG compared to the bulk control (blue). d) The measured SHG dependence on fundamental wavelength shows a resonant peak for the QW sample, whereas there is no resonance behavior from the control. The measured wavelength dependence is in good agreement with the simulations for the coupled GaAs/Al$_{0.55}$Ga$_{0.45}$As coupled QWs with a 1.8 nm tunneling barrier, 10 nm total quantum well thickness, and QW asymmetry of $s=0.42$.
• Figure 3: STEM/EDS composition profiles and measured $\chi^{(2)}$. a) Scanning transmission electron microscopy shows the asymmetric GaAs QW's coupled by the thin AlGaAs tunneling barrier. The energy dispersive spectroscopy (EDS) (colored Al, Ga, As, and profile plots) reveals the material composition profile of the layer structure. b) These plots show the measured EDS Ga and Al composition profiles, and the overlayed piecewise linear function is the composition profile to simulate $\chi^{(2)}$. c) This chart compares simulated $\chi^{(2)}$ at 1550 nm for the ideal coupled QW composition profile (gray), EDS composition profiles for Ga and Al (green and red), measured effective $\chi^{(2)}$ at 1550 nm for the GaAs and AlGaAs controls (yellow), and measured effective $\chi^{(2)}$ for the coupled QW samples (blue). The inset depicts each sample, their relative total thicknesses, and the positions of the multi-QW layers within the sample, oriented such that the sapphire substrate is at the bottom. The measured effective $\chi^{(2)}$ from the QW samples show enhancement compared to the controls, and they are in good agreement with the simulated $\chi^{(2)}$ based on the EDS composition profiles.
• Figure 4: Improved SHG by growth interruptions and large scope for designing tailored nonlinearities. a) Growth interruptions at the GaAs/AlGaAs interfaces improve SHG (gray) and the effective $\chi^{(2)}$ for the 16-period coupled-QW structures at 1550 nm fundamental wavelength. c) An enhanced $\chi^{(2)}$ up to 7 nm/V was predicted by simultaneously optimizing the number of coupled QWs and the thicknesses of the QWs and tunneling barriers. The simulated data points and inset diagrams in gray use ideal interfaces, whereas the data points and insets in red use graded interfaces as measured by STEM. Even with graded interfaces, it is possible to significantly enhance interband $\chi^{(2)}$ with more complex coupled QW designs.