Abstract

Recently large second-order nonlinearities associated with intersubband transitions have been reported in asymmetric AlGaAs/GaAs quantum well structures.¹,² In this paper we report what we believe to be the first observation of second harmonic generation (SHG) due to intersubband transitions in AlInAs/GaInAs coupled quantum wells. Previous structures were rectangular wells with applied electric field¹ or step graded asymmetric wells.² Our structure, grown lattice matched to a semi-insulating InP substrate, consists of forty coupled well periods separated from each other by 150-Å undoped AlInAs barriers and sandwiched between n⁺ InGaAs contact layers. Each period consists of a 64-Å GaInAs well, doped with Si (n ≈ 2 × 10¹⁷ cm^-3) separated from a 28-Å undoped GaInAs well by a 16-Å undoped AlInAs barrier. To achieve resonant enhancement of SHG the asymmetric structure is designed in such a way that the energy separations ΔE₁₂ and ΔE₂₃ are comparable and within the tuning range of a CO₂ laser. In addition, the composition profile is tailored to maximize the product of the dipole matrix elements of the three transitions since χ⁽²⁾ is proportional to <z₁₂> <z₂₃> <z₁₃>². We fabricated a multipass waveguide by polishing a 45° angle on both edges of the sample and illuminating one of them at normal incidence with CO₂ laser; for a primary λ = 10.5 μm we have measured χ⁽²⁾(2ω) ≈ 2.8 × 10^-8m/V at zero electric field. As a bias of the appropriate polarity is applied, the SHG increases and χ⁽⁾² reaches a maximum of 1.2 × 10^-7m/V (~300 times the bulk values in InP, GaAs, and InAs). This maximum occurs when ΔE₁₂ = ΔE₂₃ as a result of the Stark shift of the energy levels.

© 1991 Optical Society of America