Abstract

Asymmetric quantum well (QW) structures have recently attracted considerable attention as nonlinear optical materials with intersubband nonlinear susceptibilities in the ir (λ~10 μm)^1–3 many orders of magnitude higher than in bulk semiconductors.^1–3 In this paper we report the observation of a new nonlinear optical phenomenon: resonant multiphoton electron emission from a QW. This effect is conceptually similar to multiphoton ionization of a molecule, although its cross section is much larger. Our AlInAs/GaInAs sample (40 periods of a coupled-well-structure consisting of 42 Å, 20 Å and 18 Å GaInAs wells, separated by 16 Å AlInAs barriers, with only the thickest well n⁺ doped) is designed to have nearly equally spaced bound states (≃120 meV). By appropriately choosing the photon energy and Stark shifting the energy levels electrons can be ejected out of the well via a 3-photon transition into continuum states. For the experiments a linearly polarized CO₂ laser is used and the structure is processed in a two-pass 45° waveguide. The laser is initially polarized so as to maximize the component normal to the layers; the polarization is then rotated or the power is varied. The measured photocurrent at 30 K scales as ${\text{P}}_0^3{\sin }^6\theta$, where P₀ is the laser power and θ the polarization angle, giving direct evidence of 3-photon electron emission from the well. The photocurrent as a function of photon energy shows a strong peak which arises from the resonant enhancement of the transition rate by the second and third energy levels. From our data the 3-photon emission cross section is estimated. In another experiment third harmonic generation is studied as a function of electric field (magnitude and polarity) at a pump photon energy ≈ 118 meV. Two peaks are observed. The one at zero bias arises from a triple resonance of the pump photons with the three bound excited states of the QW. The second one is caused by the strong Stark shift of the energy levels and arises from a triple resonance with the first two bound excited states and the first resonant state in the classical continuum above the barriers. The measured peak nonlinear susceptibility ${\chi }_{3\omega }^{(3)}$ is $\simeq 3.5\times {10}^{-14}{( \text{m}/\text{V})}^2\simeq$ three times the value previously measured³ at 300 K and the highest ever reported in any material.

© 1992 Optical Society of America