Giant third-order nonlinear susceptibilities for in-plane far-infrared excitation of single InAs quantum wells

Third-order, free-carrier nonlinear susceptibilities, χ⁽³⁾, have been measured between 19 and 23 cm^-1 for three InAs/AlSb quantum wells with sheet densities between 2.5 × 10¹² cm^-2 and 8 × 10¹² cm^-2. We find that these wells are strongly nonlinear at far-infrared frequencies: odd harmonics ninth order have been observed at high incident intensities, and the peak value of χ⁽³⁾ reaches ∼ 1 esu. This is several orders of magnitude larger than previously reported values for χ⁽³⁾ in bulk n-GaAs (10^-4 esu)[1] and in polyacetylen (10^-7 esu)[2]. The large magnitude of χ⁽³⁾ is attributed to the high carrier density in the InAs wells, and to the strong non-parabolicity of the conduction band in InAs. However, the free-carrier χ⁽³⁾ for bulk InAs predicts a density-dependence different from that observed, and the measured decrease in χ⁽³⁾ with increasing intensity indicates non-perturbative response. We find that the anisotropy of χ⁽³⁾ displays the expected 4-fold symmetry.