Quasi-phase matched second-harmonic generation from asymmetric coupled quantum wells

Quasi-phase matched (QPM) second-harmonic (SH) generation in both insulator and semiconductor waveguides has applications ranging from the generation of coherent visible light to optical signal processing. QPM compensates the wavevector mismatch between the fundamental and SH light by periodically varying the SH susceptibility, χ⁽²⁾, and refractive index, along the SH light propagation direction. It would be very useful if quantum wells could be used to engineer materials with a large χ⁽²⁾ which can be easily varied. Large asymmetric quantum well SH susceptibilities have been measured for far infrared wavelengths, where the nonlinear response is dominated by intra-subband transitions. Although there have been several theoretical studies predicting a quantum well χ⁽²⁾ comparable to that of LiNbO₃ or larger in the near infrared and visible wavelength range, there have been no experimental measurements of χ⁽²⁾ . The authors use reflection geometry QPM to selectively enhance the SH signal from an asymmetric coupled quantum well (ACQW) superlattice. In addition they present the first measurement, to the best of their knowledge, of χ⁽²⁾ in the visible wavelength range for an ACQW