Analysis of χ⁽²⁾ of III-V quantum-well structures using transfer matrix techniques

Nano-layered quantum wells (QWs) composed of III-V semiconductors provide unexplored opportunities to engineer χ⁽²⁾ (e.g. for electro-optic and quantum information applications) by optimizing thickness, separation, and shape of individual QW layers as well as the number N of repeated layers. A digital alloys growth technique was used, which avoids phase segregation that often plagues III-V alloy growth, and also lends itself readily to nano-structuring to enhance χ⁽²⁾. Second harmonic generation (SHG) was used to probe the total nonlinear χ⁽²⁾ response of a series of N multiple-QW (MQW) layers made up of InAs QWs and AlSb barriers, sandwiched in between a GaSb oxidation cap and GaSb buffer layer, grown on GaSb substrate. The measured SHG signal is composed of a contribution from each of the layers that interfere with each other. To isolate the N-dependent SHG polarization of the MQW layer of interest, we implemented a SHG transfer matrix formalism to model the SHG signal as a coherent superposition of a variable-N MQW layer with fixed substrate and cap layer SH polarizations. Experimental results show up to 25x stronger SHG from MQW structures than from GaSb substrate. The model attributes this enhancement partly to geometrical effects and to enhanced χ⁽²⁾ of the MQW structures.