Measurement of χ⁽²⁾ of III-V quantum well structures

Nanolayered quantum wells (QWs) composed of III-V semiconductors present 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, and 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 allows 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, all based on a GaSb substrate. Spectroscopic ellipsometry was used to model an effective medium describing the MQWs and extract optical constants and thicknesses. Then, these supplied the nonlinear transfer matrix formalism employed 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 reveal up to 25x stronger SHG ∝ (χ⁽²⁾)² from MQW structures compared to a GaSb substrate. The model attributes this enhancement to geometric and interference effects in the MQW structures.