Optical index measurement of InAs-GaSb type II superlattice for mid infrared photodetection at cryogenic temperatures

The complex optical index ñ (=n + i κ) of InAs/GaSb type-II superlattice (T2SL) was determined in the 1–6 μm range from reflectivity measurements of resonant structures at 80 K. Several air/superlattice/gold cavities of various thicknesses were fabricated and their reflectivity was measured. As the large variations induced by the cavity resonances in the studied range can be accurately modeled, we were able to determine the complex optical index ñ of InAs/GaSb T2SL with an error of only a few percent. We found that the in-plane refractive index n of the T2SL lies between 3.65 and 3.77 in the mid-infrared wavelength range, and the absorption coefficient κ decreases from 0.23 to 0.001 as the wavelength increases from 1.25 to 5.65 μm. The subtractive Kramers-Kronig relation allowed us to verify our determination of ñ

APL_Indice_SR_Fig1.png APL_Indice_SR_Fig2.png APL_Indice_SR_Fig3.png

Illustration of the reflection and transmission coefficients in the Fabry-Perot resonator

Experimental (crosses) and fitted (solid lines) reflectivity spectra of air/SL/gold cavities with SL thicknesses ranging from 130 nm to 453 nm

Fitted n and k values (red) and standard deviations (black) as a function of wavelength. In inset: zoom on values of k below the SL bandgap

Contact : Jean-Luc Pelouard (jean-luc.pelouard@lpn.cnrs.fr)

Further reading : E. Steveler et al. Appl. Phys. Lett. 105 141103 (2014)