Mach-Zehnder interferometer for measuring the thermally induced optical phase shift
This protocol is extracted from research article:
Large negative thermo-optic coefficients of a lead halide perovskite
Sci Adv, Jul 19, 2019; DOI: 10.1126/sciadv.aax0786

The Mach-Zehnder–type interferometer depicted in Fig. 2A was used for the measurements. As the light source to measure the optical phase shift, the picosecond pulsed white light from a supercontinuum light source (40 MHz; SC-400-4-PP, Fianium) was used. The white light first passed through long-pass and short-pass filters with cutoff wavelengths of 475 and 875 nm and then through a linear polarizer, whose angle was set to 45° with respect to the horizontal axis of a broadband polarizing beam splitter (PBS). The PBS splits the white light into two optical beams, namely, the reference beam with horizontal polarization and the probe beam with vertical polarization. We used the same kind of optics in both paths to obtain the same spectrum and suppress the difference between the optical chirps of the two paths. To obtain the interference pattern after the second PBS, both optical path lengths were adjusted to the same value under the condition T = 300 K by using an optical delay stage and a piezoelectric actuator and controller (PAS005 and MDT694B, Thorlabs), which control the path length on the order of ~μm and ~nm, respectively. To verify the phase-shift compensation, we first measured the phase shift for a ZnSe crystal (3 mm thick, Edmund Optics) with a positive thermo-optic coefficient. The ZnSe crystal was positioned in the probe path by mounting it on a cold finger of an evacuated cryostat. Another ZnSe crystal with the same thickness as that in the probe path was placed in the reference path under ambient air. These ZnSe samples were obtained by cleaving one sample into two pieces with same thickness. The beam power at the sample position was 97 μW. We measured the spectral shape of the interference intensity using a spectrometer (USB2000, Ocean Optics) as a function of the sample temperature in the probe path and obtained the interference intensity in Fig. 2B. By using the piezoelectric actuator, we confirmed that the observed thermally induced change in the optical path length in ZnSe is positive (see the Supplementary Materials for details). Then, by using the MAPbCl3 single crystal with the negative thermo-optic coefficient, we demonstrate the compensation of the optical phase shift occurring in ZnSe. We placed 3.9-mm-thick MAPbCl3 single crystals (two single-crystal samples with same thickness) in front of each ZnSe crystal and repeated the experiment (Fig. 2D).

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