Abstract
A new method (FTIR continuous dn / dT method, n is refractive index and T temperature) for measuring the continuous thermo-optic coefficients of thin transparent films in the mid-infrared (MIR) spectral region is introduced. The technique is based on Fourier transform infrared (FTIR) transmission spectra measured at different temperatures. It is shown that this method can successfully determine the thermo-optic coefficient of chalcogenide glass thin films (of batch compositions Ge20Sb10Se70 at. % (atomic %) and Ge16As24Se15.5Te44.5 at. %) over the wavelength range from 2 to 25 µm. The measurement precision error is less than ± 11.5 ppm / °C over the wavelength range from 6 to 20 µm. The precision is much better than that provided by the prism minimum deviation method or an improved Swanepoel method.
Highlights
Chalcogenide glasses, based on one or more Group 16 elements of the Periodic Table, sulfur (S), selenium (Se), or tellurium (Te), are promising materials for mid-infrared (MIR) applications [1,2,3,4]
A more accurate method to obtain the continuous dn / dT data of a thin film is highly desired. We propose such a method based on the Fourier transform infrared (FTIR) transmission spectra at different temperatures to obtain the continuous thermo-optic coefficient of a MIR transparent thin film over the wavelength range from 2 to 25 μm, which is christened as the FTIR continuous dn / dT method
For FTIR continuous dn / dT method, the thermal expansion coefficients (TECs) of Ge-Sb-Se and Ge-As-Se-Te were determined by Thermal Mechanical Analysis (TMA) to be 22.4 ppm / °C and 16.6 ppm / °C within the temperature range from 40 to 100 °C, respectively, with an error of ± 0.5 ppm / °C
Summary
Chalcogenide glasses, based on one or more Group 16 elements of the Periodic Table, sulfur (S), selenium (Se), or tellurium (Te), are promising materials for mid-infrared (MIR) applications [1,2,3,4]. The properties of a wide range of stable glass compositions, MIR transparency, high refractive index, low phonon energy, and high optical non-linearity, make chalcogenide glasses attractive for use in planar photonic integrated circuits [7,8], fiber-based components for supercontinuum generation [9], optical amplification [10,11,12], and infrared (IR) thermal lenses [13,14] operating in the MIR region. Apart from spectroscopic ellipsometry, the continuous refractive index over the whole MIR region can be determined by an improved Swanepoel method [21]. This method can determine the refractive index with a standard deviation of precision less than 0.002, which will lead to a maximum error of 75.5 ppm / °C in determining the dn/dT when the measurement temperature range is 53 °C. A more accurate method to obtain the continuous dn / dT data of a thin film is highly desired
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