Highly sensitive measurement of photothermal effects with a Sagnac interferometer

Shiokawa Naoyuki
Highly sensitive methods for photothermal deflection spectroscopy (PDS) were developed using a Sagnac interferometer. A part of (most of, in many cases) electronic excitation energy in a material received from incident light is eventually transferred to lattices in a thermal relaxation process to generate heat. The PDS is a method to obtain information of the light absorption intensity by detecting the quantity of the emitted heat as the probe-beam deflection due to the thermally induced
more » ... ive index gradient on the photoexcited sample. The PDS is categorized as a kind of the photothermal conversion spectroscopy, as well as photoacoustic spectroscopy and thermal lens spectroscopy. These spectroscopic techniques are mainly used for evaluation of optical properties of materials, selective imaging of absorptive objects, and applications to gas sensors. The high measurement sensitivity and characteristics of non-contact and non-destructive measurement are useful in a wide range of applications. In conventional PDS, carbon tetrachloride is frequently used as the deflection medium, a laser light source as the excitation light, and a position sensitive detector (PSD) as the detector. There have been many problems in these conventional experimental conditions, namely, restriction of measurable samples as well as the excitation wavelength, and degraded sensitivity due to the vibration noise in the optical setup. In this desertation, (1) the Sagnac interferometer was incorporated into photothermal deflection spectroscopy to resolve the problems. Using a photodiode as the detector, the signal was amplified by increasing the optical path length with the Sagnac interferometer, suppressing the vibration noise. (2) Considering the thermal diffusion length, a focusing Sagnac interferometer was constructed to further amplify the signal by focusing the probe laser beam at the sample position. Based on this focusing Sagnac interferometer PDS, the following three original ideas were further successfully demonstrated. (3) Inserting an f lens into the focusing optical system, the merits of extending the optical path length and focusing the beam for signal amplification were compatibly achieved. (4) Considering the gradient of the transverse electric field of the probe beam, a higher-order Hermite Gaussian beams was utilized to amplify the signal intensity. ( 5 ) In order to suppress the probelaser intensity noise, the balance detection Sagnac PDS optical system was constructed by creating orthogonally polarized twin parallel probe beams with identical intensity fluctuations using a birefringent crystal, the S / N value was improved by suppressing the intensity noise of the probe laser light. These improvement enabled us to use air as the deflection medium and a monochromated white-light Xe lamp as the excitation light to expand kinds of measurable ABSTRACT samples and the excitation wavelength with practically no restriction. The detection limits of the beam deflection angle, the refractive index change and the temperature change in the deflection medium (air) were achieved to be θ , respectively. This scores a more than one order of magnitude higher sensitivity than that previously reported by conventional photothermal deflection spectroscopy (~10 -4 K) with a PSD and a white-light lamp as the detector and the excitation light source. The developed methods could be used for thermal relaxation spectroscopy, where thermal relaxation pathways of the excited state in a variety of samples, including the photosynthetic proteins, are elucidated by comparing photothermal deflection spectra with the absorption spectra and the fluorescence excitation spectra. In addition, the exciation sources other than the visible light, such as positron and X-ray beams, could be used for thermal relaxation spectroscopy, too.
doi:10.20604/00000230 fatcat:5dczlhi4orggvesrnebb4luafa