Abstract
Highly sensitive broadband photothermal spectroscopy with a white-light lamp as the excitation source was developed by combining a Sagnac interferometer and balanced detection with a photothermal deflection method. A probe beam was split by a birefringent crystal CaCO3 into signal and reference beams with a balanced intensity. This balanced detection enabled the measurement of photoexcited thermal relaxation spectra of materials in the air over the whole visible range in the weak excitation limit 50 µW/cm2. The photothermal excitation spectrum of Eu2+-doped CaAlSiN3 phosphors (CASN:Eu2+) with a high luminescent quantum efficiency was measured to be distinctly different from the photoluminescence excitation spectrum which reflects the absorption spectrum, revealing the thermal relaxation mechanism of the phosphor. Assuming a typical non-radiative relaxation from the higher excited states to the lowest excited state and successively to the ground state, it is demonstrated that the photoluminescence efficiency of the phosphors is readily evaluated simply by comparing the photothermal and photoluminescence excitation spectra.
Highlights
Measurement of the light energy conversion efficiency in a phosphor, a solar cell, or a plant is not easy technically even though the principle is very simple, because it requires accurate measurement [1] of irradiation light intensity and absorption efficiency, total lightAppl
It was previously shown that the dominant factor of the residual noise in the Sagnac-interferometer photothermal deflection spectroscopy (SIPDS) system was linearly correlated with the probe laser beam intensity in Reference [11], where it is suggested that subtractive detection of the probe light should be implemented for further reducing the noise in the system
The signal beam and reference beam were split by the beam splitter (BS) and one of the beams was reflected at the mirror before the Sagnac interferometer
Summary
Measurement of the light energy conversion efficiency in a phosphor, a solar cell, or a plant is not easy technically even though the principle is very simple, because it requires accurate measurement [1] of irradiation light intensity and absorption efficiency (input power), total lightAppl. As demonstrated in this paper, by contrast, if the photothermal spectrum can be measured over a wide spectral range showing the difference in shape from the absorption spectrum, the light energy conversion efficiency can be evaluated based on physically reasonable assumptions about thermal relaxation. For most substances (especially condensed matter) excluding atoms in a vacuum, most of the excitation energy relaxation rate is dominated by a non-radiative thermal relaxation rate, where the energy is released as heat to the surroundings of the sample. Since the photothermal spectrum is a measure of the heat generation rather than the luminescence amount as a function of the excitation light wavelength, it will be referred to as the photothermal excitation spectrum (PTES) in the rest of this paper as a counterpart of the photoluminescence excitation spectrum (PLES).
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