Experimental setup: Light pulses are generated and focused onto a sample. A piezo transducer is attached to the sample to measure the acoustic signal.
Typical photoacoustic signal measured with the piezo transducer, the sample is an undoped congruent lithium niobate crystal with dimensions mm3. The distance between transducer and illuminated cylinder is 40 mm, leading to a delay of τdelay = r/v = 6.5 μs. The measured quantity is the maximal voltage of the first peak U PhAc, called the photoacoustic voltage.
The photoacoustic voltage rises linearly with pulse energy, shown here for a lithium triborate crystal and y-polarized light at 1600 nm wavelength.
Photoacoustic signal for an LBO crystal, top: crystallographic axis points onto the piezo, bottom: axis points onto the piezo.
Calibration: The photoacoustic measurement is compared to the absorption spectrum measured with a FTIR spectrometer. The comparison yields the calibration coefficient, to convert the normalized photoacoustic signal into absorption coefficients.
Photoacoustic absorption spectrum of an LBO crystal (sample 4) for light polarized along the crystallographic y axis.
Comparison of photoacoustic absorption spectra of LBO crystals from different growers for light polarized along the crystallographic y axis.
Calibration: The absorption band of the fundamental OH− vibration can be measured with a standard grating spectrophotometer.
Calibration: The absorption band of the overtone of the OH− vibration can be measured with the photoacoustic spectrometer. The ratio of the oscillator strengths between fundamental and overtone vibration is known from literature.
Photoacoustic signal of a LiNbO3 crystal versus laser pulse energy at 532 nm wavelength. For high pulse energies two-photon absorption dominates linear absorption.
Absorption spectrum of an undoped LiNbO3 crystal for ordinary and extraordinary polarization, measured with the photoacoustic absorption spectrometer.
Absorption spectrum of a 5 mol.% MgO-doped LiNbO3 crystal for ordinary and extraordinary light polarization, measured with the photoacoustic absorption spectrometer.
Absorption bands of a 5 mol.% MgO-doped LiNbO3 crystal between 2000 and 2500 nm wavelength.
Absorption spectra of α-BBO measured with the photoacoustic setup and with a FTIR spectrometer.
Absorption spectrum of an α-BBO crystal for ordinary light polarization, (a) in semi-logarithmic scale, (b) in linear ordinate scale. Absorption bands at 1800, 1960, and 2090 nm are measured with the photoacoustic setup.
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