Picosecond absorption relaxation measured with nanosecond laser photoacoustics

Abstract

Picosecond absorption relaxation-central to many disciplines-is typically measured by ultrafast ͑femtosecond or picosecond͒ pump-probe techniques, which however are restricted to optically thin and weakly scattering materials or require artificial sample preparation. Here, we developed a reflection-mode relaxation photoacoustic microscope based on a nanosecond laser and measured picosecond absorption relaxation times. The relaxation times of oxygenated and deoxygenated hemoglobin molecules, both possessing extremely low fluorescence quantum yields, were measured at 576 nm. The added advantages in dispersion susceptibility, laser-wavelength availability, reflection sensing, and expense foster the study of natural-including strongly scattering and nonfluorescent-materials. © 2010 American Institute of Physics. ͓doi:10.1063/1.3500820͔Picosecond absorption relaxation is central to many optical phenomena in physics, chemistry, biology, and medicine-such as absorption saturation, 1-3 photosynthesis, 4 and photolysis. [5][6][7] Typically, relaxation times are studied via an ultrafast ͑femtosecond or picosecond͒ pump-probe technique by measuring either fluorescence in a reflection configuration or absorption in a transmission configuration. 8,9 In the reflection-based approach, the molecules are excited by a laser pulse through the transition ͉0͘ → ͉i͘. A probe pulse with a variable delay time relative to the pump pulse monitors the time evolution of the population density N i ͑t͒. 8 When applied to molecules with low fluorescence quantum yield, this approach is noisy. In the transmission-based approach, the relaxation time is extracted by measuring pump-induced changes in probe-beam transmittivity ⌬T = T͑I p ͒ − T͑0͒, where T͑I p ͒ and T͑0͒ are the probe beam transmittivities with and without a pump of intensity I p . 9 In both approaches, the time resolution is limited by the pulse width of both the pump and the probe pulses. Hence, not only is an expensive picosecond or femtosecond laser required, but also the detection is susceptible to pulse broadening in dispersive media. Alternatively, intensity correlation of nanosecond laser pulses can be used. 10,11 Working in transmission mode, this technique is not suitable for optically thick media. All of the current techniques are limited to optically thin and weakly scattering materials or require artificial sample preparation.Photoacoustic microscopy and photoacoustic computed tomography, usually based on nanosecond laser excitation, are effective functional and molecular imaging tools in vivo. In the photoacoustic phenomenon, light is absorbed by a material and converted to heat. The subsequent thermoelastic expansion generates a detectable acoustic wave. 12 Photoacoustic sensing presents an exquisite sensitivity due to its inherent background-free nature. Its relative sensitivity to absorption reaches the theoretical limit of 100%, by far the highest among all optical imaging modalities. Most quantitative photoacoustic studies have assumed a linear dependence between ...

Quantitative spectroscopic photoacoustic imaging: a review

“…As the absorption coefficient increases, the characteristic length of the absorbed energy distribution, 1∕μ a reduces until it is less than the distance the PA wave can travel during the laser pulse, and the stress confinement condition no longer holds. Danielli et al 113 used the nonlinear PA response from an absorber owing to optical saturation to determine relaxation times on the picosecond timescale.…”

Nonlinear photoacoustic spectroscopy of hemoglobin

“…[3][4][5] Generally, the amplitude of the PA signal is assumed to be linearly proportional to the excitation pulse fluence. However, as the excitation laser intensity increases, both the saturation of the optical absorption 6,7 and the temperature dependence of thermal expansion [8][9][10] result in a measurable nonlinear dependence of the PA signal on the excitation pulse fluence. PA nonlinearity has recently been used in several applications such as quantifying picosecond absorption relaxation times with a nanosecond laser, 6 differentiating optical absorbers, 10 measuring oxygen saturation in vivo, 7 and performing label-free PA nanoscopy of biological structures having undetectable fluorescence.…”

“…However, as the excitation laser intensity increases, both the saturation of the optical absorption 6,7 and the temperature dependence of thermal expansion [8][9][10] result in a measurable nonlinear dependence of the PA signal on the excitation pulse fluence. PA nonlinearity has recently been used in several applications such as quantifying picosecond absorption relaxation times with a nanosecond laser, 6 differentiating optical absorbers, 10 measuring oxygen saturation in vivo, 7 and performing label-free PA nanoscopy of biological structures having undetectable fluorescence. 11 In the presence of nonlinearity, quantitative PA measurements require a detailed analysis of the intensity-dependence of the PA signal, particularly for hemoglobin, the major intrinsic absorber in tissue for PA imaging.…”

“…For deoxygenated hemoglobin, the absorption relaxation time is short. 6,7 Thus, optical saturation (the first term in Eq. (8) negligible, and the scaled second-order coefficient (Fig.…”

“…Overall, understanding of these nonlinear phenomena provides insights into existing and future applications of photoacoustics. 6,7,9,11 For example, careful selection of wavelengths and laser pulse durations can improve the accuracy of quantitative functional PAM by minimizing nonlinear effects. In other applications, one might desire to maximize the nonlinear effects.…”