THIRD-ORDER NONLINEAR OPTICAL STUDIES USING NONLINEAR PHOTOACOUSTICS
Chandra S. Yelleswarapu
Department of Physics, University of Massachusetts Boston, MA, United States Chandra.yelleswarapu@umb.edu
Nonlinear optics (NLO) is the study of phenomena that occurs as a consequence of the modification of optical properties of materials on interaction with intense light. The advent of high power laser sources brought tremendous advantages in this area of research, enabling us to probe deeply into the various aspects of nonlinear interaction of matter with optical fields. Materials with optimized NLO properties have been of great interest from both fundamental and application point of view, including processing of optical signals, optical data storage, nanophotonics, and biophotonics [1-4]. Molecular engineering with aim of realizing new, multifunctional NLO material have been synthesized in recent times [5, 6]. In the last decade, organic materials have emerged as potentially viable NLO candidates due to the large laser-intensity-induced refraction and absorption changes [7]. The mechanisms such two-photon absorption, excited state absorption depends on the material and are studied by varying laser parameters such as pulse width and intensity.
Characterization of NLO materials is important not only to develop better materials, but also to understand the basic physics involved. The z-scan technique is often used for measuring Kerr nonlinearity of an optical material and the nonlinear absorption coefficient [8]. It is a simple technique in measuring the third order nonlinearity. Several improvements have been implemented to the conventional Z-scan technique in order to improve the sensitivity and/or to study a variety of materials. In some cases, however, it is still not convenient to use the basic Z-scan technique: the excitation laser wavelength has to be off resonance for strongly nonlinear absorption materials so that transmitted optical signal can be measured; weak nonlinear materials require intense optical beams, often resulting in damage of the material; it cannot be used to study surface properties of non-transparent materials like semiconductors.
Recently we demonstrated a novel PAZ-scan technique in which the generated nonlinear photoacoustic behavior is used to measure the third-order NLO absorption coefficients [9]. The experimental procedure is similar to conventional optical Z-scan setup; only difference being that the generated acoustic signal is measured using an ultrasound transducer instead of measuring the transmitted optical signal. As the laser pulse is incident on the sample, some of the energy delivered is absorbed and converted into heat. This produces pressure transients and thus wideband ultrasonic emission. The ultrasonic waves (PA signal) are then detected using a 10 MHz focused water immersion transducer. The strength of the PA signal is directly proportional to the optical absorption coefficient of the sample and the incident laser fluence. In general, the photoacoustic signal is linear in light absorption.
However at higher intensities, a nonlinear optical absorption of the medium generates nonlinear photoacoustic behavior. In the present case, as the photoacoustic transients are produced due to the interaction between the incident laser fluence and the nonlinear absorbing medium, the photoacoutic behavior is nonlinear. When the sample is scanned across the focused laser beam the peak incident intensity changes smoothly along the optical axis. The intensity will be optimum at focus, while decreases gradually on either sides. We studied nonlinear absorption behaviors of saturable (SA) and reverse saturable absorbers (RSA). In the case of SA materials, the absorption decreases at high intensities due to cross section of the excited state being smaller than that of the ground state. Thus the photoacoustic signal decreases and reaches a minimum value at the focus (z=0). On the other hand, for RSA materials, the absorption increases at higher intensities leading to increase in PA signal which reaches a maximum at z=0. Thus the photoacoustic behavior compliments the nonlinear absorption behavior of the sample. As the generated acoustic waves are propagated outwards in all directions, we performed PAZ-scan in reflection geometry also. This enables one to study the surface properties of non-transparent materials and living subjects.
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