Optical pump-probe method

Bernini U, Lettieri S, Maddalena P, Vitiello R, Di Francia G (2001) Evaluation of the thermal conductivity of porous silicon layers by an optical pump-probe method. J Phys Condens Matter... 

In the first reported measurements made with picosecond pulses, an optical beam splitter was used to pick off a portion of the pulse train and a variable optical delay path was introduced between the two beams [7]. The main beam was used to excite (pump) a dye sample, and the weak (probe) beam was used to monitor the recovery of dye transmission as a function of delay. Over the past two decades, this pump-probe method has been extended to a variety of measurement geometries and used to measure electronic polarization dephasing times as well as population lifetimes. 

The NO2 dissociation rate was measured by a two-color picosecond pump-probe method in which the product NO was monitored by LIF. Of particular significance in this study is that the NO2 density of states at the dissociation limit of 25,130.6 cm is relatively well established from an extrapolation of experimentally determined densities at an energy of 18,500 cm . This density (for cold samples where the rotations do not contribute significant densities) is 0.3 states per cm , (Miyawaki et al., 1993) which leads to a minimum rate constant l/h p( ) = 1 x 10 sec . The experimentally measured rate increases from 0 to 1.6 x 10 sec at the dissociation limit. It is interesting that the subpicosecond laser pulses with their transform limited resolution of about 20 cm do not excite individual NO2 resonance states (see section 8.3, p. 284) but, instead, prepare a superposition of those states that are optically accessible within the laser bandwidth. It is thought that all resonance states in this bandwidth are... 

In femtosecond experiments, as shown in Fig. 4.1, the pump-probe methods are most commonly used to study the dynamic processes in chemical compounds or materials. It should be noted that for probing, one can use the optical excitation, photoionization up-conversion, and stimulated emission [18]. From the uncertainty principle, AEAt w /2, we can see that AE depends on the pumping-pulse duration At. For short At, both population and coherence (or phase) can be created. In other words, in this case, both population and coherence dynamics have to be... 

The diatomic yttrium halides have been the topic of both ab initio and experimental studies. Fischell et al. (1980) have studied the excitation spectra of the YCl diatomic molecule using the laser-induced fluorescence (LIF) method. More recently, Xin et al. (1991) have studied the B ri-X system of YCl in high resolution. The rotational analysis of the observed bands has yielded very accurate molecular constants for the X and B states of YCl. Shirley et al. (1990) have studied the molecular-beam optical Stark spectrum of the B n(t = 0)-X (t = 0) band system of YF. The permanent dipole moment and the magnetic hyperfine parameter a for the B n state have been determined as 2.96(4) D and 146.8(3) MHz, respectively. The dipole moment of the X S state was determined as 1.82(8)D. More recently, Shirley et al. (1991) have employed the molecular-beam millimeter-wave optical pump-probe spectroscopy to study pure rotational transitions of the YF ground state. This study has yielded improved ground-state rotational constants as B = 8683.65(1) MHz and D = 0.0079(2)-MHz, respectively. 

In ultrafast, time-resolved infrared absorption measurements by the pump-probe method, the sample is first excited by an ultrashort pump pulse, and then irradiated by an ultrashort infrared pulse (probe pulse) after a certain delay time from the excitation by the pump pulse. The delay time of the probe pulse from the pump pulse is usually changed by the difference in the optical path lengths of the pump and probe pulses (a delay time of 1 ps arises from a path difference of about 0.3 mm). When the infrared spectrum of a molecule in an excited electronic state is measured, pulses in the ultraviolet to visible region are used for the pump purpose, and pulses in the infrared region are used for the probe purpose. When a vibrationally excited molecule is the target of such a measurement, pulses in the infrared region are used for both the pump and probe purposes. The transient (or time-resolved) infrared absorption spectra by this method are usually measured as the difference in absorption intensities for the probe pulses between the measurements with the pump pulses and those without the pump pulses. 

Hewakuruppu, Y. L., Dombrovsky, L. A, Chen, C., Timchenko, V., Jiang, X., Baek, S., and Taylor, R A (2013]. Plasmonic "pump-probe" method to study semi-transparent nano-fluids. Appl. Optics, 52, 6041-6050. 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

The term nonlinear optical property refers to an optical property, which can be modified by exposing the material to intense light irradiation. In this section, we focus on the cascaded first- (/ 1 ) and third-order ( / ) susceptibilities describing nonlinear absorption (ESA and 2PA) and nonlinear refraction (n2) processes. Z-scan, pump-probe, and two-photon upconverted fluorescence techniques are among the most used experimental methods for determining optical nonlinearities. 

We consider a model for the pump-probe stimulated emission measurement in which a pumping laser pulse excites molecules in a ground vibronic manifold g to an excited vibronic manifold 11 and a probing pulse applied to the system after the excitation. The probing laser induces stimulated emission in which transitions from the manifold 11 to the ground-state manifold m take place. We assume that there is no overlap between the two optical processes and that they are separated by a time interval x. On the basis of the perturbative density operator method, we can derive an expression for the time-resolved profiles, which are associated with the imaginary part of the transient linear susceptibility, that is,... 

Many methods of investigation of protein-ligand binding kinetics that are based on linear processes are of a pump-probe type. In this approach an optical pulse, called a pump, starts a photoreaction (such as dissociation of MbCO into Mb and CO), and its progress is probed a time At later. The probe could be, for example, a weak laser pulse, which detects the spectral changes in the heme during the protein-ligand recombination, or an x-ray pulse, which allows determination of the protein structure at a particular instant in time. 

Contrary to the above-described detection methods, fluorescence up-conversion and optical Kerr gate techniques readily achieve picosecond/femtosecond time resolution (Ippen and Shank 1975 Shah 1988 Takeuchi and Tahara 1998), because they are in the pump-probe measurement, in principle. 

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

TR methods were originally developed in om laboratories to study excited-state structures and dynamics of transition metal complexes such as Ru + (bpy)s and metaUoproteins. TR measurements rely on a pump-probe approach in which two separate laser pulses are used, one to excite the system and the other to probe the transient Raman spectrum. The time resolution of the experiment is determined by the width of the laser pulses (typically 7 ns for a Q-switched laser or as short as 1 ps for a mode-locked laser). The pulses are variably delayed with respect to one another to achieve time resolution, either by optically dela)dng the probe pulse with respect to the pump pulse or by electronically delaying two independently tunable lasers. Thus, two different approaches are required depending on the time scale of interest. The fastest timescale (from 10 to 10 s) requires optical delay to achieve sufficiently short separation between the pump and probe pulses. In such a scheme, the probe pulse is sent through a fixed path, but the pump pulse is sent through a variable path that can be scanned. Since hght travels about 1 ft per ns, a difference in pathlength of a few feet is sufficient. The second approach typically uses two Q-switched Nd YAG lasers that are electronically delayed with respect to one another, to access... 

The initial charge separation in PS I and PS II can be followed by what are known as ultrafast optical spectroscopy techniques. Several variations on this method exist, but they can be grouped into pump-probe absorbance difference and transient fluorescence methods (25, 26). In the first instance, the sample is irradiated with a pump pulse to initiate the electron transfer and the absorbance is measured using a probe pulse at a... 

This review describes some of the recent developments in materials which exhibit enhanced two-photon absorption that can initiate photopolymerization or up-converted emission. Various optical methods including femtosecond time-resolved pump-probe experiments to characterize the two-photon properties are discussed. Finally, the applications of two-photon processes to optical power limiting, up-converted lasing, 3-D data storage, 3-D micro-fabrication, two-photon fluorescence microscopy and bio-imaging, and two-photon photodynamic therapy are presented. 

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