Pump Beam Depletion Research Articles

Sample Concentration Affects Optical Gain Results in Colloidal Nanomaterials: Circumventing the Distortions by Below Band Gap Excitation

Ultrafast spectroscopy studies have been key to the development of optical materials, including colloidal semiconductor nanocrystals (NCs) engineered for lighting and light-harvesting technologies. Several physical processes, which are revealed by ultrafast spectroscopy in NCs, are highly dependent on the average number of excitons created per NC, including optical gain properties and multiexciton interactions. Consequently, proper considerations regarding NC populations are necessary to avoid misinterpretations. In this paper, we present an experimental and theoretical analysis of the influence of the sample optical density (OD) at the excitation energy on the results of ultrafast spectroscopy studies in NCs. We show that the pump beam depletion caused by high ODs can drastically change the results from transient absorption (TA) experiments leading to data misinterpretations, such as the overestimation of the optical gain threshold. Based on that, we propose a robust modification on the TA technique, which allows for an OD-independent characterization, free of distortions. The modification consists of pumping the sample below its band gap energy, limiting the electronic excitation to a two-photon absorption process, resulting in an effectively zero OD for the pump beam and a uniform excitation in the direction of the beam propagation. Consequently, an undistorted TA signal is produced, allowing for precise characterization of NCs, including the gain/absorption cross section, gain coefficient, and gain threshold. Furthermore, the uniform excitation allows for higher signal-to-noise ratio, independent of the sample concentration.

Theoretical and experimental study on a large energy potassium titanyl phosphate terahertz parametric source

The combination of a Stokes parametric oscillator and a Stokes parametric amplifier is used to generate high-energy terahertz pulses in KTiOPO4 (KTP) crystal at 5.70 THz. The Stokes parametric oscillator generates Stokes pulses first. And then they are amplified in the Stokes parametric amplifier and at the same time, high energy terahertz pulses are generated via stimulated polariton scattering. When the incident Stokes pulse energy is 36.6 mJ and the pump energy is 580.0 mJ, the amplified Stokes energy is 192.1 mJ and the obtained maximum terahertz pulse energy is 17.0 μJ. Compared with the reported terahertz pulse energy obtained by the KTiOPO4 terahertz parametric oscillators or generators, the terahertz pulse energy is greatly enhanced. In addition, considering the large angle non-collinear phase matching, pump beam depletion, terahertz wave surface-emitted structure, Gaussian pulse distribution, we make a theoretical simulation for the Stokes parametric amplifier through the coupled-wave equations. The theoretical results are in agreement with the experimental results on the whole.

Optimizing modulation transfer spectroscopy signals for frequency locking in the presence of depleted saturating fields

A theoretical model of modulation transfer spectroscopy (MTS) that includes pump beam depletion is presented and experimentally verified with data covering visible iodine transitions at 532, 543, and 612 nm. This model is used to determine the values for pressure, interaction length, and saturation intensity that yield maximum MTS signals for frequency locking to iodine transitions. The approach is demonstrated for iodine transitions at 532, 633, and 778 nm, with the results showing that theoretically the frequency instability scales inversely to the absorption coefficient.

Effect of Brillouin slow light on distributed Brillouin fiber sensors

The effect of Brillouin slow light on distributed Brillouin fiber sensors (DBFSs) is studied. We demonstrate Brillouin slow light for a 1.2 ns pulse with peak powers (PS) from 3.3 to 56.2 mW on depletion of the pump power (PP) ranging from 1.3 to 83.2 mW in conventional optical fibers (SMF-28). Experiments show that, when pump power depletion is not negligible, for a given PP the Brillouin gain and delay time of a pulse decrease when PS increases in a long (> or =10 km) sensing fiber. The optimum pump beam depletion resulting from strong interaction of the pump and the probe in the fiber provides accurate temperature and strain information at a high spatial resolution. Our study reveals that at low PP the spatial resolution error caused by the pulse delay for a DBFS with centimeter spatial resolution is less than 5% of the pulse length.

Modeling of end-pumped CW quasi-three-level lasers

A plane wave model has been developed to describe the dynamic behavior of end-pumped CW quasi-three-level lasers. We have derived algebraic equations and analytic expressions from a set of coupled rate equations. The analytic expression predicts that characteristics of a laser are affected not only by the Boltzmann occupation factor of the pump states, but also by that of the laser states. To illustrate the utility of the model, an end-pumped Yb:YAG laser oscillator was investigated and optimized. We have examined the threshold value of the pump beam, pump beam depletion in the gain medium, optimal crystal length, and laser output power for a given pump power. Finally, we have shown the pump and laser intensities and the population of each manifold as a function of crystal length.

Reflectivity of photorefractive cross-polarized four-wave mixing in consideration of pump depletion

We analyze the phase-conjugate reflectivity and the amplification property of cross-polarized four-wave mixing with a photorefractive BaTiO3 crystal in consideration of the pump-beam depletion. The coupled-wave equations of cross-polarized four-wave mixing with ordinary writing beams and an extraordinary reading beam are derived and solved with the grating-integral method. The results of calculations of the reflectivity and the amplification factors will be compared with the result under the undepleted-pumps approximation. In addition, to achieve high conversion efficiency, the optimum conditions for the intensity ratio and the angle configurations of the probe and pump beams are also discussed.

Laser-induced breakdown in H_2 gas at 248 nm

Measurement of optical breakdown threshold in hydrogen gas with a KrF excimer laser at 248 nm is presented. The result shows a sharp increase of the threshold intensity at a few atmospheres of gas pressure when a focusing lens with a longer focal length is used. This enhancement is partially explained in terms of the influence of pump beam depletion in stimulated Raman scattering.

Degenerate four-wave mixing in photorefractive media in the presence of biasing incoherent illumination

The effect of biasing illumination on the degenerate four-wave mixing (DFWM) in absorbing photorefractive (PR) media has been investigated. The input beams are considered to be spatially uniform. The analysis has been made both for transmission and reflection geometry. Considering pump beam depletion and the slowly varying amplitude approximation, nonlinear coupled wave equations are obtained and have been solved numerically by a shooting method. The effect of different parameters such as input beam intensities ratio, thickness of the interacting medium on the phase conjugate reflectively (PCR) has been investigated. It has been observed that PCR can be controlled to a large extent by controlling the biasing intensity.

Effect of spatial overlap in optically pumped molecular laser amplifiers

A time- and space-dependent simulation for an optically pumped molecular laser amplifier has been carried out. The influence of superfluorescent emission, self-absorption of the laser emission and pump beam depletion have been included. We have studied the effect of spatial overlap of the pump and the signal beams and their intensity profile mismatch on the amplifier extraction efficiency. Strong generation of superfluorescent emission is observed from that part of the excited amplifier volume which is not probed by the input signal when the signal and the pump beam do not overlap exactly. Thus, in order to achieve high amplifier extraction efficiency and a reduced level of superfluorescent emission, the pump and the signal beams should be overlapped spatially, and their intensity profiles matched. Also, the requirement of spatial overlap is more stringent for signal beams having uniform intensity profiles.

Two-wave mixing and fanning effect in Bi<inline-formula><sub><roman>12</roman></sub></inline-formula>TiO<inline-formula><sub><roman>20</roman></sub></inline-formula> under an alternating electric field

Numerical calculation of the two-wave mixing gain factor for arbitrary orientation of the holographic grating vector using the coupled wave theory developed for optically active electro-optic crystal enabling pump-beam depletion is presented. Two-dimensional angular dependencies of the gain factor are measured in Bi₁₂TiO₂₀ photorefractive crystal under an alternating external electric field using the fanning effect. It is found experimentally that the angular ditribution of fanning light is independent of the polarization state of the pump beam. This fact could not be explained by the existing theory.

Transmission of time modulated optical signals through an absorbing photorefractive crystal

We solve the problem of the transmission of time modulated signals in a two-beam coupling photorefractive set-up using an absorbing material. We obtain an analytical expression that describes this phenomenon in absence of pump beam depletion due to beam coupling. We use this expression to analyse the influence of absorption on the photorefractive build-up. We also describe, using this model, the photorefractive beamsplitter response. It is also showed that the π 2 phase shift between illumination and index gratings is detrimental to a linear detection of the phase variation of the signal beam.

Gain saturation in single pass and double pass optically pumped laser amplifiers

The operation of an optically pumped laser amplifier has been studied for two different geometries: the single pass and the double pass configurations. The influence of superflourescent emission, self-absorption of the laser emission and pump beam depletion have been included. Our results indicate that for a longitudinally pumped amplifier configuration, a multipass geometry is a disadvantage since it results in higher loss through enhanced superfluorescent emission and self absorption. Also, more passes do not result in increased gain length because of a substantial depletion in the pump beam energy. Thus, only for low laser input signals is the stage gain for the double pass amplifier higher than that for the single pass amplifier.

Cross and Parallel Polarization Coupling in Transmission-type Degenerate Four-wave Mixing in Compound Semiconductor Photorefractive Crystals: Orientational Dependence

Abstract Effects of crystal orientation which gives rise to parallel and cross polarization coupling have been investigated in degenerate four-wave mixing in absorbing semi-insulating compound semiconductor photorefractive crystals in transmission geometry. The dependence of cross and parallel polarization coupling, and phase conjugate beam of particular polarization, on crystal rotation has been investigated. Considering the pump beam depletion, nonlinear coupled wave equations are obtained and have been solved numerically by a shooting method. The presence of parallel coupling suppresses the cross coupling effect to a large extent and vice versa. Numerical results obtained from computer calculations are presented in graphical form.

Crystal orientation-dependent cross and parallel polarization coupling in the transmission type DTWM

Effects of crystal orientation which give rise to parallel and cross polarization coupling, have been investigated in degenerate two-wave mixing (DTWM) in absorbing semi-insulating compound semiconductor photorefractive (PR) crystals in transmission geometry. Dependence of cross and parallel polarization coupling, and output beam of particular polarization, on crystal rotation has been investigated. Considering the pump beam depletion, nonlinear coupled wave equations are obtained and solved numerically. Presence of parallel coupling suppressed the cross coupling effect to a large extent and vice versa. Numerical results obtained from computer calculations are presented in graphical form.

Effects of Cross-polarization Coupling in DFWM in Photorefractive Compound Semiconductor Crystals: Transmission Geometry

Abstract Effects of cross-polarization coupling have been investigated in the case of degenerate four-wave mixing (DFWM) in absorbing semi-insulating compound semiconductor photorefractive (PR) crystals in transmission geometry. Considering pump beam depletion, and the slowly varying approximation, nonlinear coupled wave equations are obtained and have been solved numerically by a shooting method. Numerical results obtained from computer calculation are presented in graphical form. It is seen that if all the input beams are of the same polarization, cross-polarization coupling generates a phase-conjugate beam which is polarized in a plane orthogonal to that of the input beam.

Cross-polarization coupling via reflection-type degenerate four-wave mixing in compound semiconductor photorefractive crystals

The photorefractive (PR) degenerate four-wave mixing (DFWM) of polarized electromagnetic waves in cubic isotropic crystals is considered. Effects of cross-polarization coupling have been investigated in the case of DFWM in absorbing, semi-insulating compound semiconductor PR crystals in reflection geometry. Considering the pump beam depletion, nonlinear coupled wave equations are obtained and have been solved numerically by a shooting method. Numerical results obtained from computer calculations are presented in graphical form. It is observed that the intensity of a particular polarization component can be appreciably increased by a proper choice of input beam polarization angle, crystal orientation, and crystal thickness.

Four-wave mixing oscillation in a cavity using sodium vapor

The experimental characteristics of a four-wave mixing oscillator which uses sodium vapor enclosed in a linear cavity are described. The pump beam depletion and the enhancement of the fluorescence induced by the four-wave mixing oscillation are studied. The situations of pump beams of the same frequency and of different frequencies are considered. In both cases, the output beam almost always exhibits a bistable behavior. The bistability is observed by varying the length of the cavity, the pump beams being constant, or by varying the pump beam intensity, the length of the cavity, the remaining constant. With degenerate pump beams, the phase and the frequency of the output beam is compared to those of the pump beam. In a study of the noise of such a degenerate four-wave oscillator both in the case of degenerate and nondegenerate pumps, a very important phase sensitive noise was observed. This feature was observed for nondegenerate oscillation. >

Self-oscillation of a cavity using a phase-conjugate amplifier: study of the stationary regime

A theoretical study of the stationary regime of a four-wave mixing oscillator inside a cavity is presented. The theory is developed in a situation where the phase-conjugate reflection R/sub c/ is much smaller than 1. To find the intensity emitted by this oscillator, two types of saturation mechanism, pump-beam depletion and atomic saturation, are considered. The principal characteristics of the degenerate oscillation are the following: very-low-threshold, bistable behavior, and phase locking of the oscillating beam on the pump beams. In the nondegenerate case, it is shown that the position of the phase-conjugate medium inside the cavity is an essential parameter to determine the threshold condition.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optical digital logic operations by two-beam coupling in photorefractive material

The nonlinear phenomena of signal beam saturation, pump beam depletion, and optically controlled coupling coefficient of two-beam interaction in photorefractive materials are analyzed. Applications of these phenomena to digital logic operations is discussed and supported by experimental results obtained in photorefractive BaTiO3.