Anisotropy Of Nonlinear Absorption Research Articles

Direct bleaching of a Cr4+:YAG saturable absorber in a passively Q-switched Nd:YAG laser.

In this work, the anisotropy of nonlinear absorption in a crystal Q-switch was considered when we established coupled rate equations of a passively Q-switched laser. A [100]-cut Cr4+:YAG crystal, with initial transmission T0=40%, was used as the Q-switch to evaluate the theoretical model, and the results of the simulation were in good accordance with the experiment. In order to control timing jitter of the passively Q-switched laser, an actively Q-switched Nd:YAG laser was applied to directly bleach the [100]-cut Cr4+:YAG crystal. The timing jitter was more than 1μs without bleaching light. While there was a bleaching light, the time lag between the laser pulse and the bleaching light was less than 100ns, which meant the timing jitter decreased. The pulse width of the passively Q-switched laser was found to decrease from 45 to 35ns due to the existing of bleaching light. As the peak power of bleaching light was increased, the laser pulse energy increased from 18.2 to 24.6mJ, which meant a 35% increment in the pulse energy. The increase in pulse energy can be explained by the increase of α coefficient, and the results of simulation agreed well with the experiment.

Differential Polarization Nonlinear Optical Microscopy with Adaptive Optics Controlled Multiplexed Beams

Differential polarization nonlinear optical microscopy has the potential to become an indispensable tool for structural investigations of ordered biological assemblies and microcrystalline aggregates. Their microscopic organization can be probed through fast and sensitive measurements of nonlinear optical signal anisotropy, which can be achieved with microscopic spatial resolution by using time-multiplexed pulsed laser beams with perpendicular polarization orientations and photon-counting detection electronics for signal demultiplexing. In addition, deformable membrane mirrors can be used to correct for optical aberrations in the microscope and simultaneously optimize beam overlap using a genetic algorithm. The beam overlap can be achieved with better accuracy than diffraction limited point-spread function, which allows to perform polarization-resolved measurements on the pixel-by-pixel basis. We describe a newly developed differential polarization microscope and present applications of the differential microscopy technique for structural studies of collagen and cellulose. Both, second harmonic generation, and fluorescence-detected nonlinear absorption anisotropy are used in these investigations. It is shown that the orientation and structural properties of the fibers in biological tissue can be deduced and that the orientation of fluorescent molecules (Congo Red), which label the fibers, can be determined. Differential polarization microscopy sidesteps common issues such as photobleaching and sample movement. Due to tens of megahertz alternating polarization of excitation pulses fast data acquisition can be conveniently applied to measure changes in the nonlinear signal anisotropy in dynamically changing in vivo structures.

Enhanced anisotropy of the nonlinear absorption in the individual Au nanoparticles functionalized KNbO_3 sub-microwire

High quality and high quantity ultra-long KNbO3 microwires with diameters of 200 nm to a few microns and lengths up to several milimeters have been synthesized by using a simple solvothermal method. The high purity, ideal length and good transferability make it easy to be transferred to the quartz substrate for further linear and nonlinear optical measurements. Functionalized by plasmonic Au nanoparticles, the as-prepared KNbO(3) sub-microwires exhibited enhanced anisotropy of nonlinear absorption that the polarization ratio of individual Au nanoparticles coated KNbO(3) sub-microwire (ρ=0.32) is two times more than the uncoated one. It is demonstrated that this ultra-long KNbO(3) microwire is an ideal candidate for nanolasers, polarization sensitive photodetector and photo-thermal nanodevices.

Passive Q-switching of diode pumped Nd:KGd(WO_4)_2 lasers by V^3+:Y_3Al_5O_12 crystal with anisotropy of nonlinear absorption

Use of a V(3+):Y(3)Al(5)O(12) crystal as a saturable absorber Q-switch for 1.07 and 1.35 microm Nd:KGd(WO(4))(2) diode pumped lasers shows a considerable dependence of output characteristics on the orientation of the intracavity field polarization vector regarding V(3+):Y(3)Al(5)O(12) crystallographic axes. Anisotropy of nonlinear absorption of V(3+) ions in a Y(3)Al(5)O(12) single crystal at wavelengths of 1.08 and 1.35 microm has been experimentally studied. The experimental data are analyzed within the framework of a phenomenological model when the V(3+) ions are described as three sets of linear dipoles oriented along the crystallographic axes. Ground-state and excited-state absorption cross sections at 1.08 and 1.35 mum are evaluated to be sigma(gsa)=3.4x10(-18) cm(2), sigma(esa)=3.0x10(-19) cm(2) and sigma(gsa)=5.4x10(-18) cm(2), sigma(esa)=4.8x10(-19) cm(2), respectively. Saturation fluence and intensity at 1.08 and 1.35 microm are estimated as 55 mJ/cm(2) and 1.1 MW/cm(2), respectively.

Model of passive Q switching taking account of the anisotropy of nonlinear absorption in a crystal switch with phototropic centres

A model is proposed of passive Q switching in a laser by a crystal switch with phototropic centres. Expressions for the output energy and duration of a giant pulse are derived taking account of the geometric factor representing the distribution of the orientations of these centres relative to the crystal lattice of the passive switch. The calculated results are in agreement with those obtained in an experimental investigation of passive Q switching in a neodymium laser with a switch in the form of a doped LiF :F2- or Cr4+ : YAG crystal.

Influence of the nonlinear anisotropy of absorption in a passive Cr4+ : YAG switch on the energy and polarisation characteristics of a neodymium laser

An experimental investigation was made of the polarisation characteristics of a solid-state neodymium laser with a passive Cr4+ : YAG switch. The energy and the state of polarisation of laser radiation were affected radically by the orientation and position of the Cr4+ : YAG switch in the cavity, and also by the nature of an intracavity polariser. The polarisation characteristics of the laser radiation were determined by a self-induced nonlinear-absorption anisotropy and a self-induced change in the state of polarisation of the laser radiation during growth of a giant pulse when saturation of the absorption was 'switched on' in the Cr4+ : YAG crystal.

Intrinsic and induced anisotropy of nonlinear absorption and refraction in zinc blende semiconductors

An ellipsometric picosecond pump–probe technique is used to measure the birefringence and dichroism induced in the zinc blende semiconductors ZnSe, GaAs, and CdTe by an intense linearly polarized pump pulse at 950 nm. We show that the induced birefringence and dichroism depend strongly on sample orientation (i.e., they are anisotropic). Furthermore, we demonstrate that by measuring the induced birefringence and dichroism for the pump polarized along the [100] and [110] crystal axes we can determine the sign and the magnitude of the intrinsic anisotropy both in the bound-electronic nonlinear refractive index and in the two-photon absorption coefficient; and, finally, we can extract the anisotropy in both the real and the imaginary parts of the product of the anisotropy parameter σ and the diagonal element of χ(3), the third-order susceptibility tensor. The latter product is a measure of the intrinsic anisotropy of the material. We find that the induced birefringence varies by roughly a factor of 2 with sample orientation in all three materials. The induced birefringence is, however, roughly an order of magnitude larger in size and opposite in sign for excitation above half the band gap (in GaAs and CdTe) than it is for excitation below half the band gap (in ZnSe). As expected, no dichroism is observed in ZnSe, but it is large in GaAs and CdTe, and, as with the birefringence, the dichroism varies by roughly a factor of 2 with crystal orientation. In order to test the effect of the measured anisotropy on device performance, we construct a simple on–off optical switch that exploits the measured birefringence and dichroism. Figures of merit are defined, and switch performance is investigated as a function of crystal orientation for excitation above and below the two-photon resonance (i.e., half the band gap). The figures of merit are shown to be extremely sensitive to crystal anisotropies for excitation below half the band gap and less so for excitation above half the band gap.

Z-scan measurements of the anisotropy of nonlinear refraction and absorption in crystals

We introduce a method for measuring the anisotropy of nonlinear absorption and nonlinear refraction in crystals by incorporating a wave plate into the Z-scan apparatus. We demonstrate this method by measuring the polarization dependence of the nonlinear refractive index or two-photon absorption coefficient in BaF2, KTP, and GaAs at wavelengths of 532 and 1064 nm.