T. Xia

Eclipsing Z-scan measurement of λ/10 4 wave-front distortion

We introduce a simple modification to the Z-scan technique that results in a sensitivity enhancement that permits measurement of nonlinearly induced wave-front distortion of ≃λ/104. This sensitivity was achieved with 10-Hz repetition-rate pulsed laser sources. Sensitivity to nonlinear absorption is also enhanced by a factor of ≃3. This method permits characterization of nonlinear thin films without the need for waveguiding.

Optimization of optical limiting devices based on excited-state absorption

Limiting devices protect sensitive optical elements from laser-induced damage (LID). Passive devices use focusing optics to concentrate the light through a nonlinear optical (NLO) element (or elements) to reduce the limiting threshold. Unfortunately, these NLO elements may themselves undergo LID for high inputs, restricting the useful dynamic range (DR). Recently, efforts at optimizing this DR have focused on distributing the NLO material along the propagation path z of a focused beam, resulting in different portions of the device (in z) exhibiting NLO response at different inputs. For example, nonlinear absorbers closer to the lens, i.e., upstream, protect device elements downstream near the focal plane. This results in an undesirable increase in the threshold, although the lowest threshold is always obtained with the final element at focus. Thus there is a compromise between DR and threshold. This compromise is determined by the material. We concentrate on reverse saturable absorber (RSA) materials (molecules exhibiting larger excited-state than ground-state absorption). We look at both tandem devices and devices in which the concentration of the NLO material is allowed to spatially vary in z. These latter devices require solid-state hosts. The damage threshold of currently available solid-state hosts is too low to allow known RSA materials to reach their maximum absorption, which occurs when all molecules are in their excited state. This is demonstrated by approximate analytical methods as well as by a full numerical solution of the nonlinear wave propagation equation over extremely large distances in z (up to 10(3)Z(0), where Z(0) is the Rayleigh range of the focused beam). The numerical calculations, based on a one-dimensional fast Fourier transform, indicate that proper inclusion of diffraction reduces the effectiveness of reverse saturable absorption for limiting, sometimes by more than a factor of 10. Liquid-based devices have higher damage thresholds (damage occurs to the cuvette wall) and, thus, larger nonlinear absorption. However, RSA material in liquid hosts may suffer from larger thermal lensing.

Nonlinear response and optical limiting in inorganic metal cluster Mo 2 Ag 4 S 8 (PPh 3 ) 4 solutions

We describe a series of experiments on acetonitrile solutions of an inorganic cluster molecule Mo2Ag4S8(PPh3)4 and compare them with data on a suspension of carbon particles in liquid (dilute ink). The optical-limiting behavior is measured by single-picosecond 532-nm pulses and nanosecond-long trains of these picosecond pulses. Nonlinear loss measurements are also performed with pulse trains at 1064 nm. Both materials show reduced transmittance for increasing fluence (energy per unit area). We also perform picosecond time-resolved pump–probe measurements at 532 nm, and we find that the observed pump–probe behavior is identical for the metal-cluster solution and the carbon-black suspension. We believe that the nonlinear mechanisms are the same for the two materials. Our previous studies of carbon-black suspension indicate that the primary nonlinear losses are due to scattering and absorption by microplasmas formed after thermionic emission from heated carbon black augmented by scattering from subsequently created bubbles. The conclusion of a similar limiting mechanism for the two materials is confirmed by time-resolved shadowgraphic images taken on both samples; however, a definitive conclusion concerning the role of microplasmas versus bubbles in either material is still under investigation.

HIGH DYNAMIC RANGE PASSIVE OPTICAL LIMITERS

We present results of optical limiting experiments designed to study optical geometries for increasing the dynamic range over which limiters function without incurring optical damage. Specifically, we investigate a tandem geometry with two passive nonlinear elements, one placed in the focal plane of a lens and the second placed “upstream” of the focal position to protect the material at focus from damage. To provide a proof-of-principle demonstration of this geometry, simple limiters consisting of combinations of reverse saturable absorber dyes and a carbon black suspension in thin cells were tested. Our results show that a substantial increase in device performance can be achieved by use of a tandem limiter geometry. Simple modelling predicts that the dynamic range of a separate-element tandem limiter is given by the product of the dynamic ranges of the individual component limiting elements, in agreement with our experimental results.

Purely refractive transient energy transfer by stimulated Rayleigh-wing scattering

Two-beam coupling is demonstrated in CS2 and other transparent Kerr liquids by use of frequency chirped, picosecond 532-nm-wavelength pulses with several polarization combinations. As the temporal delay between pulses is varied within the coherence time, the first pulse always loses energy while the second pulse gains this energy. The transferred energy at a fixed delay varies linearly with irradiance. The results are consistent with energy transfer from transient refractive gratings that are due to stimulated Rayleigh-wing scattering.

Measurement of the optical damage threshold in fused quartz

The damage thresholds of five different types of quartz glass used for the production of spectroscopic cuvettes for liquids were determined with single temporal and spatial mode nanosecond pulses at 532 nm. One of the glasses had a damage threshold of ≃420 J/cm(2), which was more than twice that of the other glasses.

Nonlinear absorption and refraction in CuCl at 532 nm

We report two-photon absorption in CuCl and its polarization dependence to give all three χ(3) tensor elements at 532 nm. We also report bound electronic n2 and free-carrier refraction in CuCl. The values obtained are each approximately one half the corresponding values for ZnSe. Previously, with ZnSe, the large two-photon absorption (2PA) and negative bound-electronic nonlinear refraction were combined with defocusing from 2PA-generated free carriers to provide effective optical limiting for visible picosecond input pulses. For nanosecond pulses thermal refraction is a problem because it leads to self focusing, and thus damage in ZnSe and most other semiconductors studied. However, the thermal refraction in CuCl is reported to be self-defocusing. Thus one might expect the self-protection properties of CuCl to be better than those of ZnSe for optical-limiting applications for nanosecond input pulses.

Z-SCAN AND EZ-SCAN MEASUREMENTS OF OPTICAL NONLINEARITIES

We describe the application of single beam propagation methods, namely Z-scan and EZ-scan, for the determination of nonlinear refractive indices in materials including thin films. In these experiments the transmittance of a sample is measured either through a finite aperture (Z-scan) or around an eclipsing disk (EZ-scan) placed in the far-field as the sample is moved along the propagation path (Z) of a focused beam. Both methods can also be used to separately measure the nonlinear absorption so that both the real and imaginary parts of the nonlinear susceptibility are determined along with their signs. The sensitivity to induced phase distortion depends on the sensitivity of the measuring apparatus to transmittance changes ΔT. For the 10 Hz repetition rate Nd:YAG lasers used in our experiments, we can detect ΔT≃10−3. This leads to a sensitivity to optical path length changes of λ/103 for the Z-scan and λ/104 for the EZ-scan where λ is the wavelength. This interferometric sensitivity, using a single beam, allows measurement of nonlinear refraction in thin films without the need for using a waveguiding geometry.