Tai-Huei Wei

Determination of bound-electronic and free-carrier nonlinearities in ZnSe, GaAs, CdTe, and ZnTe

We extend the application of the Z-scan experimental technique to determine free-carrier nonlinearities in the presence of bound electronic refraction and two-photon absorption. We employ this method, using picosecond pulses in CdTe, GaAs, and ZnTe at 1.06 μm and in ZnSe at 1.06 and 0.53 μm, to measure the refractive-index change induced by two-photon-excited free carriers (coefficient σr,), the two-photon absorption coefficient β, and the bound electronic nonlinear refractive index n2. The real and imaginary parts of the third-order susceptibility (i.e., n2 and β, respectively) are determined by Z scans with low inputs, and the refraction from carriers generated by two-photon absorption (an effecitve fifth-order nonlinearity) is determined from Z scans with higher input energies. We compare our experimental results with theoretical models and deduce that the three measured parameters are well predicted by simple two-band models. n2 changes from positive to negative as the photon energy approaches the band edge, in accordance with a recent theory of the dispersion of n2 in solids based on Kramers–Kronig transformations [ Phys. Rev. Lett.65, 96 ( 1990); IEEE J. Quantum Electron.27, 1296 ( 1991)]. We find that the values of σr are in agreement with simple band-filling models.

Direct measurements of nonlinear absorption and refraction in solutions of phthalocyanines

We report direct measurements of the excited singlet state absorption cross section and the associated nonlinear refractive cross section using picosecond pulses at 532 nm in solutions of phthalocyanine and naphthalocyanine dyes. By monitoring the transmittance and far field spatial beam distortion for different pulsewidths in the picosecond regime, we determine that both the nonlinear absorption and refraction are fluence (energy per unit area) rather than irradiance dependent. Thus, excited state absorption (ESA) is the dominant nonlinear absorption process, and the observed nonlinear refraction is also due to real population excitation.

Optical Limiting In Solutions Of Metallo-Phthalocyanines And Naphthalocyanines

Optical limiting measurements have been made on solutions of several metal containing phthalocyanines and naphthalocyanines. Measurements at 532nm using nanosecond pulses from a Q-switched Nd:YAG laser show limiting throughputs of 1-10 millijoules with mild focussing in alcohol solutions with nominal transmissions of 30-70%. Measurements on chloro-aluminum-phthalocyanine solutions utilizing individual 30 psec pulses or trains (spanning about 100nsec) of modelocked pulses have shown even lower limiting throughputs. Thus, the dynamic range of the limiting behavior has been shown to cover at least three orders of magnitude. Prompt limiting is attributed to strong singlet-singlet (S1-Sn) absorption, whereas the longer time limiting behavior is postulated to result from strong triplet-triplet (T1-Tn) absorption. In addition to these studies, efforts have been underway to identify materials with reduced limiting throughput and improved optical transmission characteristics.

Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

This work examines a planar cholesteric liquid crystal (CLC) cell with a negative dielectric anisotropy, doped with laser dye, as an electrically tunable one-dimensional photonic crystal laser device. The lasing wavelength is demonstrated to be tunable by applying a voltage. Additionally, lasing can be switched on and off changing the frequency of the applied voltage. Wavelength tuning caused by the shift of the reflection band of CLC is attributed to the electrohydrodynamical effect in the negative dielectric cell.

LIFETIME DETERMINATION FOR HIGH-LYING EXCITED STATES USING Z SCAN

We report a method of determining the lifetimes of the high‐lying excited singlet states of dye molecules in solution. The method is based on the Z‐scan technique and the observation of saturation of excited state absorption in dye solutions using laser pulses having Gaussian temporal and spatial distributions. This leads to the important finding that the use of laser pulses as long as tens of picosecond can resolve subpicosecond lifetimes. The method is demonstrated by measuring an upper excited singlet state lifetime of 900 fs using 30 ps full width at half maximum input pulses on a chloroaluminum phthalocyanine dye.

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.

Measurements of third order optical nonlinearities of nematic liquid crystals

Abstract In order to better understand the physical mechanisms responsible for the large observed optical nonlinearities of nematic liquid crystals, we have carried out nonlinear absorption and nonlinear refraction measurements on the pure liquid crystals 5CB and 8CB, and the liquid crystal mixture E7 using picosecond, nanosecond and millisecond pulses. We have used the recently developed Z-scan method1–2, a sensitive single-beam technique which allows the determination of the sign and magnitude of the nonlinear refraction and the magnitude of the nonlinear absorption for aligned samples3. We have performed these measurements on aligned samples at wavelengths of 514nm and 532nm. In addition, we have studied the temperature dependence of the nonlinear refractive indices. Possible mechanisms responsible for the observed nonlinearities are discussed.

Mechanism of reverse saturable absorption in chloro-aluminum phthalocyanine solution studied with Z-scan

Abstract Using the Z -scan technique with 532 nm laser pulses of dual widths in the picosecond regime, we distinguish between two-photon absorption (TPA) and excited state absorption (ESA). To account for reverse saturable absorption in chloro-aluminum phthalocyanine dissolved in methanol both TPA and ESA, rather than either one, need be invoked.

Signs of nonlinear refraction in chloroaluminum phthalocyanine solution

Opposite signs of nonlinear refraction in chloroaluminum (ClAl) phthalocyanine (Pc) dissolved in methanol are observed using the Z-scan technique with two output modes of a frequency-doubled, Q-switched, mode-locked Nd:YAG laser operating at 10 Hz. Using the single picosecond (ps) pulses switched out of the pulse trains, positive nonlinear refraction is observed and attributed to population transitions among ClAlPc’s singlet states. Using the full nanosecond pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed, and is attributed to the heat-induced temperature rise and the population relaxing to ClAlPc’s triplet states.

Studies of nonlinear absorption and refraction in C60/toluene solution

Abstract Nonlinear absorptive and refractive properties of C60/toluene solution are studied using the Z-scan technique and a 532 nm laser delivering single picosecond (ps) pulses and trains of ps pulses separated by 7 nanoseconds (ns). With single ps pulses, reverse saturable absorption and positive nonlinear refraction are observed and attributed to population transitions among the singlet states. With ps pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed and is attributed to heat-induced temperature rise and population relaxing to triplet states.