Yee-Loy Lam

Thermally tuning of the photonic band gap of SiO2 colloid-crystal infilled with ferroelectric BaTiO3

SiO2 colloid crystal infilled with BaTiO3 was synthesized by a process of self-assembly in combination of a sol–gel technique. It is found that infilling of the fully crystallized BaTiO3 into the colloid assembly can enhance the photonic band gap significantly. In the vicinity of the ferroelectric phase transition point of BaTiO3 (100–150 °C), the photonic band gap of the assembly exhibits a strong temperature dependence. At the Curie point, the band gap has a 20 nm redshift, and the optical transmittance reaches its minimum. Such a temperature-tuning effect in the photonic band gap should be of high interest in device applications.

Femtosecond Z-scan measurements of nonlinear refraction in nonlinear optical crystals

Abstract We report an investigation of third-order optical nonlinearities in several nonlinear optical crystals using the Z-scan technique with femtosecond laser pulses at 780 nm wavelength. The crystals studied include LiNbO 3 :MgO, KTiOAsO 4 , KTiOPO 4 , β-BaB 2 O 4 and LiB 3 O 5 , which are extensively used for ultrashort-pulse second-harmonic generation and optical parametric oscillation. The nonlinear refractive index n 2 in these crystals has been determined to be in the range from 10 −16 to 10 −15 cm 2 /W. No two-photon absorption has been observed. The experimental results are compared with the two-band model for the bound electronic Kerr nonlinearity. It is shown that the measured n 2 values in β-BaB 2 O 4 and LiB 3 O 5 are one order of magnitude smaller than those of LiNbO 3 :MgO, KTiOAsO 4 , KTiOPO 4 , which is in agreement with the theoretical prediction.

Photoluminescence and electroluminescence from copper doped zinc sulphide nanocrystals/polymer composite

Cu-doped ZnS nanocrystals were prepared in an inverse microemulsion at room temperature as well as under a hydrothermal condition. X-ray diffraction analysis showed that the diameter of the Cu-doped ZnS nanocrystals particles was about 9 nm. These particles showed a strong photoluminescence intensity and a broad emission band from 490 to 530 nm. The half-width of emission was about 60 nm. Cu-doped ZnS nanocrystals/polymethylmethacrylate composite as a light-emitting layer was used to fabricate a single layer structure electroluminescent device which had low turn on voltage (less than 5 V). The green light of electroluminescence was observed at room temperature. The electroluminescence and photoluminescence spectra were nearly identical at room temperature.

Optical nonlinearities and photo-excited carrier lifetime in CdS at 532 nm

Abstract Bound-electronic and free-carrier optical nonlinearities, and relaxation of photo-excited free carriers in CdS have been investigated by the use of a single-beam Z -scan technique at 532 nm. Under pulsed radiation of 35-ps duration with the input irradiances up to 4.8 GW/cm 2 , the two-photon absorption coefficient, the bound-electron nonlinear refractive index, the free-carrier absorption cross-section, and the change in the refractive index per unit carrier density are determined to be 5.4±0.8 cm/GW, −(5.3±0.8)×10 −13 cm 2 /W, (3.0±0.5)×10 −17 cm 2 and −(0.8±0.1)×10 −21 cm 3 , respectively. By using these values in the open-aperture Z -scans conducted with 7-ns laser pulses, the carrier recombination time is extracted to be 3.6±0.7 ns. The measured parameters are compared to theoretical calculations.

Femtosecond Z-scan investigation of nonlinear refraction in surface modified PbS nanoparticles

Abstract The third-order optical nonlinearities of surface modified PbS nanoparticles have been measured using the Z-scan technique with femtosecond laser pulses at 780 nm wavelength. The samples studied are the nanoparticles in microemulsion with the PbS concentration ranging from 0.3×10 −3 to 1.9×10 −3 M (mol/l). A Z-scan analysis method based on the Huygens–Fresnel (H–F) propagation integral is employed to extract the nonlinear refractive index from the experimental Z-scan data with a large nonlinear phase shift. The nonlinear refractive index of the PbS nanoparticles in microemulsion is found to increase linearly with the PbS concentration. The highest concentration microemulsion gives a nonlinear refractive index of −4.7×10 −12 cm2/W, which is approximately 3 orders of magnitude higher than those of commercially available bulk semiconductors, such as ZnS and CdS. In addition, nonlinear absorption in these samples remained unmeasurable up to 0.9 GW/cm2. The mechanisms responsible for the observed large refractive nonlinearity are discussed.

Fiber optic acoustic hydrophone with double Mach-Zehnder interferometers for optical path length compensation

Abstract We report on the development of a fiber optic acoustic hydrophone which consists of a sensing Mach–Zehnder (MZ) interferometer and a compensating MZ interferometer for optical path length compensation. This double-interferometer configuration has the following advantages: the hydrophone is a true heterodyne device; a laser source with a short coherence length can be used; the sensing interferometer is completely passive; the compensating interferometer can be located near the signal processing electronics, far away from the sensing interferometer; a conventional modulation analyzer can be used to demodulate the optical phase shift, which greatly simplifies the demodulation electronics. The performance of the hydrophone is evaluated experimentally by immersing the sensing interferometer in a water tank to detect underwater acoustic signals generated by an acoustic wave projector. Experimental results show that over the frequency range of 5 to 20 kHz, the hydrophone has an almost flat response with an average normalized phase sensitivity of −322.3 dB re 1/μPa and an average pressure sensitivity of −153.7 dB re rad/μPa. These performance figures are better than those obtained from a commercial piezoelectric hydrophone. Furthermore, we have also demonstrated that with improved signal processing techniques, the normalized phase sensitivity of the hydrophone increases to −313 dB re 1/μPa, and the pressure sensitivity increases to −136.9 dB re rad/μPa. These results indicate that the present design offers equal or better performance in terms of sensitivity over its counterparts employing conventional Mach–Zehner configurations.

Influence of the thermal effect on the TEM 00 mode output power of a laser-diode side-pumped solid-state laser

A fraction of pump power has been converted to TEM00 mode laser power for a side-pumped solid-state laser by use of a space-dependent rate equation. We investigated the pump-to-mode (TEM00) ratio when scaling laser-diode side-pumped solid-state lasers to high-power levels by including the thermal effect in the space-dependent rate equation. Based on the assumption that Gaussian pump power is the same at any cross section of a laser rod, we resolved the output power with a space-dependent rate equation; temperature distribution in the laser rod was obtained; the optical path difference distribution was derived, and we estimated the diffraction losses that result from thermally induced spherical aberration by use of the Strehl intensity ratio. We determined that thermally induced diffraction losses are strongly dependent on pump power and on the pump-to-mode ratio.

Fabrication of microlens in photosensitive hybrid sol–gel films using a gray scale mask

Abstract Hybrid sol–gel glasses, which combine the attractive features of organic polymers with those of inorganic glasses, are desirable materials for integrated optics. In this work, the synthesis and properties of an ultraviolet photosensitive hybrid sol–gel glass films are demonstrated. Multilevel diffractive lens in sol–gel film fabricated with a simple way of UV photolithography using a gray-scale mask is reported. The effects of UV light exposure and heat treatment on the properties of the film are also studied.

Design of diffractive phase elements for beam shaping: hybrid approach

Hybrid approaches that combine genetic algorithms (GA’s) with traditional gradient-based local search techniques are proposed for the optimization design of diffractive phase elements (DPE’s) for laser beam shaping. These hybrid methods exploit the global nature of the GA’s as well as the local improvement capabilities of the gradient-based local search techniques and will perform a more improved search in comparison with each of the individual approaches. The incorporated local search technique that we used here is the Davidon–Fletcher–Powell method. A cost function that can directly control the performance of the final solutions is also used. By performing the DPE design with different desired diffraction efficiencies, we obtain a set of results that approximately reflect the trade-off between the design objectives, namely, signal-to-noise ratio (SNR) and diffraction efficiency. Reasonable solutions can be chosen on the basis of the knowledge of the problem. Simulation computations are detailed for two rotationally symmetric beam-shaping systems, in which an incident Gaussian profile laser beam is converted into a uniform beam and a zero-order Bessel beam. Numerical results demonstrate that the proposed algorithm is highly efficient and robust. DPE’s that have high diffraction efficiency and excellent SNR can be achieved by using the algorithm that we propose.

Modeling pulse shape of Q-switched lasers

A general approach describing the pulse shape and temporal width of Q-switched lasers has been derived. Based on a normalized intermediate variable describing the ratio of population inversion density at the beginning of Q-switch to that at threshold, a generalized characteristic equation that governs passively and actively Q-switched lasers is derived. From the characteristic equation, the pulse symmetry property and pulse width can be described and easily calculated for any given operating parameters, and the waveform of the laser pulse can be reconstructed without solving the laser rate equations. An approach is also given for the case of intracavity frequency-doubled Q-switched lasers. Theoretical results show agreement with experiments for a diode pumped Nd:YAG laser system that is passively Q-switched by a Cr/sup 4+/:YAG absorber.