Pulsed laser induced absorption (one–four) dependence of electron mobility in CdI2 and ZnS crystals

The photoconductivity and electron mobility of CdI₂ and ZnS crystals have been studied using N₂-laser, fundamental and frequency doubled Nd:YAG laser. Low values of the electron mobilities obtained in the present case have been attributed to laser-induced-absorption. It is low in one photon excitation and increases with the order of absorption.     

Nonlinear fluorescence through intermolecular energy transfer and resolution increase in fluorescence microscopy

We investigate a novel concept to efficiently generate multiphoton induced fluorescence from organic molecules. The concept is based on frustrating the energy transfer between a fluorescent donor and one or more acceptors in conjugated molecules. The nonlinearity is not based on higher order molecular susceptibilities but entirely on their linear properties. Therefore, in contrast to nonresonant multiphoton absorption, this method does not require high local intensities. Likewise, the production of visible fluorescence does not require an infrared excitation wavelength. Hence, when applied to scanning microscopy this property is predicted to increase spatial resolution. Instead of the ∼10 GW/cm² required in non-resonant multiphoton excitation, focal intensities ∼10 MW/cm² are expected to produce an equally strong nonlinear signal. The predicted resolution is up to 30% greater than that of an ideal confocal microscope operating at the same fluorescence wavelength. The resolution improvement over non-resonant two-photon absorption microscopes is about two-fold in all directions.     

Multi-channel 80-GHz RZ pulse train generation based on parametric process in highly-nonlinear fiber

It is experimentally shown that six-channel C + L-band 80-GHz short pulse trains are generated based on parametric process in highly-nonlinear fiber (HNLF). By launching a pulsed pump and three-channel continuous wave (CW) lights into 1-km HNLF, three-channel signals are amplified and three-channel idlers are generated. The waveforms of the generated 80-GHz short pulses are measured by an auto-correlator. The bit-error-rate (BER) of each channel is analyzed numerically. In the back-to-back case, the power penalties of the generated channels are less than 1.5-dB.     

Modulation of spatial spectrum in tunable two crystal optical parametric generator

We report on theoretical and experimental investigations of two crystal traveling-wave optical parametric generator (OPG). It is shown that the efficiency and angular structure of traveling-wave tunable OPG output depends on the distance between the two crystals. Air dispersion driven efficiency modulation at different OPG output wavelengths is observed experimentally and confirmed by theoretical calculations.     

Character analysis of slow light with communication waveband in fiber optical parametric amplifier

The comprehensive characters of delay time and delay bandwidth with distortion, signal wavelength dependence and delay bandwidth product in fiber optical parametric amplifier are investigated theoretically when the pump locates at anomalous dispersion region. The delay character of single pulse, data stream with certain bit pattern and pseudo random data stream are analyzed comprehensively. In this category of fiber optical parametric amplifier, the delay time and delay bandwidth product vary from signal wavelengths and bit rates. For the same nonlinear coefficient, pump power and fiber length, the diverse dependences of bit pattern in different signal wavelengths are found by numerical simulation. The impacts of pump power and fiber length on the delay character and distortion are also discussed. These relations are conductive to the design of slow light based on fiber optical parametric amplifier in telecommunication waveband.     

The generation of terahertz radiation via optical rectification in the self-induced transparency regime

The possibility of efficient terahertz generation via optical rectification in electro-optic crystal doped with the two-level resonant impurities is demonstrated. Under conditions of the self-induced transparency regime the laser pulses slows down to achieve the phase matching condition. The terahertz generation is accompanied by periodic modulation and spectral deformation of the laser pulse. The most efficient generation occurs under condition of small negative detuning from the resonant frequency of the two-level system.     

Chaos generation in a semiconductor ring laser with an optical injection

Two chaos generation schemes are demonstrated numerically based on a semiconductor ring laser (SRL) with either a constant or a chaotic optical injection from an external distributed feedback (DFB) laser. Under a certain bias current, the chaotic output can be generated by the SRL with appropriate injection parameters. Parameter map is achieved numerically to show types of oscillation in the SRL. Bifurcation diagrams and Lyapunov exponent spectra roughly describe the routes to chaos for the SRL with disturbance. A bandwidth-enhanced chaos generation can be describes based on the chaotic optical injection scheme. The bandwidth of the bandwidth-enhanced chaos generated is about 12 GHz, which achieves approximately a two-fold increase compared with the constant injection scheme.     

Role of polytypism and degree of hexagonality on the photoinduced optical second harmonic generation in SiC nanocrystalline films

Photoinduced optiсal second harmonic generation was studied in nanocrystalline SiC films prepared by the method of direct ion deposition. For the studies were chosen three types of polytypes (with different degree of hexagonality) – 24R with degree hexagonality G=25, 27R-G=44, 33R with – G=36. The bicolor photoinduced treatment was performed by the wavelengths 1064nm/532 nm by 15 ns YAG:Nd laser. The efficiency of the output SHG was evaluated by ratio of the corresponding signal intensities with respect to the references and by the time delay between the SHG and the fundamental maxima. Explanation of the observed effect is given within a framework of the occurrence of the nano-trapping levels in the film crystalline interfaces.     

Light-shift induced by two unbalanced spontaneous decay rates in EIT (CPT) spectroscopies under Ramsey pulse excitation

Light shift is important and inevitably affects the long-term stability of an atomic clock. In this work, considering two unbalanced branches of the spontaneous decay rate in a three-level system, we studied the frequency shifts of electromagnetically induced transparency (EIT) and coherent population trapping (CPT) clocks operating under the pulse sequence regime by numerically solving the Liouville density matrix equations. The results show that the frequency shifts are larger when the two branches of spontaneous emission rate are not equal compared to the equal case. In addition, in EIT-Ramsey, the effect of the unbalanced branches of the spontaneous decay rate and relaxations of low-energy states on the frequency shift is greater than that of Rabi frequency. In CPT-Ramsey, the relaxations of low-energy states play a dominant role in frequency shift.     

The effect of two-photon absorption and optical excitation area on the generation of THz radiation

It was found experimentally that the THz radiation throughput based on optical rectification is strongly related to the optical excitation size in a ZeTe emitter. The factors affecting the THz radiation throughput are investigated in detail both experimentally and theoretically. By taking into account optical rectification, diffraction and two-photon absorption effects, a theoretical model is established to describe the emitting field intensity of THz radiation. There is excellent agreement between the theoretical results and the experimental data under high excitation power. Both theoretical and experimental results demonstrate that there is a trade-off between these three effects for THz radiation, and that in order to increase the THz radiation throughput based on optical rectification, one should choose suitable excitation size under a given pump power.     

两束离轴高斯光束干涉场中的横向光涡旋

研究了两束离轴高斯光束干涉场中的波前结构以及横向光涡旋.研究表明干涉场中的相位鞍点以及涡旋点的位置与光束的离轴参数、束腰宽度、相位以及相对振幅有关.相位鞍点既可位于涡旋点的内侧，也可以位于涡旋点的外侧，且控制参数取一定值时，相位鞍点将与涡旋点重合.对于离轴光束，XZ平面两侧的相位鞍点与涡旋点相遇时所对应的控制参数并不相同，且XZ平面上相位鞍点相遇时所对应x值，一般情况下与涡旋点相遇时所对应的x值不相同.     

Resolution and contrast in confocal optical scanning microscopes

A theoretical study is made of a simple method of obstructing an amplitude pupil which results in improved microscope resolution with only a little degradation of the image contrast. Using a confocal scanning optical microscope the resultant point spread function is calculated, from which the image of a point object is deduced. The calculations are made for two different apertures — the first has four-fold symmetry and the second has eight-fold symmetry — and are compared with full circular and annular apertures.     

掺杂比对CdGe(As11-xPx)2倍频与参量振荡的影响

 在相位匹配和可接受掺杂比理论的基础上，针对CdGe((As1_1-xP_x)₂晶体，研究了掺杂比对频率转换的影响。依据Sellmeier方程得到了倍频和Ho³⁺:YLF与Cr:Er:YSGG激光器泵浦光学参量振荡的相位匹配调谐曲线，并给出了不同掺杂下的可接受掺杂比。结果表明：掺杂晶体可实现2~9 μm范围内非临界相位匹配的倍频和2~18 μm的参量光产生；随着掺杂比的增大，可接受掺杂比近似线性减小，并随着波长改变展现出了不同变化趋势。     

Relation between different formalisms describing partially incoherent wave propagation in nonlinear optical media

It is shown that the Wigner formalism for describing propagation of partially incoherent waves in nonlinear media with slow medium response is equivalent to previously established methods based on coherent density theory, self-consistent multi-mode theory and the mutual coherence function approach.     

Tunable infrared generation in KTA and applications

Noncollinear difference frequency mixing of dye laser and Nd:YAG second harmonic (fundamental) radiation from a commercial laser system is employed for the generation of 2.7–5.3 μm (1.6–1.7 μm) radiations in a flux-grown KTiOAsO₁ crystal. The generated radiation is used to scan the methane absorption in the fundamental (v ₃) and its first overtone (2v ₃) band at pressure 90 torr in a laboratory made single pass gas cell of length 33 cm.     

Generation and amplification of dual-color femtosecond pulses by a noncollinear optical parametric up-conversion process

Noncollinear optical parametric up-conversion generation and amplification are realized in a thick β-barium borate (BBO) crystal, and a couple of visible femtosecond up-conversion laser pulses can be achieved by a femtosecond pulse at 800 nm as the pump sources. The theoretical and experimental results indicate that there exist phase-matching conditions for dual-color noncollinear parametric up-conversion generation and amplification, and their wavelengths can be tuned by rotating the BBO crystal. This parametric up-conversion generation and amplification can be attributed to three and five-wave mixing in a thick BBO crystal, and it shows the potential application on optical parametric chirped pulse amplification (OPCPA) to generate multi-color ultraviolet or visible femtosecond laser pulses pumped directly by femtosecond fundamental laser pulses without frequency-doubling or tripling.     

Entanglement properties between two atoms in the binomial optical field interacting with two entangled atoms

The temporal evolution of the degree of entanglement between two atoms in a system of the binomial optical field interacting with two arbitrary entangled atoms is investigated. The influence of the strength of the dipole–dipole interaction between two atoms, probabilities of the Bernoulli trial, and particle number of the binomial optical field on the temporal evolution of the atomic entanglement are discussed. The result shows that the two atoms are always in the entanglement state. Moreover, if and only if the two atoms are initially in the maximally entangled state, the entanglement evolution is not affected by the parameters, and the degree of entanglement is always kept as 1.     

Enhancement of second harmonic generation efficiency: Growth and characterization of magnesium(II)-doped tetrakis(thiourea)nickel(II) chloride crystals

The influence of magnesium doping on the properties of tetrakis(thiourea)nickel(II) chloride crystals has been described. The reduction in the intensity observed in powder X-ray diffraction of doped specimen and slight shifts in vibrational frequencies confirm the lattice stress as a result of doping. Surface morphological changes due to doping of the alkaline earth metal are observed by scanning electron microscopy. The incorporation of Mg(II) into the crystal lattice was confirmed by energy dispersive X-ray spectroscopy. Lattice parameters are determined by single crystal XRD analysis. The thermogravimetric and differential thermal analysis studies reveal the purity of the materials and no decomposition is observed up to the melting point. The crystal is further characterized by Kurtz powder technique and dielectric studies.