Modulation of four-wave mixing via photonic band gap

The dressed four-wave mixing （FWM） in a four-level S5Rb atomic system, experimentally demon- strated in this paper, is comprised by two coexisting processes. One is emission signal due to enhanced nonlinear via electromagnetically induced transparency （EIT）. The other is the Bragg reflection of probe beam because of the created photonic band gap （PBG）, which is affected by both linear and third-order nonlinear susceptibility. Moreover, we have demonstrated that different experimental parameters can significantly influence the measured signal with flexibly controlled PBG. These studies are found useful for understanding the fundamental mechanisms in generated FWM processing.     

A study on the optical nonlinearity due to the Z3 and Z1,2 excitons of CuCl by the reflected four-wave mixed light generation

We have demonstrated the generation of the reflected four-wave mixed light in the energy region from 3.185 eV to 3.320 eV including the highly absorptive region due to the Z₃ and Z_1,2 exciton bands of CuCl at 1.6 K. The mixed light signal is concluded to be due to the one-photon resonant contribution of the respective exciton level, from its polarization characteristics. The corresponding third order non-linear susceptibility is estimated from the data by the correction with the Fresnel factors which are calculated on the basis of the Pekar's Additional Boundary Condition.     

The electron energy loss spectrum of LiF in the band gap region

The eels of LiF has been measured in the range 0–18 eV with primary beam energies 50 eV and 1.5 keV. Four peaks are clearly resolved in the band gap region at room temperature. The amplitude of the three lower energy loss peaks was found to depend on beam exposure and temperature. It is concluded that the lowest energy peak arises at a beam induced defect, that the next two peaks arise in lithium metal liberated by the electron beam and that the peak nearest to the band edge is due to an intrinsic surface excitation.     

Parametric study of two-frequency raman conversion in 16 μm region by four-wave mixing effect

Summary In this work an experiment, projected for a simultaneous operation of two different-frequency radiations in the 16 μ range, is described according to a four-wave mixing process in a para-H₂ Raman cell. The main parameters which influence the conversion efficiency of the two primary CO₂ pump radiations are investigated and optimized to get an appreciable reliability of the system. ENEA Guest     

Investigation of energy band gap and optical properties of cubic CdS epilayers

High quality cubic CdS epilayers were grown on GaAs (1 0 0) substrates by the hot-wall epitaxy method. The crystal structure of the grown epilayers was confirmed to be the cubic structure by X-ray diffraction patterns. The optical properties of the epilayers were investigated in a wide photon energy range between 2.0 and 8.5 eV using spectroscopic ellipsometry (SE) and were studied in the transmittance spectra at a wavelength range of 400-700 nm at room temperature. The data obtained by SE were analyzed to find the critical points of the pseudodielectric function spectra, 〈?(E)〉 = 〈?₁(E)〉 + i〈?₂(E)〉, such as E₀, E₁, E₂, E^′₀, and E^′₁ structures. In addition, the optical properties related to the pseudodielectric function of CdS, such as the absorption coefficient α(E), were investigated. All the critical point structures were observed, for the first time, at 300 K by ellipsometric measurements for the cubic CdS epilayers. Also, the energy band gap was determined by the transmittance spectra of the free-standing film, and the results were compared with the E₀ structure obtained by SE measurement.     

Dynamics of four-wave mixing on excitons in a quantum optical microcavity

A theory of degenerate four-wave mixing in a semiconductor optical cavity with quantum wells is constructed. The nonlinear response of a microcavity can be four to five orders of magnitude stronger than that of an isolated quantum well. For P ² E-and P ³-type nonlinearities the damped diffracted signal oscillates with a period determined by the Rabi splitting. For a biexcitonic mechanism of nonlinearity, the signal contains damped overtones of the Rabi splitting and the biexciton binding energy. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 749–754 (10 December 1996)     

The magnetooptical properties of InSe near the band gap

We have studied the magnetooptical properties of InSe near the band gap by wavelength modulation spectroscopy at 10 K with the electric vectorE perpendicular to theC-axis of the crystal. For the first time, Landau levels have been observed from which we obtain the reduced effective mass perpendicular to $\overrightarrow{C}$, μ_⊥ = (0.090 ± 0.010)m₀. The n = 1, 2 and 3 exciton lines along with a diamagnetic shift of the n = 1 level have been resolved from which we evaluate the Rydberg constant R = 15.0 ± 0.5 meV, the diamagnetic shift σ₁ = 14 ± 2 meV/(kG)² and ε_⊥ε_∥ = 81 ± 8. Evidence of the presence of the γ and ε polytypes in the sample due to stacking faults is given.     

Doppler-free spectroscopy of neon atoms dressed by optical photons in nearly-degenerate four-wave mixing

We have investigated nearly degenerate four-wave mixing in neon on the 607 nm line. We have considered the case where one pump beam saturates the medium while the two other incident beams have a much weaker intensity. The frequency of the most intense pump beam is equal to the frequency of the probe beam. When the frequency of the second pump beam is scanned, narrow resonances are observed. These resonances correspond to transitions between the energy levels of the atom dressed by the photons of the intense pump beam.     

Influence of degenerate four-wave mixing on the performance of supercontinuum-based multiwavelength optical source

The influence of degenerate four-wave mixing (FWM) on the performance of supercontinuum-based multiwavelength optical source has been investigated in detail experimentally and theoretically. Numerical simulation results show that the degenerate FWM effect has a deteriorative influence on the spectral uniformity and the optical signal-to-noise ratio (OSNR) of supercontinuum-based optical source, and by suppressing degenerate FWM effect the performance enhancement of the supercontinuum can be achieved successfully. These results are also confirmed by our experiments. Experimentally, by suppressing degenerate FWM the crosstalk of adjacent channels to the filtered channel can be reduced by as much as 15 dB, and consequently the measured receiver sensitivity at 10 Gbit/s for the filtered optical source is improved from $-1.7$ to $-17.8$~dBm.     

Effect of Er doping on optical band gap energy of TiO2 thin films prepared by spin coating

In order to evaluate the effect of Er doping in the range of 0–1.0 mol% on optical indirect band gap energy (E_g) of the film, the Er-doped TiO₂ (Er-TiO₂) thin films were spin-coated onto fluorine-doped SnO₂ coated (FTO) glasses. Glancing angle X-ray diffraction (GAXRD) results indicated that the films whose thickness was 550 nm consisted of pure anatase and FTO substrate. The anatase (101) TiO₂ peaks became broader and weaker with the rise in Er content. The apparent crystallite size decreased from 12 nm to 10 nm with increasing the amount of Er from 0 mol% to 1.0 mol%. UV–vis spectrophotometry showed that the values of E_g decreased from 3.25 eV to 2.81 eV with the increase of Er doping from 0 to 0.7 mol%, but changed to 2.89 eV when Er content was 1.0 mol%. The reduction in E_g might be attributed to electron and/or hole trapping at the donor and acceptor levels in the TiO₂ band structure.     

Effects of the spectral inhomogeneity on the optical properties in the four-wave mixing signal

Effects of the inhomogeneous broadening on the optical responses of a two-level system in the four-wave mixing signal (FWM) were studied in this work. In this model, we introduced in the optical Bloch equations a stochastic Bohr frequency, generated as a consequence of the solute-solvent molecular collisions. These expressions are averaged in the statistical ensemble over all realizations of the random variables. Using the convolution theorem, it is possible to incorporate the saturation effects of the electromagnetic field treated at all orders of perturbation. For the averages, we have considered two distributions of linewidth. From the analytical expressions, for both linear and nonlinear responses, numerical calculations were carried out to obtain surfaces as a function of the magnitude of the pump field and the detuning factor. Saturation effect is shown in the linear and the coupled nonlinear responses. In this work, we have already considered the inhomogeneous spectral line and studied the cases in which all the fields propagate through the medium.     

Wavelength conversion by use of four-wave mixing in a novel optical loop configuration

A novel loop configuration for generation of four-wave mixing (FWM) and suppression of input pump wave, signal wave, and amplified spontaneous emission is proposed and experimentally demonstrated. The novel loop configuration is very simple and functions independently of the wavelengths of the pump and the signal waves. By use of the novel loop configuration, nonreturn-to-zero wavelength conversion at 10 Gbits/s is achieved. The FWM-to-pump ratio, the FWM-to-signal ratio, and the signal-to-noise ratio are improved by 17.9, 18.8, and 8.2 dB, respectively. A principle experiment of wavelength conversion of four simultaneous channels is demonstrated.     

基于光纤四波混频波长转换和色散的慢光实验研究

对基于光纤四波混频（FWM）波长转换和色散的慢光实现进行了详细和系统的实验研究.首先,实验测定了高非线性光纤中FWM带宽约为40 nm,从而确定了慢光的可调谐带宽;接着,在普通单模光纤和色散补偿光纤（DCF）中针对500 MHz正弦信号和100 ps短脉冲信号分别实现了34和198 ns的脉冲延迟,在DCF中还实现了209 ns的脉冲提前.讨论了增大延迟量的方法,指出随着宽带FWM波长转换的实现和大色散光纤的应用有望获得微秒量级的大延迟量,从而为高性能光纤延迟线和全光缓存器等应用提供支持. 关键词： 慢光 四波混频 色散     

布里渊增强四波混频时域特性的理论研究

通过对布里渊增强四波混频(BEFWM)瞬态耦合波方程的数值求解，描述了共轭输出光的时域调制行为.同时，对能量耦合过程中起决定性作用的声学波的时空演化特征进行了模拟与分析，由此给出BEFWM时域特性物理机理的解释.研究表明，共轭光输出强度的波动与介质内声波场的分布和起伏相关联，共轭光波形调制现象的强弱取决于介质声子寿命、信号及抽运光强度、有效作用长度等诸多参数.     

Generation of monochromatic visible light in microstructured optical fiber by nondegenerate four-wave mixing

We exploit nondegenerate four-wave mixing to generate narrow-bandwidth visible light from blue to orange in microstructured optical fiber. The process is excited by nanosecond dual-wavelength pump pulses (532/1064 nm) produced by a frequency-doubled passively Q-switched Nd:YAG laser. A linearly polarized, fundamental mode signal wave is generated in the visible, accompanied by an idler wave in the near-infrared. The signal and idler wavelengths are adjustable by the design of the microstructure.     

Femtosecond optical diagnostics of interlevel energy relaxation in the band gap of silver iodide nanocrystals

The kinetics of the relaxation processes in silver iodide nanocrystals synthesized in a nanoporous silicate glass matrix and excited by femtosecond laser pulses has been experimentally studied. A short-lived characteristic transmission band is found to be formed in the spectral range of exciton absorption of nanocrystals. The specific features of photoexcited-carrier relaxation, which are due to the energy redistribution of captured carriers over levels in the band gap of silver iodide nanocrystals, are considered. The energy transfer relaxation rates are estimated and possible mechanisms of the energy transfer are discussed.     

Classical theory of non-degenerate sum-frequency generation by incoherent nonlinear optical mixing of coherent and chaotic radiations

The incoherent nonlinear optical sum-frequency generation with non-monochromatic initially uncorrelated sub-frequency input radiations in a dispersive medium is studied in this paper. The efficiency of the process is calculated in the second approximation, whilst the spectral distribution of generated radiation is merely described in terms of the first approximation of the iterative method. The calculations of the efficiency of nonlinear process and the spectral distribution of sum-frequency radiation are performed for one coherent and the other chaotic input radiations, and for both chaotic input radiations, respectively.This work was partially supported by Research Project C.P.B.P. 01. 07.     

Classical theory of non-degenerate sum-frequency generation by coherent nonlinear optical mixing of coherent and chaotic radiations

Classical solution of coherent nonlinear optical sum-frequency generation process based on the averaging of stochastic quantities over the initial photocount distributions in sub-frequency radiations is presented in this paper. The evolution of second-order light statistics in sum-frequency generation with coherent and chaotic input radiations is calculated for arbitrary values of the time or space parameter.The author would like to thank Dr. M. Krková from the Computer Centre of Palacky University for performing the numerical calculations.