Magneto-phonon polaritons in two-dimension antiferromagnetic/ion-crystalic photonic crystals

Magneto-phonon polaritons in a two-dimension photonic crystal (PC) are discussed. This PC is constructed by embedding a periodical square lattice of ionic-crystal cylinders into an antiferromagnet. The two media are dispersive, with their individual resonant frequencies near each other. We first set up an effective-medium method to obtain the effective magnetic permeability and dielectric permittivity of the PC, followed by the dispersion relations of surface and bulk polaritons. There are a number of new surface polaritons, and two new distinctive bulk polariton bands in which the negative refraction and left-handedness can appear. The numerical calculations are based on the example, FeF₂/TlBr PC.     

Optical properties of antiferromagnetic/ion-crystal superlattices

Transmission, refraction and absorption properties of an antiferromagnetic/ion-crystal superlattice are investigated. The transmission spectra based on FeF₂/TlBr superlattices reveal that there exist two intriguing guided modes in a wide stop band. Additionally, FeF₂/TlBr superlattices possess either the negative refraction or the quasi left-handedness, or even simultaneously hold them at certain frequencies of two guided modes, which require both negative magnetic permeability of antiferromagnetic layers and negative permittivity of ion-crystal layers. Frequency regimes of the guided modes will be dependent on the magnitude of the external magnetic field. Therefore, handedness and refraction properties of the system can be manipulated by modifying the external magnetic field. Absorption spectra exhibit that absorption corresponding to guided modes is noticeable.     

A monochromatic and frequency stabilized cw source in the 5 μm region obtained by doubling the frequencies of a cw laser in tellurium

In this paper we describe the construction of a monochromatic and frequency stabilized continuous wave (cw) source in the 5 μm region. It is built up by a cw single line, single-mode frequency and amplitude stabilized CO₂ laser. This primary CO₂ laser source is then frequency-doubled in a tellurium crystal using second harmonic generation (SHG). The obtained emission is monochromatic (half-width 50 kHz). The frequency can be varied discreetly from 4.6 μm to 5.5 μ depending on the CO₂ laser line chosen.     

Voigt位型下电介质/反铁磁/电介质结构二次谐波生成非倒易性研究

利用非线性传递矩阵方法研究了Voigt位型下电介质/反铁磁/电介质 结构二次谐波生成的非倒易性. 研究发现外加静磁场反向和电介质层排序翻转均对二次谐波输出产生影响, 出现了二次谐波生成的非倒易性. 二次谐波生成非倒易性频率区域在反铁磁共振区, 此区间正处于THz频段. 随着入射角度的增加, 非倒易性的效果越来越明显. 研究二次谐波生成的非倒易性, 可为反铁磁器件的设计加工提供理论支持.     

Thermally induced second harmonic generation in PbO-B2O3 glasses

Second harmonic generation (SHG) was observed in PbO-B₂O₃ glasses after heat treatment at elevated temperature followed by rapid quenching in water at room temperature (about 25 °C). According to the results of thermally stimulated depolarization current (TSDC) measurement and XRD patterns, no depletion layer or crystals was found in the rapidly quenched glass samples. The thermally induced nonlinear layer is located in the regions extended from the surface and 8 μm into the bulk. The possible mechanism responsible for the SHG is that the bond deformation of glass structure induced by stress gradient breaks the inversion symmetry of glass. The influencing factors of the SH intensity are also discussed.     

Theoretical analysis of the TE mode Cerenkov type second harmonic generation in ion-implanted X-cut lithium niobate channel waveguides

Optical second harmonic generation (SHG) in the form of Cerenkov radiation from ion-implanted lithium niobate (LiNbO₃) channel waveguides is analyzed by directly resolving the wave equations. Useful formula of the SHG efficiency is derived and expressed in terms of waveguide parameters. Numerical examples are plotted for LiNbO₃ crystals. The results enable the optimization of waveguide design for efficient second harmonic generation in the Cerenkov configuration.     

Effects of weak localization of photons in nonlinear optics: Second harmonic generation

The angular distribution of the second harmonic generation (SHG) in disordered nonlinear media is shown to exhibit sharp peaks due to the weak localization of photons. The shape of these peaks is related to the spatial structure of the electric field for strongly scattered second harmonic (SH) radiation and is sensitive to the Anderson localization effects. The angular distribution of the second harmonic energy flux is calculated for the case of steady state and pulse generation in a slab of thickness d ≫ l (l is the elastic mean free path). For the case of pulse generation the time dependence of the SHG is discussed.     

Structure and NLO property relationship in a novel chalcone co-crystal

Single crystals of a chalcone co-crystal (C₁₈H₁₉NO₄/C₁₇H₁₆NO₃Br; 0.972/0.028) have been grown by slow evaporation from solution. The powder second harmonic generation (SHG) efficiency of this chalcone co-crystal is 7 times that of urea. The dependence of second harmonic (SH) intensity on particle size revealed the existence of phase matching direction in this crystal. The large SHG efficiency observed is mainly due to the unidirectional alignment of molecular dipoles, in which the dipole moment of each molecule adds to establish a net polarization. The weak N–H⋅⋅⋅O hydrogen-bond interactions help to stabilize the noncentrosymmetric crystal packing and also contribute partly to the SHG. The better thermal stability, transparency and high laser damage resistance (>1.5 GW cm⁻² at 532 nm, 8 ns) of this chalcone co-crystal indicate that it is a promising material for frequency doubling of diode lasers down to 470 nm. This molecule also shows a third-order NLO response and good optical limiting property of 8 ns laser pulses at 532 nm. The mechanism for optical limiting in this chalcone was attributed to two-photon induced excited state absorption that leads to reverse saturable absorption. The structure–property relationship in this chalcone and related compounds is discussed based on the experimental results and semiempherical hyperpolarizability calculations.     

Efficient Nd:Y0.5Gd0.5VO4-KTiOPO4 green laser under diode pumping into the emitting level 4F3/2

We report a green laser at 532 nm generation by intracavity frequency doubling of a continuous wave (cw) laser operation of a 1064 nm Nd:Y_0.5Gd_0.5VO₄ laser under diode pumping into the emitting level ⁴ F _3/2. A KTiOPO₄ (KTP) crystal, cut for critical type-II phase matching at room temperature is used for second harmonic generation (SHG) of the laser. At an incident pump power of 17.8 W, as high as 4.21 W of cw output power at 532 nm is achieved. The optical-to-optical conversion efficiency is up to 23.6%, and the fluctuation of the green output power was better than 2.8% in the given 30 min.     

Development of coherent tunable source in 2–16 μm region using nonlinear frequency mixing processes

A very convenient way to obtain widely tunable source of coherent radiation in the infrared region is through nonlinear frequency mixing processes like second harmonic generation (SHG), difference-frequency mixing (DFM) or optical parametric oscillation (OPO). Using commonly available Nd:YAG laser and its harmonic pumped dye laser radiation as parent beams, we have been able to generate coherent tunable infrared radiation (IR) in 2–16 μm region using different nonlinear crystals by DFM and OPO. We have also generated such IR source in the 4–5 μm region through SHG of CO₂ laser in different infrared crystals. In the process we have characterized a large number of nonlinear crystals like different borate group of crystals, KTP, KTA, LiIO₃, MgO:LiNbO₃, GaSe, AgGaSe₂, ZnGeP₂, AgGa_1?x In _x Se₂, HgGa₂S₄ etc. To improve the conversion efficiencies of such frequency conversion processes, we have developed some novel schemes, like multipass configuration (MC) and positive optical feedback (POF). The significance of the obtained results lies in the fact that to get the same conversion in SHG or DFM, one now requires fundamental input radiation with much lower intensity.     

Structures responsible for quasisynchronism in second-harmonic generation in BaTiO3:Fe

Second harmonic generation (SHG) of light in iron-doped BaTiO₃ crystals occurs primarily at an angle of about 7° to incident λ=1.06 μm radiation. It is shown that the quasisynchronism is accounted for by a 90° domain structure with a spatial wave vector q∥[011] and a spatial period of about 3 μm. This result may serve as a basis for interpretation of the anomalous SHG enhancement near the phase transition. Fiz. Tverd. Tela (St. Petersburg) 41, 1076–1079 (June 1999)     

Efficient generation of a continuous-wave,tunable 780 nm laser via an optimized cavity-enhanced frequency doubling of 1.56 µm at low pump powers

We present the strict design parameters of the experiment for the 780 nm tunable continuous-wave second harmonic (SH) generation by the nonlinear resonator containing a MgO doped periodically poled LiNbO₃ (MgO:PPLN) crystal. Optimization of such critical parameters, including focusing and impedance matching, more than 84% SH conversion efficiency and 3.1 W available output power at 780 nm were obtained from the fundamental wave at 1560 nm with two different input couplers. The thermal saturated behavior of the SH output power has been observed in the experiment. The beam quality factor M² of the generated SH wave is 1.04 (1.03), and the RMS power stability is 1.29% in 3 h. The SH wave was further used to detect the D ₂ transitions of Rb atom, exhibiting a fine tunable characteristic. Such laser source can be a suitable candidate in the atomic physics and quantum optics.     

在BBO中获得230.8—223.2nm高功率和频输出

用Q-YAG泵浦的Rh·6G染料激光在一块45°切割的β-BaB₂O₄(BBO)晶体中倍频,产生294.8—282.5nm范围的连续调谐输出,其能量为8mJ(在285nm处)。用这个倍频光与泵浦染料后剩余基波(1064nm)在另一块45°切割的BBO中和频,已获得230.8—223.2nm范围的连续调谐输出,其能量为120μJ,相应的峰值功率为12kW。还简述了获得高功率和频输出的关键技术。     

Frequency doubling of phase-modulated femtosecond laser pulses in periodically poled and chirped nonlinear crystals

The second harmonic generation (SHG) at a wavelength of 0.8 μm by 50-and 10-fs pulses with and without phase modulation (PM) was systematically studied in LiNbO₃ crystals with regular domain structure and linearly varied domain thickness. The main results were obtained by numerical method, taking into account the difference between group velocities of interacting pulses and the group-velocity dispersion. In the approximation of the given field of the fundamental radiation, an analytical expression was derived for the spectral density of the second harmonic in the periodically poled nonlinear crystal (PPNC) under nonstationary excitation conditions. It was numerically found that the conversion efficiency of about 90% can be achieved by doubling the frequency of 50-fs laser pulses without PM in the LiNbO₃ PPNC. The maximum conversion efficiency for the SHG by PM pulses is achieved at a certain optimum chirp step in the crystal domain length, which depends on both the value and sign of frequency modulation.     

Surface second harmonic generation from coumarin 343 dye-attached TiO<Subscript>2</Subscript> nanoparticles at liquid–liquid interface

The nonlinear optical properties of coumarin 343 (C343) dye-attached TiO₂ nanoparticles in the size range 5–8 nm adsorbed at the interface of water/1,2-dichloroethane have been studied by using the surface second harmonic generation technique. No second harmonic (SH) response was observed from the bare TiO₂ nanoparticles adsorbed at the interface, however, a strong SH response was measured from the dye molecules attached at the surfaces of the nanoparticles. The increase in the SH intensity with the increase of TiO₂ nanoparticle concentration in the aqueous solution of C343 is mainly due to the pre-alignment of the dye molecules at the surfaces of nanoparticles and is partly due to the third-order polarization contribution of the nanoparticles to the observed total SH response.     

Yellow light generation by frequency doubling of a diode-pumped Nd:YAG laser

We demonstrate the generation of TEM₀₀ mode yellow light in critically type II phase-matched KTiOPO₄ (KTP) with intracavity frequency doubling of a diode-pumped Nd:YAG laser at room temperature. After a 150 μm thick etalon have been inserted into the cavity, the stability and beam quality of the second harmonic generation (SHG) is enhanced. A continuous wave (CW) TEM₀₀ mode output power of 1.67 W at 556 nm is obtained at a pump level of 16 W. The total optical to optical conversion efficiency is about 10.44%. To the best of our knowledge, this is the first Watt-level yellow light generation by frequency doubling of Nd:YAG laser.     

A method of optical implementation of frequency encoded all optical logic gates based on multiphoton processes in non linear material

A novel frequency encoded all optical logic gates are proposed exploiting multiphoton processes in non linear optical medium. In the frequency encoding of the information the ‘0’ is represented by a frequency ω and ‘1’ is represented by another frequency 2ω. The gates proposed are NOT, OR, AND, NAND and NOR among which NAND and NOR are universal. Using these gates one can generate other important gates and logical function generating all optical devices. Two main three-photon processes, second harmonic generation (SHG) and parametric light generation (PLG) are used to implement the gates and the corresponding appropriate non linear material is LiB₃O₅ (LBO) which has wide operating and transparency range in the wavelength 350–3,200 nm. The source of optical frequency encoded signal may be derived from an external cavity diode laser generating a wavelength 1,560 nm for ω (‘0’ state of information) and its second harmonic 780 nm for 2ω (‘1’ state of information).     

Diode-pumped Nd:LGS/LBO laser at 452 nm

We report a blue laser at 452 nm generation by intracavity frequency doubling of a continuous wave (cw) laser operation of a 904 nm Nd:LGS laser under 808 nm diode pumping. A LiB₃O₅ (LBO) crystal, cut for critical type I phase matching at room temperature is used for second harmonic generation (SHG) of the laser. At an incident pump power of 17.8 W, as high as 1.14 W of cw output power at 452 nm is achieved. The optical-to-optical conversion efficiency is up to 6.4%, and the fluctuation of the blue output power was better than 4.1% in the given 30 min.