Efficient single-pass third-harmonic generation from 1560 nm to 520 nm for pumping doubly-resonant OPO

A ~545 mW single-frequency tunable 520 nm green laser has been demonstrated using a periodically-poled potassium titanyl phosphate (PPKTP) bulk crystal based on single-pass third-harmonic generation (THG) of a 1560 nm laser via single-pass second-harmonic generation (SHG) followed by single-pass sum-frequency generation (SFG). In single-pass SHG, two cascaded periodically-poled magnesium-oxide-doped lithium niobate (PPMgO:LN) crystals were used, and ~3.5 W of 780.25 nm doubled laser output is produced, corresponding to maximum doubling efficiency of 26.8%. The system can provide a pump source (520 nm) for an optical parametric oscillator for two-colour entangled continuous-variable optical field generation at 1560 and 780 nm and two-colour local oscillators for homodyne detection.     

Nonresonant recirculating type II second-harmonic generator

We show an experimental proof of concept for a nonresonant recirculation method to increase the conversion efficiency of second-harmonic generation (SHG) with type II phase matching. As much as a factor-of-4 efficiency increase compared with that of single-pass SHG is possible, provided that the recirculation length is within the coherence length of the pump laser. Nonresonant recirculating SHG may be valuable in systems in which intracavity doubling is not practicable, such as high-power cw bulk solid-state or fiber lasers.     

Tunable optical frequency comb generation in a periodically poled lithium niobate waveguide based on stimulated Brillouin scattering

We propose and demonstrate a technique for the generation of an optical frequency comb (OFC) in a periodically poled lithium niobate (PPLN) waveguide based on stimulated Brillouin scattering. A single continuous wave laser is sent to a multi-wavelength Brillouin erbium fibre laser for generating a set of coherent and phase-locked multi-wavelength spectral lines. They are injected into a PPLN waveguide to obtain an OFC. We investigate the influence of the cavity structure on the OFC property in our two different schemes. The number of comb lines is affected by the 980 pump current and Brillouin pump power. The OFCs are tunable in a large range by changing their central wavelength.     

Probing and modeling of carrier motion in organic devices by optical second harmonic generation

We here report a novel optical second harmonic generation (SHG) measurement that allows an electric field formed in organic solid to be probed. We examined the SHG intensity profile that changes depending on a space charge field caused by carrier injection. Experiments making use of time-resolved SHG technique has revealed dynamic changes of SHG intensity profiles arising from pentacene, and that carrier transport in OFET was diffusion-like. Calculations using drift-diffusion equation well accounted for the visualized carrier motion probed by time-resolved SHG. That is, interface charge propagation process dominates the carrier transport, which is regulated by applied gate-source voltage.     

Large enhancement of second harmonic generation in highly cerium doped lead-phosphate glass

The influence of lead and cerium concentration on photoinduced second harmonic generation (SHG) in lead-phosphate glasses has been studied. It has been shown that SHG can be considerably enhanced by increasing of lead and cerium concentration. We also have found that cerium greatly extends the induced nonlinearity lifetime. Physical mechanism of the phenomenon is discussed.     

Broadband second harmonic generation in an imperfect nonlinear photonic crystal with random defects

In this paper, we study broadband second harmonic generation (SHG) in an imperfect nonlinear photonic crystal in which defects are introduced with random lengths. We show that the efficient SHG output is obtained when the length of each defect varies near certain specialized values. The bandwidth of the SHG output broadens with the increasing randomness of defect length. Moreover, the SHG bandwidth is nearly unaffected only when the total length of the whole structure is long enough. The disordered structure also exhibits good tolerance to the fabrication error, which provides a way to control SHG intensity and bandwidth separately.     

二次谐波在二维材料结构表征中的应用

二次谐波作为非线性光学的重要分支, 逐渐成为表征晶体结构的重要手段之一。在众多表征方法中, 二次谐波因其无损检测、高稳定性、可调谐性、超快响应、偏振敏感性、通用性、操作简单等特点被广泛应用于二维材料结构表征, 为二维材料的物性研究和功能应用提供了重要信息, 大大推动了二维材料基础研究的快速发展。本文综述了近几年二次谐波在二维材料结构表征中的研究, 简述了二次谐波产生原理, 介绍了飞秒激光器接入共聚焦拉曼光谱仪产生二次谐波测试装置, 分别讨论了二次谐波在二维材料的层间堆垛层数、层间堆垛角度、单层二维材料晶界及晶体取向表征方面的应用。同时, 本文还介绍了采用二次谐波强度直接、灵敏地检测晶体中应变幅度以及通过二次谐波信号变化跟踪材料中的缺陷变化, 接着讨论了二次谐波与拉曼光谱、光致发光的多维度关联分析在材料全面深度表征方面的重要性。最后展望了二次谐波未来在材料结构表征中的潜在研究方向。     

Non-linear optical effects in layer-by-layer alternate films of polycations and an azobenzene-containing polyanion

The technique of polycation/ polyanion layer-by-layer adsorption was applied to fabrication of non-linear optical films. Optical second-harmonic generation (SHG) was clearly observed from the films prepared by alternate adsorption of polycations and an azobenzene-containing polyanion (PAZO) on quartz substrate modified with polyion precursor. Regular film growth during adsorption cycles was proved by means of UV spectroscopy and quartz crystal microbalance technique. The SHG signal increased with the number of PAZO layers up to four layers, while further assembly reduced the signal. The SHG decreased considerably by heating the film up to 120 °C.     

Optical second harmonic generation at platinum phthalocyanine-modified platinum electrodes

Optical second harmonic generation (SHG) has been used to observe changes within electronically conducting platinum phthalocyanine (PtPc) films deposited on polycrystalline platinum electrodes as the film undergoes electrochemical modification. PtPc-modified electrodes produced enhanced SHG responses over bare platinum surfaces. This is proposed to be due to an electric quadrupole contribution from the PtPc molecule. Examination of the polarization dependence of the SHG response reveals that electroactive PtPc molecules exist in environments both parallel and perpendicular to the electrode surface. It has been possible to observe redox processes occurring within the films by monitoring the magnitude of the SHG response with variations in potential. The decrease in SHG signal has been shown empirically to be proportional to the charge removed from the film during oxidation of the phthalocyanine (Pc) ring.     

Near-Unity Polarization of Valley-Dependent Second-Harmonic Generation in Stacked TMDC Layers and Heterostructures at Room Temperature

With unique valley-dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second-harmonic generation (SHG) in 2D TMDCs monolayers has shown valley-dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion-symmetry-breaking (3R-like phase) TMDCs (WSe₂, WS₂, MoS₂) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular-polarization-resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley-dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.     

Numerical simulation of cascaded third harmonic generation for both phase matched and mismatched schemes

Numerical solutions to the nonlinear coupling-wave equations of second harmonic (SH) and third harmonic (TH) generators are investigated for both phase matched and phase mismatched configurations. For phase mismatched TH generation, several kinds of schemes (the phase mismatch either in second harmonic generation (SHG) or sum-frequency generation (SFG) process) are considered and analyzed. The physical nature corresponding to the different ratios of the coupling coefficients is discussed.     

Stable high-power green light generation with a periodically poled stoichiometric lithium tantalate

Periodically poled stoichiometric lithium tantalate was used for an efficient high-power coherent green light generation based on second-harmonic generation (SHG) using a Q-switched Nd:YVO₄ laser. Achieved conversion efficiency was 69% that corresponds to SHG power density of 21.6 MW/cm² at the maximum input fundamental peak power density of 31.5 MW/cm². The SHG power fluctuation was less than 3.4% for 35 h, which was mainly caused by input power fluctuation of 2.6%. Obtained SHG powers of 4.4 and 3.4 W were sufficiently stable without spatial beam distortion at room temperature and a phase matching temperature adjustment was required precisely at high input power.     

Second harmonic generation from a ferroelectric film

We derive an expression for transmittivity (T_SHG) of second harmonic generation (SHG) signals from a ferroelectric (FE) film. Intensities of up and down fields in the medium are investigated in relation to T_SHG. The derivations are made based on undepletion of input fields and nonlinear wave equation derived from the Maxwell equations. We present two cases: film without mirrors and with partial mirrors. Expressions for the newly derived nonlinear susceptibility coefficients of SHG for real crystal symmetry [J. Opt. Soc. Am. B 19 (2002) 2007] are used to get more realistic results. Variations in T_SHG with respect to film thickness are illustrated.     

Nonlinear optical responses in two-dimensional photonic crystals

A two-dimensional (2D) highly nonlinear lithium niobate (LN) photonic crystal (PhC) waveguide is fabricated with the aim of studying its nonlinear optical properties. We show a large enhancement of the second-harmonic generation (SHG) in the 2D LN PhCs, originating from resonance between the external pump laser field and a photonic band mode. The SHG enhancement results agree well with the experimental photonic band structure obtained by an angle-dependent optical reflectivity and the theoretical band structure generated by three-dimensional finite-difference time-domain calculations. These results open new possibilities for the use of 2D LN PhC waveguide in integrated nonlinear optical applications.     

Second harmonic generation investigation on electric poling effects in fused silica

The influence of electric of poling conditions on the optical nonlinearity of fused silica has been investigated by the second harmonic generation (SHG) technique. The SHG intensity of the poled fused silica increased monotonically with increasing the poling voltage or poling time, and reached a maximum at a poling temperature of T = 250°C, but the SHG intensity decreased quickly as the thermal erasure time increased. The experimental results have been explained using an electric field induced dipole orientation model.     

Hole trapping in polydiacetylene field effect transistor studied by optical second harmonic generation

Hole trapping in polydiacetylene field effect transistor (PDA-FET) was studied by the electric field induced second harmonic generation (EFISHG). Response of SHG signal from PDA-FET with an application of external voltage was monitored. Applying positive voltage to source and drain electrodes with respect to gate electrode, SHG signal was not observed during bias application, whereas the signal was enhanced after turning off the bias. Since positive bias promotes hole injection from source and drain electrodes, electric field formed by trapped holes in PDA layer activated the SHG signal. Microscopic SHG measurement implies that the trapped holes are concentrated around source and drain electrodes.     

Perturbation analysis of optical anisotropy in organic molecular films

Dipping-induced in-plane anisotropy in organic molecular films is characterized by use of second-harmonic generation (SHG) measurement and analyzed by use of a new perturbation method. We derived theoretical formulas for the dependence of SHG intensity on the polarizations of an incident fundamental beam, an output second-harmonic beam, and an azimuthal angle of samples, which agreed well with our experimental measurements.     

Multi-wavelength Q-switched Erbium-doped fiber laser with photonic crystal fiber and multi-walled carbon nanotubes

A simple multi-wavelength passively Q-switched Erbium-doped fiber laser (EDFL) is demonstrated using low-cost multi-walled carbon nanotubes (MWCNTs)-based saturable absorber, which is prepared using polyvinyl alcohol as a host polymer. The multi-wavelength operation is achieved based on non-linear polarization rotation effect by incorporating 50?m long photonic crystal fiber in the ring cavity. The EDFL produces a stable multi-wavelength comb spectrum for more than 14 lines with a fixed spacing of 0.48?nm. The laser also demonstrates a stable pulse train with the repetition rate increasing from 14.9 to 25.4?kHz as the pump power increases from the threshold power of 69.0?mW to the maximum pump power of 133.8?mW. The minimum pulse width of 4.4?μs was obtained at the maximum pump power of 133.8?mW while the highest energy of 0.74 nJ was obtained at the pump power of 69.0?mW.