Second harmonic generation in In2O3–SiON films doped by Al and Sn

Photoinduced optical and second-order non-linear optical effects in the interfaces separating In₂O₃–SiON (O/N ratio equals 1) films doped with A1, Sn and glass substrates were investigated using the photoinduced optical second harmonic generation. The photoinduced effective second-order optical susceptibility d_eff (at λ=1.76 μm) shows a good correlation with the linear optical susceptibility, particularly with the shift of the absorption edge. The maximal response of the photoinduced optical response signal was observed for the pump–probe delaying time of 34 ps. The performed experimental measurements indicate that the observed effects are mainly caused by the interface potential gradients on the border glass–In₂O₃–SiON film and by additional polarization due to insertion of the Al and Sn atoms. The observed phenomenon may be proposed as a sensitive tool for investigation of thin semiconducting interfacial layers and simultaneously such films may be used as materials for non-linear optical devices.     

Second harmonic generation in a disk shaped quantum dot in the presence of spin-orbit interaction

The second harmonic generation (SHG) coefficients for a disk shaped quantum dot (DSQD) in the magnetic field are studied in the presence of spin-orbit interactions (SOI). The spin-orbit terms we have used in our calculations are both Rashba and Dresselhaus. We have shown that the presence of SOI modifies the SHG terms. In addition, it has been shown that SOI coupling terms influence the spectrum of DSQD resulting in defined changes in the harmonic generation.     

Second harmonic generation: Effects of the multiple reflections of the fundamental and the second harmonic waves on the Maker fringes

The Maker fringes technique is commonly used for the determination of nonlinear optical coefficients. In this article, we present a new formulation of Maker fringes in parallel-surface samples, using boundary conditions taking into account the anisotropy of the crystal, the refractive-index dispersion, and the reflections of the fundamental and the second harmonic waves inside the material. Complete expressions for the generated second harmonic intensity are given for birefringent crystals for the case of no pump depletion. A comparison between theory and experimental results is made, showing the accuracy of our theoretical expressions.     

Second harmonic generation by femtosecond Yb-doped fiber laser source based on PPKTP waveguide fabricated by femtosecond laser direct writing

The frequency doubling of femtosecond pulses from an Yb-doped fiber laser source was demonstrated in a PPKTP waveguide fabricated by femtosecond laser direct writing. The PPKTP waveguide contains a fixed period of 8.9 μm and the feomtosecond fundamental pulses have a central wavelength of 1044 nm. A maximum SHG power of 406 mW was produced, yielding a conversion efficiency of 5.6%. Numerical simulations were carried out to investigate the property of frequency doubling for femtosecond pulses. The results show that the SHG process proceeds even the quasi-phase-matching (QPM) condition is not well satisfied, which is significantly different from that of “long” pulses or CW light and is accorded with the experimental results.     

飞秒脉冲的二次谐波理论和实验研究

本文分析了小信号情况下飞秒脉冲的倍频特性,在非耗尽近似下,得出了飞秒脉冲二次谐波波形及效率的解析解,在时间域内,对一般条件的耦合波方程进行了数值解,分析了相位失配对飞秒脉冲倍频波形及效率的影响。 用0.5mm厚的一类匹配LBO晶体对碰撞锁模激光器产生的80fs超短脉冲进行了腔外倍频实验,实验结果与理论计算符合得很好。     

Spontaneous 2D modulation instability in second harmonic generation process

Two-dimensional modulation instability (2DMI) is experimentally demonstrated in a classical second harmonic generation setup. The spatial spectrum is measured and reveals typical 2DMI bands, in agreement with the analytical MI model. These observations are confirmed by (2 + 1)D numerical simulations.     

Second harmonic generation in layered TunS2 in the vicinity of the low-temperature phase transitions

Second-harmonic generation in the ternary layered semiconductor TlInS₂ excited with the wavelength λ = 1.06 μm of YAG: Nd^{3 +} laser is investigated in the temperature range corresponding to the low-temperature phase transitions. It is shown that the intensity of the second-harmonic signal corresponding to the non-linear coefficients d_eff, d₂₁, d₂₂ reveals peculiarities close to the commensurate-incommensurate phase transitions. The temperature hysteresis of the second-harmonic signal in the low-temperature region (below and close to T_i2 = 206K) is explained assuming that an incomplete lock-in transition in TlInS₂ takes place at T_c1 =204K within the temperature range between a ferroelectric (T_c2 = 201K) and an incommensurate (T_i2 = 206 K) phases. It is shown that the monoclinic point group symmetry C₂ is preserved also at temperatures lower than the phase transition temperature Tc4 = 79K to a weak ferroelectric state.     

Unidirectional second harmonic generation based on electromagnetic induced transparency-like phenomenon derived from standing waves

The nonlinearity and one-way response associated with an electromagnetically induced transparency-like (EIT-like) phenomenon based on a standing wave are theoretically analyzed and experimentally measured. The dark-meta-atom's strong energy localization results in strong second harmonic generation (SHG), while input for the opposite direction elicits no EIT-like response and a 20 dB decrease in SHG strength. The second-order standing wave can be tuned to affect the efficiency of SHG. The results presented here may represent a novel approach to designing nonreciprocal devices such as high signal-to-noise ratio electromagnetic diodes.     

Computing for second harmonic generation in one-dimensional nonlinear photonic crystals

A numerical study of second harmonic generation (SHG) in one-dimensional nonlinear photonic crystals based on full nonlinear system of equations, implemented by a combination of the method of finite elements and fixed-point iterations, is reported. This model is derived from a nonlinear system of Maxwell’s equations, which partly overcomes the known shortcoming of some existing models relied on the undepleted-pump approximation. We derive a general solution of SHG in one-dimensional nonlinear photonic crystals structures. The convergence of our method is fast. Numerical simulations also show the conversion efficiency of SHG can be significantly enhanced when the frequencies of the fundamental wave are located at the photonic band edges or are assigned to the designed defect states.     

Two-photon resonance in optical second harmonic generation from quantum well states in ultra-thin Ag films grown on Si(1 1 1) surfaces

We studied optical second harmonic generation (SHG) oscillations during the growth of Ag films on Si(1 1 1) 7 × 7 clean and H-terminated surfaces. In the growth on the 7 × 7 surfaces at room temperature, the second and third peaks of the oscillation shift towards the thinner side with an increase in pump photon energy. Our analysis revealed that these peaks are caused by two-photon resonant transitions from the n = 1 and 2 occupied quantum well states (QWSs) in the Ag film to the Ag/Si interface at 1.9 eV above the Fermi level (E_f). In Ag growth on the hydrogen-terminated surfaces, the SHG oscillation was similar to that on the 7 × 7 surfaces at room temperature. However, the QWS-related peak was suppressed in the growth at 300 °C. This is attributed to an inhibited intrusion of the interface state into the Ag layers.     

Second harmonic generation from closed packed hexagonal structure of nano-nickel arrays

A closed packed hexagonal structure of nickel nanoparticles was fabricated by deposition of thin nickel films on self-assembled polystyrene micron spheres, which were made of nanospherical lithography in conjunction with left-off the micro spheres. The optical second harmonic generation from these periodically arrayed nickel nanoparticles reveals intensity with a value much larger than that generated from a uniform nickel-film of the same thickness. Quasi phase matching contributed from Bragg wave vectors of the periodic arrayed nickel nanoparticles can satisfactorily express this enhancement.     

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.     

Optical second harmonic generation in asymmetric double triangular quantum wells

The second harmonic generation (SHG) in the asymmetric double triangular quantum wells (DTQWs) is investigated theoretically. The dependence of the SHG coefficient on the right-well width of the DTQWs is studied, and the influence of the applied electric field on SHG coefficient is also taken into account. The analytical expression of the SHG coefficient is analyzed by using the compact density-matrix approach and the iterative method. Finally, the numerical calculations are presented for the typical GaAs/Al_xGa_1−xAs asymmetric DTQWs. The results show that the calculated SHG coefficient in this coupled system can reach the magnitude of 10⁻⁵ m/V, 1–2 orders of magnitude higher than that in step quantum well, and that in double square quantum wells. Moreover, the SHG coefficient is not a monotonic function of the right-well width, but has complex relationship with it. The calculated results also reveal that an applied electric field has a great influence on the SHG coefficient. Applying an appropriate electric field to a DTQW with a wider right well can induce a sharper peak of the SHG coefficient due to the double-resonant enhancement.     

Electrical second harmonic generation by TGSe and TGS-Se in the autostabilized state

Second harmonic generation and TANDEL effect studies on ferroelectric triglycine selenate and triglycine sulphate-Selenate crystals near the Curie temperature indicate that the generated second harmonic is linear for low biasing fields with a zero off-set, while it decreases sharply at higher biasing fields. In the autostabilized state, the TANDEL elements adjust their impedance against the variation of a.c. field. The experiments on annealed crystals establish that the zero offset is due to the internal bias that owes its origin to the defect structure.     

Broadband second harmonic generation of an optical frequency comb produced by four-wave mixing in highly nonlinear fibers

We demonstrate broadband second harmonic generation of low-energy pulses produced by injecting two single-frequency lasers into a highly nonlinear fiber. Full nonlinear conversion of the corresponding spectra, consisting of broadband (∼200 nm) optical frequency combs at ∼1580 nm, were obtained by using conventional birefringence phase-matching in two BIBO crystals (2-mm and 100-μm long) with a normal incidence configuration. The crystals were not tilted and the pulses were not compressed. This broadband conversion results from the large phase-matching bandwidth of the nonlinear BIBO crystals at ∼1550 nm, but also seems to be a consequence of a fundamental comb with small spectral phase variation.     

Blue laser light generation by intracavity frequency doubling of Cesium vapor laser

Blue laser light with a wavelength 447 nm was generated by intracavity frequency doubling of a Cs laser in PPKTP crystal. A continuous wave power of 600 mW was obtained at an optical-to-optical efficiency of 4%. In the pulsed operation, when thermal effects were reduced, the efficiency obtained was 14% and the peak blue power was 2.1 W.     

Third harmonic generation by hot electrons in metals

The weak nonlinear response of a metal to the action of low-intensity laser radiation was studied under the normal skin-effect conditions. We have shown that temperature variations with a double frequency emerging during the electron heating lead to the generation of the third harmonic of the fundamental wave. The density of the radiation flux at the tripled frequency was calculated. By measuring this flux one can determine the frequency of the electron-electron collisions with the umklapp of the quasimomentum. Talk presented at the oral issue of J. Russ. Laser Res. dedicated to the memory of Professor Vladimir A. Isakov, Professor Alexander S. Shumovsky, and Professor Andrei V. Vinogradov held in Moscow February 21–22, 2008.     

上海深紫外自由电子激光的非线性谐波辐射研究

 自由电子激光中的非线性谐波辐射能达到较高的谐波功率,可以用来得到短波长辐射或者降低第4代先进光源对电子束团品质的严厉要求。基于3维自由电子激光软件,深入详细地研究了上海深紫外自由电子激光装置的非线性谐波辐射,并且提出了谐波辐射实验和测量建议。研究表明上海深紫外自由电子激光装置3次非线性谐波辐射的功率可以达到基波功率的2%水平。     

Bohmian mechanics to high-order harmonic generation

This paper introduces Bohmian mechanics (BM) into the intense laser-atom physics to study high-order harmonic generation. In BM, the trajectories of atomic electron in an intense laser field can be obtained with the Bohm--Newton equation. The power spectrum with the trajectory of an atomic electron is calculated, which is found to be irregular. Next, the power spectrum associated with an atom ensemble from BM is considered, where the power spectrum becomes regular and consistent with that from quantum mechanics. Finally, the reason of the generation of the irregular spectrum is discussed.