Second harmonic generation in InSe

Experimental evaluation of non-linear susceptibility coefficient d₂₂ for InSe, which is responsible for second harmonic generation in this material at 1.06 and 10.6 μm, is reported and the value compared with GaSe. At the CO₂ laser frequency of oscillation, phase matching has been observed and a value of d₂₂ almost equal to d₁₅ for CdSe has been found. The InSe high transparency in the wavelength range 1.1–22 μm, the sizeable value of d₂₂ and the possibility of reaching phase matching condition in the IR range make InSe a potential device for far IR frequency conversion.     

Second harmonic generation in graphene

Second harmonic generation in graphene has been theoretically studied. Phenomenological analysis based on symmetry arguments is carried out. It is demonstrated that second harmonic generation in ideal graphene samples is possible only if the radiation wave vector or its magnetic field is taken into account. Microscopic theory is developed for the classical regime of radiation interaction with electrons, where the photon energy is much lower than the characteristic energy of charge carriers. It is demonstrated that the emitted radiation can be strongly circularly polarized for the linearly polarized incident wave.     

Second harmonic generation in order-disorder ferroelectrics

We investigate the temperature dependence of second harmonic generation in order-disorder ferroelectrics. We obtain the qualitative temperature dependence of the second harmonic amplitude; the amplitude goes to zero when the temperature reaches the Curie point. We take into account the disruption of wave number harmony when the temperature is changed. We show the second harmonic is largest at the temperature where the wave numbers are synchronized and there is no detuning. The theoretical conclusions reached here are confirmed by experimental data. Volgogradskii State University, Volgograd. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 55–58, January, 1999.     

Second harmonic generation in squaric acid

From the temperature-dependent optical second harmonic generation in squaric acid we conclude that no local symmetry breaking occurs in the high temperature phase of this material.     

Second harmonic generation in centro-symmetric semiconductors

Second harmonic generation is observed in Si and Ge at electron-hole plasma densities of 10²¹-10²² cm^?3 and found to depend strongly on crystal orientation. We suggest that a lower crystal symmetry within the absorption volume is consistent with a model in which depletion of covalent bond charge, and subsequent lattice softening, is brought about by laser excitation of a high density of electrons into antibonding states.     

Second harmonic generation in deposited clusters

We analyse theoretically the generation of second and higher harmonics for sodium clusters deposited on an insulating surface. To this end, we use the time-dependent local-density approximation solved on a three-dimensional grid. We explore the impact of the various laser parameters (intensity, frequency, polarisation) on the efficiency of second harmonic generation. The success sensitively depends on a proper tuning of these parameters. We find optimum conditions for which the laser frequency is half the resonance frequency of the system, if the polarisation is directed orthogonal to the surface of the substrate, and if the intensity is large but safely below the critical value for destruction of the electron cloud. Received 22 October 1999 and Received in final form 14 December 1999     

Second harmonic generation of fluorine-doped zinc oxide thin films grown on soda-lime glass substrates by a chemical spray technique

Second harmonic generation (SHG) studies of fluorine-doped zinc oxide (ZnO:F) thin films deposited on soda-lime glass substrates from an aged solution in conjunction with zinc pentanedionate, using the chemical spray deposition technique were carried out. The and independent tensorial components of the quadratic nonlinear optical susceptibility of the ZnO:F thin films were evaluated. Scanning electron microscopy and X-ray diffraction investigations revealed a homogeneous distribution of nanoparticles of similar size and morphology for various samples deposited at different substrate temperatures (ranging from 400 to 525 °C). The SHG-technique revealed a clear dependence of the nonlinear optical response with the deposition temperature. Typical optical transmittance and photoluminescence (PL) studies were also performed, from which a bandgap (E_g) of 3.3 eV was evaluated in films deposited under optimal conditions of conductivity and transmittance.     

Second harmonic generation in birefringent optical fibers

Second harmonic generation in optical fibers is investigated. A scheme to combine material, modal and geometrical dispersion to achieve phase matching is proposed. Conversion efficiency, as a function of the fiber's parameters and the incoming electric field strength, is computed.     

Second harmonic generation in Ga nanoparticle monolayers

Ga nanoparticle monolayers formed by evaporation-condensation in ultrahigh vacuum and embedded in a transparent SiO_x matrix generate second harmonic (SH) signals in transmission and reflection when illuminated by a 150 fs, 800 nm laser pulses. The observed SH light exhibits a critical dependence on input and output polarizations, angle of incidence and azimuthal orientation of the samples. The results lead to a consistent picture of shape and orientation of the nanoparticles. Linear transmittance spectra in the visible range support these findings and the observed size dependence of the SH signal. Received 29 November 2000     

Second harmonic generation in planar chiral nanostructures

The effect of the chirality of planar spiral nanostructures on optical second harmonic generation (SHG) is studied in the paper. It is shown that the intensity of left and right circularly polarized SHG differs for structures of different handedness and the direction of the SHG polarization rotation plane is opposite for enantiomers.     

Second harmonic generation in additively coloured NaCl

Optical second harmonic generation has been observed in additively coloured NaCl crystals. The dependence of the effect upon the annealing temperature of the sample and upon the polarization of the exciting beam suggest that small anisotropic colloids are responsible for the nonlinear scattering.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

Second harmonic generation from alkali metals

Nonlinear optical constants of the alkali metals, responsible for the second harmonic generation, are calculated using the Drude model. It is found that the electric quadrupole term of the nonlinear polarization is larger than that of a previous theory of Jha by a major factor of ω²_p/ω². By considering a more realstic electron density distribution, we also obtain an additional dipole term due to symmetry breakdown at the surface. The new nonlinear polarization gives the reflected second harmonic intensity in good agreement with experiments.     

Second harmonic generation at metal surfaces

The nonlinear response of a metal to optical fields is derived for a hydrodynamic model of electron dynamics. The results are used to analyse recent experiments on harmonic generation from a prism-silver film-α quartz sample.     

Second harmonic generation with noncollinear focusing

The propagation of off-axis Gaussian beams through an optical system is described in the paraxial approximation. The formulae for the second harmonic power for stationary focused noncollinear SHG is given. Numerical results and analytical approximations for limiting cases are provided. The theory is applied to the problem of background of noncollinear SHG in pulse duration measurement techniques.     

Second harmonic generation in a polarized Dirac vacuum

The Dirac vacuum, polarized by a strong electric field E₀, is discussed as nonlinear medium for laser radiation (LR). It is shown that such a medium leads to (a) a LR refractive index appearing, (b) LR polarization plane rotation, (c) LR second harmonic generation. It is proposed to use these effects for E₀ field diagnostics.     

Second harmonic generation in the centrosymmetric antiferromagnet NiO

In spite of the fact that inversion is a symmetry operation of both the crystalline and the magnetic lattice of NiO, second harmonic generation (SHG) has been observed below the Néel temperature. A spectroscopic study shows that the signal is due to combined magnetic-dipole and electric-dipole transitions between the (3d)(8) levels of the Ni(2+) ion in the crystal field. The SHG is resonant in both the incoming and the outgoing light waves and thus greatly enhanced. A quadratic coupling of the nonlinear polarization to the order parameter was found. This allows the investigation of individual domains.     

Second harmonic generation in generalized Thue-Morse ferroelectric superlattices

In this paper the second harmonic generation (SHG) in generalized Thue-Morse (GTM(m, n)) ferroelectric superlattices is studied. Under the small-signal approximation, the SHG spectra in both real and reciprocal spaces are investigated. It is found that: (1) only when the structure parameters l, l_A, and l_B are all chosen to be proper, can SHG in GTM(m, n) ferroelectric superlattices be generated; (2) for Family A of generalized Thue-Morse, GTM(m, 1) ferroelectric systems, with the increase of parameter m, the intense peaks of SHG concentrate on the long wavelength (the fundamental beam (FB) wavelength is within ), but for Family B of generalized Thue-Morse, GTM(1, n) ferroelectric superlattices, with the increase of parameter n, the intense peaks of SHG concentrate on the middle wavelength ; and (3) for GTM(m, 1) ferroelectric superlattices, the bigger the m, the stronger the relative integral intensity (RII) of SHG would be, but for GTM(1, n) ferroelectric systems, the bigger the n, the weaker the RII of SHG would be.     

Second harmonic generation from alternating langmuir-blodgett films

We report here the investigation of the second harmonic generation from a non-centrosymmetric alternating Langmuir-Blodgett film of ω-tricosenoic acid and highly polarisable merocyanine dye. Results for films containing one, two and three layers of the merocyanine dye are described.     

Second harmonic generation by cold-deposited silver films

The second harmonic generation (SHG) of light diffusely scattered by cold-deposited silver films is negligible with respect to the specular intensities. Therefore, a cold-deposited silver film is well approximated by a homogeneous effective medium. The intensities of SHG at 1.06 μm and the optical absorption at 0.53 μm depend on both the square of the internal effective field at 0.53 μm and a change in the same way when annealing the film. This can be understood by the assumption that SHG is also mainly following the electric field within the metal phase and surface contributions to SHG interfere destructively. This is corroborated by the observation that the SHG intensity does not show the ‘chemical first layer effects’ seen by surface-enhanced Raman scattering (SERS).