Photoinduced Second Harmonic Generation Signal Research Articles

Photoinduced second harmonic generation of LaFe4Sb12 near spin fluctuated critical points

AbstractThe temperature dependence of the resistivity, the Seebeck coefficient and photoinduced second harmonic generation (PISHG) are studied near the quantum critical point in the skutterudite compound LaFe4Sb12, possessing increased spin fluctuations. We observed a large maximum of the PISHG at a temperature of about 15 K. The PISHG signal increases substantially below 35 K. We found a correlation between the temperature dependences of PISHG, resistivity and Seebeck coefficient. We proposed a phenomenological explanation for the occurrence of the PISHG signal in LaFe4Sb12 implying strong spin fluctuations exist in this system, which may present some interest for the study of other spin fluctuation systems. Physical insight into the phenomenon observed is grounded in the participation of anharmonic electron–phonon and electron–paramagnon interactions stimulated by inducing light in the interactions with the photoexcited dipole moments. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

IR-stimulated second harmonic generation in Sb2Te2 Se-BaF2-PbCl2 glasses

Theoretically predicted and experimentally observed infraredinduced second-harmonic generation of glasses in the mid-infrared spectral region can be described by fifth-order nonlinear optical susceptibility. The effect is observed in the mid-IR region when the value of the electronic energy gap is comparable to the energies of actual phonons participating in the anharmonic (non-centrosymmetric) electron-phonon interactions. As subjects for investigation, chalcohalide Sb2Te2Se-BaF2-PbCl2 glasses were chosen. They are transparent, over a spectral range of 1.1–10.9 μm. The second-harmonic generation (SHG) output signal within the 1.5–4.8 μm spectral range has significant spectral dependence. Correlation of the SHG spectral maxima positions with spectral positions of anharmonic phonon frequencies confirms that the fifth-order steady-state process occurs due to cascading processes and IR-induced charge density non-centrosymmetry. A maximum value of the SHG is achieved at a pump-probe delay time of 18–36 ps, which is typical for anharmonic electron-phonon interactions. As temperature rises, the values of the photoinduced SHG signal susceptibility increases up to 3 × 10−39 m4/V4. The SHG signal reaches a saturation point for the IR-pump power densities of about 0.8 GW/cm2, which corresponds to the output SHG intensities of about 6 × 10−4 with respect to fundamental one. The values of the diagonal fifth-order tensor component χ(2ω) xxxxx are at least one order of magnitude larger than the off-diagonal tensor components.

Manifestation of hexagonal-like structure in the second-order nonlinear effects in the GaN nanocrystallites

Contribution of the hexagonal-like structural components to the photoinduced second harmonic generation (SHG) in GaN large-sized nanocrystallites (with sizes about the 10– 30 nm ) incorporated into the polyvinylalcohol photopolymer matrices is revealed. The SHG measurements were done using pulsed Nd:YAG laser beam ( λ=1.06 μm ; pulse duration τ=15– 50 ps , laser power about 30 MW ) as a fundamental ones and a picosecond nitrogen pulsed laser (P=10 MW; λ=0.377 μm ; pulse time duration τ=10– 25 ps ) as a photoinducing one. We have found that with increasing pumping power density the SHG output signal increases and achieves its maximum value for the power density about 2.6 GW/ cm 2 per pulse. The maximal output photoinduced SHG signal was achieved for parallel directions of the pumping and fundamental beam polarizations. The maximal values of the second-order nonlinear susceptibilities were equal to about 1.09 pm/ V . We have observed an increase of the output SHG below 30 K and for pump-probe delaying time about 18 ps . Substantial contribution to the SHG of wurtzite-like (hexagonal) structural fragments is shown.

Photoinduced optical second harmonic generation in Fe–Co metallic spin glasses

For the first time, second-order nonlinear optical effects have been observed in metallic spin glasses. Photoinduced second harmonic generation (PISHG) in reflected light was used as a particular nonlinear optical method. We have varied degree of long-range spin ordering. PISHG is measured immediately after the thermo-treatment using thermoannealing treatment (up to 823 K) of Fe–Co glasses. As a photoinducing beam, Q-switched nitrogen laser ( λ=337 nm) was used. YAG–Nd pulse laser ( W=30 MW, τ=30 ps, frequency repetition about 12 Hz) was applied as a probe. Only the presence of the photoexcitation stimulates an appearance of the PISHG with values up to 0.128 pm/V. Influence of surface, phase synchronism conditions, fluorescence, etc. is discussed. We have revealed almost a linear correlation between degree of spin ordering and the output PISHG signal. Molecular dynamics geometry optimization and self-consistent quantum chemical simulations have been done. The observed phenomenon can be explained by contributions of long-range ordered spins to electronic dipole moments determining second-order nonlinear optical susceptibilities. Specially performed X-ray diffractometry structural and magnetically induced nonlinear optical effects show that the observed effects are caused by contribution of collective spin interactions.

Non-linear optical phenomena in the large-sized nanocrystallites

Photoinduced second harmonic generation (PISHG) in SiC nanocrystallites (with sizes ranging from 10 to 30 nm) incorporated into an oligoetheracrylate photopolymer matrix was studied using an experimental photoinducing non-linear optical technique and ab initio molecular dynamics simulations. PISHG measurements were made using a pulse YAG:Nd laser ( λ=1.06 μm ; duration time varying from 4 to 850 ps, laser power about 14 MW) as the probing beam and a nitrogen pulse laser ( P=25 MW; λ=0.377 μm ) as the pumping beam synchronized with a probing beam. We found that with an increase in the nitrogen laser power, the PISHG output signal increased and achieved its maximum value at nitrogen laser photon flux of about 2.6 GW/ cm 2 . Maximum output PISHG signals were observed for parallel polarization of the photoinducing and probing light beams. The second order non-linear optical susceptibility observed at the maximum PISHG was about 1.2 pm/V. The PISHG signals increased as the sample temperature decreased. This increase became marked when the temperature fell below 30 K. Time-dependent probe-pump measurements showed the existence of a PISHG maximum at a time delay of about 20–25 ps. As a result of total energy structure optimization, a good correlation between the size dimensions and the degree of hexagonality within the nanocrystallites was observed. An increase in the degree of hexagonality was also found to cause an increase in the PISHG signal. At a temperature of about 4.2 K, and with a time delay of 25 ps, the maximum PISHG was observed for a degree of hexagonality of approximately 0.76. Our calculations suggest that hexagonal substructures play a key role in the photoinduced effects observed. Several discrepancies between theoretical and experimental data may be caused by specimen in-homogeneity which is usually present in such kinds of materials.

Photoinduced non-linear optical diagnostic of SiN xO y/Si〈111〉 interfaces

Anisotropic (elliptically polarized) photoinduced second harmonic generation (PISHG) in SiN x O y/ Si〈1 1 1〉 films was proposed for contact-less monitoring of specimens with different nitrogen to oxygen (N/O) ratios. As a source for the photoinducing light, we used a nitrogen Q-switched pulse laser at wavelengths of 315, 337 and 354 nm as well as doubled frequency YAG–Nd laser wavelength (λ=530 nm) . The YAG : Nd pulse laser (λ=1.06 μm; W=30 MW; τ=10 – 50 ps) was used to measure the PISHG. All measurements were done in a reflected light regime. We found that the output PISHG signal was sensitive to the N/O ratio and the film thickness. Measurements of the PISHG versus pumping wavelengths, powers, incident angles as well as independent measurements of the DC-electric field induced second harmonic generation indicate the major role played in this process by axially symmetric photoexcited electron–phonon states. The SiN x O y films were synthesized using a technique of chemical evaporation at low pressures. Films with thickness varying between 10 and 30 nm and with an N/O ratio between 0 and 1 were obtained. Electrostatic potential distribution at the Si〈1 1 1〉 –SiN x O y interfaces was calculated. Comparison of the experimentally obtained and quantum chemically calculated PISHG data are presented. High sensitivity of anisotropic PISHG to the N/O ratio and film thickness is revealed. The role of the electron–phonon interactions in the dependencies observed is discussed. We have shown that the PISHG method has higher sensitivity than the traditional extended X-ray absorption fine structure spectroscopic and linear optical method for films with the N/O ratio higher than 0.50.

Photoinduced optical second-harmonic generation of SiNO films deposited on Si〈111〉 substrate

Optical photoinduced second-harmonic generation (PISHG) of SiON films deposited on Si〈111〉 substrates has been studied. Nitrogen laser (λ=337 nm) was used as a source of pumping light. We have found that with an increase of photoinducing power and nitrogen-to-oxygen (N/O) ratio, the PISHG for probe YAG-Nd laser (for the doubled frequency λ=0.53 μm) signal increases and achieves its maximum value at photoinducing power about 0.96 GW/cm2. The maximal value of the PISHG was equal about 1 pm/V. With decreasing temperature, the PISHG signal strongly increases below 28 K. Femtosecond probe-pump measurements indicate the existence of the PISHG maximum at pump-probe time delay about 0.4 ps. We explain these dependencies within a framework of the quantum chemical approach and molecular dynamics interface structure optimization. Role of photoinduced anharmonic electron-phonon interaction is demonstrated. We have revealed that Si–N chemical bonds play a key role in the observed photoinduced nonlinear optics effect.

Photoinduced nonlinear optical phenomena in Sb2Se3-BaCl2-SnCl2 glasses

Photoinduced nonlinear optical phenomena have been studied in amorphous Sb2Te3-CaCl2-SnCl2 glasses using photoinduced optical second harmonic generation (SHG). The photoinduced SHG signal was measured for double frequency of a CO2 laser (λ = 5.3 μm) using a photoinducing CO laser (λ = 5.5 μm). We have found that the SHG signal intensity increases with increase of the CO laser photoinducing exposure and achieves its maximum value after 1.5 h. Absolute values of the SHG signals were more than one order in magnitude smaller comparing to a χ222 tensor component for the ZnSe single crystals. The SHG signal strongly increases if temperature decreases from 39 to 16 K. Femtosecond probe-pump measurements indicate existence of the SHG maximum at pump-probe time delay of about 43 ps. It is suggested that Sb-Se tetrahedra play a key role in the observed photoinduced nonlinear optical effects. Degree of noncentrosymmetry of corresponding bonds depends on time of the CO-laser illumination, temperature and mechanical stresses. Simultaneously, the investigated glasses can serve as promising materials for a femtosecond IR quantum electronics.

Magnetically induced nonlinear diagnostic features of Fe–Co–Si–B metallic glasses

We investigate the influence of an external magnetic field on optical photoinduced second-harmonic generation (PISHG) in Fe(18)Co(60)Si(9)B(13) metallic glass. A correlation between the output PISHG signal and degree of crystallinity (varied by thermoannealing) is observed. The applied magnetic field essentially suppresses the relaxation time of the PISHG. Strong angular dependence of the output PISHG signal reflects the peculiarities of phase synchronism conditions that exist in certain types of glass as a result of light scattering on grains. Saturation of the PISHG at magnetic fields higher then 0.75 T indicates correlation between the magnetically induced ordering and noncentrosymmetry electron-charge density appearance. Comparison of PISHG with x-ray diffraction results indicates that the PISHG results have higher sensitivity to hyperfine magnetic ordering. A possible origin of the observed phenomena is explained on the basis of phenomenological and microscopic approaches.

Photoinduced second harmonic generation of tellurite glass

Second harmonic generation (SHG) in materials with inversion symmetry such as glasses should be forbidden according to nonlinear optics. However, Osterberg et al. [1] reported SHG from GeO2 doped SiO2 glass fibers after the fibers were illuminated with intense 1.06μm laser beam. Later Lawandy et al. [2–5] reported SHG from bulk glasses irradiated with 1.06μm and frequency-doubled laser beams simultaneously. It is commonly considered that the laser irradiation causes second-order nonliner polarization within the glasses, but a thorough clarification of the mechanism for the effect has not been obtained yet. In this letter, the observation of photoinduced SHG from tellurite glass is presented. Tellurite glasses 75TeO2·12.5Li2O·12.5Nb2O5 were prepared by the melting-quenching process. Using TeO2(AR), Li2CO3(AR), Nb2O5(AR) as starting materials, the glass mixture in Al2O3 crucible was melted in an electric furnace at 800∼ 900 ◦C for 0.5 h under stirring. After the glass melt had been cleared at 900 ◦C for 0.5 h, it was cast in a steel mold. The glasses were annealed at 390 ◦C for 3 h and then cooled with the stove. The glasses obtained were pale yellow and transparent. The glasses were fabricated into 1.5 mm-thick platelets with optical quality for SHG measurement. The SHG signal from the glasses was detected through the experimental setup illustrated schematically in Fig. 1. The fundamental laser beam with 10 Hz repetition rate, 40∼ 45 ps pulsewidth and 13 mJ pulse−1 at 1.06μm was obtained from a mode-locked Nd : YAG laser. The fundamental beam was partially transformed into a frequency-doubled beam by a KDP crystal. The dichromatic beam was focused to a spot on the sample by a lens (L1). After irradiating from several minutes to hours, only the fundamental laser beam was allowed to illuminate the sample by removing the KDP crystal in order that the photoinduced SHG signal could be measured. The SHG signal at 0.532μm was received in transmission by a photodiode (D) and displayed by a storage oscilloscope. Two mirrors (HR1, HR2) with high reflection of 1.06μm and a 0.532μm interference filter (IF) were inserted to ensure that only second harmonic radiation was received. Photoinduced SHG signal from the sample was clearly observed. Fig. 2 is the waveform of SHG signal displayed by the storage oscilloscope. The peak value of the signal indicates the relative intensity. No signal was observed when the sample was removed, so the signal was undoubtedly produced from the sample. It was found that the SHG intensity increased with the irradiating time and approached a saturation gradually. Fig. 3 is the time evolution of the SHG intensity for the sample exposed to the dichromatic beams with average power of 120 mW and 10 mW at 1.06μm and 0.532μm, respectively. The irradiating process was interrupted periodically to permit readout of the second harmonic power. The result shows that the SHG intensity rose with the irradiating time and approached a saturation at the time of 0.5 h under our experimental conditions. So far several models have been proposed to explain the origin of the photoinduced second-order nonlinearity in glasses. The most plausible mechanism is considered to be the DC field model [6]. The mechanism involves a zero-frequency polarization produced from

Low temperature anomalies in Ga oxide glasses

Abstract Low temperature anomalies in PbO-Bi2O3-Ga2O3 glasses were observed. Nonlinear optical methods, including photoinduced second harmonic generation (SHG) and piezooptics, were applied. Nitrogen (λ = 337 nm) and ruby (λ = 694 nm) lasers were used as sources of the photoinduced changes. A temperature split of the photoinduced SHG signal was observed at wavelength λ = 1.06μm. Investigations of hyper-Raman light scattering were also performed. A strict correlation between the temperature dependence of the hyper Raman scattering and the nonlinear optical results was found. To investigate the contribution of spontaneous polarization and deformation we performed dielectric and dilatometry measurements. The photoinduced changes can be explained by the strong electron-phonon interaction in such systems.