Photoinduced Optical Second Harmonic Generation Research Articles

Optoelectronic operation in ferroic [NH2(C2H5)2]2CuxCo1−xCl4 nanocomposites

The complex studies of optically induced piezooptics and second harmonic generation (SHG) are performed for the [NH2(C2H5)2]2CuxCo1−xCl4 nanocomposites in a form of films and bulks. The corresponding nanoparticles (NP) were embedded into different polymer matrices with content of NP varying from 1 % up to 9 % in weight units. The changes of the photoinduced optical SHG were caused by simultaneous illumination of two bicolour coherent laser beams and simultaneously applied dc-electric field at different temperatures, above glass transition temperature. The fundamental 1,064 nm Nd: YAG laser duration was equal to 12 ns. A comparison of the same features with the analogous crystals doped with Fe2+ was carried out. The samples doped by Fe3+ ions have shown substantially less photo induced SHG up to one order. The time kinetics of the photoinduced changes after the switching off the external laser fields was studied. Temperature dependences of the piezooptics and the SHG near the incommensurate phase existence were explored.

Optical Operation by Chromophores Featuring 4,5-Dicyanoimidazole Embedded within Poly(methyl methacrylate) Matrices

We have studied photoinduced absorption, birefringence, and optical second-harmonic generation in poly(methyl methacrylate) (PMMA) films doped by organic chromophores featuring 4,5-dicyanoimidazole in the weight content equal to 5%. The chromophores indicated as IM1-IM6 were synthesized from 2-bromo-1-methylimidazole-4,5-dicarbonitrile by either nucleophilic substitution or Suzuki-Miyaura cross-coupling reaction. The samples were obtained as films of several micrometers thickness by the spin-coating method on a quartz substrate. Measurements of the optically induced birefringence were done by the Senarmont method at wavelength 1150 nm, and photoinduced absorption was studied in the spectral range 250-700 nm under optical treatment by 300 mW cw 532 nm laser. Photoinduced optical effects were studied by bicolor 1064 and 532 nm coherent laser pulses. The maximal changes were observed for the ratio between fundamental and writing beam intensities equal to about 7:1. To interpret the observed experimental measurements, theoretical simulations of photoinduced optical properties were performed by quantum chemical computational methods.

All-optical operation by optical susceptibilities of Ca4NdO(BO3)3:Yb single crystals

We have found substantial enhancement of the optical second harmonic generation (SHG) in Ca4NdO(BO3)3:Yb (1.9% of Yb in weight units) crystals after 10 min optical treatment by the continuous wave (cw) laser at 532 nm and after 15 min treatment by blue 405 nm cw laser. Treatment by a red laser (660 nm) leads to the suppression of this effect. Principally the effect is only slightly reversible and the successive illumination of the same sample leads to the enhanced SHG, which correlates with changes in the principal absorption bands around 300–330 nm and 400–430 nm corresponding to the optical transitions between the energy levels of Nd3+ ions. The effect does not depend on the photoinducing light polarization. The principal origin of the observed photoinduced optical SHG is caused by photopolarization of the Nd localized levels and formation of the stable polarized states.

Non-linear optical monitoring of melting processes in (TEA)–CoCl 2Br 2 ferroelectric nanocrystallites incorporated into polymer matrices

Photoinduced optical second harmonic generation (PISHG) and differential scanning calorimetry (DSC) methods were used for study of tetraethyl-ammonium (TEA)–CoCl 2Br 2 ferroelectric nanocrystallites (about 50 nm) near melting point. It was shown that PISHG is more sensitive to melting temperature compared to the DSC. This is caused by sensitivity of PISHG due to contribution both of ground electron states as well as of excited ones contrary to the DSC method where principal role is played by ground states.

Nonlinear optical effects in Bi 12TiO 20 nanocrystallites embedded within a photopolymer matrix

Photoinduced nonlinear optical effects in large-sized (up to 20 nm) Bi 12TiO 20 (BTO) nanocrystallites incorporated within olygoether photopolymer matrix have been studied. Photoinduced optical second harmonic generation (PISHG) was measured. Nd:YAG pulsed laser ( λ=1.06 μm) was used as a source of photoinducing light. As a fundamental light source for the SHG and two-photon absorption, Er:LiYF 4 laser ( λ=2.065 μm) was used. We have found that with increasing IR pump power density, the output doubled frequency SHG signal ( λ=1.03 μ m) increases and achieves its maximum value at the pump power density about 1.4 GW/cm 2 and nanocrystallite (NC) sizes about 15 nm. The values of second-order optical susceptibilities were almost 15% smaller compared to d eff tensor of the KDP single crystals. With decreasing temperature below 22 K, the SHG signal increases. Pump–probe measurements indicate the occurrence of the SHG maximum at pump–probe delaying time of about 29 ps. This effect is explained within a framework of photoinduced anharmonic electron–phonon interaction due to the creation of electrostricted phonons. Two-photon absorption was also investigated. Quantum chemical simulations have shown that the TiO 4 structural fragments play a crucial role in the observed NLO effects. The obtained results show that the mentioned nanocomposites can be used as a powerful tool for creation of IR-operated nonlinear optical devices.

Optically-induced non-linear optical effects in indium–tin oxide crystalline films

We have studied thin nanolayers (with thicknesses of about 1–2 nm) between In2O3:Sn (indium–tin oxide (ITO)) crystalline films and glass substrates using the photo-induced optical second harmonic generation (PISHG) method. The maximal PISHG response observed for the pump–probe delay times was about 26 ps. The performed experimental measurements and quantum chemical theoretical simulations show that the observed effects are caused by two factors. The first is caused by the film–glass interface potential gradient and the second is a consequence of additional trapping levels appearing in the energy gap, particularly due to Sn doping. PISHG may be considered as a sensitive tool for such kinds of semiconductors.

Non-linear optical anomalies near the phase transitions in polysiloxane chromophore incorporated within an oligoetheracrylate photopolymer matrix

The photoinduced optical second-harmonic generation (PISHG) in polysiloxane (PX) chromophore incorporated within an oligoetheracrylate photopolymer matrix was investigated as a function of temperature within the 4.2–300 K temperature range. The addition of PX chromophore leads to an enhancement of the local non-centrosymmetric charge density distribution determining optical second-harmonic generation. Two maxima of the PISHG signals were observed at temperatures of ~ = 20 and 126 K. The DSC data give the positions of the maxima at 36 and 132 K. This temperature discrepancy may be a consequence of different contributions of the electron–vibration sub-system to the observed phase transformations. An explanation of the observed temperature anomalies is given within a quantum chemical approach.

Photoinduced nonlinear optics methods as new methods of monitoring the superconducting films

A new method of monitoring the superconductivity in the Hg-1201 films is proposed using the photoinduced optical second harmonic generation (PISHG). In the present work the experimental PISHG results are presented for the Hg-1201 films deposited on the MgO substrates. We have found that the PISHG is more sensitive compared with the traditional methods (resistivity, magnetic susceptibility, etc.). Using of the pump–probe technique allows obtaining additional parameters of the carrier relaxation responsible for the superconductivity. The proposed method has an advantage compared with the others because they are contactless and non-invasive and give a possibility to detect the phase transitions with higher precision.

Photoinduced optical second-harmonic generation in Fe-Co-Si-B metallic spin glasses

For the first time the second-order nonlinear optical effect has 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 the degree of long-range spin ordering detected by magnetoresistance measurements. PISHG is measured immediately after thermotreatment using thermoannealing (up to 823 K) of Fe18Co60Si9B13 glass. As a photoinducing beam a Q-switched nitrogen laser (λ = 337 nm) was used. A YAG:Nd pulse laser (W = 30 MW, τ = 30 ps, and frequency repetition of about 12 Hz) was applied as a probe. Only the presence of the photoexcitation stimulates the appearance of the PISHG with a value up to 0.128 pm V-1. The influence of the surface, phase matching conditions, fluorescence etc, is discussed. We have revealed an almost linear correlation between the degree of spin ordering and the PISHG output signal for different gaseous media. Molecular dynamics geometry optimization and self-consistent quantum chemical simulations have been performed. The observed phenomenon can be explained by contributions of long-range-ordered spins to electronic dipole moments determining second-order nonlinear optical susceptibilities.

Non-linear optical diagnostics of phase transitions in 1,4- cis-polybutadiene polymers

The photoinduced optical second harmonic generation (PISHG) in 1,4- cis-polybutadiene has been investigated as a function of temperature in the vicinity of the glassing and crystallisation temperatures. Both the addition of centrosymmetric tetraphene molecules as well as the trans-isomers lead to an enhancement of the local non-centrosymmetric charge density distribution determining optical second harmonic generation. Using ab initio quantum chemical calculations with the inclusion of electron–vibration anharmonic interactions and combined with experimental PISHG data we show that increasing molecular localisation favours increasing non-centrosymmetry detected by PISHG. Two maxima of the PISHG signals have been observed at temperatures of ≅52 and 153 K for the specimens doped with p-terphenyl molecules and those doped with isomer trans-polybutadiene phase. Good correlation of the experimental data is obtained with theoretical calculations. The main mechanism of the observed temperature anomalies is connected with the enhancement of electron–vibration anharmonicity during glassing and crystallisation structural rearrangements.

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.

Photostimulated optical SHG in BEDO‐TTF polyiodide/polycarbonate composites

Measurements of photoinduced optical second-harmonic generation (PISHG) for the bis(ethylenedioxy)tetrathiofulvalene (BEDO-TTF)/Iodine complex in a polycarbonate matrix were performed. Two kinds of samples were studied: as-obtained, grey-violet material, showing a metal-like temperature dependence of conductivity down to liquid helium temperature (system 1), and electrochemically decoloured, transparent (slightly grey) material, showing metallic conductivity down to about 30 K and semiconductive properties below this temperature (system 2). An increase in the PISHG signal with decreasing temperature was observed below 10 K for both specimens. In the temperature range 40-88 K, an increase then a decrease in PISHG with increasing temperature was observed for specimen 2, while practically no PISHG signal was recorded for sample 1. The dependences obtained indicate unambiguously the essential influence of the BEDO-TTF(l) dopant on the PISHG of polycarbonate composites. Simultaneously, competition was demonstrated between the optical second- and third-harmonic signals at photoinducing fluxes near 1015 photons cm−2. A strong increase in the PISHG signal at low temperatures was observed for both systems (below 11 K for sample 2 and below 7 K for sample 1). In the temperature range 40-88 K a maximum of the PISHG was observed only for specimen 2. At higher temperatures the intensity of the PISHG was very low in both kinds of samples.