Anharmonic Electron-phonon Interaction Research Articles

Nonlinear IR absorption in europium- and thulium-doped oxide glasses

Infrared (IR)-induced two-photon absorption (TPA) in synthesized PbO–Ga2O3–Bi2O3–CdO (PGBC) glasses doped by Eu3+ and Tm3+ rare earth (RE3+) ions was investigated. Temperature-dependent measurements of TPA were carried out in the mid-IR spectral range. A CO pulsed laser (λ = 5.5 μm, energy power density up to 3.8 GW/cm2 per pulse with picosecond pulse duration) was applied as the source of photoinducing IR radiation. An optical parametrical generator cut from a proustite crystal was used as the source of the fundamental TPA beam within the 3–7.5 μm spectral range. Absolute values of the TPA coefficients were more than 13% higher than corresponding values obtained for other similar glasses (As2Te3–CaCl2–PbCl2 [1] or Sb2Se3–BaCl2–PbCl2 [2]). The PGBC system investigated possessed a shorter time response (about 12 ps) compared with other IR nonlinear optical glasses. It was also established that all nonlinear optical susceptibilities were dependent on the type of RE3+ ion. A maximum value of the TPA was achieved for the glasses doped by Tm3+. The TPA values increased significantly below 45 K. Ab initio molecular dynamics and quantum chemical simulations were performed in order to evaluate the possible role of electron–phonon anharmonic interactions in the observed phenomena. The influence of the external CO photoinduced pump beam through the photoinduced electrostricted anharmonic electron–phonon interactions was simulated. A decrease of picosecond delaying time response compared with other glasses was achieved. To obtain independent confirmation of the observed dependencies, measurements were carried out of χxxxx(E) during the external IR pumping. The measured and theoretically calculated dependencies of the third-order susceptibilities were also compared. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical response, precursor effects, charge transfer and competing interactions in perovskite ferroelectrics

Ferro- and anti-ferroelectricity observed in perovskite oxides has been shown to stem from the unusual polarizability of the doubly negatively charged oxygen ion O 2− which is unstable as a free ion. In a crystal this instability is only partially compensated by the Madelung potential of the surrounding ions, which induces a strong volume, temperature and structure dependent polarizability of the oxygen ion. The giant changes observed in the polarizability have been shown to stem from the delocalisation of the outer p-electrons by means of which dynamical hybridisation effects occur also in nominally ionic systems. Modelling these compounds within an anharmonic electron–phonon interaction Hamiltonian leads to new interesting effects as precursor domain formation, competing length and time scales, structural and electronic anomalies which are discussed in the following.

Giant increase of the second harmonic radiation's absorption during optical poling of oxide glass

A giant reversible growth (≥ 103 times) of the absorption of second harmonic radiation by optical poling of oxide glass was discovered. The observed absorption exerts an influence on the optical poling process and leads to a restriction of the maximum value of the photoinduced second harmonic generation in oxide glass. A theoretical model of the observed phenomenon is given. The growth of absorption is considered as a result of special anharmonic electron-phonon interactions caused by photoinduced electrostricted phonons. Exploration of the total absorption indicates that the effects of photodarkening do not play a key role in the observed phenomena.

UV-induced kinetics of second harmonic generation in CdI 2–Cu layered nanocrystals

Substantial increase (up to 0.40 pm/V) of the second-order susceptibility was observed in the CdI 2–Cu single layered nanocrystals for the Nd:YAG laser beam λ=1.06 μm. The Cu impurity and nano-sized layer’s thickness of the cleaved crystals (about several nanometers) seem to play a crucial role in the observed phenomenon. The temperature dependence of the optical second harmonic generation (SHG) together with its correlation with Raman spectra of low-frequency modes indicate a key role of the UV-induced anharmonic electron–phonon interactions in the observed effect. The maximal output UV-induced SHG was achieved for the Cu content about 0.5% and at liquid helium temperatures.

Optically induced second-harmonic generation in CdI2–Culayered nanocrystals

A large enhancement (up to 0.40 pm V−1)of the second-order optical susceptibility was observed in CdI2–Cusingle-layered nanocrystals for the Nd:YAG fundamental laser beam λ = 1.06 µm.The Cu impurity content and nanolayer thickness of the cleaved layers (aboutseveral nanometres) play a crucial role in the observed effect. The temperaturedependence of the optical second-harmonic generation (SHG) together with itscorrelation with Raman spectra of low-frequency modes indicate a keyrole for the UV-induced anharmonic electron–phonon interactions in theobserved effect. The maximal output UV-induced SHG was achieved for aCu content of about 0.5% and at liquid helium temperatures.

Mid-infrared light-induced second-harmonic generation in specific glasses

We have theoretically predicted and experimentally confirmed a possibility of optically induced second-harmonic generation (SHG) for glasses in the middle-infrared (IR) spectral region described by fifth-order nonlinear optical susceptibility. This effect may be observed in the middle-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 (EPIs). As a subject for the investigations we have chosen Sb2Te3–BaF2–SrBr2 glasses which are transparent in the range of 0.8–10.1 µm. The SHG output within the 1.5–4.8 µm spectral range demonstrates a crucial spectral dependence. Correlation of the SHG spectral dependences with spectral positions of the anharmonic phonon mode density confirms that a fifth-order steady-state process occurs from both the cascading processes and IR-induced charge density non-centrosymmetry. A maximal value of the photoinduced SHG is achieved at a pump–probe delaying time of about 20 ps which is typical for the anharmonic EPI.

Low-temperature anomalies of photoinduced second harmonic generation inskutterudites

Photoinduced second harmonic generation (PISHG) was found in skutterudite compounds of CeFe4Sb12 and Ce0.7Fe3.5Ni0.5 Sb12.Measurements versus temperature, pump–probe delaying time andexternal magnetic field were performed. The studied compoundsbelong to moderate heavy fermion compounds (HFC) in the groundstate. The PISHG signals appear at 6.8 and 4.9 K for CeFe4Sb12 and Ce0.7Fe3.5Ni0.5Sb12,respectively. We suspect that these signals are due to anharmonic electron–phononinteractions creating a charge density non-centrosymmetry. The observed effectsare caused either by a possible phase transition or by drastic changes inthe electron structure of the HFC with decreasing temperature.

Photoinduced nonlinear optics in Ca4GdO(BO3)3 nanocrystallites embedded within a polymer matrix

The photoinduced nonlinear optical effects in large-sized (up to 20 nm) Ca4(GdO(BO3)3) GdCOB nanocrystallites incorporated within an oligoether photopolymer matrix have been studied. The photoinduced optical second harmonic generation (SHG) was measured at the fundamental wavelength 1.06 μm. A nitrogen pulsed laser (λ=0.337 μm) was used as the source of photoinducing light. We found that with increasing ultraviolet (UV) illumination power density, the output doubled frequency SHG signal (λ=0.53 μm) increases and achieves its maximum value at a UV power density about 1.4 GW/cm2 and nanocrystallite size about 5.5 nm. The absolute SHG values were about 15% less than for the d222 tensor of potassium dihydrogen phosphate single crystals. With decreasing temperature below 18 K, the SHG signal increases. Probe-pump measurements indicate the occurrence of a SHG maximum at a pump-probe delay time of about 27 ps. This effect is explained within the framework of the anharmonic electron-phonon interaction due to creation of photoinduced electrostricted phonons. Third-order two-photon absorption was also investigated. We showed that the BO3 structural fragments play a crucial role in the observed photoinduced effects. The results obtained show that the mentioned composites can be used as a powerful tool for creation of UV-operated nonlinear optical devices.

Thermoelectric power of MgB 2

Thermoelectric power (TEP) of MgB 2 and its changes with the superconducting transition temperature ( T c) have been investigated. MgB 2 samples synthesized with different techniques––sample 1 by the encapsulation method and sample 2 by the high pressure and high temperature technique––showed different T c ( T c of sample 1= 38.7 K, and T c of sample 2 = 40.4 K). The characteristic features of the TEP of these samples––low temperature linear behavior, deviation from the low temperature linear behavior at a certain temperature, and an almost saturating value at ∼300 K––were found to vary systematically with T c, consistent with the previously reported TEP changes in Mg 1− x Al x B 2. These changes of the TEP behaviors with T c in the superconducting diborides system are discussed in comparison with those of high T c cuprates and bismuthates. The common feature of the saturating TEP at high temperatures suggests the importance of an anharmonic electron–phonon interaction.

Synthesized rare-earth doped oxide glasses for nonlinear optics

Photostimulated nonlinear optical effects in synthesized PbO–Ga2O3–Bi2O3–CdO (PGBC) glasses doped by Yb3+, Er3+, and Dy3+ rare-earth (RE3+) ions were discovered. Temperature-dependent measurements of optical photoinduced second-harmonic generation (PISHG) and two-photon absorption were performed in the infrared (IR) range. CO pulse laser (λ=5.5 μm, energy power density up to 3.8 GW/cm2 per pulse) was applied as a source of IR-photoinducing and probing (fundamental) light. Absolute values of the PISHG were more than 22% higher than corresponding values obtained for other glasses: As2Te3–CaCl2–PbCl2 [1] or Sb2Se3–BaCl2–PbCl2 [2] type. The investigated PGBC system possesses a shorter time response (about 18 ps), compared with other IR nonlinear optical glasses. We have also established that all nonlinear optical susceptibilities are dependent on the type of RE3+ ion. A maximal value of the PISHG is achieved for the glasses doped by Yb3+. The PISHG values increase significantly below 25 K. We have carried out ab initio molecular dynamics and quantum chemical simulations in order to evaluate the possible contribution of electron–phonon anharmonic interactions in the observed phenomena. We have modeled the influence of the external CO photoinduced beam through the photoinduced anharmonic electron-phonon interactions. A decrease of the delaying time response is achieved. To obtain independent confirmation of the observed dependencies, we have carried out measurements of the ε2(E) during the external IR pumping. We have also compared the measured and theoretically calculated dependencies of the IR-induced effects.

Charge-density-wave formation in TiSe2 driven by an incipient antiferroelectricinstability

Recently reported temperature-dependent synchrotron x-ray thermal diffusescattering data (Holt M, Zschack P, Hong H, Chou M Y and Chiang T C 2001Phys. Rev. Lett. 86 3799) are analysed within an anharmonic electron–phononinteraction model, where the observed zone boundary phonon softening is relatedto an incipient antiferroelectric instability. The parameter-free model reproducesthe temperature dependence of the soft phonon mode in excellent agreement withthe experimental data. Predictions for the charge-density-wave-induced gap aremade and consequences for the electronic structure changes are discussed.

Optimized structure of SiNxCy–Si⟨111⟩ interfaces

SiNxCy films were synthesized using a technique of chemical evaporation at low pressure. We have carried out quantum chemical and molecular dynamics simulations of the film structure versus the nitrogen/carbon ratio. The film thickness varied between 8–24 nm. The stoichiometry of these films was measured after their deposition on Si[111] substrates using an extended x-ray absorption fine structure (EXAFS) method. A comparison was made of the experimentally obtained and molecular dynamics optimized structure. At the same time, pressure derivatives of the total energy within the calculated local density approach agree well with the experimental data. Sufficient agreement between the experimental EXAFS data and the simulated structural data indicates the crucial role of anharmonic electron–phonon interactions.

IR-poled second-harmonic generation in glass

Infrared (IR)-induced second-harmonic generation in the chalcogenide glasses is observed. A phenomenological approach of IR picosecond non-linear optical (NLO) response in glass is developed for the middle IR spectral range (5– 15 μm ). The observed effect is explained within the framework of fifth-order NLO susceptibilities. A model that reproduces the basic characteristics of the experimental data, in which the optical non-linearities caused by photoinduced electron–phonon anharmonic interactions, is proposed. The role of the IR-induced phase matching conditions in the observed phenomenon is discussed.

Photoinduced second harmonic generation in Bi 2Se 3–CaBr 2–PbCl 2 optical fibers

Photoinduced second harmonic generation (SHG) in Bi 2Se 3–CaBr 2–PbCl 2 optical fibers have been studied using experimental and theoretical quantum chemical and molecular-dynamics methods. Infra-red (IR) induced SHG was measured in the IR region within the 5.5–21 μm . CO laser ( λ=5.5 μm ) was used as a source of the photoinducing IR light. The glasses had diameters varied within the 5–35 μm . The lengths of the fibers varied from 0.5 cm up to the 1.4 m. We have found that with increasing photoinducing power, the SHG for the doubled frequency 5.3 μm of the CO 2-laser increases and achieves its maximum value at photoinducing power 1.35 GW/ cm 2 per pulse. The maximal by achieved values of the second-order nonlinear optical susceptibilities d 222 was equal about 1.5 pm/V. With decreasing temperature, the SHG signal strongly increases within the 75–220 K temperature range. Picosecond probe–pump measurements indicate on the occurrence of the SHG maximum at pump–probe time delay about 25 ps. These dependencies we explain within a framework of the quantum chemical approach taken into acount photoinduced anharmonic electron–phonon interactions. We have found that Bi–Se tetrahedral bonds play a key role in the observed IR photoinduced nonlinear optical (NLO) effects. The obtained results show that the mentioned glasses may be applied as materials for IR picosecond–second quantum electronics.

Phonons, charge ordering and high- Tc superconductivity in cuprates

High temperature superconducting copper oxides are modelled within an anharmonic electron–phonon interaction Hamiltonian, where the anharmonicity is shown to stem from the coupling of the different structural elements as, e.g. c-axis and planar components. The linear and higher order electron–phonon interaction terms induce dynamical incommensurate nanoscale correlated displacement patterns in the oxygen ion displacements, which, in turn, lead to dynamical charge density formation prior to superconductivity and coexistence of both. The high superconducting transition temperatures T c are shown to result from the effect of the charge density wave induced gap on the single particle energies together with multiphonon induced interband interactions.

Photoinduced electronic structure and nonlinear susceptibilities in Y–Ba–Cu–O ceramics

Photoinduced second harmonic generation in Y–Ba–Cu–O ceramics was investigated. An analysis of the observed effect was performed based on quantum chemical modeling taking account of electron–phonon anharmonic interactions. Using band energy structure, calculations we have revealed a correlation between photoinduced electron charge density redistribution within the Cu–O octahedra of the Y–Ba–Cu–O ceramic and appropriate second-order nonlinear optical susceptibilities. Other structural fragments play only a minor role in the observed photostructural changes. The possibility of using the photoinduced optical second harmonic generation for detection of superconducting phase transition is demonstrated.

CsLiB 6O 10 crystallites embedded into the photopolymer matrices as promising materials for acoustically induced electrooptics

Acoustically induced linear electrooptics effects (LEOE) in CsLiB 6O 10 (CLBO) nanocrystallites incorporated into the olygoether photopolymer matrices have been revealed at liquid helium temperature (4.2 K). The LEOE has been measured for the source wavelength of a He–Ne laser ( λ=0.63 μm). With increasing acoustical power, the LEOE signal increases and achieves its maximum at an acoustical intensity of about 18.5 W cm −2 and a frequency of about 14 kHz. The value of the LEOE r 222 tensor component was comparable with that for the traditional nonlinear optical crystals, such as KH 2PO 4, KTiOPO 4 and LiNbO 3. With decreasing temperature (below 25 K), the acoustically induced LEOE signal strongly increases. A correlation between the acoustically nonlinear optical response and low-frequency Raman intensities has been found. The maximal acoustically induced SHG has been observed for the nanocrystallite concentration of about 3.1% (in weight units) and nanocrystallite sizes lying within 35–40 nm. The LEOE tensor coefficients were larger than for the bulk CLBO crystals (at least 21%). Advantages and drawbacks of the presented model were discussed. The origin of the observed phenomena is explained on the ground of the ab initio band energy calculations taking into account anharmonic electron–phonon interactions.

Infrared-induced nonlinear optical effects in chalcogenide glasses

The second-order nonlinear optical effects such as second harmonic generation (SHG) and linear electrooptics (LEO) in the middle infrared spectral range (5–15 μm) are experimentally observed for the first time. We have found that the novel As2Te3–BaBr2–BiCl2 chalcogenide glasses, possessing transmission windows within the 3–45 μm spectral range, have both photoinduced SHG equal to 0.0012 pm/V and comparable values of photoinduced electrooptics effect at wavelength of excitation (λ=10.6 μm). The photoinduced anharmonic electron–phonon interaction play the major role in the observed phenomena. A good correlation between the IR-induced nonlinear optical susceptibilities and the photoinduced anharmonic electron-quasi-phonon interactions is demonstrated. The pump–probe experiment shows that the intensity-dependent maximum of the LEO is shifted towards the higher delaying times comparing with the SHG ones. Comparison of theoretical simulations with experimental measurements indicate dominant role of the electron–phonon anharmonic subsystem in the observed effects.

Isotope-Induced Ferroelectricity

Although it is well known that for hydrogen-bonded ferro- and antiferroelectric systems the replacement of hydrogen by deuterium induces a large isotope effect on the ferroelectric transition temperature Tc, similar findings have not been reported for perovskite type ferroelectrics until recently when it has been shown experimentally that the incipient quantum paraelectric SrTiO3 can be driven to a ferroelectric state by replacing O16 by its isotope O18 [Phys. Rev. Lett. 28, 3540 (1999)]. The experimental data have been analyzed within an anharmonic electron–phonon interaction model and consistent quantitative agreement with the corresponding experiment is achieved. In addition, it is shown that the origin of the isotope effect in perovskite type quantum paraelectrics differs substantially from the one observed in hydrogen-bonded systems.

Optical second harmonic generation in CsLiB6O10 nanocrystallites incorporated to the olygoether photopolymer matrices induced by acustical field

Optical second harmonic generation in CsLiB6O10 (CLBO) large-sized nanocrystallites (15–40 nm) incorporated into the olygoether photopolymer matrices has been found at liquid helium temperature (LHeT). The second harmonic generation (SHG) has been measured for the source wavelength of the YAG : Nd laser (λ = 1.06 μm). Increasing acoustical power (up to 18.1 W/cm2) and acoustical frequencies at 14 kHz give maximal values of the output SHG. The measured SHG χ222 tensor component was comparable with the one for the traditional nonlinear optical crystals such as KH2PO4, KTiOPO4, LiNbO3. With decreasing temperature below 28 K the acoustically induced SHG signal strongly increases. A correlation between the acoustically induced SHG and low-frequency Raman modes has been found. The maximal acoustically induced SHG has been observed for the nanocrystallite concentration about 3.1% (in weighting units) and the crystallite sizes lying within the 35–40 nm. The SHG tensor coefficients were higher than for the proper CLBO crystals at least on the 13%. Advantageous and drawbacks of the presented model are discussed. The theory of the observed phenomena is explained on the ground of ab initio band energy calculations with taking into account of anharmonic electron-phonon interactions.