B.P. Antonyuk

All optical poling and second harmonic generation in glasses: theory and experiment

Abstract Theoretical and experimental evidence of light driven electron–hole self-organization resulting in amplification of seed static electric field is presented. The amplification allowed us to perform all optical poling of conventional commercial “heavy flint” silica glass. The χ (2) -grating is optically recorded in the initially centrosymmetric glass and the second harmonic generation (SHG) is observed in poled noncentrosymmetric material. The fundamental and frequency doubled output of a YAG:Nd 3+ laser starts the poling process creating rectified DC-electric field. Then single wavelength radiation (either fundamental or frequency doubled output) proceeds with optical recording of nonlinear grating amplifying this initial DC-electric field.

Charge transfer excitons in Ge-doped silica fibres and their response to static electric field

Absorption in the region of green light and the corresponding broad band luminescence in Ge-doped silica fibres are measured and interpreted. The spectra obtained show that new type of excitations arises in Ge-doped silica fibres which is absent in pure silica. They are charge transfer excitons. The concentration dependence reveals charge transfers from Ge centers to their hosts as well as transfers between different Ge centers. The orientation ordering of the exciton dipole moments results in positive feedback in response to static electric field. This produces the strong phase-matched electric field allowing the effective second harmonic generation.

All optical poling of glasses

Amplification of rectified static electric field by electron transitions between different traps is possible in wide class of glass materials with low carrier mobilities. It allows to create a phase matched polarization resulting in effective second harmonic generation.

Charge transfer excitons in Ge-doped silica fibers: Self-organization and second harmonic generation

Abstract A Raman and a first hyper-Raman study of pure silica and Ge-doped silica fibers is performed. The Raman spectra of a doped fiber are accompanied by a broad luminescence band corresponding to charge transfer excitons of Ge-centers. Self-organization (orientational ordering) of these excitons under light pumping results in an instability of the system with respect to the weak static phase matched field ∼χ (3) E 2ω E ω ∗ E ω ∗ found by Stolen and Tom. A fiber makes a transition into a polarized state accompanied by the strong phase matched field Edc ∼ ei(k2ω − 2kω)z allowing the second harmonic generation (χ(2) = χ(3)Edc, Edc ∼ 105 V/cm) to occur.

High efficient second harmonic generation in Ge-doped silica fibers

Interaction of fundamental, second harmonic and generated static electric fields in Ge-doped silica fibers is considered. Frequency doubling for stationary conditions is found to be efficient only at the stage of preparation and is switched off when the fiber is full prepared. If the phase of the incident fundamental wave varies in time in a proper way (nonstationary conditions) frequency doubling becomes highly efficient and does not decay.

Second-harmonic generation in Ge-doped silica fibres : experiment and theory

Abstract Raman and hyper-Raman studies of pure silica and Ge-doped silica fibres reveals charge transfer from the Ge centre to silica in the fibres under pumping by the second harmonic. Self-organization of these chargetransfer excitons results in a strong phase-matched static electric field breaking the inversion symmetry and allowing frequency doubling. Owing to the longrange electron transfer the response of the medium is very slow and a considerable retardation takes place during the preparation of the fibre by light. This retardation makes the second-harmonic amplitude grow at the stage of preparation. The retardation vanishes in the stationary state and the secondharmonic amplitude tends to its initial magnitude in the fully prepared fibre. Despite the fact that the strong phase-matched electric field is still present, the frequency-doubling mechanism is now switched off.

Light driven structuring of glasses

Abstract Theoretical and experimental evidence of light driven structuring of glasses is presented. We show that light overcomes Coulomb repulsion and effective electron–electron interaction in glasses under strong light pumping becomes attractive. As the result homogenious distribution of trapped electrons gets unstable and macroscopic electron bunches are grown. At different conditions ordered structures with period 2 μm ± 0.2 μm determined by internal properties of the material are formed. These structures were observed in ablation: surface profile after laser treatment reveals ordered patern corresponding to the light induced electron domains.

Influence of an external electric field on the second harmonic generation in Ge-doped silica fibres☆

Abstract A theoretical model that explains the main experimental results on frequency doubling in fibres is developed. Special attention is paid to the recent experiment [P.G. Kazansky, V. Pruneri, Phys. Rev. Lett. 78 (1997) 2956] on the electrically stimulated second harmonic generation, which is crucial for different theories. We show that any “shake” of the system results in the burst of the second harmonic signal. This is the reason why bumps in the signal were observed after turning the strong external electric field on and off.

Electron-hole self-organization and refractive index fluctuations in amorphous semiconductors

Abstract Strong correlations between trapped electron and hole positions in amorphous semiconductors under light pumping are investigated. Computer simulation shows that the correlation radius strongly increases when pumping decreases. Electron (hole) density fluctuations (and, hence, the refractive index fluctuations) for the volume of an arbitrary size l have a universal spectrum F 1 (ω)= α 1 ω , where α l ∼l − 3 2 .

Electron-hole ordering and 1ω noise in amorphous semiconductors☆

Abstract Theoretical and experimental studies of amorphous semiconductors As 2 S 5 show that photogenerated trapped electrons and holes are strongly correlated. The Fourier transform of the electron (hole) density time fluctuations (and, hence, of the refractive index variations) indicates that universal 1 ω noise is present in an ordered state, while disordered electron-hole systems give white noise. We give experimental evidence of this 1 ω noise in AS 2 S 5 in support of the idea of electron-hole self-organization under light pumping.