Exponential Gain Coefficient Research Articles

Polarization and angle dependent subwavelength coupling and dramatic reflection enhancement/reduction from an ITO–LiNbO3 interface

Two-dimensional electron gases (2DEGs) were demonstrated in indium–tin–oxide (ITO)-coated lithium niobate (LN) slabs by monitoring the very first reflection (VFR). Being dictated by light–matter interaction within the half-a-wavelength range, two-fold enhancement of the VFR was observed at one incidence angle, while two-and-half-fold reduction was obtained at another angle, when illuminating a Z-cut ITO–LN slab with two laser beams at 532 nm from the same side. In terms of exponential gain coefficient, such VFR enhancement/reduction corresponds to a range from −7.416 × 104 cm−1 to +4.68 × 104 cm−1, three orders of magnitude higher than that expected from the conventional photorefractive (PR) theory. The nanometer-thick 2DEG and the coupling role played by the evanescent field of the surface plasmon polaritons (SPPs) impose a much looser coherency requirement for the interacting light beams, which was verified by the solid energy coupling observed while rotating the polarization state of one laser beam. As much as 18.4 mW of the side diffraction mode associated with the light track formed on the slab surface, which sheds some light on the dramatic coupling dynamics. All these findings above were far beyond the reach of conventional PR theory, yet well consistent with the physical picture of 2DEG-supported SPPs and their interactions at the ITO–LN interface, offering a good opportunity to conceive and design future photonic/electronic devices for optical modulators.

Subwavelength coupling and ultra-high exponential gain coefficient originating from 2D electron gas at ITO/LiNbO3 interface

Two dimensional electron gases (2DEGs) formed at interfaces between two oxides have been drawing growing attention for their intriguing magnetic, superconducting, and optical properties. Remarkable anisotropic transmission was observed from an indium-tin-oxide (ITO) coated lithium niobate (LN) slab, implying that a 2DEG is formed at one of the ITO/LN interfaces, and this is seemingly behind the anisotropic transmission. To optically probe 2DEG formation at one of the ITO/LN interfaces, the first reflected beam was monitored with one (two) laser beam(s). Reflective dynamics as large as 5%-15% were observed, pointing unambiguously to a subwavelength coupling and corresponding to, conservatively, exponential gain coefficients of −26 800 to +2700 cm−1 with half a wavelength as the coupling range. All observations are far beyond the reach of conventional bulk photorefractive effects, but align well with a picture of surface plasmon polariton excitation based on 2DEG formation. The 2DEG proximal to the LN substrate is a promising candidate for designing nonlinear plasmonics based nanometric waveguides, rectifiers, modulators, and sensors, which are compatible with current photonic circuits.

Two photon induced amplified spontaneous emission at low threshold from Styryl 7 dye incorporated DNA template

Studies on biophysical interactions between DNA-surfactant complexes and dye molecules are necessary to fully exploit its potential in optoelectronic devices. In this work investigations on nonlinear optical absorption and amplified spontaneous emission (ASE) of Styryl 7 dye (LDS 750) incorporating DNA-CTMA (deoxyribonucleic acid blended with cationic surfactant molecule, cetyltrimethyl-ammonium chloride) and PMMA (Poly methyl methacrylate) is reported. The nonlinear optical studies on the sample carried out using Z scan technique exhibits reverse saturable absorption with DNA -CTMA complex but with PMMA the sample behaves as a saturable absorber. We observed ASE at low pump energy for Styryl 7–DNA-CTMA film, when excited with a pulsed nanosecond Nd: YAG laser (λ = 532 nm) whereas ASE was not observed from dye incorporated with PMMA even with high pump energy. ASE characteristics were studied as a function of excitation energy density for the evaluation of threshold energy and exponential gain coefficient. The enhanced fluorescence and low threshold for ASE make Styryl 7 dye –DNA template a promising candidate for novel optoelectronic applications.

Subwavelength coupling strengthened optical amplification in nematic liquid crystal cells

We observed strikingly different first reflection dynamics of two counter-propagating laser beams passing through a wedge-shaped C60-doped liquid crystal cell sandwiched between two ZnSe-coated ITO glass plates without applied electric field, suggesting a strong subwavelength energy coupling between light beams. Exponential gain coefficient as high as 10 574 cm−1 was obtained from the 1.1 μm-thick portion of the cell under applied voltage U0 = 3.0 V, consisting with the subwavelength coupling picture. Surface plasmon polariton (SPP) supporting layer is identified by considering dipolar properties of liquid crystal molecules that are well aligned. The specific features in energy coupling dynamics and 2D diffraction patterns perceived suggest that SPP mediated coupling is responsible for all the findings.

Large exponential gain coefficient in polymer assisted asymmetric liquid crystal cells originating from surface effect

As large as 4607cm−1 gain coefficient in two beam coupling experiment was obtained by introducing PVK:C60 film to ZnSe assisted liquid crystal system. As short as 5.0ms holographic recording time was reached when probing the grating formation process, showing great potential in real time applications. Systematical two beam coupling and grating probing experiments were performed in studying the mechanism behind the high photorefractive (PR) performance. Unusual energy transfer direction change and gain coefficient fluctuation were observed when the voltage polarity and incidence side were altered in the related two wave coupling experiments.

Impact of thickness of liquid crystal layer on response rate and exponential gain coefficient with assistance of ZnSe film.

A response time as short as 5.4 ms and an exponential gain coefficient as large as 1795.0 cm(-1) were obtained in C(60) doped 4,4'-n-pentylcyanobiphenyl liquid crystal cells sandwiched with two indium tin oxide glass plates coated with nanoscale photoconductive ZnSe films, which is believed to be facilitating charge-carrier generation and transportation and, hence, to be responsible for the fast response rate. The surface-mediated photorefractive effect and the ZnSe interlayers were both behind the high gain coefficients. The two-dimensional diffraction patterns observed in our system are also discussed.

Impact of surface plasmon polaritons on photorefractive effect in dye doped liquid crystal cells with ZnSe interlayers.

Great impact of surface plasmon polaritons (SPPs) on photorefractive effect in ZnSe/liquid crystal interface was observed and studied in dye pyrromethane 597 doped 4,4'-n-pentylcyanobiphenyl (5CB) liquid crystal (LC) cells sandwiched with ZnSe coated ITO glass plates. Locally electrostatic modification of ZnSe in charge carrier density makes possible visible light excitation of SPPs in the LC/ZnSe interfaces. A tentative physical picture of SPP mediation was proposed in elucidating associated findings, including photoinduced scattering enhancement at low electric field and then reduction at high field, stepwise up- and down-turns in exponential gain coefficient, and 2D diffraction patterns. This work may open a new way toward tunable low-loss visible excitation of SPPs for plasmonic applications, specifically for organic plasmonics.

IMPROVEMENT OF NONVOLATILE BLUE PHOTOREFRACTIVE PROPERTIES IN In:Ce:Cu:LiNbO3 CRYSTALS

The In : Ce : Cu : LiNbO 3 crystals used in this work were grown by Czchralski method. The key parameters: the exponential gain coefficient Γ, the diffraction efficiency ηS, the response time τw, the dynamic range M/#, the sensitivity S and the maximal refractive index change Δn max were measured with conventional two beam coupling. The optimal values of Γ, ηS, M/#, S and Δn max achieved are 44.7, 68.6%, 6.02, 0.471 cm/J and 5.37 × 10-5, respectively. It was found that the blue light photorefractive properties of Γ, ηS, M/#, S and Δn max increased with increasing In 3+ ions concentration, while the red light photorefractive properties of Γ, ηS, Δn max decreased with increasing In 3+ ions concentration. The nonvolatile holographic storage properties were measured by both the dual wavelength and two color techniques. The maximal diffraction efficiency and fixing diffraction efficiency using the dual wavelength technique were doubled compared to those obtained in using the two color technique. The sensitivity and nonvolatile sensitivity were one order of magnitude higher than those by the two color technique. The photorefractive property enhancement by the dual wavelength technique was investigated.

Amplified spontaneous emission in polymethyl methacrylate doped with 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP)

Luminescence and amplified spontaneous emission (ASE) phenomenon of 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) dissolved in a polymethyl methacrylate (PMMA) matrix have been studied. We analyzed luminescence from thin films of DCNP–PMMA excited by the neodymium doped yttrium aluminum garnet (Nd:YAG) nanosecond pulsed laser doubled and tripled in frequency. We report on ASE in function of different excitation pulse energy densities (ASE threshold was estimated to be 10 and 13mJ/cm2 for pumping beams wavelengths of 532 and 355nm, respectively). We observed the photo-degradation process of a dye and performed exponential gain coefficient measurements (up to 10cm−1) in the system.

Two-beam energy exchange in a hybrid photorefractive-flexoelectric liquid-crystal cell

We develop a semiquantitative theory to describe the experimentally observed energy gain when two light beams intersect in hybrid organic-inorganic photorefractives. These systems consist of a nematic liquid-crystal (LC) layer placed between two photorefractive windows. A periodic space-charge field is induced by the interfering light beams in the photorefractive windows. The field penetrates into the LC, interacting with the nematic director and giving rise to a diffraction grating. LC flexoelectricity is the principal physical mechanism driving the grating structure. Each light beam diffracts from the induced grating, leading to an apparent energy gain and loss within each beam. The LC optics is described in the Bragg regime. In the theory the exponential gain coefficient is a product of a beam interference term, a flexoelectricity term and a space-charge term. The theory has been compared with results of an experimental study on hybrid cells filled with the LC mixture TL 205. Experimentally the energy gain is maximal at much lower grating wave numbers than is predicted by naïve theory. However, if the director reorientation is cubic rather than linear in the space-charge field term, then good agreement between theory and experiment can be achieved using only a single fitting parameter. We provide a semiquantitative argument to justify this nonlinearity in terms of electric-field-induced local phase separation between different components of the liquid crystal.

Spectrum evolutions of spontaneous and pump-depleted stimulated Brillouin scatterings in liquid media

A theoretical model for calculating spontaneous and stimulated Brillouin scattering(SBS) spectra is described. An empirical formula for the Stokes output spectral linewidth, a function of spontaneous Brillouin linewidth and the exponential gain coefficient, is obtained by the calculated data fitting. The formula holds true for two cases involving pump undepletion and depletion. The lineshape change from spontaneous to highly pump-depleted SBS spectra is also investigated. The result shows that for the pump power below the SBS threshold, the Stokes output spectral lineshape evolves from Lorentzian to approximately Gaussian as the pump power increases. For the pump power near or beyond the threshold, the SBS spectrum is in the form of a steady Gaussian profile, and the spectral linewidth comes to a certain value about 7 times narrower than the spontaneous one. The theoretical results are experimentally demonstrated by using several common liquid media.

Photorefractive features of non‐stoichiometry codoped Hf:Fe:LiNbO3 single crystals

AbstractHf(2mol%):Fe(0.05wt%):LiNbO3 crystals with various [Li]/[Nb] ratios of 0.94, 1.05, 1.2 and 1.38 have been grown. The photorefractive resistant ability increases with the accretion of [Li]/[Nb] ratio. When the ratio of [Li]/[Nb] is 1.20 or 1.38, the OH‐ absorption band shifts to about 3477cm‐1. The mechanisms of the photorefractive resistant ability increase and the absorption band shift have been discussed. The exponential gain coefficient (Γ) of the crystals was measured with two‐beam coupling method and the effective charge carrier concentration (Neff) was calculated. The results show that Γ and Neff increase with the accretion of [Li]/[Nb] ratio. The temperature effect of codoped Hf:Fe:LiNbO3 crystals was also studied, it was found that the exponential gain coefficient increase dramatically at about 55°C, 70°C and 110°C, this is due to the inner electric field which is resulted from structure phase change. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Properties of Ru-doped near-stoichiometric lithium niobate crystals produced by vapor transport equilibration

Ruthenium (Ru)-doped near-stoichiometric lithium niobate crystals (Ru:SLN) were prepared by the vapor transport equilibration (VTE) technique from Ru-doped congruent lithium niobate (CLN) grown using the Czochralski method. Increasing the duration time of the VTE treatment caused the ratio of [Li]/[Nb] in the crystal to increase, which in turn caused the absorption edges to shift to violet. The absorption coefficient in the UV/VIS absorption spectra at 530 nm decreased because of the change in the valence of the Ru ions cause by the VTE treatment. The OH − absorption spectra showed that the broad band located at 3482 cm −1 gradually decreased while another band located at 3466 cm −1 gradually increased and the shape became sharper as the VTE treatment time increased. When the VTE treatment time reached 200 h, Ru-doped near-stoichiometric lithium niobate crystals were obtained. Two-beam coupling examination with a 532 nm laser showed that the Ru-doped near-stoichiometric lithium niobate crystals had a larger exponential gain coefficient, lower diffraction efficiency, faster response time, and higher sensitivity than did the Ru-doped CLN crystals.

Effect of post treatment on the photorefractive properties of Ru‐doped lithium niobate

AbstractThe effect of post treatment on the photorefractive properties of Ru‐doped lithium niobate was studied. The absorption spectra examination of Ru‐doped LiNbO3 crystals with different post treatments showed that the absorption coefficient at 530 nm increased after the reduced treatment was employed and the absorption edge of the reduced crystal shifted towards the infrared band. On the contrary, the trend reversed after the oxidized treatment was employed. In addition, the photorefractive properties were investigated with the two‐beam coupling method conducted via a 532 nm solid state laser. It was found that the oxidized Ru:LiNbO3 had smaller exponential gain coefficient and diffraction efficiency because the charges in the shallow level were exchanged to the deep level. On the other hand, the reduced Ru:LiNbO3 crystals had larger exponential gain coefficient and diffraction efficiency due to the increase of the Ru3+ which existed in the shallow level. The response times of both oxidized and reduced Ru:LiNbO3 were longer than those of the as‐grown ones. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photorefractive Properties of Potassium Lithium Niobate Crystals with CeO 2 and Nd 2O 3

KLN and Ce:Nd:KLN crystals were grown by Czochralski method and polarized into single ferroelectrics domain along c-axis. The properties of KLN and Ce:Nd:KLN crystals, such as Curie temperature, Raman spectra, exponential gain coefficient ( T) and thin crystal sheet effect, were measured. The results showed that the two spectra resembling Ce:Nd:KLN crystal were of tetragonal tungsten bronze structure, the exponential gain coefficient of Ce:Nd:KLN crystal was higher than that of KLN crystals and Ce:Nd:KLN crystal had thin crystal sheet effect, for its exponential gain coefficient increasing with crystal sheet thinning. The thin crystal sheet effect of Ce:Nd:KLN crystal was also discussed.

Effect of Li/Nb ratio on structure and photorefractive properties of Zn:Fe:LiNbO3 crystals

Zn:Fe:LiNbO3 crystals with different Li/Nb ratios in the melts (Li/Nb = 0.946, 0.97, 1.00, 1.10, 1.20, 1.44) have been grown for the first time. The UV–Vis absorption spectra, exponential gain coefficient, diffraction efficiency and response time of the crystals were measured. With the ratio of Li/Nb increasing, the absorption edge shifts to a shorter wavelength, the exponential gain coefficient and response speed increase, but the diffraction efficiency decreases.

Fast photorefractive response in LiNbO 3 crystal co-doped with Fe/Sm ions

Sm:Fe:LiNbO 3 crystal was grown by doping Sm 2O 3 in Fe:LiNbO 3 crystal. The exponential gain coefficient, diffraction efficiency and photorefractive response time were measured by two-wave mixing experiment, respectively. It was found that the response speed was greatly improved by doping Sm ions in Fe:LiNbO 3 crystal. Our analysis also showed that photoconductivity enhancement was responsible for improving the response speed in the Sm:Fe:LiNbO 3 crystal.

Growth and photorefractive properties of an Fe-doped near-stoichiometric LiNbO3 crystal

A near-stoichiometric LiNbO3 crystal with 0.02 wt% Fe2O3 doping was grown from a Li-rich melt (Li/Nb = 1.38, atomic ratio) by the Czochralski method in air atmosphere. The OH− absorption band was characterized by infrared transmittance spectra. The appearance of the 3466 cm−1 absorption band (2.89 µm) manifests that the composition of the grown crystal is close to the stoichiometric ratio. The photorefractive properties were measured by a two-wave coupling experiment. The measured results show that the Fe-doped near-stoichiometric LiNbO3 crystal has a larger exponential gain coefficient than the Fe-doped congruent LiNbO3 crystal. The remarkable gain can be attributed to the photovoltaic field being comparable with the effective limiting space-charge field.

The growth and photorefractive effect of co‐doped Ce:Cu:BSO crystals

AbstractIn the paper Ce:Cu:BSO crystal has been grown by Czochralski method for the first time with doping CeO2 and CuO into BSO(Bi12SiO20) crystal. The exponential gain coefficient and respond time of Ce:Cu:BSO crystal are measured by two‐wave‐coupling technology. The results indicate that exponential gain coefficient of Ce:Cu:BSO is more than two times as that of non‐doped BSO and the response time exhibits in microsecond level. Furthermore its exponential gain coefficient improves greatly compared with Ce:BSO's at the same doping level of Ce, while its response time is less than Ce:BSO's. The improvement mechanism of photorefractive effect of Ce:Cu:BSO crystal is investigated systematically. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photorefractive Effect of Fe:KLN Crystal

We have grown KLN and Fe:KLN crystals using the TSSG method. The crystal lattice constants are measured by the X-ray diffraction technique and the crystal optical absorption edges are characterized by an ultraviolet-visible absorption spectrum measurement. The results show that the absorption edge of the Fe:KLN crystal shifts to the longer wavelengths compared with that of the KLN crystal. The exponential gain coefficient and erasure time are examined by two-beam coupling experiment. The results indicate that the exponential gain coefficient of Fe:KLN crystal is twice as large as that of the KLN crystal, and the photoconductivity of the Fe:KLN is three orders of magnitude higher than that of KLN. The enhancement of photorefractive properties of Fe:KLN crystal is also discussed.