Light-induced Refractive Index Changes Research Articles

Experimental study on the chirped structure of the white-light continuum generation by femtosecond laser spectroscopy

The chirped structure of the white-light continuum generation (WLCG) pulse produced by focusing 800nm laser pulse with a pulse duration of 150fs (FWHM: full-width-at-half-maximum) onto a 2.4 mm thick sapphire plate was investigated by the optical Kerr gate technique with normal hexane as the optical Kerr gate medium. The observed WLCG was positively chirped, the measured anti-Stokes spectrum of WLCG ranges from 449 to 580nm with a temporal span of 2.56ps. When using metal reflecting mirrors to eliminate the group velocity dispersion (GVD) effect, we found that a span of 1.3ps still remained, indicating that the chirped pulse cannot be accounted for simply by GVD of the pulse propagation in the dispersive media. Our results suggest that the light-induced refractive index change due to the third-order nonlinear optical effect leads to an additional positive group velocity dispersion, which contributes to an important portion of the observed temporal broadening of the chirped WLCG. In addition to using reflective optical elements instead of dispersive optical elements, an effective way of reducing the chirp is to minimize the optical path length of the WLCG medium.

Optically induced photorefractive waveguides in KNSBN:Ce crystal

Abstract We present our experiments on fabricating fiber-like and planar photorefractive waveguides in a KNSBN:Ce crystal solely by laser irradiation with the wavelength of 632.8 nm and 532 nm, respectively. The high refractive index regions induced by thin cylindrical beams or thin sheet beams propagating through the crystal are used as waveguide structures. The refractive index changes in the waveguide region are measured by a Mach–Zehnder interferometer using the phase-stepping method, and the refractive index profiles are mapped. The optically induced distributions of refractive index changes observed in our experiments are discussed according to the transport mechanism of the photo-induced charge carriers in the crystal. The asymmetry of the light-induced space charge fields in the absence of external electric field may cause the asymmetry of the refractive index distributions. The guidance of the waveguides fabricated in our experiments shows that the probe beams are trapped in the waveguide regions. It is shown that fabricating photorefractive waveguides by use of light-induced refractive index changes in KNSBN:Ce crystals is feasible.

Photoinduced reversible optical gratings in photochromic diarylethene-doped polymeric thin films

We report on our investigation of the potential for all-optical reversible photostructuring of the photochromic molecule 1,2-bis(2-methylbenzo[b]thiophen-3-yl)perfluorocyclopentene (BFCP). The light-induced isomerization of BFCP was studied in solution as well as in polymeric thin films. The absorption change that is due to the photochromic reaction of BFCP was determined by conventional spectroscopic measurements. The absolute values of the refractive indices of doped polymer films were measured by the waveguide grating coupling technique for the two states of the photochromic molecule. The dynamics of the reactions and the light-induced refractive-index changes Δn of doped polymer samples were investigated in holographic grating experiments. Changes of as much as Δn=(3.5±0.5)×10-4 were obtained at 806 nm for 41.16-wt. % doping. Finally, a modified two-beam coupling experiment permitted the separation of absorption and refractive-index contributions to the grating buildup and the determination of the phase shifts of these gratings with respect to the intensity grating.

Protein-based integrated optical switching and modulation

The static and dynamic response of optical waveguides coated with a thin protein film of bacteriorhodopsin was investigated. The size and kinetics of the light-induced refractive index changes of the adlayer were determined under different conditions of illumination. The results demonstrate the applicability of this protein as an active, programmable nonlinear optical material in all-optical integrated circuits.

Measurement of laser-induced refractive index changes of Mg-doped congruent and stoichiometric LiNbO3

Light-induced refractive index changes and nonlinear absorption of undoped and Mg-doped LiNbO3 with congruent and stoichiometric composition were investigated by the Z-scan method using an all-visible argon-ion laser with intensities up to the MW/cm2 level. Charge coupled device recordings demonstrate that the Z-scan method is suitable for quantitative study of Mg-doped samples, while it can be used only for qualitative measurements of undoped samples because of beam fanning in the latter case. Stoichiometric LiNbO3 doped with 5 mol % Mg shows positive refractive index changes opposite to those in cases of undoped congruent, undoped stoichiometric and 5 mol % Mg-doped congruent LiNbO3. The refractive index change of Δne=2.5×10−5 measured at 0.37 MW/cm2 light intensity for stoichiometric LiNbO3:Mg is six times smaller than that of congruent LiNbO3:Mg at 0.1 MW/cm2.

Picosecond Z-scan measurements on a glass-ceramic containing sodium niobate nanocrystals

The third-order nonlinearity of a glass-ceramic containing sodium niobate (NaNbO 3) nanocrystals was studied using the Z-scan technique. The measurements were performed at 532 nm with 70-ps pulses at 5 Hz. Two-photon absorption coefficients and light-induced refractive index changes were determined. The results show dependence of the nonlinear parameters on the volume fraction of NaNbO 3 crystallites.

Study of UV-written channels in lead silicate glasses

Light-induced refractive index change was investigated in two lead-silicate (SF57 and F2) glasses by irradiation using a pulsed 248 nm KrF excimer laser and a CW frequency doubled argon ion laser at 244 nm. UV–Vis spectra were recorded after each exposure to the excimer laser to study the change in absorption. The exposures were cumulative. The results showed that, with increasing number of pulses, there was an increase in the attenuation of the samples. Large photosensitivity was observed from direct 244 nm UV-written channels without the use of a phase mask. The refractive index change measured at 633 nm for SF57 glass was to 2.9×10 −2 and a channel waveguide loss of 4.8 dB/cm and for F2 glass was to 7×10 −3 and a loss of 5 dB/cm. The large refractive index changes measured here are achieved at the expense of large induced losses. The channel waveguides were thermally stable up to 300 °C.

Photorefractive optical damage in protonated LiNbO3 waveguides

The photorefractive optical damage in annealed proton-exchanged LiNbO3 planar waveguides is studied. The pfotorefractive thresholds for the wavelengths 514.5 and 647.1 nm are determined. Also, the light-induced refractive index change in a wide range of powers is determined. An interesting and unexpected photorefractive behaviour of the samples in q-state (metastable at room temperature) and s-state (steady-state) is described. An analysis of the light self action in s-type waveguides is performed and used for estimating the waveguide conductivity.

Investigation of titanium- and copper-indiffused channel waveguides in lithium niobate and their application as holographic filters for infrared light

Monomode channel waveguides for infrared light around 1550 nm are fabricated in lithium niobate by successive indiffusion of titanium stripes and thin evaporated layers of copper. The fabricated waveguides are optically characterized and investigated with an atomic force microscope. To evaluate the photorefractive properties holograms are recorded with green light of an argon ion laser and read in the various channels in reflection geometry with infrared light. The saturation values of the light-induced refractive-index change and the photoconductivity in the channels are measured. The results are consistent with a one-centre model for the charge transport. To demonstrate the multiplexing capabilities of the photorefractive waveguides three reflection holograms with 90% diffraction efficiency and a channel spacing of only 0.8 nm are superimposed in one sample.

Neutron diffraction from holographic gratings in PMMA

In most photorefractive materials light-induced refractive index changes are accompanied by changes of the density of the material. Hence, gratings capable to diffract cold neutrons can be produced by standard holographic techniques. We compare measurements on the angular dependence of the diffraction efficiency with calculations. The conclusions are used to improve the time resolution of neutron diffraction experiments aiming at an investigation of the development process of holographic gratings in PMMA.

Photorefractive properties of highly-doped lithium niobate crystals in the visible and near-infrared

Light-induced refractive-index changes, bulk-photovoltaic current densities, and photoconductivities of photorefractive iron-doped lithium niobate crystals (iron concentrations between 0.02 and 0.17 wt. %) are investigated in detail using visible and near-infrared light. It turns out that the one-center model predicts the material performance correctly for small iron concentrations (cFe<0.06 wt. % Fe2O3), only. A strong increase of the photoconductivity for higher doping levels limits the space-charge fields. Refractive-index changes up to 7×10-4 for green and 2.8×10-4 for near-infrared ordinarily polarized light are obtained. The corresponding hologram multiplexing numbers are 11 for green and 5 for near-infrared light.

Reduction of light-induced refractive-index changes by decreased modulation of light patterns in photorefractive crystals

In holographic experiments many photorefractive crystals show refractive-index changes much smaller than one would expect from the known electro-optic coefficients and space-charge fields. We show that degradation of the interference pattern is the origin of this effect. The degree of modulation of a light grating in photorefractive crystals is measured by three methods and compared with the modulation of the grating of the incident light. All methods, measurement of the amplitudes of fundamental and second-order gratings, of the grating amplitudes in a crystal with and without another crystal in front of it, and of the drift currents through an inhomogeneously illuminated sample, yield consistent results: In lithium niobate there is almost no degradation of the light pattern. However, in our barium titanate and potassium tantalate–niobate samples the degree of modulation is smaller, and the reduction factor is 0.55 and 0.60, respectively. Inhomogeneities of refractive-index changes are shown to be the origin of the effect.

Holographic Recording in Planar Cu:H:LiTaO3 Waveguides

Photorefractive planar waveguides in LiTaO 3 are fabricated by a proton exchange and a successive copper exchange from melts, containing either Cu + or Cu 2+ ions. The influence of different fabrication steps on refractive index profiles, hydrogen content, optical absorption, and waveguide transparency is investigated. With holographic methods dark and photoconductivity, holographic sensitivity, and light-induced refractive index change are measured. By the additional copper exchange the steady-state diffraction efficiency of holographic gratings in the waveguides is increased from 0.005 to 81%. Here the crucial influence of both, Cu valence state and hydrogen concentration on the holographic sensitivity is demonstrated.

Image holography in CdF 2:Ga crystals

The recording of an image hologram in CdF 2:Ga crystals at room temperature is reported. Writing the phase holographic pattern utilizes a unique property of Ga and In doped CdF 2 crystals, namely, the bistability of these dopants. In contrast to CdF 2:In crystals, in which there is only one bistable center, the holographic experiment reveals two different bistable centers in CdF 2:Ga. Room temperature bistability is related to Ga, while the low temperature center resembles very much the properties of In, likely a non-intentional In dopant of the CdF 2:Ga crystals. The holographic sensitivity of doped CdF 2 crystals compares favorably with sensitivity of standard photorefractive materials. The main advantage for some applications of using the novel mechanism is the inherently local nature of the light-induced refractive index changes.

Holographic Measurement of Dark Conductivity in LiNbO3:Ti:Fe Planar Optical Waveguides

LiNbO3 is a promising photorefractive material for many applications [1]. Phase holograms formed in strip and planar waveguides may be used for phase conjugation, narrow-band holographic filters, or light amplifiers [2]. In most cases Fe2‡=3‡ serves as the photorefractive center. On the one hand, the maximum value of the light-induced refractive index changes has been found to be proportional to the concentration of Fe3‡ [3]. On the other hand, there are several hints that iron doping also has a significant influence on the dark conductivity of the material [4], thereby limiting the storage time. In waveguides the Fe concentration can be easily varied by indiffusion. In this letter we will show that the dark conductivity of heavily iron-doped planar waveguides in LiNbO3 increases exponentially with the iron concentration, and this correlation may also be applicable to samples with a much lower doping. Planar photorefractive waveguides are fabricated by two subsequent surface diffusions using commercially available optical grade y-cut LiNbO3 wafers of congruently melting composition. At first, an 80 nm-thick evaporated titanium layer is indiffused at 1000 C for 24 h in wet argon atmosphere to increase the surface refractive indices of LiNbO3. In a second step, evaporated iron layers of different thicknesses (30, 40, 60, 80, 100, and 120 nm) are indiffused at 1000 C for 18 h in wet oxygen to increase the photorefractive sensitivity of the samples. For comparison, we investigate a titanium-diffused waveguide (80 nm-thick Ti layer, indiffused at 1000 C for 18 h in wet oxygen) on a homogeneously doped LiNbO3 : Fe substrate [5], too. The concentration profiles of both, titanium and iron, can be well described by Gaussian functions, ci ˆ ci exp yy2=2r2†, with penetration depths rTi ˆ 3:6 mm and rFe ˆ 10:7 mm, respectively [6]. All waveguides are multimode with typical depths of the nearly Gaussian refractive index profiles of rwg 5 mm. The relative iron concentration at the surface is measured with the help of an electron microprobe, where we demonstrate that cFe is proportional to the thickness of the evaporated iron layer. The absolute value of the surface concentration of the indiffused iron, cFe y ˆ 0†, is determined by X-ray photoelectron spectroscopy (XPS) [6]. Only Fe3‡ can be detected, while the concentration of Fe2‡ is below the measuring accuracy. Thus we estimate a concentration ratio cFe2‡=cFe3‡ well below 0.01. The dark conductivity in the waveguide is measured with a holographic two-wave mixing technique. Both, a TE and a TM mode are excited with the help of a rutile prism to propagate collinearly along the x-axis of the sample, and the transmitted powers are measured. Modulated photovoltaic currents are generated by these orthogonally polarized waves, and a periodic space charge field builds up which leads to a perturbation of the dielectric constant via the electrooptic effect. Details of the formation of the gratings can be found in [7,8]. When the phase gratings are written to saturation, the two writing beams are switched off, and the decay of the grating in the dark is monitored by subsequently measuring the diffraction efficiency for short time intervalls (50 ms) with a low-intense reading beam. We define the diffraction efficiency as the ratio of diffracted to total read-out light intensity. The dark conductivity sd is calculated from the relaxation time td of the gratings via sd ˆ ee0=td, where e0 is the vacuum permittivity and e is the static dielectric constant of LiNbO3. In Fig. 1 the measured value of dark conductivity is shown as a function of the iron concentration cFe cFe3‡ at the surface of different waveguides. Samples with iron contents larger than 2 1020 cmy3 are produced by indiffusion of iron, whereas the sample with the lowest iron concentration (cFe ˆ 0:13 1020 cmy3) is fabricated on a LiNbO3 : Fe substrate that has been homogeneously doped in the melt. For the different modes of a waveguide, i.e., for different propagation depths, we obtain almost the same dark conductivity. This is expected from the nearly constant iron concentration in the waveguiding layer, and points to a minor influence of titanium on dark conductivity, because the titanium concentration strongly decreases with increasing propagation depth [6]. The measured dependence ln ‰s cFe†Š can be fairly well approximated by a straight line, including the sample prepared on the melt-doped substrate, thus obeing the relation sd cFe† ˆ sd exp cFe=cFe†, Rapid Research Notes R3

Holographic recording at 633 nm in the garnet Ca_3Mn_2Ge_3O_12

Elementary holographic gratings are recorded in Ca3Mn2Ge3O12 at 633 nm with orthogonal, linearly polarized writing beams at temperatures below 180 K. Refractive-index amplitudes of as high as 1.5×10-4 are achieved at temperatures below 100 K. We analyze the dependence of the diffraction efficiency on the polarization of the recording beams. The time constant for writing at 100 K with a total intensity of 540 mW/cm2 is 15.5 s. The temperature dependence of the holographically determined light-induced refractive-index changes can be described by Δn(T)=Δn(T=0){1/[1+r exp(-EA/kT)]}, with an activation energy of 0.16 eV.

Electrooptic Modulators in KTiOPO4 for the Short Visible Wavelength Region

Single-mode channel waveguides at short visible wavelengths have been fabricated by Rb ↔ K ion exchange in KTiOPO 4 . The channel waveguides show a typical attenuation of 1.5 dB/cm and negligible light-induced refractive index changes. The electrooptic properties as well as dc-drift phenomena have been investigated by using integrated electrooptic phase- and Mach-Zehnder-interferometer modulators.

Light-induced refractive index changes in singlemode channel waveguides in KTiOPO 4

Light-induced changes of the effective refractive index in Rb ↔ K ion-exchanged singlemode channel waveguides in KTiOPO 4 have been investigated in the visible wavelength region dependent on time, guided optical mode intensity and temperature. A hypothesis for the explanation of the light-induced effects is suggested. Thermooptic and pyroelectric effects are discussed. The light-induced refractive index changes do not restrict the function of integrated-optic components in the visible.

Characterization of photorefractive LiNbO3 waveguides fabricated by combined proton and copper exchange

Photorefractive planar waveguides in LiNbO3 are fabricated by a combined proton and copper exchange. The dependence of refractive index profiles, optical absorption, and electrooptic coefficients on different fabrication steps is investigated. With holographic methods dark and photoconductivity, holographic sensitivity, and light-induced refractive index change in the waveguides are measured. By the additional copper exchange the steady state diffraction efficiency of holographic gratings in our proton-exchanged waveguides is increased from 0.01 to 65%.

Optimization of Tetragonal KTa1−xNbxO3: Fe crystals as Storage Materials for Double-Exposure Holographic Interferometry

Abstract Double-exposure holographic interferometry with as-grown and oxidized iron-doped tetragonal potassium tantalate-niobate (KTN: Fe) crystals as a photorefractive storage material is presented. The influence of oxidation treatments on the light-induced refractive index change and decay time constant is investigated for different external electric fields and spatial frequencies. The experimental findings can be explained in terms of a charge transport model which assumes one kind of photorefractive centres with hole conductivity only. The fringe contrast and exposure time of double-exposure holographic interferograms are determined. The displacement of a test target is evaluated from interferograms utilizing a quasi-heterodyne technique. For that purpose a fully automatically working computer program is developed.