K. Peithmann

Stabilized recording and thermal fixing of holograms in photorefractive lithium niobate crystals

Holograms are thermally fixed in photorefractive lithium niobate crystals, i.e., they are recorded at temperatures between 130 and 180 °C. The setup is actively stabilized during recording against movements or vibrations of the interference pattern which especially occur during long-period writing at enhanced temperatures. Two different techniques are investigated: (1) Interference of the recording beams using one crystal surface as a beamsplitter yields a signal for stabilization. (2) Alternatively, one of the beams is periodically phase modulated and the beam-coupling signal is used for stabilization. Reproducible refractive index changes of thermally fixed holograms up to 7.5×10−4 are obtained with both stabilization techniques. However, the second method is advantageous for multiplexing experiments, because no readjustment of the beam-coupling stabilization system is required if the angles of the recording beams are changed.

Modification of the refractive index of lithium niobate crystals by transmission of high-energy 4He2+ and D+ particles

We report reductions of the refractive index of congruently melting lithium niobate crystals of up to 6×10−3 by exposure of z-cut samples with high-energy 4He2+ and D+ particles which are transmitted through the crystals.

Low-spatial-frequency refractive-index changes in iron-doped lithium niobate crystals upon illumination with a focused continuous-wave laser beam

Iron-doped lithium niobate crystals are illuminated with a single continuous-wave (cw) focused green laser beam. Surface deformations, temperature distributions, and changes of the refractive index of the material are investigated by means of interferometric techniques. It turns out that light absorption causes pronounced temperature profiles in the samples, which induce pyroelectric fields. Electronic space-charge fields that compensate these pyroelectric fields remain in the crystals after the focused light is switched off and modulate, together with bulk-photovoltaic fields, the refractive index by means of the electro-optic effect. These low-spatial-frequency effects must be taken into account when focused light beams are utilized, e.g., for high-speed holographic data storage or two-beam coupling, because the effects determine an upper limit of the highest usable cw light intensities.

Incremental holographic recording in lithium niobate with active phase locking.

Angular-multiplexed hologram recording in iron-doped lithium niobate crystals was carried out with near-infrared light. An incremental recording schedule with active phase locking of the light pattern onto the hologram was used. Continuous and reproducible recording of holograms of equal efficiency was achieved, and a hologram multiplexing number, M/#=2 , for a 5-mm-thick crystal was obtained at a 760-nm wavelength of light.

Domain reversal properties and refractive index changes of magnesium doped lithium niobate upon ion exposure

Irradiation of optical damage resistant, magnesium doped lithium niobate crystals with fast, high-energy He2+3 ions changes important material properties. In the interaction region, where the ions transmit through the material, the ferroelectric coercive field EC is diminished from 6.0kVmm−1 down to 5.0–5.4kVmm−1 after transmission of 41MeV He2+3 particles. This enables easier domain reversal in irradiated crystals compared to untreated material. Besides, large changes of the refractive index of the crystals on the order of 6×10−3 are induced by the treatment. Moderate annealing treatments do not diminish Δn, but refresh the coercive field.

Dynamics of holographic recording with focused beams in iron-doped lithium niobate crystals

Holograms are recorded with focused beams in an iron-doped lithium niobate crystal. The diffraction efficiency shows a maximum after several seconds of recording, unlike in the case of writing with two homogeneous plane waves in the same crystal. This behavior can be attributed to a compensation field caused by incomplete illumination of the crystal. The field finally stops the bulk photovoltaic effect, which is the main driving force of the process. Based on this assumptions, we derive an analytical expression for the evolution of the diffraction efficiency which correctly fits the experimental data.

Refractive index changes in lithium niobate crystals by high-energy particle radiation

Irradiation of lithium niobate crystals with 41 MeV 3He ions causes strong changes of the ordinary and extraordinary refractive indexes. We present a detailed study of this effect. Small fluence of irradiation already yields refractive index changes about 5×10−4; the highest values reach 3×10−3. These index modulations are stable up to 100°C and can be erased thermally, for which temperatures up to 500°C are required. A direct correlation between the refractive index changes and the produced lattice vacancies is found.

Photorefractive lithography with synchrotron light in poly(methyl methacrylate)

Synchrotron light changes the refractive index of poly(methyl methacrylate) (PMMA). Refractive-index enhancements as well as reductions depending on dose and energy of the radiation used can be observed. This effect allows the manufacturing of, for example integrated-optical components, as is demonstrated by realization of waveguides in PMMA.

Electrical conductivity and asymmetric material changes upon irradiation of Mg-doped lithium niobate crystals with low-mass, high-energy ions

Radiation damage in magnesium-doped lithium niobate crystals, created by low-mass, high-energy ions which have transmitted the entire crystal thickness, leads to an enhanced electrical dark conductivity as well as an enhanced photoconductivity. Experimental results on the electrical properties after ion exposure are given, and an asymmetric dependence of the conductivity as well as refractive index changes on the irradiation geometry with respect to the ferroelectric axis is revealed.

Optimization of iron-doped photorefractive lithium niobate crystals

Summary form only given. Photorefractive iron-doped lithium niobate crystals (LiNbO/sub 3/:Fe) are still the best choice for many applications, such as holographic data storage, outstanding interference filtering, or wavelength division multiplexing. Iron occurs in LiNbO/sub 3/ in the valence states Fe/sup 2+/ and Fe/sup 3+/, only. Holographic investigations are performed by an (argon-ion laser) and with near-infrared light (wavelength 760 nm, Ti:sapphire laser). Gratings are recorded by interference of two plane waves. One of the recording waves is blocked and the other one is diffracted from the grating. The amplitude of the refractive-index modulation is calculated from the measured diffraction efficiency.