Iron-doped Lithium Research Articles

Highly reduced iron-doped lithium niobate for optoelectronic tweezers

We investigate the applicability of highly reduced lithium niobate samples doped with iron for the use as optoelectronic tweezers. Increasing the reduction degree of Fe-doped lithium niobate is well known to increase the photoconductivity and reduce the writing time of internal space-charge fields. Based on our measurements of the photorefractive properties, we determine the optimal conditions for dielectrophoretic trapping and present the application of Fe-doped lithium niobate as optoelectronic tweezers. For higher reduction degrees, an unexpected decrease in the photovoltaic current density and the saturation space-charge field is reported.

Effect of impurity iron ions on the formation of photoinduced lattices and domain structures in lithium niobate

Processes and mechanisms of the formation of lattices and periodic domain structures in iron-doped lithium niobate monocrystals under the effect of laser beams are examined. The role of Jahn-Teller Fe2+ ions in the formation of photoinduced periodic structures is demonstrated.

Direct Laser Writing: Versatile Tool for Microfabrication of Lithium Niobate

We demonstrate versatility of femtosecond direct laser writing (DLW) technique for microstructuring of iron-doped and as-grown lithium niobate crystals. Formation of extended, metastable, and re-writable refractive index modulation structures is achievable via fast local photorefractive modification of refractive index at the focus of the writing laser beam, with a modulation of up to ∆n ~ 10 -3 , without permanent structural damage to the host crystal. Non-destructive photomodification allows total or partial erasure or modification of the previously recorded structures. This capability may enable realization of optically reconfigurable diffractive optical elements in the same volume of lithium niobate crystal. Flexibility of the DLW technique can be exploited for threedimensional microstructuring of periodic and aperiodic patterns with controllable index modulation profiles.

Fabrication of large area two-dimensional nonlinear photonic lattices using improved Michelson interferometer

We demonstrate an approach for easy fabrication of large area two-dimensional nonlinear photonic lattices in photorefractive crystal using the laser interference technique with an improved Michelson interferometer. The experimental setup of this method is relatively simple and inexpensive. It can be applied in almost any optical laboratories. Large area (about 28.2mm2) two-dimensional square nonlinear photonic lattices have been produced in an iron-doped lithium niobate photorefractive crystal. We analyze the induced nonlinear photonic lattices by plane wave guiding, Brillouin-zone spectroscopy and far field diffraction pattern imaging. The period of the induced photonic lattices can be dominated easily. The induced photonic lattices can exist stably for a long time in the photorefractive crystal.

Optically induced two-dimensional photonic quasicrystal lattices in iron-doped lithium niobate crystal with an amplitude mask

We fabricate two-dimensional photonic quasicrystal lattices in iron-doped lithium niobate photorefractive crystal by a single amplitude mask. The experimental setup of this method is very simple and flexible without complicated optical adjustment system. It can be applied in almost any optical laboratories. We analyze the photonic quasicrystal lattices by plane wave guiding, Brillouin-zone spectroscopy and far field diffraction pattern imaging. The induced photonic quasicrystal lattices can exist stably for a long time in the photorefractive crystal. This method can be easily extended to generate more complex quasi-periodicity microstructures by designing the amplitude mask properly.

A simple method for fabricating two- and three-dimensional photorefractive photonic lattices microstructures

We demonstrate an approach for easy fabrication of two- and three-dimensional optically induced nonlinear photonic lattices microstructures in photorefractive crystal by applying spatial filter and amplitude mask. The experimental setup of this method is very simple and flexible without complicated optical adjustment system. It can be applied in almost any optical laboratories. Two-dimensional hexagonal, square and three-dimensional hexagonal nonlinear photonic lattices microstructures have been produced in an iron-doped lithium niobate photorefractive crystal. The period of the induced photonic lattices microstructures can be dominated easily. This method is easily extended to generate more complex photonic lattices microstructures in photorefractive crystals, such as quasicrystal lattice.

Optical fabrication of three-dimensional photonic lattices in LiNbO 3:Fe crystals with a single amplitude mask

We fabricate three-dimensional nonlinear photonic lattices in iron-doped lithium niobate photorefractive crystal for the first time by a single amplitude mask. The experimental setup of this method is very simple and flexible. The period of the lattices can be dominated easily. We analyze the three-dimensional photonic lattices by plane wave guiding and far field diffraction pattern imaging. The induced photonic lattices can exist stably for a long time in the photorefractive crystal.

Diffusion of iron in lithium niobate: a secondary ion mass spectrometry study

Iron-doped X-cut lithium niobate crystals were prepared by means of thermal diffusion from thin film varying in a systematic way the process parameters such as temperature and diffusion duration. Secondary Ion Mass Spectrometry was exploited to characterize the iron in-depth profiles. The evolution of the composition of the Fe thin film in the range between 600°C and 800°C was studied, and the diffusion coefficient at different temperatures in the range between 900°C and 1050°C and the activation energy of the diffusion process were estimated.

Optically induced three-dimensional nonlinear photonic lattices in LiNbO 3:Fe crystal

We fabricate three-dimensional nonlinear photonic lattices in self-defocusing iron-doped lithium niobate photorefractive crystal for the first time by an improved Michelson interferometer device. The experimental setup is simple and inexpensive. The period of the induced photonic lattices can be dominated easily. We analyze the three-dimensional nonlinear photonic lattices by plane wave guiding and far field diffraction pattern imaging. The induced nonlinear photonic lattices can exist stably for a long time in the photorefractive crystal.

Tumour cell death induced by the bulk photovoltaic effect of LiNbO3:Fe under visible light irradiation

This work reports a pioneer application of the bulk photovoltaic effect in the biomedical field. Massive necrotic cell death was induced in human tumour cell cultures grown on a bulk photovoltaic material (iron-doped lithium niobate, LiNbO(3):Fe) after irradiation with visible light. Lethal doses (≈100% cell death) were obtained with low-intensity visible light sources (10-100 mW cm(-2) irradiances) and short exposure times of the order of minutes. The wavelength dependence to induce the lethal effect observed is consistent with that corresponding to the bulk photovoltaic effect generation in LiNbO(3):Fe. Necrosis also occurred when cultured tumour cells were exposed to LiNbO(3):Fe microparticles and visible light.

Direct Laser Writing of Refractive Index Micro-Structures in Lithium Niobate

We report on micro-structuring of iron-doped lithium niobate using femtosecond direct laser writing (DLW) technique. Formation of extended, stable index modulation micro-structures, such as diffraction gratings and linear waveguides is achieved by translation of a focused femtosecond laser beam inside lithium niobate crystals. From diffraction efficiency of the recorded gratings refractive index modulation amplitude of n ~ 10-3 is estimated. It is demonstrated that the recorded structures can be optically erased and re-written.

Light-induced scattering of femtosecond laser pulses in iron-doped lithium niobate crystals

Self-amplification of weak scattered coherent light waves in photorefractive crystals leads to losses, known as light-induced scattering or holographic scattering. We find with 532 nm light that it is reduced in LiNbO3:Fe for femtosecond laser pulses as compared to cw laser light. Light-induced scattering of pulses is completely absent in samples with sufficiently small Fe2+ content, in contrast to the scattering of cw light. Additional differences include a slower buildup time, a weaker Bragg selectivity, and a narrower angular distribution of the scattered light for pulsed illumination. The differences can be attributed mainly to the smaller temporal coherence of pulses.

Photocarrier transport in iron-doped potassium lithium tantalate niobate studied by time-of-flight measurement

The photocarrier mobility of Fe 0.03 wt%-doped potassium lithium tantalate niobate (K 0.95Li 0.05Ta 0.61Nb 0.39O 3) was investigated by time-of-flight (TOF) measurement. The longitudinal photocarrier response due to pulsed excitation leads to values of the drift mobility of μ h = 1.45 × 10 −2 cm 2/V s for holes, μ e = 0.325 × 10 −2 cm 2/V s for electrons, and a value for the range of holes ( μτ) h = 4.38 × 10 −5 cm 2/V at room temperature and at low field 3 KV/cm. The response time of holes and electrons (or the relaxation time) is determined to be 3.02 × 10 −3 s and 3.74 × 10 −3 s, respectively. The mobility of holes strongly depends on the field strength, and is observed to decrease with increasing bias field.

Gap solitons in defocusing lithium niobate binary waveguide arrays fabricated by proton implantation and selective light illumination

Photovoltaic photorefractive binary waveguide arrays are fabricated by proton implantation and selective light illumination on top of an iron-doped near stoichiometric lithium niobate crystal. Linear discrete diffraction and nonlinear formation of gap solitons were investigated by single-channel excitation using Gaussian light beams coupled into either wide or narrow waveguide channels. The results show that, at low power, linear light propagation leads to discrete diffraction, whilst for higher input power the focusing mechanism dominates, finally leading to the formation of gap solitons in the binary waveguide arrays. Our simulation of light propagation based on a nonlinear beam propagation method confirms the experimental findings.

Photorefraction in LiNbO3:Fe crystals with femtosecond pulses at 532 nm

Photorefractive index gratings are written into iron-doped lithium niobate crystals using femtosecond laser pulses and cw light, both having a wavelength of 532 nm. The saturation value of the refractive index changes in as-grown samples is found to decrease with increasing peak pulse intensity. Furthermore, in oxidized crystals, writing with femtosecond pulses is much faster than with cw light and retains about the same writing speed than in as-grown crystals. We propose a charge transport model that addresses the special case of recording with high intensity femtosecond pulses.

Holographic phase conjugation through a sub-wavelength hole

Holographic phase conjugation is analyzed as a method to create a photo-refractive lens with high numerical aperture. For this purpose a sub-wavelength hole is drilled into a metal surface directly on top of an iron-doped lithium niobate crystal. An interference pattern generated by the light coming from this point source and a plane reference wave is recorded. By using the phase-conjugated reference wave for read-out, a light wave being focused onto the former point source is reconstructed. In principle, a focusing system close to the theoretical diffraction limit could be implemented by this method. The performance of this arrangement is mainly determined by properties of the lithium niobate crystal, especially the crystal symmetry. Experimentally, the tight holographic focusing is demonstrated. The focus width of the reconstructed wave is shown to be below 1.2 μm, which is our spatial resolution. The diffraction efficiency obtained, however, is just 3 x 10- 5 compared to 3 x 10- 2 in the plane-wave case. This can be explained by experimental reasons, the inhomogeneous light intensity and limitations originating from the crystal symmetry. We estimate that the diffraction efficiency for phase conjugation through a sub-wavelength hole can be improved by three to four orders of magnitude by addressing the above-mentioned issues.

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.

Suppression of photorefractive damage with aid of steady-state temperature gradient in nominally pure LiNbO3 crystals

Photorefractive damage (PRD) in as-grown, oxidized, and slightly reduced nominally pure LiNbO3, and iron-doped crystals with different compositions has been studied with the closed-aperture Z-scan and pseudo-Z-scan techniques at uniform temperature distribution, as well as at a steady-state temperature gradient ∇T, created by two external thermoelectric elements in the transverse direction to the light beam trajectory. The most important experimental finding consists of the demonstration of the possibility of a full PRD suppression in nominally pure crystals with the aid of a relatively small temperature gradient (12.5–84 K/cm), if ∇T is above a threshold value specific for each crystal studied. The threshold ∇Tth decreases significantly with the increase in the so-called bipolaron absorption band (center at 470–500 nm) in the optical spectra, and it is not correlated with the composition of the crystals within the studied range of compositions ([Li]/[Nb]=0.946–0.983). No any partial suppression of PRD was observed in iron-doped ([Fe]≥0.01 wt %) lithium niobate crystals, even at the largest temperature gradient (85 K/cm) used in our study. To explain these experimental results, we use theoretical model taking into account local changes of spontaneous polarization (i.e., polarization charge field) at photoionization of intrinsic defects (polarons, bipolarons, and hole polarons). It has been shown that a steady-state temperature gradient may induce a local thermoelectric current of light-induced charge carriers and their instant recombination, resulting in a decrease in the polarization space-charge field and hence, a light-induced refractive index change. The latter mechanism may induce the full PRD suppression, if macroscopic charge separation (space-charge field effect) gives insignificant contribution to the total light-induced electric field, as in as-grown and reduced nominally pure LiNbO3 crystals.

Depth Resolved Study of the Composition and Polaron Luminescence of Fe:LiNbO3 Diffused Crystals

Iron-doped lithium niobate crystals attract great attention for practical applications such as holographic storage systems, all-optical photonic devices and reconfigurable narrow band holographic filters. In this work we present some preliminary experimental results on the investigation of compositional and photorefractive properties at the surface of iron doped lithium niobate crystals prepared by thermal diffusion in oxygen atmosphere of a thin film deposited by magnetron sputtering. In particular both the Secondary Ion Mass Spectrometry and confocal polaron microluminescence are exploited.

Effect of Jahn-Teller distortions on the structural and magnetic ordering in perovskite-type transition-metal oxides

The effect of local and cooperative distortions of oxygen octahedra containing Jahn-Teller ions on the elastic, electric, magnetic, and optical properties of lightly doped lanthanum strontium manganites and irondoped lithium niobate ferroelectric has been experimentally investigated. The formation of nano-and microscale clusters and domains around, respectively, Mn3+ and Fe2+ Jahn-Teller ions at phase transitions and in magnetic or electric fields were studied. A model is proposed to consistently explain the results of the investigations of Jahn-Teller ions in magnetic and/or electrically ordered materials.