In this paper, the two-center recording with 633nm recording and 465nm sensitizing is performed in doubly-doped LiNbO3:Fe:Ru and LiNbO3:Ce:Ru crystals, respectively. The investigation for transmission spectra shows that the better transmission can be found from 400nm to 700nm in LiNbO3:Ce:Ru crystals. Performance measures, such as diffraction efficiency, recording sensitivity, are calculated and compared according to the time evolution curves of diffraction efficiency. The results show that the high recording sensitivity and dynamic range can be obtained in LiNbO3:Fe:Ru crystal. The scattering noise in LiNbO3:Ce:Ru and LiNbO3:Fe:Ru is investigated, the results show that the lower noise can be observed in LiNbO3:Ce:Ru crystal.
The absorption characteristic and nonvolatile holographic storage are investigated for the first time in Ce and Ru
codoped LiNbO3 crystals in this paper. The absorption spectra examination shows that there is a good optical
transmission in the blue and violet wavelength range in the as-grown and oxidized samples. An absorption peak around
530nm induced by Ru can be found. A photochromic effect is observed through UV illumination experiment.
Nonvolatile holographic recording is performed with different sensitizing and recording wavelength strategies. The
results show that the highest recording sensitivity (0.050cm/J) and dynamic range (2.7 cm-1) can be obtained with 514nm
recording and 405nm sensitizing.
One-color photorefractive holography has been investigated in the oxidized LiNbO3:Fe:Ru crystal with recording wavelength 632.8nm. The crystal shows a high asymmetric behavior in grating buildup and readout erasure rates. This remarkable property shows that the recorded grating is quasi-nonvolatile during readout. In this paper, the effect of the pre-sensitizing process and the readout intensity on the asymmetric behavior is investigated experimentally, the results show that the recording sensitivity can be increased by the pre-sensitizing process, and the grating lifetime can be extended by increasing readout light. At last, the reason for the high asymmetry is explained qualitatively in terms of a multiple center model.
A new design and fabrication of flat optical lens by use of 90-degree volume holographic grating recording geometry is presented. It is recorded by the interference of a plane wave and a spherical wave in a Fe-doped LiNbO3. The flat optical lens can laterally transfer and focus the plane wave which perpendicularly incidents on the planar crystal surface. The optimal switching from recording to thermal fixing is taken into consideration in order to obtain the nonvolatile hologram with maximum fixing efficiency. The flat optical lens with different recording and reconstructing wavelength has also been discussed. The flat optical lens has the advantages of light weight, laterally transferring and focusing, small duty ratio, and easy for the microstructure integration. The testing results measured verify that the flat optical lens can successfully be used for the free space optical communication.
The effect of dopants in doubly-doped LiNbO3 crystals on persistence of two-center recording is investigated experimentally and theoretically in this paper. Six kinds of doubly-doped LiNbO3 crystals are used in experiments with 633nm recording and 404nm sensitizing, the results show that LiNbO3:Ce:Cu crystal has the largest persistence. The dependence of persistence on bulk photo-voltaic coefficient and photo-excitation coefficient of deep center for recording light (K12 and S12) is discussed based on jointly solving the two-center material equations and the coupled-wave equations. The results show that both the increase of S12 and the decrease of K12 will result in the decrease of the persistence. When a holographic storage system is designed, the appropriate dopant will help to obtain the high persistence and dynamic range simultaneously.
The effect of variable sensitizing intensity on photorefractive performance in two-center recording is investigated
through jointly solving two-center material equations and the coupled-wave equations. Four kinds of schemes for
varying sensitizing intensity are designed, the relationship between the sensitizing intensity and recording time is simply
assumed to be linear, and at the same time the recording intensity is stable in recording process. The temporal evolution
of the space charge field (SCF) is obtained in each scheme. The results show that SCF increase monotonously no more in
the recording process. The increase of sensitizing intensity will reduce the peak value of SCF during recording process.
However, the decay of sensitizing intensity can enhance the peak value of SCF. The slow decay of sensitizing intensity is
also useful to the buildup of high SCF, but the recording time will be long. At last, the effect of the decay mode and the
initial value of sensitizing intensity on recording process are discussed. Both of recording sensitivity and the peak value
of SCF can be enhanced in decay scheme with the high initial sensitizing intensity. In a real application an appropriate
sensitizing light scheme needs to be designed.
Two-wave coupling in LiNbO3:Fe:Ru crystals is investigated experimentally and theoretically in this paper. The
dependence of gain coefficient on doped concentration, as well as the light intensity ratio between pump beam and signal
is experimentally investigated, the maximal gain coefficient 9.2cm-1 is found while the ratio is 2.2, light gain cannot be
observed in the reduced sample because of fan effect. Effect of phase difference between light modulation and the
refractive index grating on two-wave coupling is investigated based on jointly solving the two-center material equations
and the coupled wave equations, the variation of phase difference is obvious in the crystals with reduced process or high
doped concentration.
Holographic scattering noise in LiNbO3:Fe:Ru crystals is investigated in this paper. The effect of doped concentration
and the incident light intensity on the holographic scattering noise is investigated. The experimental results show that the
transmitted light intensity is fluctuated in the initial stage of scattering noise evolution, and the sample with highly doped
concentration has the lowest scattering noise level among three LiNbO3:Fe:Ru samples. The results are analyzed on the
basis of multi-wave coupling amplification mechanism and the saturated space charge field.
High diffraction efficiency of each grating is desired to reduce light intensity loss when gratings are integrated to realize
miniaturization for 3-D optical instruments in LiNbO3 crystals. Based on jointly solving the two-center material
equations, oscillating behavior of diffraction efficiency during recording and erasing process is investigated theoretically
to realize maximized diffraction efficiency for integrated volume grating instruments in weak oxidized doubly-doped
LiNbO3:Fe:Mn crystals. Two nonvolatile gratings are integrated and nonvolatile diffraction efficiency for each grating
exceeds 60%. These methods depend on the dopant elements and its concentrations, annealing (or oxidation-reduction)
in doubly-doped LiNbO3 crystals, and recording and sensitizing intensities and wavelengths. The experimental
investigation will be performed to verify the calculated results.
The recording and readout characteristics in doubly-doped LiNbO3 crystal with one-color scheme are investigated based
on jointly solving material equations and coupled-wave equations. Asymmetry between grating buildup and readout
process is found when electrons in deep centers can be excited by recording light; the grating is quasi-nonvolatile. The
shorter recording wavelength, and dopant in deep centers with a closer energy level to Fe in LiNbO3 crystal, can
strengthen such asymmetry. The further investigation shows that two aspects induce the quasi-nonvolatile behavior in
doubly-doped LiNbO3 crystal, one is the beam coupling between incident and diffracted beams, the other is two grating
form in both centers. This research provides a possible method to prolong the lifetime of grating in doubly-doped
LiNbO3 crystals.
LiNbO3:Fe:Ru crystal is the effective recording media with high recording sensitivity for two-center recording, and the
physical mechanism for the high recording sensitivity is investigated theoretically and experimentally. The results show
that the energy level of Ru perhaps is closer to that of Fe than that of Mn in LiNbO3 crystal, the electrons in Ru center
can be excited more effectively into the conduction band in the same sensitizing conditions, which can induce the
improvement of the recording sensitivity. The recording sensitivity 0.044cm/J in LiNbO3:Fe:Ru crystal, which is ten
times larger than that obtained in LiNbO3:Fe:Mn reported early. However, the elevation of energy level of deep center
will induce that the electron excitation from deep center by the recording light become more effective, and the
persistence decreases with the recording sensitivity increase, the grating in Ru center can be erased by red light obviously.
In practical application people must take a trade off between the recording sensitivity and persistence.
An oscillatory characteristic of diffraction is observed during holographic recording period in an oxidized LiNbO3:Cr:Cu
crystal with 514 nm green light as the recording light and 390 nm UV light as the sensitizing light. The optimal
switching time from the recording step to the fixing step for high diffraction of a fixed hologram is studied. It is shown
that switching after the first diffraction maximum leads to higher fixed diffraction efficiency. The theoretical explanation
is presented according to time-space dynamic theory of the nonvolatile holographic recording in doubly-doped LiNbO3
crystals.
The domain inversion and electrochromism in congruent RuO2:LiNbO3 crystals are investigated at room temperature, and the relations between them are proposed. During electric poling process, the electrochromism accompanies the ferroelectric domain inversion simultaneously, and is localized to the same region with domain inversion. As domain inversion, the electrochromism is completely reversible when the domain is inverted from the reverse direction. The properties, coincidence and localization, between domain inversion and electrochromism are proved solidly by the digital holographic interferometry, the real-time measurement of optical transmittance change and poling current, and the micrographic analysis after being etched in hydrofluoric acid. The influences of annealing conditions on electrochromism are also discussed. We provide the reasonable assumptions and analyses that the charge redistribution within the crystal caused by domain inversion is the source for electrochemically oxidation and reduction of Ru ion, and the intervalence electron transference between Ru4+ and Ru3+ plays a key role in the spectrum shift within different spectral range by the change of the photon absorption.
The 90-degree recording geometry has special advantages in suppressing holographic scattering and can be used for designing of compact volume grating components, but the main drawbacks are low diffraction efficiency and small sensitivity. In this paper, we optimized the components and oxidation states of LiNbO3:Fe crystals for high diffraction efficiency during 90-degree holographic recording. The experimental results show that the samples with moderately doped components of Fe ions and high oxidation states have high diffraction efficiency.
Holographic recording experiments of doubly-doped LiNbO3:Fe:Ni crystals were conducted by three kinds of different two-color recording schemes. The results show that the saturation diffraction efficiency, the fixing diffraction efficiency, and the recording sensitivity of oxidized LiNbO3:Fe:Ni crystal are higher than those of other reported doubly-doped LiNbO3 crystals. Based on the doped energy-band diagram, the effect of microcosmic optical parameter of the deep trap center on holographic recording properties of doubly-doped LiNbO3 is analyzed theoretically. LiNbO3:Fe:Ni has the potential of being a new highly efficient nonvolatile holographic recording material.
In this paper two-beam interference experiments are performed to investigate the diffraction characteristics of volume gratings in Fe:LiNbO3 crystals (0.15wt% Fe2O3), the results show that the dependence of the diffraction characteristics on the oxidization-reduction state and the oscillatory diffraction can be found in the reduced crystals. The theoretical analysis shows that the strongest space charge field can be formed in the reduced crystals, which induce the strong refractive index change, and because of the sin2 relation between refractive index change and diffraction efficiency, the oscillatory diffraction can be found in the reduced crystals. In application for high diffraction efficiency the oxidized treatment should be performed for Fe:LiNbO3 crystals so that the depth integral of saturated refractive index change is equal to π/2.
Dopants in doubly doped lithium niobate crystals are crucial for properties of nonvolatile holographic recording. In our experiments, a series of possible doubly doped congruent LiNbO3:Fe:X (X=Mn, Cu, Rh, Ru, Ni) are proposed and investigated for nonvolatile holographic recording. The experimental results demonstrate that the dopants with different distances of energy band to conduction band have different recording efficiency. Further analysis approve that the dopants which have moderate energy-band distance to conduction band can perform recording with both high diffraction efficiency and long lifetime. The compared results show that LiNbO3:Fe:Ni is promising for nonvolatile holographic recording, and its recording conditions are optimized.
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