The method we developed both theoretically and experimentally in this thesis is called two-center holographic recording. 132 4.3.1 Effect of carrier mobility in normal holographic recording 133 4.3.2 Effect of carrier mobility in two-center holographic recording.
List of Figures
In this calculation, we assumed that the recording takes place in the simultaneous presence of a sensitizing beam and two red beams (wavelength 633 nm, intensity of each beam 300 mW/cm2). The reading is performed by one of the recording beams without the presence of UV light.
List of Tables
Chapter 1 Introd uction
- Holography: basic idea and applications
- Holographic recording materials: organic and
The wave vectors of incoming and diffracted waves must match a lattice vector in the hologram (conservation of momentum). Bragg mismatch can be achieved by rotation of the sample or of the reference beam (angular multiplexing), by changing the wavelength of the readout light (wavelength multiplexing), or by several other techniques.
InorganIc
Persistent holographic recording methods: copy- ing and all-optical schemes
The techniques can be grouped into two categories: (1) Copying the electronic space charge pattern to. The domain pattern created is a replica of the space charge pattern and the polarization changes are compensated by a modulated concentration of electron sources and traps [32, 33].
Outline of the thesis
We then explain the main disadvantage of two-step absorption as the short lifetime of the electrons in the shallow polaron levels. Finally, the main conclusions of the thesis, together with guidelines for future research into the various aspects of two-center uptake, are summarized in Chapter S.
Chapter 2 recording
Two-step holographic
Introduction
Having a reliable model is very helpful in understanding the physical mechanisms responsible for two-step uptake and explaining the experimental observations. We present in this chapter a complete theoretical analysis of the two-step absorption processes in congruent iron-doped lithium niobate.
Experiments
Next, we use some approximations to derive an analytical solution to the model equations. The angle of intersection of the infrared pulses and the wavelength of the light determine the edge spacing A.
Two-center model
An additional beam splitter splits the infrared light into two coherent waves of equal intensity. The diffraction efficiency is defined as the ratio of the intensities of the diffracted and total transmitted light.
He-Ne laser
Nd:YAG
Electrons can be excited from Fe2+ by light either into the conduction band or into Nb~;- forming Nbi;-. However, infrared light (wavelength 1064 nm) has a smaller photon energy that is sufficient to excite only electrons from Nbi;- into the conduction band.
Conduction Band
Valence Band
Numerical solution
- Algorithm
- Shape and evolution of the space- charge field
Thus, the amplitude of the space charge field modulation can be easily determined from a sinusoidal fit to the calculated data. The evolution of the amplitude of the space charge field during recording and erasing is shown in Figure 2.5.
Recording Erasure
- Intensity and concentration dependences
We experimentally investigated the dependence of the saturation value of the generated modulations of the refractive index and the time constant on the intensity of green and infrared light [41, 43]. Only two parameters remain free and can be varied to explain all these dependencies, the photon absorption cross section qX,IRSX,IR and the bulk photovoltaic coefficient K:x,IR of the Nb~i/5+ center for infrared light.
Analytic solution
- Assumptions
- Fourier development
- Solution of the zeroth order equations
- Solution of the first order equations
- Saturation space-charge field
- Time-dependence of space- charge field
- Simplified formulas
- Comparison with numerical solution
Equation (2.47) clearly shows the dependence of the saturation space charge field (and thus the strength of the saturation hologram) on the sensitization and recording intensities. This formula does not show the variation of the space charge field within the individual pulses.
Explanation of the experimental observations
Although the physical mechanisms in two-step holographic recordings with high-intensity pulses are similar to those of normal recordings, the intensity dependence of the saturation space charge and the recording rate are different in the two cases. This explains the experimentally observed dependence of the uptake rate on Ia and lIRa, shown in Figure 2.12. Therefore, at very high intensities we can observe a quadratic dependence (alIa + a2n) of the uptake rate with Ia.
The intensity dependence of the strength of the saturation hologram in two-step recording (space charge field or .6.n) is puzzling, as it is very different from that in normal recording.
Application of the model to two-step record- ing with cw light
In summary, the simple model, based on equations (2.68) and (2.67), gives us a complete understanding of the physical mechanisms involved in two-step holographic recordings with high-intensity pulses, and helps us to understand and exploit the experimental observations explain those that were not. all explained before. Furthermore, it helps us understand the main drawbacks of the method and suggests ways to improve it.
Discussion
Another reason for this large intensity requirement is the weak population mechanism of the shallow traps by direct electron transfer from deep traps. This is due to the fact that electrons in this case are transferred from the shallow traps to the deep traps via the conduction band. This assumption (neglecting the direct depopulation of the shallow traps) is of course unphysical as it only considers electron transfer from the deep traps to the shallow traps and not the reverse transfer.
Depopulation of the shallow traps in this case is performed by the read light while reading through the passband.
Conclusions
Chapter 3 recording
Two-center holographic
Introduction
Two-center holographic recording
Persistent holographic recording requires that the final hologram be stored in the Mn centers to persist against further reading with red light. Consequently, it is essential for persistent recording that all Fe traps are empty and only a portion of the Mn traps are filled. This ensures that the final hologram can be recorded in the Mn trap after a sufficient readout.
Experiments
- Experimental setup
- Sensitization and bleaching experiments
- Holographic recording experiments
To investigate the dynamics of sensitization, we monitor the absorption of red light by the crystal. UV sensitization fills some of the Fe traps resulting in the dark appearance of the crystal as shown in Figure 3.4. Bleaching with red light depopulates the Fe traps resulting in the transparent appearance of the bleached part of the crystal as shown in Figure 3.4.
Then the center of the sensitized crystal is bleached with a strong red beam (wavelength 633 nm, intensity 300 m vV / cm2; ordinary polarization).
Theory
- Two-center model
- Parameters of the model
- Comparison with the experimental results
- Effect of sensitizing and recording intensities
- Importance of sensitizing light
Most values are determined using the referenced experimental data curves in the literature. The complication in the theoretical calculation is due to the variation of Ni within the thickness of the crystal (as a result of the large UV absorption). Therefore, we get a broad peak in the variation of the final diffraction efficiency with UV intensity while the recording intensity is fixed.
Any change in one of the gratings results in a corresponding change in the other.
Optimization of two-center recording
- Effect of Fe concentration
- Effect of Mn concentration
- Effect of annealing
As Figure 3.13 shows, the final modulation depth of the electron concentration in the Mn traps is much smaller than that before readout. In this calculation, we assumed that all Fe traps are initially empty and 90% of the Mn traps are occupied by electrons. This explains the appearance of a maximum in the variation of the M/# with the concentration of the Mn traps.
In the simulation, it is assumed that all Fe traps are empty, and 90% of the Mn traps are filled with electrons.
Effect of sensitizing wavelength
Discussion
In this section, we discuss the performance of the two-center holographic recording from a system point of view. In Chapter 4 we will discuss a more accurate definition of J\l1/# for two-center holographic recording. However, the absorption of the sensitizing beam in two-center holographic recording is typically much greater than that of the recording beams.
Using two-center imaging we can record holograms in slices or even small spots of the crystal.
Conclusions
This property is the result of the sensitization process, i.e. holograms cannot be recorded without the presence of a sensitizing beam. The sensitizing beam (UV) and the reference beam (red) are focused by a cylindrical lens to illuminate only the part of the material in which the hologram is recorded. The signal beam does not significantly affect the holograms recorded in other wafers due to the insensitivity of the deeper traps (Mn centers) to the recording light (red).
Using two-center recording, we can also record holograms in small spots of the crystal by focusing the sensitizing beam to a small spot.
Doubly doped crystal
Chapter 4 System issues in two-center holographic recording
- Introduction
- Improving sensitivity in two-center recording by using a shorter recording wavelength
- Two-center recording experiments using 514 nm light for recording
- Partial loss of persistence III recording with 514 nm light
- Effect of carrier mobility in holographic record-
- Effect of carrier mobility in normal holographic record- mg
- Effect of carner mobility III two-center holographic recording
- Summary
- Reduction of fanning in two-center recording
- Multiplexing holograms In two-center record-
- Dynamics of recording and erasure in two-center record-
- Hologram multiplexing experiments
- Hologram multiplexing using incremental recording
- Conclusions
It is caused by the recording of unwanted holograms during both recording and reading of the stored information. As Figure 4.1 shows, the absorption coefficient of the crystal (which is linearly proportional to the absorption cross-section of the Fe traps) is much smaller at red than at blue (wavelength 488 nm). Readout of each hologram was performed by only one of the recording beams with the UV beam blocked.
Much faster sensitization (stronger UV) results in strong erasure of the hologram with UV light, while much faster bleaching (stronger green or red intensity) results in strong bleaching of the Fe traps. However, the variation of the persistent lVJ / # with /-1 in two-center recording is totally different from that in normal recording, as Figure 4.6 (b) shows. It can be seen from Figure 4.10 that the quality of the reconstructed image in two-center recording after 7 hours of readout is still comparable to the original quality.
Chapter 5 Comparison of two-step and two-center holographic recording methods
- Introduction
- General two-center model
- Theoretical comparison of two-step and two- center recording methods
- Effect of the concentrations of deeper and shallower traps
The main mechanisms we need to explain are sensitization, registration and dark depopulation of shallow traps (transfer of electrons from shallow to deep traps without light interference). The relative strengths of the holograms recorded in deeper and shallower traps depend on the properties of the traps as mentioned for the sensitization processes. In two-center recording, the strength of the hologram recorded in shallower traps is comparable to that recorded in
This is mainly due to the lack of dark depopulation of the shallow traps in the two-center recording.
