Since it was discovered that hippocampus plays a vital role in human memory, enormous investigations on brain memory have been carried out in the last a few decades. However, so far, there has been no report demonstrating how and in what form that the memory is encoded, stored and retrieved. In this paper, we propose a holographic model with theoretical analysis to describe the mechanism of memory encoding, storage and retrieval in brain. The model suggests that memory encoding in brain is achieved through the process of hologram generation in synapses via the interference of the signal wave carrying the outside information and the energy wave emitted by neurons of hippocampus. The holograms are formed by modulated synapses with the structures of the interference patterns. In memory retrieval process, hippocampus emits the same energy wave as that used for encoding with consciousness to induce the reconstruction of the signal wave from the hologram. Experiment was carried out to simulate hologram encoding and retrieval. The discussion presented in the paper shows that the proposed model can reasonably explain some phenomena related to brain memory.
Fabricating 2-dimensional (2D) photonic crystals (PCs) on InSnO2 (ITO) layers of LEDs can largely enhance light
extraction efficiency. The paper presents the theoretical analysis for determining the optimal lattice constants for
PC-LEDs to acquire more efficient light extraction. The equations for obtaining optimal hexagonal lattice constants
corresponding to different LED wavelengths were derived. Calculation shows that for 450 nm GaN-LEDs, the optimal
lattice constant is about 500 nm, and for the lattice constants in the range from a few micron to 500 nm, light extraction
increases with the reduction of lattice constant. In the experiment, hexagonal PCs with 1.8 micron, 1.5micron and 1.2
micron lattice constants were fabricated in ITO layers of GaN-LEDs using holographic 3-beam interference approach
and wet etching, resulting in 6%, 12% and 22.5% output increase respectively under 20 mA current injection. The
experimental result demonstrates that higher light extraction is acquired with the lattice constants closer to the optimum,
which is consistent with the theoretical analysis.
A single-exposure holographic technique for fabricating 2-dimensional photonic crystals (PCs) with hexagonal lattices is
presented. A specially made holographic optical element (HOE), which consists of three holographic gratings, is used to
generate three interference beams simultaneously under the illumination of a single laser beam. Theoretical analysis
indicates that by selecting appropriate grating period and illuminating wavelength, the hexagonal lattice patterns formed
by the interference of the three beams can have the lattice constants in the range from micron to sub-micron. Moreover,
to obtain uniform lattice pattern with high contrast, the gratings in the HOE must have proper grating shape and depth. In
the experiment, hexagonal lattices with lattice constants from 5 μm to 620 nm were fabricated in photoresist using the
method. The patterned photoresist layers can be used as the masks to generate PCs in semiconductor materials through
dry or wet etchings.
Small-angle beam deviation has many applications and is conventionally performed by wedge prisms. The usual method
suffers from a disadvantage that the manufacturing tolerances limit the resolution to about an arc second. The paper
presents a new technique of using low frequency gratings to realize small-angle beam deviation with higher resolution.
By rotating a grating placed in the laser beam path, the deviation of the diffracted light beams can be controlled. Using
the proposed method, very small beam deviation angle can be achieved under large grating rotation angle, resulting in
high reduction ratio. Theoretical analysis shows that as long as the grating period is large enough, the reduction ratio can
be much higher than that given by a wedge prism. The theoretical result has been verified experimentally using a
holographic grating with 7.25 μm period placed in a Mach-Zehnder interferometer.
2-channel multiplexer/demultiplexer (Muxer/Demuxer) is a key component for bidirectional data traffics applied for
optical communication. Up to date various types of Muxer/Demuxer have been proposed and demonstrated. A grating
coupler diffracts light into substrates or waveguides, along which light beam propagates by total internal reflection. In
addition, one can exploit the dispersive and filtering characteristics of gratings, for dropping or separating one or several
wavelengths from one another. When a laser beam containing two wavelengths is striking the surface of the grating with
an incident angle within certain range, four diffracted beams will be generated. If two diffracted beams, corresponding to
different wavelengths, meet the condition of total internal reflection, they will propagate inside the glass substrate
(performs as a waveguide). While the third one cannot meet total reflection condition, and the last one should become
the evanescent wave. Therefore it can separate two signals and couple signals to different waveguides. These functions
are suited for WDM application and directional couplers. For convenience sake, the visible lights at 458nm and 633nm
were used as the incident laser beams. To give a simple sample for 1×2 demultiplexing system, a holographic grating
was recorded, with the period around 441nm which was chose discretionally within the certain range. The primary
experimental results indicate that the two-wavelength signal can be separated and coupled into the respective waveguide
as long as the grating is recorded and operated complying with the certain condition. The average insertion loss and
crosstalk of the device were presented in this paper.
Using 2-dimensional (2D) photonic crystal with hexagonal lattice to extract guided waves inside slab waveguides is
investigated in the paper. Since 2D hexagonal lattice structure is circularly symmetrical, the effect of light extraction can
be regarded as the same for guided waves propagating toward different directions on the plane of the slab. Theoretical
analysis was carried out under the assumption that a hexagonal lattice consists of many groups of gratings with different
periods and orientations. Based on the assumption, the simulations of the extracted light-spot distributions on the
waveguides for the guided waves with different reflection angles inside the slab were obtained. The simulations are
consistent with the experimental result, verifying that the observed spot array above the slab surface, where 2D
hexagonal lattice was fabricated, was generated by the diffraction of the lattice. The light extraction method can be used
to separate a laser beam into many distributed beams for optical network interconnection applications and to extract
trapped light inside LEDs. Theoretical analysis indicates that to obtain efficient light extraction via hexagonal lattice,
lattice constant should be properly chosen. Method for choosing desired lattice constant is provided.
A new method for reducing surface reflectance of multicrystalline silicon wafers is presented. The method using
holographic lithography and wet etching to fabricate 2-dimensional (2D) photonic crystal (PC) on the surface of
multicrystalline silicon. 2D hexangonal PC structures with micron scale lattice constant exhibit significant reduction of
the surface reflectance. In the method, 2D hexagonal PC structure is firstly recorded in photoresist, which is coated on
the surface of Si wafer, using holographic 3-beam interference technique. After exposure and development the wafer is
put into acid solution to transfer the lattice structure into the silicon. Experiments with different exposure angles and
etching times were carried out to form different lattice period and structure depth for obtaining optimal lattice parameters.
PC with 1.3 μm lattice constant and 0.5 μm depth has achieved a reflectance below 6%. The holographic technique used
in the work allows large-area lattice fabrication with only one process. The proposed method has the advantages of low
production cost and high throughput, enabling industrial mass production of Si solar cells.
A new type of holographic screen used to display projected 3-dimensional (3D) images with large viewing
angle and relatively high brightness is presented. The screen is 2-dimensional hexagonal lattice relief in
micron-scale lattice period, fabricated in plastic sheet using holographic technique and pattern replication
method so that the fabrication of the screen is fast and cost effective and large-size screen can be obtained.
Theoretical analysis and experimental results demonstrate that larger lattice period generates more diffracted
rays thus provides larger viewing anlge and brighter image. 45° horizontal and 30° vertical viewing anlges
have been achieved and the images with parallax have been observed. The screen has the potential of being
applied for 3D holographic movie display and 3D art exhibition.
In this paper, we propose an improved hologrpahic technique for fabricating 2D hexagonal and 3D hexagonal packed
photonic crystals (PhCs). The key element in the fabrication system is an off-axis holographic lens group (OHLG)
consisting of three off-axis holographic lenses. Under the illumination of an expanded laser beam, three collimated
beams can be generated by the OHLG to form a uniform hexagonal lattice pattern without using a collimating lens. The
utilization of OHLG in the system eliminates the noise in the interference pattern resulted from the collimating lens so
that lattice quality is enhanced. The system is simple and stable and can be used to fabricate large-size PhCs.
One of the more intriguing of solar collection techniques, holographic concentration, has been addressed in many ways,
but it has so far met with only limited success. The major attraction of holography is that unlike conventional
concentrators, the holographic concentrator can track the sun across the sky while remaining completely stationary. A
simple system using holographic Fresnel lenses (HFLs) was proposed in this paper. We present the theoretical analysis
and the technique for fabricating HFLs used for solar concentrator. Several HFLs fabricated using corresponding
recording geometries, resulted in a combined device that collects light at any angle, with each lens responsible for
gathering sunlight for a certain portion of the day. Three HFLs with different fringe pattern were achieved, which
operates in the morning, at noon and in the afternoon, respectively. The primary experimental results of the concentrator
were presented, which effectively concentrates the visible spectrum over a 60 degrees daily angular variation. Several
advantages of the holographic concentrator we obtained include longer lifetime, low cost, and the small size of a flat
piece of film compared to a mirror or lens. The dispersive characteristics applied to photovoltaic power generation would
offer a novel type of generator with high conversion efficiencies.
Light-emitting diodes (LEDs) have the potential to become the main light source for the advantages of low energy
consumption, being environmental friendly and long lifetime. However so far the brightness of LEDs is not sufficient for
many applications due to low light extraction efficiency resulted from total internal reflection of the emitted light at
semiconductor/air interface. To overcome this problem, fabricating 2-dimensional photonic crystals (PhCs) on the
surfaces of LEDs is considered as one of the most effective ways. At present, the method of fabricating PhCs is primarily
through e-beam lithography and ion-beam etching. Such processes are complicated and unsuitable for low cost mass
production. In this paper we propose a novel method of using holographic and wet etching processes to fabricate PhCs in
p-contact ITO layers of LEDs. The PhC patterns are firstly fabricated in photoresist layers coated on LEDs, using
holographic approach, then acid solution are used to transfer the patterns into ITO layers. The experiments of adopting
different etching solutions and etching rates were carried out and the results were compared, in order to find out the best
etching condition. Experimental results demonstrate that solution HCl : HNO3 : H2O made less damage to photoresist
mask and can better control the etching depth in ITO. PhCs with different lattice constant and depth were fabricated to
obtain the best PhC parameters for higher light extraction. 30% increase of external quantum efficiency has been
achieved in the experiment.
Hot embossing has become a key technique in the replication of microstructures. In particular, the hot embossing of
diffractive gratings into thermoplastic has been widely used in the mass production of holograms. The process involves
the transfer of a surface relief structure from a plate to a substrate by high temperature and pressure applied to an
intermediate film. So in this paper a novel printing process for replication of holographic lens used as
multiplexer/demultiplexer has been demonstrated. Advantages of this technique are the low cost and ease of processing.
Some analysis indicates that for WDM in fiber communication, the thermoplastic polycarbonate is a good choice. In the
primary experiments, the optimum temperature and force for embossing were 130 degrees and 5Mpa. The optimal
thickness of a nickel shim used in the mesne process was established to be 60 micron. The lens arrays were uniformly
embossed over the entire area of the thermoplastic which was typically 8×8. Minimum diffraction efficiency of the
embossed lens in thermoplastic layer is about 50% (-3dB). High quality replicas of device have been fabricated using
this process. Printing via an embossing process offers the possibility of mass production of demultiplexer easily and
cheaply compared with conventional demultiplexer.
A technique using a holographic optical element to split one incident laser beam into hundreds is proposed. The holographic optical element is fabricated with hexagonal packed lattice structure using 4-beam interference method. When the element is illuminated by a single laser beam with normal incidence, hundreds of beams are generated by diffraction. The element has the potential to be used as the device for interconnection and clock distribution in optical and electronic systems.
A simple optical system for generating large-size hexagonal lattice used as the mask for photonic crystal LED
fabrication is proposed. The key element in the system is a holographic diffractive optical element consisting of three
phase gratings made with holographic means. Under the illumination of a single plane wave, three plane waves can be
generated and interfere to form a uniform hexagonal pattern. Theoretical analysis demonstrates that equal intensity of
the three plane waves can be obtained if groove depth of the gratings is about 400 nm, which can be achieved by
controlling the development time in grating processing. Mask on LED with lattice area up to 15 cm2 has been obtained
with the method.
A technique for fabricating optimized holographic diffractive optical element (HDOE) used for fabricating photonic crystals is proposed in this paper. The HDOE consists of three identical transmission gratings symmetrically placed around the center with 120° between each other. Usually the polarization of the diffracted beam of a grating will be changed from that of the incident beam depending on the orientation of the grating. As the orientations of the three gratings in a HDOE are different, the polarizations of the diffracted beams and their diffraction efficiencies are different, resulting in low contrast of the interference pattern. This problem can be solved by controlling the parameters in grating fabrication. Rigorous coupled-wave analysis is used to calculate the influence of grating depth and polarization angle on diffraction efficiency and polarization of diffracted beams. Calculated results demonstrate that under 458 nm illumination, if the three sinusoidal gratings in a HDOE have 0.729 μm period and 0.33 μm depth, the polarization and diffraction efficiencies of the three first-order diffracted beams will be equal. The experimental results are presented to prove the practicability of the technique.
Multiplexer/demultiplexer for WDM system based on off-axis holographic Fresnel lens was fabricated. The principle and fabricating technology of the demultiplexer with the central working wavelength of 1310 nm and the channel spacing of 25 nm were described. The reduction of third-order aberrations was the main consideration. The theoretical analysis indicates that third-order aberration can be effectively reduced if the condition tanαC/tanαR=-λC/λO is applied in geometry design and long object distance is selected. The experiment method and results were presented in this paper, and results verified the theoretical analysis.
A simple holographic 3D photonic crystal fabrication system is presented. The key element in the system is a HDOE made with holographic approach. The HDOE can generate four beams with single illuminating beam to form a 3D interference pattern. The incident angle of the outer beams and beam intensity ratio of the central beam to the outer beam can be adjusted in HDOE fabrication process. Computer simulation demonstrates that to obtain properly connected lattice structure in photoresist, beam intensity ratio and exposure dosage must be considered simultaneously. Experimental result has verified the effectiveness of the technique.
The two-step holographic technique of fabricating blazed gratings has been successfully applied to common positive photoresist. Influence of three fabrication parameters in the second step on groove profile of blazed gratings was studied experimentally. The three fabrication parameters are exposure, incident angle of laser beam and development time. The exposure distribution on the cross-section of one groove stripe of grating was modeled. Results of experiment and modeling with various exposures reveal that the technique is effective only when the symmetrical contour lines of exposure in the first step fade out with increasing second exposure. With other parameters fixed, experiments were also carried out with the beam incident angle varying from 10° to 80° and with the development time varying from 1 s to 9 s. Blazed gratings with quasi-triangular are obtained when incident angle reaches 40° and when development time reaches 5 s. Scanning electron micrographs of the results are presented. Blazed gratings with observed blaze angles of 20° to 50° were obtained by changing the incident angle. Efficiencies of different diffraction orders for gratings with various incident angles are also given.
Photonic crystal template can be fabricated by holographic lithography in photoresist. A method for enhancing the quality of holographic photonic crystal template is mentioned in this paper. Because light energy is absorbed by photoresist while propagating through the material, the exposure decreases with the depth of the photoresist layer. In order to compensate for the light energy losing in exposure process, the recording plate is flipped to let the interference beams incident on the side of glass substrate instead of the surface of the material. During the development, the etching is proceeding quickly into the lower part of the material while the structure on the surface is maintained. Experimentally, face centered cubic lattice structures were fabricated in the AZ-4620 positive photoresist using an Ar+ laser with wavelength of 458 nm. Good 3D lattice structures have been obtained using this method. Theoretical analysis, computer simulation and experimental results obtained with the interference beams incident on different sides of the photoresist plate are presented in the paper.
A novel holographic technique for fabricating three-dimensional photonic crystals (PhCs) by two-beam interference is presented in this paper. The optical setup in this method is much simpler and more flexible compared with other multi-beam interference methods, and large and uniform PhCs are easier to be obtained. In PhCs' fabrication, two coherent laser beams interfere and generate a set of two-dimensional interference fringes, which are recorded on a plate of photosensitive material. One laser beam is incident on the plate in normal direction and the other beam with an angle to the normal. Then with the laser beams maintained in a fixed relative position, the plate is rotated by 120 degrees about an axis through the center of the plate and a second recording is made. This procedure is repeated one more time, producing finally a superposition of three sets of interference fringes at angles of 120 degree to each other. After the chemical treatment, a three-dimensional PhC is fabricated in the material with a particular lattice structure that depends on the detail of the basic interference pattern. PhC with fcc lattice structure was fabricated in the experiment with the angle of 38.9 degrees between the two interference beams, verifying the effectiveness of the technique.
Off-axis holographic Fresnel lenses were designed and fabricated, with the aim of using them as multiplexer/demultiplexer in optical networks. The reduction of third-order aberrations was the main consideration. The theoretical analysis indicates that third-order aberration can be effectively reduced if the condition tanαC/tanαR = -λ2/λ1 is applied in geometry design and long object distance is selected. The experimental results verify the theoretical analysis.
A two-step technique of fabricating blazed holographic grating is presented. The grating fabrication includes two exposures and two developments. In the first step, two collimated laser beams with symmetrical incidence about the normal of the holographic plate generate a grating with quasi-sinusoidal groove profile after the first development. In the second step only one collimated laser beam irradiates on the plate with a selected incident angle, resulting in a change of groove profile into quasi-triangular after the second development. A blazed holographic grating with 65% relative diffraction efficiency under non-auto-collimation was obtained in the experiment. The scanning electron micrographs clearly show the quasi-triangular groove profile of the grating. The factors that affect the groove profile are discussed.
A novel technique is proposed to fabricate coded holograms based on dot-matrix holograms for security applicaitons. Fourier transform and inverse Fourier transform are adopted in the coding process. The principle of the coding is to introduce a phase function into the reconstructed wave function of a dot-matrix hologram and record a new hologram. As the result of the coding, the new hologram not only preserves the pattern of the original dot-matrix hologram but also has 3D effect, which is resulted from the phase distribution on the hologram plane. Theoretical analysis and experimental results are presented.
The parameters used for recording 3D rainbow holograms are analyzed theoretically. The analysis indicates that the object distance in recording Fresnel hologram is a key parameter for diminishing field curvature and image blur due to wavelength spread and extended source size. To obtain sharp image under extended illumination, the object distance must be reasonably large. The experimental results verify the theoretical analysis.
It is verified by the source-to-fiber coupling experiment that micro holographic lenses (MHLs) can be used as the outside coupler the light emitted form a laser diode to a fiber. Properly arranging the recording geometry using the equation presented in the paper to fabricate MHLs, the performance of a single MHL can replace a complicated optical coupling system, thus resulting in compact configuration, easy alignment and low cost. Proposals for further increasing the coupling efficiency are provided.
A new compact hologram display system is presented. An LED is used to illuminate the hologram directly in air with around 85 degrees incidence. No intermediate element is used between the light source and the hologram so that the system is extremely compact and the hologram recording process is simple. The mechanism of the generation of harmful interference fringes on the hologram is analyzed, and the proposed method eliminates the harmful fringes effectively.
The novel method of using holographic optical element (HOE) to realize optical clock distribution for optical interconnection is presented. Polarization holographic technique is adopted to fabricate the HOEs. 4-fan-out clock distribution has been obtained. Methods for obtaining multiple fan-out clock distribution using polarization holographic technique is proposed.
Theoretical analysis indicates that for micro holographic Fresnel lenses with the same diameter, the spatial frequency of the interference fringes of the MHFL recorded with two spherical waves is higher than that recorded with a spherical wave and a plane wave. Therefore two-spherical- wave arrangement results in higher diffraction efficiency. Experimental results have verified the theoretical analysis. To obtain an MHFL with small diameter and high spatial frequency of interference fringes, lense with large f-number are preferred in the recording geometry.
Techniques of making visible and concealed holographic secret codes based on optical moire phenomenon are described. Theoretical analysis, experimental results and the sample holograms with the secret codes are presented.
In this paper the demand for anti-counterfeiting holograms, the case examples of brand authentication, the current anti- counterfeiting techniques in China and the role of China Government in anti-piracy efforts are reported.
The paper introduces the methods of making secret codes in the holograms for the purpose of anti-counterfeiting, especially the production of two kinds of visual holographic secret codes. The optical arrangements for recording are presented, and the effective results from using the visual secret codes into holographic trade marks are reported.
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