Use of the circular Dammann grating in angle measurement

We describe a novel method of angle measurement by borrowing the concept of the circular Dammann grating (CDG). A three-order CDG is employed in this experiment. The displacement of the tilted angle can be determined accurately by measuring the projection from the distorted CDG image. This method is controlled only by the initial radius of the image and the converging ratio of the lens. Compared with conventional techniques, this technique has the advantages of a simple design with superior resolution to within 1 degree, low cost, and compactness. A theoretical analysis together with experimental results is presented.     

Three-dimensional complex image coding using a circular Dammann grating

Recently, optical image coding using a circular Dammann grating (CDG) has been proposed and investigated. However, the proposed technique is intensity based and could not be used for three-dimensional (3D) image coding. In this paper, we investigate an optical image coding technique that is complex-amplitude based. The system can be used for 3D image coding. The complex-amplitude coding is provided by a circular Dammann grating through the use of a digital holographic recording technique called optical scanning holography. To decode the image, along the depth we record a series of pinhole holograms coded by the CDG. The decoded reconstruction of each depth location is extracted by the measured pinhole hologram matched to the desired depth. Computer simulations as well as experimental results are provided.     

Simple focal-length measurement technique with a circular Dammann grating

A novel technique for focal-length measurements with a circular Dammann grating is presented. In the back focal plane of the lens under test, a one-order circular Dammann grating with limited aperture will produce double-humped radial rings. The separation between the two lobes varies with the displacement of the observed plane from the focal plane of the lens. By searching for the position at which the separation is minimal, the focal point of the lens can be located and hence the back focal length can be determined. Experimental results demonstrated that this method is efficient and can be used effectively for a quick check of focal length.     

All-optical switchable holographic Fresnel lens based on azo-dye-doped polymer-dispersed liquid crystals

Fabrication of an all-optical switchable holographic liquid crystal (LC) Fresnel lens based on azo-dye-doped polymer-dispersed LCs is reported using a Michelson interferometer. It is found that, upon circularly polarized photoirradiation, the diffraction efficiency of the fabricated Fresnel lens was increased significantly in a reversible manner. We believe this is due to the anisotropy induced by reorientation of the LC molecules coupled with azo-dye molecule orientation due to trans-cis-trans photoisomerization, which modulates the refractive index of the LC-rich regions. We also studied the effect of azo dye on the polarization dependency of the fabricated lens.     

Circular Dammann grating under high numerical aperture focusing

Circular Dammann grating (CDG) under high numerical aperture (NA) focusing is described based on Richards-Wolf vectorial diffraction theory in this paper. Several CDGs are presented under the condition of NA=0.9 with the illumination of circularly polarized plane-wave laser beams. Numerical results show that the sizes of these circular patterns with equal-intensity are in the wavelength scale, and doughnut-shaped central spots and dark rings are in the subwavelength width. To verify this kind of CDG, a binary pure-phase three-order CDG is fabricated to produce a dark center pattern surrounded by three concentric bright rings. The corresponding intensity distribution of the pattern on the focal plane of a high-NA objective (NA=0.9) is measured, and the results agree well with theoretical simulations. This kind of CDG with annular patterns of equal-intensity in the wavelength scale should be highly interesting for its potential applications in optical trapping, stimulated emission depletion (STED) microscopy, and the study of singular optics, as well as annular array illumination.     

Design of circular Dammann gratings by employing the circular spot rotation method

A new concept based on the theory of Dammann gratings is proposed for the generation of circular optical beams. This grating shows that it can achieve equal intensity and equal spacing with acceptable efficiency that is controlled by set of transition points. A numerical solution is also presented together with the fabrication of 4-order circular Dammann grating by e-beam lithography. Experimental results agree well with the scalar diffraction theory. This grating has the potential to be further developed into practical applications.     

Two-dimensional wavelet transform achieved by computer-generated multireference matched filter and Dammann grating

A two-dimensional wavelet transform is optically performed in real time by use of a new multichannel system that processes the different daughter wavelets separately. The system, which is able to handle every wavelet function, relies on a Dammann grating for generating a multichannel array. All channels are processed in parallel by a conventional two-dimensional correlator. Experimental results applying Morlet-wavelet decomposition are presented.     

Splitting of femtosecond laser pulses by using a Dammann grating and compensation gratings

When a Dammann grating is used to split a beam of femtosecond laser pulses into multiple equal-intensity beams, chromatic dispersion will occur in beams of each order of diffraction and with different scale of angular dispersion because the incident ultrashort pulse contains a broad range of spectral bandwidths. We propose a novel method in which the angular dispersion can be compensated by positioning an m-time-density grating to collimate the mth-order beam that has been split, producing an array of beams that are free of angular dispersion. The increased width of the compensated output pulses and the spectral walk-off effect are discussed. We have verified this approach theoretically and validated it through experiments. It should be highly interesting in practical applications of splitting femtosecond laser pulses for pulse-width measurement, pump-probe measurement, and micromachining at multiple points.     

Polarization-dependent diffraction efficiency of a photorefractive volume grating and suppression of this efficiency

In terms of refractive-index ellipsoid of a uniaxial crystal, the relationship between the diffraction efficiency of a volume grating and the polarization state of a readout beam is theoretically analyzed. The direction of a refractive light beam and the corresponding refractive-index modulation will both be changed by a variation of the polarization state. In the polarization state of the readout beam, which may lead to a strong variation in the diffraction efficiency of the volume grating. This kind of polarization-dependent diffraction efficiency of a volume grating in an anisotropic crystal is extremely disadvantageous for some applications. A method to suppress the polarization-dependent diffraction efficiency by use of double volume gratings is presented, and experiments with LiNbO3:Fe crystal are also demonstrated. The experimental results indicate that this method can well suppress the polarization-dependent diffraction efficiency of a volume grating. Furthermore, the diffraction properties of the double volume gratings are almost independent of the polarization state of the readout beam. The relative values of the diffraction peaks are calculated on the basis of the relationship between index modulation and the state of polarization. The experimental values are in good agreement with the theoretical analyses.     

Polarization-dependent angle filters based on one-dimensional photonic crystals using mu-negative materials

We study transmission properties of one-dimensional photonic crystals consisting of mu-negative and positive index materials by using transfer matrix methods. The results show that transmission properties of the transverse electric waves depend on permittivity (?), while transmission properties of the transverse magnetic waves depend on permeability (μ); there exists a transmission band inside the single-negative gap in this periodic structure without defects, and the transmission band is insensitive to the incident angle for the transverse electric waves but sensitive for the transverse magnetic waves. This property can be used to make polarization-dependent angle filters.     

Three-dimensional sensing by using a lenslet, a Dammann grating, and a combination

In the last two decades, three-dimensional sensors based on misfocusing have been suggested. This research addresses the question of measuring several focal depths simultaneously. Two options for generating the necessary array of spots are analyzed: the use of a lenslet array and the use of a Dammann grating. Finally, a combination of the two approaches is proposed. Such a combination enables tailoring the system performance to the exact needs of the user.     

Manipulation of photonic nanojet using liquid crystals for elliptical and circular core-shell variations

We present theoretical studies on a tunable photonic nanojet (PNJ) created by adapting a shell and liquid crystal (LC) core architecture. The shell is made of indium tin oxide and the core is infiltrated with nematic LCs. The application of an external static electric field to the LCs modifies their refractive index, and this allows tuning the PNJ effect in the proposed system. In addition to nonresonant excitation, resonant PNJ excitation is also obtained from a hybrid structure. Both nonresonant and resonant internal field excitations of circular and elliptical PNJ configurations are examined by using a high-resolution finite-difference time-domain method. The calculated results indicate that the proposed PNJ configurations with tunable refractive indices lead to significant changes in some parameters such as decay length, focal distance, full width at half maximum and electric field intensity. Such PNJ designs can be employed in high-resolution optical sensors, optical trapping, and high-density data storage.     

Structured light spots projected by a Dammann grating with high power efficiency and uniformity for optical sorting

We report a method for microfluidic multiple trapping and continuous sorting of microparticles using an optical potential landscape projected by a Dammann grating, enabling a high power-efficient approach to forming a composite two-dimensional spots array with high uniformity. The Dammann grating is fabricated in a photoresist by optical lithography. It is employed to create an optical lattice for multiple optical trapping and sorting in a mixture of polymer particles (n = 1.59) and silica particles (n = 1.42) with the same diameters of 3.1 μm. In addition to the exponential selectivity by the projected optical landscapes, the proposed microfluidic sorting system has advantages in terms of high power efficiency and high uniformity due to the Dammann grating.     

Photorefractive liquid crystals

Photorefractive liquid crystals represent the newest class of photorefractive materials. The low electric fields required for reorienting liquid crystals, combined with their high birefringence, results in the observation of photorefractivity with very low optical intensities and low applied fields. Photoinducing efficient charge transport over macroscopic distances is the primary hurdle for producing the space-charge field required for photorefractivity.     

Photoconducting liquid crystals

Major recent advances: ‘High’ mobility photoconduction in the columnar mesophases of disc-shaped (discotic) liquid crystals in which the charge carriers are holes or electrons was discovered in 1995. Prior to this photoconduction in liquid crystals was attributed to photo-generated ions and associated with ‘low’ mobilities. Over the last 7 years our understanding of the mechanism of carrier generation and transport in these novel, self-assembling systems has progressed to the point where we are able to design and manufacture organic semi-conductors with well-defined electronic and physical properties. Serious commercial devices incorporating conducting liquid crystals are finally on the horizon.     

Absolute strain measurements made with fiber bragg grating sensors

A strain sensor system based on optical fiber Bragg gratings (FBGs) is proposed with a new matched-filter design. The strain variation on the sensor FBG is continuously followed and matched by a filter FBG by use of a feedback control loop that produces an identical strain condition on the filter FBG. The matched strain on the filter FBG is then determined from the resonance vibration of the fiber piece embedding the filter FBG. The implementation and the performance of the proposed system are described. It is demonstrated that the proposed system can distinguish strain variation on the sensor FBG with resolution of one microstrain.     

Bioinspired supramolecular liquid crystals

A brief account on the historical events leading to the discovery of self-assembling dendrons that generate self-organizable supramolecular dendrimers, or supramolecular polymers, and self-organizable dendronized polymers is provided. These building blocks were accessed by an accelerated design strategy that involves structural and retrostructural analysis of periodic and quasi-periodic assemblies. This design strategy mediated the discovery of porous helical supramolecular structures that self-assembled from dendritic dipeptides. Helical porous columns are the closest mimics of biologically related structures, such as tobacco mosaic virus coat, porous transmembrane proteins, porous pathogens and antibiotics. It is expected that this concept will allow one to investigate the structural origin of functions in synthetic supramolecular materials.     

Sinusoidal shearing of liquid crystals

Two types of processes are shown to be possible, without external forces, in nematic liquid crystals. One is a spatially sinusoidal unsteady planar flow with spatially uniform temperature and the other is a temporally sinusoidal flow with temperature fleld constant in time. Specific solutions are obtained for a particular constitutive law proposed by Leslie; a shearing flow which tends to Ericksen's universal static orientation pattern and a temporally sinusoidal standing twist wave between parallel plates. The latter solution suggests an experimental means of determing one of the material parameters.