Optical grating patterns

A nanoimprint-based patterning technique that can be applied in a continuous roll-to roll process to drastically increase the patterning speed is thus an attractive proposition. Continuous roll-to-roll nanoimprint processes are capable of replicating 300 nm line width optical grating patterns on both hard glass and flexible plastic substrates have been demonstrated [60, 61]. 

Pseudostochastic random binary sequences are noise-like time patterns. They are defined at times n A t and assume only two different values, corresponding to the grating amplitudes -1 and +1, if 180°-phase modulation is used for switching off the optical grating. Only software modifications, and no changes in the hardware of the TDFRS setup, are necessary in order to utilize pseudostochastic excitation sequences. The timing for heterodyne/homodyne separation is identical to the one already described for pulsed excitation. 

Figure 1 Phase relationship between the optical interference pattern and the space-charge field. For liquid crystals, this example illustrates mobile anions migrating into the nulls of the interference pattern. The application of an applied electric field Ej is usually required to observe a phase-shifted photorefractive grating.

Fixed spatial phase in the grating pattern also facilitates experiments with multiple excitation pulses (20). A second, delayed pulse incident on the diffractive optic is split in the same manner as the first and results in a second excitation pattern with the same peak and null positions. Thus, multiple excitation gratings, delayed temporally and shifted spatially if desired, can be used for excitation of phonon-polaritons whose coherent superposition is well controlled. A preliminary experiment of this type has been reported (21). 

In order to produce surface-relief electro-optic gratings, Munakata et compared two fabrication methods of SRG inscription. In the first, the SRG was produced with an interference pattern of cw laser, with relatively modest intensities. The gratings so recorded were photo- and thermally erasable, and efficient writing was polarization dependent. In the second method, a phase mask was employed to provide the periodic intensity modulation of a pulsed laser, the 3rd-order harmonic (at 355 nm) of a Nd YAG laser. The SRG was produced with a single laser pulse, allowing a very short fabrication time (less than Is). The direshold for ablation was 500 mj/(em pulse), and the amplitude of the SRG increased with pulse energy. A depth of up to 300 rim could be achieved, leading to a smooth but not sinusoidal surface modulation. 

Besides microscopy, diffraction is another possibility for analyzing periodic patterns. The modulation of the refractive index is equivalent to an optical grating, thus illuminating with a laser beam the fringe pattern of the diffraction can be detected at a distant screen. This allows determination of the pattern wavelength as well as monitoring the pattern amplitude via the fringe intensities (see [14] and references therein). 

ABSTRACT. Over the past 15 years, a number of transient optical grating techniques have been developed for measurements of the transport properties of materials. Such methods have been used to measure the tracer diffusion coefficients of polymer molecules which have been labeled with photochromic or fluorescent dyes. The present paper describes the common features of these techniques, and gives an example of how Fluorescence Redistribution After Pattern Photobleaching has been used to study the diffusion of polymer molecules in the melt. 

Holographic polymer dispersed liquid crystals (H-PDLCs) are obtained by photopolymerisation of a hquid crystal-prepolymer mixture in the interference field of two or more laser beams [14,15]. This induces phase separation in which hquid crystalline material predominantly congregates in dark regions of the optical interference pattern [16,17]. As a result, holographic gratings with extremely high refractive-index contrast are formed. In addition, the diffraction efficiency of these gratings is electrically switchable the apphcation of an electric field results in reorientation of the nematic director field within... 

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