Transmission Phase Gratings Research Articles

Holographic Recording of Unslanted Volume Transmission Gratings in Acrylamide/Propargyl Acrylate Hydrogel Layers: Towards Nucleic Acids Biosensing.

The role of volume hydrogel holographic gratings as optical transducers in sensor devices for point-of-care applications is increasing due to their ability to be functionalized for achieving enhanced selectivity. The first step in the development of these transducers is the optimization of the holographic recording process. The optimization aims at achieving gratings with reproducible diffraction efficiency, which remains stable after reiterative washings, typically required when working with analytes of a biological nature or several step tests. The recording process of volume phase transmission gratings within Acrylamide/Propargyl Acrylate hydrogel layers reported in this work was successfully performed, and the obtained diffraction gratings were optically characterized. Unslanted volume transmission gratings were recorded in the hydrogel layers diffraction efficiencies; up to 80% were achieved. Additionally, the recorded gratings demonstrated stability in water after multiple washing steps. The hydrogels, after functionalization with oligonucleotide probes, yields a specific hybridization response, recognizing the complementary strand as demonstrated by fluorescence. Analyte-sensitive hydrogel layers with holographic structures are a promising candidate for the next generation of in vitro diagnostic tests.

Multiparametric, Label-Free Analysis of Microfluidic Droplets via a Dynamic Phase Grating Approach.

Droplet-based microfluidic systems possess many fundamental advantages as a platform for the analysis of chemical and biological species. However, whereas on-chip operations have rapidly developed over the past decades, approaches for analyzing target molecules within droplets have largely remained limited to methods requiring bulky and expensive instrumentation. In this work, we describe a droplet analysis approach whereby the droplet train itself is the sensing construct. Specifically, the droplet train is interrogated as a transmission phase grating, allowing high-throughput, label-free, solution-phase, and multi-parametric analysis of droplet contents. Importantly, three distinct properties of generated droplets can be simultaneously extracted using this conceptually simple and experimentally straightforward measurement approach. Under constant droplet generation conditions, measurement of droplet viscosity is achieved by monitoring changes in zero order to first order peak separation in the far-field diffraction pattern, with a sensitivity of 2.28 × 10-4 cSt per μm change in peak separation. In parallel, measurement of droplet refractive index (RI) is achieved by measuring changes in the ratio of the zero order to first order peak intensity, with a sensitivity of 2.14 × 10-4 RI units per unit change in a diffracted peak intensity ratio. Finally, droplet generation frequency is determined from the time-varying oscillation of the peak height ratio, yielding comparable results to an expensive high-speed camera commonly used for droplet imaging. Importantly, the experimental strategy for this approach is straightforward and does not require expensive instrumentation; therefore, it may find utility in affordable and portable analysis approaches applied to diverse droplet microfluidic assays.

Polymer-Based High Diffraction Efficiency and High Resolution Volume Holographic Transmission Gratings

We report on the optical characterization of very high-efficiency and high-resolution holographic volume phase transmission gratings. The gratings are recorded in a new photo-polymerizable mixture made by epoxy-resin and multi-acrylate. The epoxy-resin used is known to make tenacious acrylate-based films. The holographic mixture contains two photo-initiators, the synergic effect of which enables a reliable photo-polymerization process in the visible region of the electromagnetic spectrum. The recorded holograms are mechanically stable, show long-term temporal stability and very high values of diffraction efficiency, coupled with good angular selectivity due to a relatively narrow band of wavelengths. We measured the intensity of the transmitted beam and calculated the intensity of the diffracted beam at different wavelengths, deriving the refractive index modulation and the grating pitch by fitting the experimental data with a slightly modified theoretical approach. These kind of mixtures can be used in several fields of application, such as chemical or bio-sensors, high resolution optical sensors, high-density optical data storage, encryption and security.

HKUST-1-Doped High-Resolution Volume Holographic Gratings

We report on transmission holographic gratings doped with metal organic frameworks (MOFs). As a first attempt, we focused on MOF-199, also known as HKUST-1, which is an efficient adsorbent of VOCs. HKUST-1 is not soluble in the pre-polymerized holographic mixture. For this reason, samples containing HKUST-1 show high light scattering. In this work, the recording of HKUST-1-doped one-dimensional transmission phase gratings is demonstrated. The optical properties of the recorded structures, such as diffraction efficiency and average refractive index changes, are reported by using angular analysis measurements. A first attempt to demonstrate the possibility of using the doped gratings as sensors is also reported.

The effect of transverse wavefront width on specular neutron reflection.

In the analysis of neutron scattering measurements of condensed matter structure, it normally suffices to treat the incident and scattered neutron beams as if composed of incoherent distributions of plane waves with wavevectors of different magnitudes and directions that are taken to define an instrumental resolution. However, despite the wide-ranging applicability of this conventional treatment, there are cases, such as specular neutron reflectometry, in which the structural length scales of the scattering object require that the wavefunction of an individual neutron in the beam be described by a spatially localized packet - in particular with respect to the transverse extent of its wavefronts (i.e. normal to the packet's mean direction of propagation). It is shown in the present work that neutron diffraction patterns observed for periodic transmission phase gratings, as well as specular reflection measurements from patterned thin films with repeat units of the order of micrometres, can be accurately described by associating an individual neutron with a wave packet and treating a beam as a collection of independent packets. In these cases, accurate analysis requires that the transverse spatial extent of a neutron packet wavefront be accounted for in addition to the angular divergence of the beam that is characterized by a distribution of packet mean wavevector directions. It is shown how a measure of the effective transverse spatial extent of the neutron packet - over which its wavefronts are of sufficient uniformity to produce coherent scattering - can be determined by employing reference diffraction gratings and patterned thin films of known structure and composition.

Photopolymerization kinetics of a thiol–enol composition determined via recording/playback of a transmission holographic diffraction grating

The kinetics of thiol–enol photopolymerization of a hybrid composition based on a tetraacrylate monomer and a thiol-siloxane oligomer was studied with the use of a holographic recording of elementary transmission phase gratings. The degrees of conversion of double bonds in the tetraacrylate monomer after the polymerization in air and in an inert atmosphere of SF6 were measured via IR spectroscopy. It is shown that the use of the thiol-siloxane oligomer efficiently suppresses oxygen inhibition of the photopolymerization. When the photoinitiator concentration is increased to more than 10–2 mol/L, the photopolymerization rate levels off. An increase in the thiol-siloxane oligomer concentration leads to an extremal dependence of the photopolymerization rate on the oligomer concentration; the maximum rate is reached at an oligomer concentration of about 0.07 mol/L. The kinetic scheme of photopolymerization in the hybrid photopolymer composition was analyzed, and an analytical expression for the photopolymerization rate was obtained. The correlation between the kinetic constants of the thiol–enol photopolymerization was evaluated on the basis of the obtained parameters of the kinetic model.

Terahertz emission based on large‐area photoconductive emitters illuminated via beam interference

The use of beam interference in combination with a large-area photoconductive emitter for the generation of pulsed terahertz (THz) radiation is presented. An interference pattern with a period twice that of the THz emitter is generated with a transmission phase grating, placed directly in front of the photoconductive emitter. This way, efficient THz generation is achieved with a single metallisation layer and a single lithography step in the fabrication technology of the THz emitter.

Highly asymmetric near infrared light transmission in an all-dielectric grating-on-mirror photonic structure.

We demonstrate a photonic structure, composed of a dielectric quarter-wavelength stack topped with a transmission phase grating, designed to exhibit a significant asymmetry in the near infrared light transmission for waves propagating in opposite directions. The asymmetry, defined as the difference between the intensity transmission coefficients, reaches 0.72 ± 0.06 for a single wavelength and exceeds 0.2 over a spectral range spanning from 700 to 850 nm for one incident polarization and 750-800 nm for both polarizations. The experimental results are consistent with the numerical model of light propagation in the structure.

Transmission phase gratings fabricated with direct laser writing as color filters in the visible

Simple diffraction structures having the form of a regular grid of pillars can generate a significant range of hues in white light transmission due to color-dependent diffraction into higher orders. We present the fabrication of such submicrometer scale structures by three dimensional laser two-photon photolithography, results of their optical properties measurements and compare the latter with numerical simulations.

Humidity and temperature effect on properties of transmission gratings recorded in PVA/AA-based photopolymer layers

This paper explores the effects of humidity on gratings recorded in a polyvinylalcohol–acrylamide photopolymer medium. Investigation of the behaviour of transmission gratings exposed to high humidity is of significant interest for two reasons, firstly because the grating’s sensitivity to humidity can be exploited for the development of irreversible humidity indicators, secondly because too much sensitivity to humidity can limit the use of these materials in applications where an environmentally stable hologram is needed. In this paper we focus on the effect of high humidity on the properties of volume phase transmission gratings recorded in PVA/AA photopolymer layers in the temperature range 8–24 ° C. It has been found that although exposure to humidity changes the diffraction efficiency and Bragg angle of gratings, the effects are fully reversible if the temperatures are kept low. For example, when gratings were subjected to relative humidities of 80% and 90% at a temperature of 8 ° C the observed changes were fully reversible. However, irreversible changes in diffraction efficiency, thickness, refractive index modulation and Bragg angle were observed when the temperature during the humidity exposure was higher than 16 ° C. The magnitude of the irreversible changes depends strongly on the ambient temperature during the humidity exposure, the humidity level and also on the duration of the humidity exposure.

Stand-alone diamond binary phase transmission gratings for the EUV band

We report on the development of true free-standing phase transmission gratings for the extreme ultraviolet band. An ultra-nanocrystalline, 300 nm thin diamond film on a backside etched silicon wafer is structured by electron-beam lithography to periods of 1 μm. In this way, flat and stable gratings of 400 μm in diameter are fabricated. First-order net efficiencies up to 28% are obtained from measurements at a synchrotron beamline within a wavelength range from 5.0 nm to 8.3 nm, whereas the 0th order is suppressed to 1% near 6.8 nm. Higher diffraction orders up to the 3rd one contribute less than 7% in sum to the far-field pattern.

Efficient extreme ultraviolet transmission gratings for plasma diagnostics

We report on a theoretical study of free-standing phase transmission gratings for high-resolution extreme ultraviolet (EUV) and soft x-ray spectroscopy and investigate their properties. Designed for wavelengths between about 2 and 40 nm, the devices may provide a first order diffraction efficiency beyond 30%. We use rigorous coupled wave analysis methods in order to optimize the grating design parameters and discuss features of segmented grating arrays. Elemental, as well as compound, materials such as Be, Mo, LiF, and poly-(methylmethacrylate) are considered with respect to their potential and practical limitations in terms of feasibility and sensitivity to radiation damage. Simulations are performed for several samples on the radiation produced by a table-top EUV plasma source and applications to astrophysical problems are considered.

Sensitization of photopolymer materials by halogenated derivatives of xanthene and thioxanthene dyes in the pulsed recording mode

A holographic photopolymer material based on a poly(vinyl alcohol) matrix, the acrylamide monomer, and triethanolamine as an initiator was sensitized by the xanthene dyes erythrosine; thioeryth-rosine; eosin; thioeosin; thiofluorescein, 4-iodo-, 4,5-diiodo-, and 2,4,5-triiodofluorescein; and 4-bromo-, 4,5-dibromo-, and 2,4,5-tribromothiofluorescein. The quantum yields of photobleaching of these dyes in the polymer matrix were determined. Transmission phase gratings were recorded in the pulse mode (λ = 532 nm, tp = 10 ns). The levels of the diffraction efficiency of holograms were measured, and the 4-bromo- or 4,5-dibromothiofluorescein-triethanolamine system was found to be the most effective photoinitiator.

Fabrication of transmission phase gratings on porous anodic alumina

The transmission phase grating on anodic alumina membrane (AAM) was fabricated by a lithography and chemical etching method to demonstrate the feasibility of fabricating diffractive optical elements on AAM matrix. The monochromatic light diffraction property of the sample grating was tested using lasers with wavelengths of 532 and 650 nm, respectively. The measured diffraction efficiencies of zeroth and first orders were in good agreement with the calculated results. This method may offer an approach to prepare surface pattern on alumina and could be used in fabricating diffractive optical components such as linear gratings.

Transmission and reflection phase gratings formed in azo-dye-doped chiral nematic liquid crystals

Transmission and reflection gratings were simultaneously formed in azo-dye-doped chiral nematic liquid crystals (N∗LCs) with planar alignment. The formation process is based on a phototuning of the Bragg reflection band of the N∗LC. The helical pitch of the photoreactive N∗LC was spatially controlled with intensity variation of interference light. The resultant periodic structure showed both transmissive and reflective diffractions due to the spatially modulated light intensities. The observed dependence of diffraction efficiencies on the polarization states of the probe beam was well explained by considering a spatial modulation of the helical pitch.

Design and fabrication of gold transmission phase gratings for soft X-rays

The diffraction efficiency of a gold transmission phase grating is simulated as a function of the depth of grooves and the duty cycle on the basis of the scalar diffraction theories. It is shown that the +1 order diffraction efficiency of a transmission phase grating with appropriate groove depth and duty cycle can beup to 21.9% of the incident light，whereas it is no greater than 10% for a conventional amplitude-mode transmission grating. Transmission gratings with area of 20mm×5mm，period of 1μm，duty cycle of 0.55 and 200nm thick gold bars supported by a membrane of 300nm thick polyimide have been fabricated by combining holographic lithography and electroplating. Its diffraction efficiency has been measured in national synchrotron radiation laboratory，and the maximum efficiency in +1 order is about 16% at λ=7.425nm.

Angular selectivity of liquid crystal composite gratings

Angular selectivity of volume phase transmission gratings formed in a liquid crystal composite for s-and p-polarized light is investigated in the absence of external electric field.

Analysis on diffraction properties of the transmission phase grating

Diffraction properties of the transmission phase grating are analyzed in terms of the Fourier optics theory. The analysis results show that the diffraction intensity distribution of the transmission phase grating is closely related to the optical thickness of the grating and the wavelength of the incident light, and not only determined by the grating's period and slit width. For monochromatic incident light, more than 80% of the diffraction energy will concentrate to the first diffraction order under certain conditions. For white incident light, the energy of the first order of diffraction spectrum may be much higher than other orders. Based on these results, some possible applications of the transmission phase grating are discussed.

Optical characterization of polymer dispersed liquid crystals for holographic recording

In this study we present an extended investigation made on holographic polymer dispersed liquid crystals. A spectroscopic study of the photosensitive mixtures during the polymerization and phase separation process was performed, and an optical characterization of the recorded phase gratings was carried out using two alternative methods. The behavior of the diffraction efficiency of the transmission phase gratings versus time, angle of incidence, and wavelength was experimentally determined, and the results were successfully analyzed using the available theories for anisotropic phase gratings. Finally, the electro-optical properties concerning the beam deflection and response times were measured.

Volume Phase Transmission Gratings and Compact Configurations for Coarse Wavelength Division Multiplexing and Demultiplexing

Volume phase transmission gratings, that have high and uniform diffraction efficiency regardless of polarization of the incident light over a broad wavelength range, have been designed and recorded. Such gratings were incorporated successfully into a novel compact configuration of a coarse wavelength division demultiplexer. Calculated and experimental results of such gratings, and a compact four-channel demultiplexer operating in the 1500 nm to 1600 nm range are presented.