Holographic Gratings Research Articles

Tuning the Δn and scattering in Bayfol® HX based holograms

Self-developing photopolymers are crucial materials for making high efficiency volume holograms. A key feature is the tuning of the refractive index modulation (Δn), but at the same time, it is crucial to minimize the light scattering which reduces the efficiency and increase the unwanted light. Here, we study Bayfol® HX materials in terms of scattering formation and control of the Δn by means the realization of Volume Phase Holographic Gratings (VPHGs), which are intended for the application in spectroscopic instrumentation. We highlight the dependency of the Δn and scattering with the laser light intensity, showing that the increase of the intensity reduces the amount of scattering. In addition, the fine tuning of the Δn with the incoherent light pre- and co-exposure is studied and the full dynamic range can be addressed controlling the scattering. A modified diffusion model is also used to describe and predict the experimental results.

Holographic grating and smartphone-based optical spectrograph for educational research

The efficiency of students’ educational activity in a physical laboratory increases if students independently assemble experimental installations and use them to study physical phenomena. This paper describes a simple installation for the manufacture of a sinusoidal grating by the holographic method. Such a grating is used in many demonstration and laboratory experiments. It is used, for example, in a simple device for making photos of optical spectra on a smartphone, and this device is also described in this paper. First, students study the elementary theory of a holographic diffraction grating. They use this theory to calculate the parameters of the installation that allows them to obtain a grating with the required period and use an experiment to verify the validity of the theory. Then they calculate the parameters of the spectroscope on the basis of the studied theory and again make sure that the theory is valid. Thus, it is possible to organize a learning process in which the theory is fully confirmed by the experiment.

Microchip Nd:YAG and Nd:YVO4 Lasers Pumped by VHG Wavelength-Stabilized Laser Diode

We adopted a single-mode, single-wavelength volume holographic grating (VHG) wavelength-stabilized wavelength laser diode (LD) as a pumping LD for an end-pumped microchip Nd:YAG and Nd:YVO4 lasers we developed during CW and pulse operations. Higher optical-optical and slope efficiencies during CW operation have been obtained than when using a VHG LD experimentally. Output laser power is insensitive to the temperature of the LD when using a wavelength-stabilized LD and can remain stable and almost constant until the temperature of LD increases up to 40°C. The improved optical-optical conversion efficiency of 58% for the Nd:YVO4 laser has been obtained and calculated the output laser power during CW operation and compared it with the experimental results. We found that the output laser power of the Nd:YVO4 laser using the VHG wavelength-stabilized LD was more than twice as high as that using an LD without VHG. When the ambient temperature increases, the difference in output laser power should be large. In the future, a low-cost end-pumped microchip laser that does not require a temperature control should be developed.

Vortex Smith–Purcell radiation generation with holographic grating

Smith–Purcell radiation (SPR) is the electromagnetic wave generated by free electrons passing above a diffraction grating, and it has played an important role in free-electron light sources and particle accelerators. Orbital angular momentum (OAM) is a new degree of freedom that can significantly promote the capacity of information carried by an electro-magnetic beam. In this paper, we propose an integrable method for generating vortex Smith–Purcell radiation (VSPR), namely, SPR carrying OAM, by having free-electron bunches pass on planar holographic gratings. VSPRs generated by different electron energies, with different topological charges of the OAM, radiation angles, and frequencies are demonstrated numerically. It is also found that, for high-order radiation, the topological charge of the OAM wave will be multiplied by the radiation order. This work introduces a new way to generate SPR with OAM and provides a method to achieve an integratable and tunable free-electron OAM wave source at different frequency regions.

Second-harmonic generation and manipulation in lithium niobate slab waveguides by grating metasurfaces

Nonlinear optical processes in waveguides play important roles in compact integrated photonics, while efficient coupling and manipulations inside the waveguides still remain challenging. In this work, we propose a new scheme for second-harmonic generation as well as beam shaping in lithium niobate slab waveguides with the assistance of well-designed grating metasurfaces at λ = 1064 nm . By encoding the amplitude and phase into the holographic gratings, we further demonstrate strong functionalities of nonlinear beam shaping by the metasurface design, including dual focusing and Airy beam generation. Our approach would inspire new designs in the miniaturization and integration of compact multifunctional nonlinear light sources on chip.

RECORDING OF DIFFRACTION GRATINGS WITH AN ACTIVE STABILIZATION SCHEME OF INTERFERENCE FIELD

The paper describes a modernized optoelectronic system for stabilizing the interference pattern during exposure of a holographic grating. The results of applying the extreme control system to stabilize the interference field when recording holographic gratings of high optical quality are presented. Thin chalcogenide layers of the composition As2S3 were used as the recording medium. When using the stabilization system, a monotonic increase in the diffraction efficiency of the gratings from the recording time is observed and a low level of spurious scattering is recorded for the recorded gratings. The scheme was experimentally tested when recording holographic gratings up to 60x60 mm2 in size and exposure time up to 30 minutes. The use of an active stabilization scheme will make it possible to record large-size gratings using low-power lasers without the use of additional complex systems for protecting optical circuits from vibration.

Single- and two-photon recording of holograms at combined cationic and free-radical polymerization photoinitiated by thioxanthenone derivatives

Single- and two-photon holographic recordings were performed in a photopolymer material consisting of two types of monomers: an acrylamide (1,4-bis(acryloyl)piperazine) and an epoxide ((bis(4-oxiran-2-ylmethoxy)phenyl) sulfide). Thus, cationic ring-opening polymerization and free-radical polymerization of the monomers were simultaneously initiated. The arylsulfonium salt (2-(N,N,N-triethylammonium)methyl-9-oxo-10-(4-heptyloxyphenyl)-9H-thioxanthenium dihexafluorophosphate) was chosen as the photoinitiator. Effective photopolymerization conditions were found for both (single- and two-photon) photoinitiation modes depending on the recording light intensity. The concentrations of the components (monomers and photoinitiator) in the photopolymer was optimized, and a refractive index change of as much as 0.0015 was achieved. Single- and two-photon holographic recording was investigated in photopolymer material consisting of two types of monomers: acrylamide (BAP) and epoxy (BOPS). Thus a cationic ring-opening polymerization and free-radical polymerization of monomers were initiated simultaneously. The arylsulfonium salt—cationic derivative of thioxanthenonium (CDTX) was chosen as the photoinitiator. The refractive index change n1 and maximum recording rate (dn1/dt)max of holographic gratings were found in two-photon photoinitiation modes depending on the concentration of BAP (the molar fraction of BAP) in photopolymer composition.

Holographic reflective Bragg gratings of gain and their role in the operation of high-power pulsed lasers

The influence of reflective Bragg gratings that are formed in an active medium due to the spatial periodic modulation of gain on the spectral properties of lasing are investigated by an example of a pulsed Nd : YAG laser. It is shown that, at a certain choice of the formation regime of reflective gratings of gain, the lasing bandwidth can be reduced by a factor of 3 – 5, with a corresponding increase in the pulse energy and significant improvement of spatial beam quality.

Reconfigurable Reflective Colors in Holographically Patterned Liquid Crystal Gels

Thin film holographic gratings have attracted significant attention due the tailorability of their photonic properties. The ability of holography to create materials with unique photonic properties is particularly promising when combined with stimuli-responsive liquid crystals. Holographic polymer-dispersed liquid crystals (H-PDLCs) are an example of such a system where nonuniform irradiation is used to template alternating polymer-rich and polymer-poor regions with a periodicity related to the illumination wavelength. These systems typically use nonmesogenic monomers at a concentration range where a significant volume fraction of the final system is polymer. Here, we present a reconfigurable material that was generated by holographically polymer stabilizing liquid crystals (H-PSLCs) using liquid crystalline monomers, thereby enabling both alternating regions to be liquid crystalline. The “stabilizing effect” of the liquid crystalline polymer leads to a material with regions of periodic transition temperatures. As written, the polymer-rich regions have a higher isotropic transition temperature than the polymer-poor regions. Upon heating these samples beyond the isotropic temperature of the polymer-poor regions, a vivid wavelength specific reflection appears due to the refractive index mismatch between the ordered liquid crystalline polymer-rich regions and the disordered isotropic polymer-poor regions. When these materials are further heated to the isotropic temperature of the polymer-rich regions, the samples again return to a transparent state because the refractive index modulation between the alternating isotropic regions is negligible. By tailoring the liquid crystalline isotropic transition temperature to be near-room temperature, modest AC-fields can be used as an Ohmic heat source to drive the thermal color change.

Frequency Tunable Surface Acoustic Wave Actuators for Adjustable Pitch Diffraction Grating

Diffraction gratings are commonly used in optical instruments such as in spectrometers, monochromators, and wavelength division multiplexing devices. The diffraction efficiency is directly related to the ratio of the grating spacing or pitch to the wavelength of the light. In a sinusoidal grating (or holographic grating) the efficiency is generally lower than a blazed grating with a saw tooth profile, but the spectral coverage can be broader. To address a large spectral range, advanced spectrometers use a number of diffraction gratings with a fixed pitch. In this paper, we show for the first time, the use of a tunable-frequency surface acoustic wave (SAW) delay line as a broad-band sinusoidal diffraction grating. To tune the frequency, we tune the effective spacing of the interdigitated transducers (IDTs) by applying specific phases to each IDT finger. The prototype SAW grating covers frequency range of 86 MHz to 360 MHz corresponding to optical range of $1.3~\mu \text{m}$ to $9~\mu \text{m}$ . [2020-0189].

Improved TDM scheme and data extracting algorithm for polymerization evaluation.

To quickly evaluate holographic photopolymers with different formulations, the most effective method is to record a volume holographic grating in the samples and detect the grating's diffraction in real time. Since the volume grating is highly sensitive to incident angle, existing schemes need to precisely control many space-related parameters. This study proposes an improved scheme, in which two different sized spots are used to reduce the requirements for the overlap of the two spots and the installation precision of the samples. Transmittances, diffractive efficiencies and diffractive asymmetries are obtained at a high sampling rate, through a specifically designed algorithm with the data from uncalibrated high-speed photodiodes. The experimental results show that the proposed scheme performance well in evaluating holographic photopolymer.

On-chip generation of the reconfigurable orbital angular momentum with high order.

In this manuscript, the generation of an optical vortex beam with high order and reconfigurable orbital angular momentum (OAM) is studied. Multi-waveguide holographic gratings (MWHG) are deployed to generate OAM beams with high order. The generation of the OAM beam with an order l from +4 to +8 is demonstrated by numerical simulations, and the generated OAM order is manipulable and configurable by incident phase. The working bandwidths of the MWHG for different OAM orders are at the level of 40 nm. This work could provide valuable references for practical implementation of OAM in integrated optics.

Polarization holographic characteristics of TI/PMMA polymers by linearly polarized exposure

Polarization gratings can be recorded in photo-induced anisotropic materials. Here, we examined the anisotropy of TI/PMMA (IRGACURE 784 doped polymethyl methacrylate) polymer by investigating the maximum refractive index modulation of TI/PMMA polymer due to photo-induced birefringence which reached 9.06 × 10−5. Based on the linearly polarized light theory, the diffraction efficiency and polarization direction of the diffracted grating in TI/PMMAs were investigated with two different polarized incident beams. When the polarization angle difference between two incident beams increased from zero to π/2, the diffraction efficiency gradually decreased, while the polarization direction of the diffracted beam was different from that of recording beam. When the angle difference of polarization between two incident beams was π/2, namely, orthogonal polarization recording, the polarization direction of diffracted grating would be consistent with that of recording beam. In the case of orthogonal polarization recording, the diffraction efficiency of 1–3 mm TI/PMMA polymer was obtained by 5%, 12% and 16%, respectively. In the experiment, when the intersection angle difference between two incident beams was π/2, the polarization multiplexing storage of TI/PMMA polymer was realized with the dynamic range in 1–3 mm samples of 0.16, 0.22 and 0.26 by orthogonal polarization recording, respectively. The angular and polarization multiplexing storage of holographic grating was realized in TI/PMMAs. The research on polarization holographic storage could provide the experimental basis and feasibility for multi-dimensional multiplexing storage in TI/PMMA polymer.

Reconfigurable polymer-templated liquid crystal holographic gratings via visible-light recording.

Polymer-templated nematic liquid crystal (LC) holographic gratings via visible-light recording are presented in the presence of reactive mesogens (RMs) and rose bengal (RB)/N-phenylglycine (NPG) photoinitiation systems. By optimizing the concentration of RMs in the polymer-templated LC gratings, the template after being washed out can be refilled with suitable fluidic components. And the dependence of the first-order diffraction efficiency (DE) on the concentration of RB and NPG molecules was discussed in detail. The polarization-dependency of diffraction properties was also investigated. It is revealed that the diffractive behaviors of polymer-templated LC gratings can be dynamically reconfigured by varying temperature or refilling organic solutions with different refractive index (RI). Furthermore, the potential for recording holograms using green light is explored. We expect that the reconfigurable polymer-templated LC gratings fabricated via visible-light interference would provide a facile approach to regulate the diffraction properties of holographic gratings apart from electric field, thus paving a way towards a class of novel anti-counterfeiting devices.

Modified monomer diffusion model for volume holographic grating formation in photopolymers.

Monomer diffusion models are important in describing the formation mechanism of the volume holographic grating in photopolymers. The most representative of these models is the first-order diffusion model, which neglects the influence of the exposure intensity threshold and dissolved oxygen in a polymer. This model causes a significant deviation between the theoretical simulation and the experimental results. Therefore, we propose a modified monomer diffusion model, which is more consistent with experimental results on refractive index modulation than other models.

Low-stray-light concave holographic gratings processed by combining the methods of photoresist hot-melting and oxygen-ion ashing.

The principal motivation for this paper is to reduce stray light and line roughness in concave holographic gratings (CHG). Compared with other previously reported line-smoothing grating techniques such as dynamic-exposure near-field holography, we successfully improve the line smoothing of CHG to approximately 10 nm from 2 nm. Our method uses optimization technologies and a combination of photoresist hot-melting (PHM) and oxygen-ion ashing (OIA), thereby improving the degree of stray light before and after optimization by one order of magnitude; the level processed by OIA, PHM, and OIA successively is ${7.85} \times {{10}^{ - 5}}$7.85×10-5. Combining the two technologies, we achieve lower stray light and straighter groove lines for the concave gratings, which is more effective, easy to implement, and incurs a low cost.

Design and development of volume phase holographic grating based digital holographic interferometer for label-free quantitative cell imaging.

In this paper, a volume phase holographic optical element based digital holographic interferometer is designed and used for quantitative phase imaging of biological cells [white blood cells, red blood cells, platelets, and Staphylococcus aureus (S. aureus) bacteria cells]. The experimental results reveal that sharp images of the S. aureus bacteria cells of the order of ${\sim}{1}\;{\unicode{x00B5}{\rm m}}$∼1µm can be clearly seen. The volume phase holographic grating will remove the stray light from the system reaching toward the grating and will minimize the coherent noise (speckle noise). This will improve the sharpness in the image reconstructed from the recorded digital hologram.

A simple extended-cavity diode laser using a precision mirror mount.

We have developed an extended-cavity diode laser (ECDL) with a simple design by using a commercial precision mirror mount with minor modifications. Our design allows tuning of the external cavity configuration by tweaking the volume holographic grating without troublesome changes in the beam path of the laser output. The mode-hop-free tuning range of the presented ECDL is about 8 GHz with a linewidth of 475 kHz.

All-optically phase-induced polarization modulation by means of holographic method

Phase-induced polarization modulation has been achieved experimentally by means of the all-optical holographic method. An extra spiral phase is added to a Gaussian beam and then a holographic grating is recorded through the interference of a Gaussian beam and the phase-vortex beam with the same linear polarization state in an azobenzene liquid-crystalline film. We report here that the polarization state of the diffraction light from the recorded grating is different from that of the incident light, while no polarization variation occurs for the holographic grating recorded by two Gaussian beams. The phase-induced polarization modulation is mainly attributed to the formation of birefringence in the film generated by phase vortex, which is investigated through the ripple patterns resulting from the competition between photoinduced torques and analysed by the Jones matrix. The experimental results could enrich the connotation between optical parameters and offer a method to realize polarization modulation through phase control.

Study of the Effect of Methyldiethanolamine Initiator on the Recording Properties of Acrylamide Based Photopolymer.

The use of Holographic Optical Elements (HOEs) in applications, such as in light shaping and redirection, requires certain characteristics such as a high Diffraction Efficiency, low angular selectivity and stability against UV damage. In order to maximize the performance of the HOEs, photosensitive materials are needed that have been optimised for the characteristics that are of particular importance in that application. At the core of the performance of these devices is the refractive index modulation created during holographic recording. Typically, a higher refractive index modulation will enable greater light Diffraction Efficiency and also operation with thinner devices, which in turn decreases the angular selectivity and the stability of the refractive index modulation introduced during recording, which is key to the longevity of the device. Solar concentrators based on volume HOEs can particularly benefit from thinner devices, because, for a solar concentrator to have a high angular working range, thinner photopolymer layers with a smaller angular selectivity are required. This paper presents an optimisation of an acrylamide-based photopolymer formulation for an improved refractive index modulation and recording speed. This was achieved by studying the effect of the concentration of acrylamide and the influence of different initiators in the photopolymer composition on the diffraction efficiency of holographic gratings. Two initiators of different molecular weights were compared: triethanolamine (TEA) and methyldiethanolamine (MDEA). A fivefold increase in the rate of grating formation was achieved through the modification of the acrylamide concentration alone, and it was also found that holograms recorded with MDEA as the initiator performed the best and recorded up to 25% faster than a TEA-based photopolymer. Finally, tests were carried out on the stability of the protected and unprotected photopolymer layers when subjected to UV light. The properties exhibited by this photopolymer composition make it a promising material for the production of optical elements and suitable for use in applications requiring prolonged exposure to UV light when protected by a thin melinex cover.