Relief Gratings Research Articles

Increase of SRG modulation depth in azopolymers-nanoparticles hybrid materials

Thin films of azopolymer-nanoparticles hybrid materials were fabricated with poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl] (PAZO) and different concentrations of Ag and AgAu nanoparticles (NPs). By illuminating the films with polarized interference patterns, surface relief gratings (SRGs) were recorded. It was found that for some concentrations of NPs their modulations and diffraction efficiency were higher than the obtained for PAZO films without NPs. The effect was mainly explained by the increase of the free volume available for the photoisomerization for certain concentrations of NPs. The dependence of the diffraction efficiency on concentration was directly related to changes in modulation depth. When doping with NPs, the maximum efficiency increases more than two times the efficiency without NPs.

Spatially-resolved insulator-metal transition for rewritable optical gratings

Optical relief gratings are usually composed of physical grooves with a constant periodicity, and typically suffer from light scattering, are mechanically fragile and are single function. Here, we develop WO3-based gratings by using a recently reported electron-proton synergistic doping route under ambient conditions. This doping strategy is compatible with conventional ultraviolet photolithography, and we show that it induces a selective insulator-metal phase transition and coloration in WO3, with spatial-resolution up to micron-scale. Due to the electrochromic-induced-contrast, a WO3 volume phase grating without grooves and a WO3 relief grating with tunable periodicity are demonstrated. Both gratings can be rewritten after a reset procedure by annealing in air. Our experiments demonstrate WO3–based gratings and an attractive technique for rewritable oxides.

Evaluation of Local Mechanical and Chemical Properties via AFM as a Tool for Understanding the Formation Mechanism of Pulsed UV Laser-Nanoinduced Patterns on Azo-Naphthalene-Based Polyimide Films.

Aromatic polyimides containing side azo-naphthalene groups have been investigated regarding their capacity of generating surface relief gratings (SRGs) under pulsed UV laser irradiation through phase masks, using different fluencies and pulse numbers. The process of the material photo-fluidization and the supramolecular re-organization of the surface were investigated using atomic force microscopy (AFM). At first, an AFM nanoscale topographical analysis of the induced SRGs was performed in terms of morphology and tridimensional amplitude, spatial, hybrid, and functional parameters. Afterward, a nanomechanical characterization of SRGs using an advanced method, namely, AFM PinPoint mode, was performed, where the quantitative nanomechanical properties (i.e., modulus, adhesion, deformation) of the nanostructured azo-polyimide surfaces were acquired with a highly correlated topographic registration. This method proved to be very effective in understanding the formation mechanism of the surface modulations during pulsed UV laser irradiation. Additionally to AFM investigations, confocal Raman measurements and molecular simulations were performed to provide information about structured azo-polyimide chemical composition and macromolecular conformation induced by laser irradiation.

Modeling of Nonlinear Optical Phenomena in Host-Guest Systems Using Bond Fluctuation Monte Carlo Model: A Review.

We review the results of Monte Carlo studies of chosen nonlinear optical effects in host-guest systems, using methods based on the bond-fluctuation model (BFM) for a polymer matrix. In particular, we simulate the inscription of various types of diffraction gratings in degenerate two wave mixing (DTWM) experiments (surface relief gratings (SRG), gratings in polymers doped with azo-dye molecules and gratings in biopolymers), poling effects (electric field poling of dipolar molecules and all-optical poling) and photomechanical effect. All these processes are characterized in terms of parameters measured in experiments, such as diffraction efficiency, nonlinear susceptibilities, density profiles or loading parameters. Local free volume in the BFM matrix, characterized by probabilistic distributions and correlation functions, displays a complex mosaic-like structure of scale-free clusters, which are thought to be responsible for heterogeneous dynamics of nonlinear optical processes. The photoinduced dynamics of single azopolymer chains, studied in two and three dimensions, displays complex sub-diffusive, diffusive and super-diffusive dynamical regimes. A directly related mathematical model of SRG inscription, based on the continuous time random walk (CTRW) formalism, is formulated and studied. Theoretical part of the review is devoted to the justification of the a priori assumptions made in the BFM modeling of photoinduced motion of the azo-polymer chains.

Cicada Wing Inspired Template-Stripped SERS Active 3D Metallic Nanostructures for the Detection of Toxic Substances.

This article introduces a bioinspired, cicada wing-like surface-enhanced Raman scattering (SERS) substrate based on template-stripped crossed surface relief grating (TS-CSRG). The substrate is polarization-independent, has tunable nanofeatures and can be fabricated in a cleanroom-free environment via holographic exposure followed by template-stripping using a UV-curable resin. The bioinspired nanostructures in the substrate are strategically designed to minimize the reflection of light for wavelengths shorter than their periodicity, promoting enhanced plasmonic regions for the Raman excitation wavelength at 632.8 nm over a large area. The grating pitch that enables an effective SERS signal is studied using Rhodamine 6G, with enhancement factors of the order of 1 × 104. Water contact angle measurements reveal that the TS-CSRGs are equally hydrophobic to cicada wings, providing them with potential self-cleaning and bactericidal properties. Finite-difference time-domain simulations are used to validate the nanofabrication parameters and to further confirm the polarization-independent electromagnetic field enhancement of the nanostructures. As a real-world application, label-free detection of melamine up to 1 ppm, the maximum concentration of the contaminant in food permitted by the World Health Organization, is demonstrated. The new bioinspired functional TS-CSRG SERS substrate holds great potential as a large-area, label-free SERS-active substrate for medical and biochemical sensing applications.

Modelling of the diffraction efficiency of surface relief gratings with varying spatial frequency, height and shape of the relief

An important goal in the development of diffractive optical elements is to achieve high diffraction efficiency. One class of these elements are the surface relief gratings recorded by polarization holography. For recording media, we used azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly denoted as PAZO. The aim of the present work is to simulate a highly efficient surface relief grating formed in the azopolymer PAZO. The model is based on experimental results already obtained by our group. The tasks of the research are related to the analysis of the diffraction efficiency in the spectral range from 300 nm to 800 nm and for: 1) range of grating spatial frequencies from 1200 ln/mm to 1800 ln/mm, 2) surface relief height varying from 100 nm to 400 nm and 3) different shapes of the formed relief.

Polarization holographic gratings in PAZO polymer films doped with particles of biometals

The paper presents a study of the diffraction efficiency of polarization holographic gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with Cu(II) 3-amino-5,5′-dimethylhydantoin (CLP) and Ni(II) 3-amino-5,5′-dimethylhydantoin (NLP) at three different concentrations, namely 1, 2 and 5 wt.%. The influence of the dopants composition and concentration on the parameters of the polarization holographic gratings recorded in the thin composite films has been discussed. The gratings are recorded with a He-Cd gas laser with wavelength 442 nm. The polarization of the recording beams was left and right circular and the recording angle was 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Diffraction efficiency (η) higher than 30 % was achieved for the hybrid samples, as well as 585 nm surface relief height.

Microfabrication and Surface Functionalization of Soda Lime Glass through Direct Laser Interference Patterning.

All-purpose glasses are common in many established and emerging industries, such as microelectronics, photovoltaics, optical components, and biomedical devices due to their outstanding combination of mechanical, optical, thermal, and chemical properties. Surface functionalization through nano/micropatterning can further enhance glasses’ surface properties, expanding their applicability into new fields. Although laser structuring methods have been successfully employed on many absorbing materials, the processability of transparent materials with visible laser radiation has not been intensively studied, especially for producing structures smaller than 10 µm. Here, interference-based optical setups are used to directly pattern soda lime substrates through non-lineal absorption with ps-pulsed laser radiation in the visible spectrum. Line- and dot-like patterns are fabricated with spatial periods between 2.3 and 9.0 µm and aspect ratios up to 0.29. Furthermore, laser-induced periodic surface structures (LIPSS) with a feature size of approximately 300 nm are visible within these microstructures. The textured surfaces show significantly modified properties. Namely, the treated surfaces have an increased hydrophilic behavior, even reaching a super-hydrophilic state for some cases. In addition, the micropatterns act as relief diffraction gratings, which split incident light into diffraction modes. The process parameters were optimized to produce high-quality textures with super-hydrophilic properties and diffraction efficiencies above 30%.

Single-beam inscription of plasmon-induced surface gratings

The formation of gratings on gold nanoprisms arrays by plasmon-mediated reduction of a diazonium salt is investigated. Nanosphere lithography (NSL) is used to produce large surfaces of gold nanoprisms that are effective at reducing diazonium salts by producing hot electrons through excitation of localized surface plasmon resonances (LSPRs). Using single beam irradiation, we report here on the formation of periodic structures formed from the diazonium salts and that follow the NSL structures. On plasmonically active nanoprism substrates, the electric field enhancement promotes chemical reduction and hence modifies the grafting direction and grating properties of the ripples. The nanoprisms act as a plasmon guide which widens the pitch of the self-organized gratings and can even alter it from straight lines into a crisscross pattern. • Single beam irradiation of diazonium salt forms surface relief grating. • Light-induced reduction of diazonium salts solution yields polyaryl thin films. • Combination with plasmonic substrate enable to control the growth direction and shape of the diffractive elements. • Plasmon-mediated self-developing surface gratings enables creation of a variety of surface structures.

Investigation of surface relief gratings on azo-copolyimide films using atomic force microscopy

Azo-copolyimide films were explored regarding the surface relief gratings formation under the action of pulsatory laser irradiation. The azo-copolyimides have been synthesized by the polycondensation reaction of benzophenonetetracarboxylic dianhydride and a mixture of two diamines, one of which contains substituted phenyl groups and the other one contains two pendent substituted azobenzene group, taken in the molar ratio 3 : 1, respectively. Atomic force microscopy was used to investigate the surface relief gratings characteristics in different conditions and for different structures of the azo-copolyimide films.

Tunable narrowband plasmonic light emission from metallic crossed surface relief gratings.

Plasmonic crossed surface relief gratings were fabricated using interference lithography. Their topographies were studied by AFM as a function of laser exposure time and their surface plasmon resonance at a gold-air interface was measured between crossed polarizers in transmission and in reflection modes. Both modes resulted in emitted plasmonic light at specific wavelengths related to the grating pitch, with the reflectance SPR having a much higher intensity than the transmittance SPR. The use of these gratings as plasmonic sensors was examined and their sensitivities were measured in the reflectance and transmittance modes to be 601 nm/RIU and 589 nm/RIU, respectively.

Three-Dimensional Photoengraving of Monolithic, Multifaceted Metasurfaces.

Metasurfaces present a potent platform to manipulate light by the spatial arrangement of sub-wavelength patterns with well-defined sizes and geometries, in thin films. Metasurfaces by definition are planar. However, it would be highly desirable to integrate metasurfaces with diverse, spatially programmed sub-wavelength features into a 3D monolith, to manipulate light within a compact 3D space. Here, a 3D photoengraving strategy is presented; that is, generation of such composite metasurfaces from a single microstructure via the irradiation of multiple interference laser beams onto different facets of the parent azopolymeric microstructure. Through "photofluidization,"thistechnique enables independent inscription and erasing of metasurfaces onto and from individual facets of 3D monoliths with arbitrary shapes and dimensions, in a high-throughput fashion (over approximately a few cm2 at a time). By engraving discrete sub-wavelength 1D surface relief gratings of different pitches on different facets of an inverse pyramidal array, a multiplexing structure-color filter is demonstrated.

Polarization holographic gratings with enhanced parameters recorded in azopolymer based nanocomposite materials

We present study of the parameters of polarization holographic gratings recorded in thin films of the azopolymer PAZO (Poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]) and in a composite PAZO-based organic/inorganic material with incorporated TiO2 nanoparticles with size 21 nm and different concentrations. The gratings are inscribed by two plane waves with left and right circular polarization from a He-Cd gas laser (442 nm) at recording angle 20°. Along with the anisotropic grating in the volume of the media, surface relief is also formed. Gratings with different spatial frequencies are obtained by varying the recording angle. The diffraction efficiency kinetics is probed at 635 nm and the height of the relief gratings is determined by AFM. Our results indicate that the diffraction efficiency for the hybrid samples is enhanced with respect to the pure azopolymer films. Comparison is also made between polarization response of samples deposited from water and from methanol solution.

Blue and Deep‐Blue‐Emitting Organic Lasers with Top‐Layer Distributed Feedback Resonators

AbstractAll‐solution processed surface‐emitting organic distributed feedback lasers are attractive devices for low‐cost applications. Here, lasers emitting in the spectral region between 375 and 475 nm, in which both active material and resonator (1D relief gratings) are based on solution‐processable polymer films, are reported. Ten different organic compounds dispersed in polystyrene are used as active layers of the prepared devices. They include various carbon‐bridged oligo(p‐phenylenevinylene) (COPVn, with n = 1,2) derivatives and two terfluorene compounds. The synthesis and complete optical and amplified spontaneous emission properties of one of the COPV1 compounds, COPV1(Me)‐t‐Bu, designed for deep‐blue emission, are also included. The feasibility of the resonator fabrication, performed by holographic lithography with a dichromated gelatine photoresist over the active film, is successfully demonstrated for all devices. Remarkably, no resolution limitations are found even for the lowest grating period (235 nm) required for the fabrication of the laser based on COPV1(Me)‐t‐Bu. It is also demonstrated that the rectangular grating profile with duty cycle 0.75:0.25 (hill:valley) is very convenient to optimize the resonator efficacy.

Formation of half-period surface relief gratings in azobenzene containing polymer films

We study the peculiar response of photo-sensitive polymer films irradiated with a certain type of interference pattern where one interfering beam is S-polarized, while the second one is P-polarized. The polymer film, although in a glassy state, deforms following the local polarization distribution of the incident light, and a surface relief grating (SRG) appears whose period is half the optical one. All other types of interference patterns result in the matching of both periods. The topographical response is triggered by the alignment of photo-responsive azobenzene containing polymer side chains orthogonal to the local electrical field, resulting in a bulk birefringence grating (BBG). We investigate the process of dual grating formation (SRG and BBG) in a polymer film utilizing a dedicated set-up that combines probe beam diffraction and atomic force microscopy (AFM) measurements, and permits acquiring in situ and in real-time information about changes in local topography and birefringence. We find that the SRG maxima appear at the positions of linearly polarized light (tilted by 45° relative to the grating vector), causing the formation of the half-period topography. This permits to inscribe symmetric and asymmetric topography gratings with sub-wavelength period, while changing only slightly the polarization of one of the interfering beams. We demonstrate an easy generation of sawtooth profiles (blazed gratings) with adjustable shape. With these results, we have taken a significant step in understanding the photo-induced deformation of azo-polymer films.

Dual Amplified Spontaneous Emission and Lasing from Nanographene Films.

Chemically synthesized zigzag-edged nanographenes (NG) have recently demonstrated great success as the active laser units in solution-processed organic distributed feedback (DFB) lasers. Here, we report the first observation of dual amplified spontaneous emission (ASE) from a large-size NG derivative (with 12 benzenoid rings) dispersed in a polystyrene film. ASE is observed simultaneously at the 685 and 739 nm wavelengths, which correspond to different transitions of the photoluminescence spectrum. Ultrafast pump-probe spectroscopy has been used to ascertain the underlying photophysical processes taking place in the films. DFB lasers, based on these materials and top-layer nanostructured polymeric resonators (i.e., one or two-dimensional surface relief gratings), have been fabricated and characterized. Lasers emitting close to either one of the two possible ASE wavelengths, or simultaneously at both of them, have been prepared by proper selection of the resonator parameters.

Photo-triggered large mass transport driven only by a photoresponsive surface skin layer

Since the discovery 25 years ago, many investigations have reported light-induced macroscopic mass migration of azobenzene-containing polymer films. Various mechanisms have been proposed to account for these motions. This study explores light-inert side chain liquid crystalline polymer (SCLCP) films with a photoresponsive polymer only at the free surface and reports the key effects of the topmost surface to generate surface relief gratings (SRGs) for SCLCP films. The top-coating with an azobenzene-containing SCLCP is achieved by the Langmuir–Schaefer (LS) method or surface segregation. A negligible amount of the photoresponsive skin layer can induce large SRGs upon patterned UV light irradiation. Conversely, the motion of the SRG-forming azobenzene SCLCP is impeded by the existence of a LS monolayer of the octadecyl side chain polymer on the top. These results are well understood by considering the Marangoni flow driven by the surface tension instability. This approach should pave the way toward in-situ inscription of the surface topography for light-inert materials and eliminate the strong light absorption of azobenzene, which is a drawback in optical device applications.

Azo-polysiloxanes spontaneous surface relief grating by pulsed laser irradiation

In the herein paper, the possibility of obtaining spontaneous surface relief gratings (SSRGs) on azo-polymeric film surfaces, using a single laser beam of 1.13 mJ energy in pulse mode, was investigated. Chain rigidity is essential to have SSRG of reasonable quality so three azo modified polymers, with different main-chain rigidity, were considered: poly(chloromethyl styrene) (PCMS), a linear polysiloxanes and a cyclic polysiloxane. The AFM studies on azo-polysiloxanes revealed unregulated networks with amplitudes of only 1 nm, while on PCMS orderly SSRGs were observed. These results showed that the more rigid azo-polymer, PCMS, had better capacity to generate SSRGs of high quality. The SSRG periodicity was correlated with the number of laser pulses, i.e. higher pulse number led to lower periodicity. The SSRGs formation was associated with polymeric chains self-organization favoured by photo-fluidisation.

Polarization-contrast surface plasmon imaging.

Silver-coated Crossed Surface Relief Gratings (CSRGs) are fabricated and used for imaging and detecting localized refractive index variations in inhomogeneous water-based solutions via polarization-contrast Surface Plasmon Resonance (SPR). When placed in between crossed polarizers, incident light was transmitted at intensity levels directly and precisely related to the local refractive index value. This occurred due to the in-plane polarization conversion of the SPR light in between the orthogonal grating vectors. When viewed with a camera, SPR light from a monochromatic source enabled the acquisition of polarization-contrast microscopy images of a water and silicon oil mixture placed over the CSRG's surface.

Relaxation Polarization Dependence of Circular Vector Gratings in Azobenzene Glassy Molecular Films

Holographic grating recording and relaxation is studied in different azobenzene molecular glassy films by circularly orthogonally polarized 532 nm laser beams L and R. The readout was made by circularly polarized (R or L) 632.8 nm laser beam. Sandwich-type samples (glass-film-glass) were also studied. Maximum diffraction efficiency of 81% has been achieved in sandwich-type AR-173 film. The following relaxation features have been found: after reaching diffraction efficiency (DE) maximum no DE decay took place; DE read out by R-polarized beam was always higher than that by L-polarized beam; in sandwich-type samples DE decayed until zero when read out by R-polarization whereas DE was zero when read out by L-polarization. 50% relaxation times varied from 4 to 44 minutes, and they mainly decreased when grating period was increased. The observed relaxation peculiarities can be understood if one assumes that volume birefringence grating (VBG) is recorded followed by volume density grating (VDG) and surface relief grating (SRG) recording. R-polarization "feels" all gratings whereas L-polarization only VDG and SRG. At large exposures VDG and SRG dominate. These results confirm the conclusion made by J.Mikelsone in her 2018 PhD thesis that birefringence grating recording in azobenzene materials is a neccessary condition for SRG appearance.