Birefringence Gratings Research Articles

Single polarization, narrow linewidth hybrid laser based on selective polarization mode feedback

We demonstrate a single polarization, narrow linewidth hybrid laser by using a semiconductor gain chip and a birefringence silica-on-silicon waveguide Bragg grating. The hybrid laser shows good linewidth and single polarization characteristics with the selective polarization mode feedback of high birefringence grating. The temperature phase modulation on the characteristics of the potentially lasing modes is displayed. In addition, the performances of different mode power, side mode suppress ratio, spectrum and linewidth during temperature tuning are characterized. We based on the rate equation method and the chirp factor reduction theory analysis the variation of laser linewidth with power and lasing wavelength. The best linewidth achieves 4.36 kHz and the best polarization extinction ratio is 38.6 dB. The output power in fiber is 6.58 mW.

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.

Physico-Chemical and Light-Induced Properties of Quinoline Azo-dyes Polymers.

We present investigation of optical and photochromic properties as well as of surface quality of thin films of novel methacrylic polymers with 8-hydroxyquinoline azo-dyes in side-chain. Additionally, thermal stability of polymer powders was examined and their glass transition temperature was determined. Optical properties (extinction coefficient and refractive index) were determined by spectroscopic ellipsometry (SE) combined with absorbance measurements. Photoresponsive behavior was investigated by determination of photoisomerization rates under irradiation with unpolarized 365 nm light, as well as by conduction of holographic grating inscription experiment. Thin film quality was determined by atomic force microscopy (AFM) measurements. Thermal analysis was performed by thermogravimetric (TG), derivative thermogravimetric (DTG) and differential scanning calorimetry (DSC) measurements. We found that optical properties as well as photoisomerization rates of investigated polymers are dependent on the substituent in the para position of the phenyl ring. Surface relief grating inscription was successfully generated only for materials with chromophores containing dimethylamino (N(CH3)2) and methyl (CH3) substituents, but all materials exhibited birefringence grating in the bulk. Surface of most thin films was very smooth, but its quality was impaired by neutral (H) as well as carboxyl (COOH) substituent. Thermal stability of copolymers with side-chain chromophores was improved compared to pure poly(methyl methacrylate) (PMMA).

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.

Terahertz nonreciprocal isolator based on a magneto-optical microstructure at room temperature.

We investigated THz nonreciprocal circularly polarized transmission in thin longitudinally magnetized InSb film, especially focusing on its non-eigen nonreciprocal transmission mechanism at room temperature. Then, based on this effect, we presented a THz isolator for linear polarized waves. The nonreciprocal transmission of the InSb film in this device is converted and enhanced by a pair of orthogonal artificial birefringence gratings. After the optimization, the isolation reaches 24dB, and the insertion loss is <0.5 dB at room temperature and a low magnetic field.

Methacrylate copolymers with substituted azobenzene side groups: synthesis, characterization and photo-induced birefringence properties

AbstractWell-defined copolymer of methyl methacrylate (MMA) and 2-methylacrylic- acid-2-{ethyl-[4-(4-methoxy-phenylazo)-3-methyl-phenyl]-amino}-ethyl ester (MAMP) bearing pendent azobenzene groups, was synthesized via reversible addition-fragmentation chain transfer polymerization (RAFT). Copolymers with different contents of azobenzene moieties, e. g. from 12 mol % to 100 mol %, were obtained by varying the feed molar ratios of MAMP and MMA. The reactivity ratios of these two monomers were measured and calculated via the modified Kelen- Tüdos method. Glass transition temperature of the copolymers was measured by differential scanning calorimetry (DSC). Photoinduced trans-cis isomerization of azobenzene moieties in the copolymer was studied at room temperature. Films cast from the functional copolymers were characterized by photo-induced birefringence and surface relief grating (SRG).

Large birefringence and polarization holographic gratings formed in photocross-linkable polymer liquid crystals comprising bistolane mesogenic side groups

Polarization gratings with large birefringence are formed in photoreactive polymer liquid crystals with bistolane moiety and terminal cinnamic acid moiety by the use of polarized ultraviolet interference light and subsequent annealing. The polarized ultraviolet light causes the axis-selective photoreaction between the cinnamic acid groups and subsequent annealing induce the reorientation of peripheral molecules without cross-linking along the cross-linked groups. Long bistolane mesogenic moiety exhibits large birefringence in comparison with a biphenyl mesogenic moiety, the value of the induced birefringence in the bistolane mesogenic liquid crystalline (LC) polymer is strongly dependent on both the grating constant and the wavelength of the reconstruction light.

Spacer-Free Ionic Dye−Polyelectrolyte Complexes: Influence of Molecular Structure on Liquid Crystal Order and Photoinduced Motion

A series of spacer-free ionic azobenzene-containing stoichiometric complexes was prepared from monosulfonated azo dyes and cationic polyelectrolytes [methylated poly(4-vinylpyridine) (PVP) and poly(dimethylaminoethyl methacrylate) (PDM)]. Their thermal and structural characteristics and optical responses, particularly photoinduced birefringence (PIB) and surface relief grating (SRG) inscription, were investigated as a function of selected molecular parameters. All of the complexes have high apparent Tg values, 180−210 °C, and show liquid crystal (LC) order of the single-layer SmA type from ambient to very high temperature, usually to degradation. The range of LC order is greater in the complexes of chromophores with longer alkyl tails and shorter in the complex of the chromophore with an OH end group and the complex of methyl orange (MO) with PDM. PIB, SRG quality, and diffraction efficiency were all shown to depend in a similar way on molecular structural features: the more rigid the molecular structure,...

Azo polymers with electronical push and pull structures prepared via RAFT polymerization and its photoinduced birefringence behavior

Two methacrylate monomers containing azo and electronical push and pull structure, e.g. 2-Methyl-acrylic-acid- 2-{(4-(4-cyano-phenylazo)-3-methyl-phenyl)-ethyl-amino-ethyl ester (MACP) with cyano substituted and 2-Methyl- acrylic-acid-2-{ethyl-(4-(4-methoxy-phenylazo)-3-methyl-phenyl)-amino}-ethyl ester (MAMP) with methoxy substituted, were synthesized and polymerized using 2-cyanoprop-2-yl dithiobenzoate (CPDB) as chain transfer agent and 2,2'-azobi- sisobutyronitrile (AIBN) as initiator. The results showed that the polymerization displayed characteristics of 'living'/con- trolled free radical polymerization. Thus, the obtained polymers, polyMACP (pMACP) and polyMAMP (pMAMP), had controlled molecular weights and narrow molecular weights distribution. The chain extension experiments of pMACP and pMAMP using styrene as the second monomer were successfully carried out. The photo-induced trans-cis-trans isomeriza- tion kinetic of pMACP and pMAMP in chloroform solution were described. Marked differences in rate for the trans-cis and cis-trans isomerization of pMACP and pMAMP were observed in chloroform solution due to the different electronic effects in these two polymers. Photoinduced birefringence and surface relief grating (SRG) of the pMACP and pMAMP were investigated in thin film state.

Optically inscribed volume or surface structures in azobenzene polymer films

Volume birefringence or surface relief gratings are optically inscribed on the surface of azopolymer thin films using polarized light from a moderate power laser. The inscription is done in a one step process and the images are stable over extended periods of time. Multiple holographic images can be written locally and these erasable sub-wavelength structures are used to produce a variety of optical devices.

Femtosecond laser-induced birefringence and transient grating in lead(II) phthalocyanine-doped hybrid silica gel glasses

Ultrafast nonlinear optical properties of lead(II) phthalocyanine (PbPc)-doped silica gel glasses were investigated using a femtosecond optical Kerr shutter (OKS) setup at wavelength of 800 nm. The nonlinear response time of the PbPc-doped silica gel glasses was measured to be less than 90 fs. Measurements for the dependence of Kerr signals on the polarization angle between pump and probe beams showed that the Kerr signals induced by 30-fs pulses arose mainly from photoinduced birefringence effect and not from a laser-induced transient grating as observed when using a 200-fs pulse laser.

Optical manipulable polymer optical fiber Bragg gratings with azopolymer as core material

Polymer optical fiber (POF) with a core of azopolymer has been fabricated. Based on the photoinduced birefringence of azopolymer materials, a mechanism for writing POF gratings is presented. After writing in azopolymer optical fiber (APOF) by two beams of mutually orthogonal polarizations, a birefringence grating was formed, and then the gratings could be erased almost totally for a short time by the circularly polarized beam at the same wavelength. By manipulating the APOF gratings, the transmittance through APOF at 632.8nm can be adjustable as revealed by the transmittance change during write-erase-write procedure.

Molecular weight effect on the photoinduced birefringence and surface relief gratings formation of a methacrylate azopolymer

Samples of a methacrylic side-chain azopolymer were synthesized by a radical copolymerization of methyl-methacrylate with the methacrylic derivative of the commercial dye Disperse Red-13 (DR13). Copolymers with different molecular weight were obtained by varying the reaction time and the concentration of initiator. Samples with molecular weight averages ( M w) from 3 to 8 × 10 4 g/mol and 1.4–1.7 polydispersity were obtained. The glass transition temperature of the samples increased linearly from the lower to the highest molecular weight. Optical quality cast films were prepared for all samples and photoinduced birefringence was successfully carried out in all samples as well as surface relief gratings could be inscribed in the cast films.

Fast and stable recording of birefringence and holographic gratings in an azo-polymethacrylate using a single nanosecond light pulse

Pulsed light-induced recording in azobenzene polymers has recently been studied due to its potential use in optical storage applications. In this paper we study the photoinduced birefringence (Deltan) and holographic grating recording in an azobenzene side chain liquid-crystalline polymethacrylate irradiating with a single 4 ns light pulse at 532 nm. For some irradiation conditions, Deltan grows in less than 50 ns reaching an essentially stable value of about 10(-2). Holographic gratings have been registered using intensity and polarization patterns. Fast response and stability, similar to those of Deltan, was observed in the holographic recording process. Both light-induced anisotropy and relief contributions have been found in the case of gratings recorded using intensity patterns, relief being the dominant contribution at high recording energies. Polarization gratings have been recorded using two orthogonally circularly polarized beams. The resultant gratings showed stable efficiencies up to 0.8% (measured at 633 nm in 1-mum-thick films) and no measurable relief was observed.

Liquid Crystalline Ionomers Containing Azobenzene Mesogens: Phase Stability, Photoinduced Birefringence, and Holographic Grating

Liquid crystalline (LC) ionomers containing azobenzene mesogens and three transition metal ions (Cu2+, Zn2+, and Mn2+) were prepared and characterized. Metal ions were found to exert significant effects on the photoinduced orientation of azobenzene mesogens and on the diffraction efficiency of holographic gratings. Using methacrylic acid for neutralization with metal ions, as compared with the copolymer with no metal, ionomers of Zn and Mn showed increased photoinduced birefringence and higher diffraction efficiency of birefringence gratings obtained by excitation with spatially modulated light polarization. By contrast, ionomers with Cu showed the opposite effects. As a whole, the behavior of photoinduced birefringence and holographic grating appears to be correlated with the thermal stability of the LC phase formed by azobenzene mesogens in the ionomer. Some new thoughts are discussed regarding the effect of metal and their ionic aggregates on the clearing temperature of LC ionomers. The interest for fu...

Analysis of Photosensitivity of Copolymer Optical FibrePreform

The photosensitivity of copolymer optical fibre preform is analysed incomparison with the doped one. The effects of write conditions such aspump power and pump time have been studied. Then, the preform is drawninto single mode polymer optical fibre with core refractive index of1.499, and core-cladding refractive-index difference of 0.008.Long-period birefringence gratings with period of 120 um are fabricatedin the fibre. The duty cycle is 50%, and the refractive index changein the exposed area is about 1×10−3.

Holographic grating recording in azobenzene polymer films

Abstract The photo-induced birefringence grating and the surface grating in azobenzene polymer films were studied. The photo-induced birefringence (about 0.02 at 633 nm) was investigated at various intensities of laser (532 nm) beam. The effect of laser-induced heating has been introduced to the processes of photo-induced reorientation in azo polymer. The holographic grating period and the peak-to-peak value of the sinusoidal surface profile are 0.8 mm and 40 nm, respectively. The maximum diffractive efficiency of the grating is 2%. The mass diffusion responsible for the formation of the surface grating due to the temperature rise is analyzed.

Silica-based photorefractive sol–gel films for holography

Photorefractive gratings have been obtained with 632.8nm writing beams in organic–inorganic SiO2-based films. The hybrid glass is prepared by a sol–gel technique starting from organic Si precursors and contains Disperse Red 1 (DR1), carbazole units and 2,4,7-trinitro-9-fluorenone (TNF). A photorefractive gain has been determined through an asymmetric energy exchange by two-beam coupling (2BC) measurements. Polarization-sensitive gratings have been obtained at 488nm through birefringence induced by photoisomerization. The birefringence gratings have been characterized at different polarization configurations. Pure polarization holographic gratings (sp, rl) show diffraction efficiencies higher than those obtained by ss or pp writing beams.

Photoinduced Birefringence and Surface Relief Gratings in Polyurethane Elastomers with Azobenzene Chromophore in the Hard Segment

A series of polyurethane elastomers containing aromatic azobenzene chromophore in the hard segment was synthesized by a one-step method. AFM surface study showed that particles on the order of 100 nm to several micrometers appeared after annealing the polymer films. Birefringence was induced in these polymer elastomers. The stability of the photoinduced birefringence could be improved significantly by annealing the polymer films. This also induced microphase separation. The effects of azo contents and molecular weight of soft segments were investigated. Dynamic study shows that photoinducing rate decreased while relaxation rate increased after annealing. Surface relief gratings could be induced only on the pure polyurethane and the elastomer containing 75% bis(azo)−diol. Only a reversible volume hologram was observed for the other elastomers.

Molecular engineering strategies to control photo-induced birefringence and surface-relief gratings on layer-by-layer films from an azopolymer

The interactions governing adsorption of layer-by-layer (LBL) films from an azopolymer, PS-119®, and poly(allylamine hydrochloride) (PAH) have been controlled by modifying the pH of the solutions used to form the films. The thickness per bilayer was varied by one order of magnitude, from 10 to 240 Å, as the pH changed from 4 to 10. Thick layers were formed at higher pHs because in this case the PAH is only partially charged adopting a loopy conformation. This change in molecular conformation caused the kinetics of isomerization and mass transport to be much slower than at lower pHs. The writing time defined as the time to achieve 50% of maximum birefringence dropped from 110 to 18 min for films prepared from solutions with pH 10 and pH 4, respectively. This decrease is probably due to the higher free volume for isomerization in the films prepared at lower pHs, in which PAH molecules are less coiled than at higher pHs. For the same reason, the rate of inscription of surface-relief gratings with an interference pattern of p-polarized light was also much slower at higher pHs.