Interfering Laser Beams Research Articles

Mass migration on a polymer surface caused by photoinduced molecular rotation

We demonstrated the formation of a photoinduced surface relief grating using thin films comprising a photochromic molecular motor, 9-(2-phenyl-2,3-dihydro-cyclopenta[a]naphthalen-1-ylidene)-9H-fluorene. Results show that mass migration occurred by patterned light irradiation prepared from interfered laser beams and a photomask.

Light driving force for surface patterning on azobenzene-containing polymers

In this paper, we investigated the effect of light driving force induced surface deformation on azobenzene-containing polymers. The surface deformation is attributed to light-induced mass migration inside the polymers. Circular cap arrays are firstly fabricated by high power laser ablation via polarization controlled three-beam interference. The circular caps are subsequently exposed to polarization controlled two-beam interfering field. The results illuminate that when the interfering laser beams are both set to P polarization, the circular caps are deformed. While the laser beams are of other interfering modes like (S, S) and (+45° , -45°), the caps are seldom deformed. The circular caps are also exposed to single intensity-homogeneous linearly polarized laser beam. The deformation of the caps keeps the same direction as the irradiating polarization. A model based on the focusing effect of the circular caps is addressed to explain the origin of the light driving force for mass migration in azopolymers. The all-optical approach for the production of deformed caps can be used to generate aspherical lens, which may be applied to many domains.

Fabrication of Scaffolds and Micro-Lenses Array in a Negative Photopolymer SZ2080 by Multi-Photon Polymerization and Four-Femtosecond-Beam Interference

Multi-photon polymerization using interference of several laser beams is a promising technique for mass fabrication of 3D periodic micro-structures over large areas, which can be used in micro-biology (artificial scaffolds), photonics (photonics crystals), micro-optics (micro-lenses). While the direct laser writing (DLW) approach is a relatively slow process, use of the interference field can be a significantly faster alternative route of parallel processing when periodic structures are produced by a single laser exposure. We present examples of scaffolds and micro-lens arrays fabricated over a large area by multi-photon polymerization in a negative ultra low shrinkage and biocompatible photopolymer SZ2080 (ORMOSIL) using four-beam interference of a femtosecond laser.

Fabrication of complex periodic micro/nanopatterns by multiple interfered femtosecond laser pulses on thin solid films

Complex periodic micro/nanopatterns have been fabricated on zinc acetate thin films by the irradiation of three or two interfered 800 nm femtosecond laser beams. Three interfered laser beams produce 2D periodic microflower structures with petal sizes as small as 100 nm, which is much smaller than the laser wavelength and its diffraction-limited scale. While two interfered femtosecond laser beams lead to 2D arrayed grating-like patterns with a long period of ∼3.0 µm and a short period of ∼150 nm in two orthogonal directions, respectively. Theoretical calculation indicated that the long periodicities of flower/grating-like patterns were attributed to the interferential intensity distribution, whereas petal structures of microflowers or short periodicities of grating-like patterns were determined by the interferential polarisation distribution.

Facile synthesis and patterning of silver nanoparticles for surface plasmon generation

A facile single step strategy for the synthesis and large area patterning of silver nanoparticles via photodecomposition of a photoactive silver nitrite aliphatic amine complex was developed. Decomposition of the silver complex was facilitated using either a UV or an Argon ion laser (514.5 nm) irradiation. The synthesis was performed both in solution and thin films. The silver nanoparticles displayed characteristic localized surface plasmon resonance in the absorption spectrum. Utilizing laser beam interference, silver nanoparticles were synthesized and, as well, patterned in a single step.

Numerical investigation of optically induced microconvection in thin ferrofluid layers

We consider a periodic concentration grating induced in a thin layer of ferrofluid by an optical grating of interfering laser beams under the action of an externally applied uniform magnetic field. The direction of the applied field is parallel to the concentration gradient. Under certain conditions microconvective instability may develop during relaxation of the grating, driven by the internal demagnetizing field due to the nonhomogenous distribution of concentration. A series of numerical simulations has been performed to determine the role of microconvection in the dissipation of the concentration grating.

Large-area patterned formation of intermetallic phases on Ti/Al multilayer systems by laser interference metallurgy

Titanium and aluminium multilayer thin films with thicknesses of about 300 nm were heat treated using Laser Interference Metallurgy. By the interference of laser beams at the sample surface, a local periodic arrangement of reacted and unaffected regions at the micron scale was achieved. The intention of this treatment was the production of a thin film composite material which benefits from the combination of reacted and unaffected microstructures. In the reacted zones, a mixture of solidified titanium and various aluminide intermetallic phases has formed, whereas the pristine multilayer persisted in the not heat treated regions. Instead of a surface limited reaction as in the former experiments, a complete reaction along the thickness dimension could be obtained using Laser interference metallurgy. The focus of this research lies on the structural and chemical analysis of the laser interference irradiated multilayer system. The existence of the aforementioned intermetallic phases was studied by grazing incidence X-ray diffraction and transmission electron microscopy. A local chemical analysis of the different heat affected regions was carried out by 3D atom probe tomography. The phase formation present in this case was compared to theoretical models related to this topic.

A novel photoactive hyperbranched poly(aryl ether ketone) with azobenzene end groups for optical storage applications

A novel hyperbranched poly(aryl ether ketone) terminated by azobenzene chromophores (azo-HPAEK) was prepared using 1,3,5-tris[4-(4-fluorobenzoyl)phenoxy] benzene (B 3) as a core molecule, 4,4′-(hexafluoroisopropylidene) diphenol as an A 2 monomer and 4-(4-hydroxyphenylazo) benzonitrile as an end-capping reagent. It is shown that azo-HPAEK exhibits a high glass transition temperature (Tg), excellent thermal stability and homogeneous photochromic behavior. By exposing the spin-coated film to a laser beam interference pattern, azo-HPAEK can be used for rapid (within 30 s) fabrication of surface relief gratings (SRGs), and the azo-HPAEK gratings cannot be totally erased by heating, even at 300 °C. In particular, azo-HPAEK shows a large photoinduced birefringence intensity and good reversible optical storage upon irradiation with 532 nm Nd:YAG laser.

Characteristics and New Measurement Method of NCSFs of Individual Color Mechanisms of Human Vision

We propose a new method for determining neural contrast sensitivity functions (NCSFs) of isolated color mechanisms based on the measurements of wave-front aberrations and isoluminant color contrast sensitivity functions (CSFs). Compared with the traditional method, this technique avoids the coherent noise and speckle noise, which are brought by the interference of laser beams, and has great flexibility for the measurements of NCSF of different color mechanisms. Our experiments indicate that the mean NCSF and CSF of the red mechanism are higher than those of the green mechanism, respectively, while those of the blue mechanism are the lowest. However the relative heights of the peak of NCSF and CSF between red and green mechanisms vary in subjects. There are some individuals whose peak values of NCSF and/or CSF of green mechanism are higher than that of the red mechanism. The NCSFs and CSFs of isolated color mechanisms all exhibit the similar characteristics and variation tendency. With the statistical average, the NCSFs of the red, green and blue mechanisms are higher than the corresponding CSFs in the whole spatial frequency. Compared with the corresponding CSFs curves, the peaks of the NCSFs of isolated color mechanisms shift toward higher spatial frequencies, especially for that of blue mechanism which has a largest shift of 3.9c/deg.

Archimedean-like colloidal tilings on substrates with decagonal and tetradecagonal symmetry

Two-dimensional colloidal suspensions subjected to laser interference patterns with decagonal symmetry can form an Archimedean-like tiling phase where rows of squares and triangles order aperiodically along one direction (J. Mikhael et al., Nature 454, 501 (2008)). In experiments as well as in Monte Carlo and Brownian dynamics simulations, we identify a similar phase when the laser field possesses tetradecagonal symmetry. We characterize the structure of both Archimedean-like tilings in detail and point out how the tilings differ from each other. Furthermore, we also estimate specific particle densities where the Archimedean-like tiling phases occur. Finally, using Brownian dynamics simulations we demonstrate how phasonic distortions of the decagonal laser field influence the Archimedean-like tiling. In particular, the domain size of the tiling can be enlarged by phasonic drifts and constant gradients in the phasonic displacement. We demonstrate that the latter occurs when the interfering laser beams are not ideally adjusted.

Enhanced melt pool stirring in welding with dynamic polarised laser beam

The stirring of the molten material during CO2 laser welding with a dynamic polarised laser beam (DP) was studied. The key element of the set-up is a high power interference laser beam modulator for CW power rating up to 4 kW. Metallographic cross-sections and EDX analyses of the realised welds in St37 using the contrast material nickel proved the enhancement of melt pool stirring due to DP. Moreover, roughness measurements confirmed a smoothing of the top bead surface as well as a correlation between melt pool stirring and the formation of the top bead ripple structure.

Generation of a dynamic polarized laser beam for applications in laser welding

The temporal control and optimization of laser beam parameters, e.g., the beam power, can be employed to enhance the quality and reliability of the welding process. Until now, the influence of a time-dependent beam polarization has been less investigated in welding. The publication describes a new experimental arrangement allowing the fast variation of the polarization of CO2 laser radiation [“dynamic polarization” (DP)]. The key element of the setup is a high power interference laser beam modulator for cw laser power rating of up to 4 kW. Weld seams were produced with different process parameters in the technically relevant steels St37 and Ck45 and in the precoated steel 22MnB5 (USIBOR 1500P). The welds were subsequently characterized by roughness measurements of the top bead surfaces and by the preparation of cross sections and their macroscopic analysis. The experiments on welding with DP showed a significant influence on the keyhole and melt pool dynamics, especially a reduction in the top bead roughness at certain DP frequencies up to a factor of about 3. In addition, a threshold behavior for this effect was observed. It was shown that the cross sectional shape and thus the degree of energy coupling to the material was unaffected by the DP. Finally, during welding USIBOR 1500P with DP an enhanced melt pool stirring was achieved. The results show that the temporal modulation of energy coupling to the material caused by DP is suitable to enhance the quality and reliability of the welding process.

Picosecond Laser Beam Interference Ablation of Thin Metal Films on Glass Substrate

Picosecond Laser Beam Interference Ablation of Thin Metal Films on Glass Substrate

Organic dye lasers with distributed Bragg reflector grating and distributed feedback resonator

The paper presents polymeric waveguide dye laser with distributed Bragg reflector (DBR) grating or distributed feedback (DFB) resonator. DBR grating and DFB resonator were fabricated on a surface of SU-8 2002 photoresist polymer using interference of laser beams. Polystyrene (PS) and poly(vinyl butyral) (PVB) waveguides dispersed with laser dye of 3-(2-benzothiazolyl)-7-(diethylamino)-coumarin (Coumarin 6) and tris(8-quinolinolate)aluminum (Alq) as a host were used. Single mode of lasing with TE mode was measured from the polymeric waveguide with DBR grating and DFB resonator. Threshold of lasing with DBR grating is lower than that with DFB resonator. For PVB/Alq/Coumarin 6 waveguide, 0.1 mJ cm(-2) pulse(-1) of threshold was measured with DBR grating and 0.3 mJ cm(-2) pulse(-1) with DFB resonator. Slope efficiency between 0.06 and 0.09 % was measured for PS/Alq/Coumarin 6 waveguide and that between 0.07 and 0.15 for PVB /Alq/Coumarin 6 waveguide.

Emergent crystallinity and frustration with Bose–Einstein condensates in multimode cavities

We propose that condensed-matter phenomena involving the spontaneous emergence and dynamics of crystal lattices can be realized using Bose–Einstein condensates coupled to multimode optical cavities. It is known that, in the case of a transversely pumped single-mode cavity, the atoms crystallize at either the even or the odd antinodes of the cavity mode at sufficient pump laser intensity, thus spontaneously breaking a discrete translational symmetry. Here we demonstrate that, in multimode cavities, crystallization involves the spontaneous breaking of a continuous translational symmetry, through a variant of Brazovskii’s transition, thus paving the way for realizations of compliant lattices and associated phenomena, such as dislocations, frustration, glassiness and even supersolidity, in ultracold atomic settings, where quantum effects have a dominant role. We apply a functional-integral formalism to explore the role of fluctuations in this correlated many-body system, to calculate their effect on the threshold for ordering, and to determine their imprint on the correlations of the light emitted from the cavity. Optical lattices, generated by interfering laser beams, provide a platform for observing condensed-matter phenomena in ultracold-atom systems. By extending the lattice idea to a multimode cavity, it should be possible to observe even more complex effects, such as frustration, crystallization, glass phases and supersolidity.

Preparation and characteristics of new chiral photochromic copolymers

The copolymers consisting of chiral (S)-(−)-1-(4-nitrophenyl)pyrrolidin-2-yl]methyl 2-methacrylate ((S)-PYM) and azobenzene methacrylic co-monomers: 2-(methyl{4-[(E)-(5-methylisoxazol-3-yl)diazenyl]phenyl}amino)ethyl 2-methacrylate (ISM) or 2-{[4-[4-{(2,6-dimethylpirymidin-4-yl)amino]sulfonyl}phenyl) diazenyl]phenyl}-(methyl)amino]ethyl 2-methacrylate (SUM) were synthesized by radical polymerization. The quantum chemical calculations were carried out to predict spectral and nonlinear optical properties of the materials in question. The copolymers showed reversible photochromic properties under illumination with light by change of UV–vis spectra and ellipsometric parameters. The ellipsometric measurements made it possible to determine the change of refractive index under illumination that was in the range between 0.017 and 0.041 depending on copolymer type. Photochromic gratings were inscribed on the surface of polymer films in two kinds of experiments, leading to interesting conclusion that the surface relief grating could be recorded by two interfering laser beams as well as by a single linearly polarized laser beam. The chirooptical properties of the materials were investigated by circular dichroism spectroscopy.

Anderson localization of ultracold atoms

To study localized matter waves, two experimental groups hold a Bose–Einstein condensate in the grip of a disordered but tunable optical potential formed by interfering laser beams.

On continuous-line acoustic array behavior in a laser Doppler sound pressure measurement system.

Continuous-line acoustic array behavior in a laser doppler anemometry (LDA) based sound pressure measurement system is considered. Traditional LDA ultrasound measurement systems intentionally minimize the laser beam intersection or interference fringe volume to better achieve point measurements of the sound field. The large laser beam diameters due to beam spreading in remote sensing applications from space or airborne platforms make this approach impractical. Since the sound pressure signal is integrated over the interference fringe volume, continuous-line acoustic array behavior is expected to be observed proportional to the length of the laser light path through the interference fringe volume. This behavior is analyzed from a theoretical perspective and an experimental setup to test for continuous-line acoustic array behavior in a LDA sound pressure measurement system is presented.

Limits for reduction of effective focal volume in multiple-beam light microscopy

Employing interference patterns for illumination has been shown to reduce the focal volume in fluorescence microscopy. For example, the 4Pi technique employs two interfering laser beams and significantly decreases the focal volume, as compared to conventional microscopy. We study theoretically the effect of using multiple interfering laser beams on the focal volume. In realistic setups with three or four beams, the focal volume is about half of that from the 4Pi case. This improvement reaches a limit quickly as more beams are added, and for the idealized case of an infinite number of beams the focal volume is rather close to the three- or four-beam cases. Thus, our study suggests a limit for the possible reduction of the focal volume in a purely optical far-field setup.

Tunable acoustic resonator on a photoinduced grating in lithium niobate

It is experimentally demonstrated that photoinduced gratings in lithium niobate single crystals can be used as tunable high-frequency resonators for the surface and bulk acoustic waves. The resonance frequency can be tuned within broad limits by rewriting gratings with interfering laser beams.