Ultrafast laser processing: New options for three-dimensional photonic structures

Abstract

Using tightly focused ultrashort laser pulses allows the direct writing of three-dimensional photonic structures in different glasses and also crystalline media. One of the main drawbacks of this technology is, however, the limited writing speed achieved so far. In this paper we shall review our recent advances in the direct writing of three-dimensional integrated-optical devices and discuss a new approach using a fibre-based femtosecond laser system producing 300 fs pulses with pulse energies of 0.6 μJ at 2 MHz repetition rate. Using this laser system we fabricated low-loss waveguides (less than 0.5 dBcm^?1) at writing speeds of 100 mms^?1 for the first time. The influence of the writing speed on the produced structures as well as their optical properties will be discussed in detail.     

Controllable assembly of silver nanoparticles induced by femtosecond laser direct writing

Abstract

We report controllable assembly of silver nanoparticles (Ag NPs) for patterning of silver microstructures. The assembly is induced by femtosecond laser direct writing (FsLDW). A tightly focused femtosecond laser beam is capable of trapping and driving Ag NPs to form desired micropatterns with a high resolution of ～190 nm. Taking advantage of the ‘direct writing’ feature, three microelectrodes have been integrated with a microfluidic chip; two silver-based microdevices including a microheater and a catalytic reactor have been fabricated inside a microfluidic channel for chip functionalization. The FsLDW-induced programmable assembly of Ag NPs may open up a new way to the designable patterning of silver microstructures toward flexible fabrication and integration of functional devices.     

High-efficiency binary phase-transmission-grating using e-beam lithography

Abstract

Rigorous coupled-wave analysis indicates that diffraction efficiencies greater than 99% are feasible for dielectric phase-gratings with a rectangular profile. The use of rectangular rather than blazed profiles implies simplified fabrication techniques, since deep-etch submicrometre structuring techniques can be utilized. We identify several parameter combinations with high diffraction efficiencies using rigorous coupled wave analysis, demonstrate experimentally diffraction efficiencies of 94% in a Bragg mount configuration at a wavelength of 543 nm and discuss the influence of substrate thickness on the transmitted light intensities. A direct writing electron-beam lithography fabrication process gives wide flexibility in design realization.     

Accurate interferometric system to perform the substrate focusing and alignment in laser writing processes

Abstract

A simple, low cost system useful in laser writing processes to set the focal spot on the substrate surface with micrometre precision is described. Evaluation of its sensitivity is also presented. The lack of essential critical adjustments accounts for the utility of this system and its performance.     

Observation of self-diffraction effect in acid blue 7 dye soaked gelatin films

Abstract

Permanent self-diffraction gratings are formed in red sensitive Acid blue 7 dye soaked gelatin films under illumination of an interference pattern by two linearly polarized beams from a He-Ne laser at 632.8 nm. Growth of the self-diffraction grating is monitored by measuring intensities of various diffraction orders. Systematic studies are carried out to investigate the influence of various parameters involved in diffraction efficiency of the grating such as time of exposure, concentration of dye in the gelatin-coated plate, intensity of interfering beams and intensity ratio of interfering beams. Efficient gratings with ten diffracted orders are formed. Several interesting observations are made by blocking one of the writing beams and an attempt is made to analyse these results.     

A VUV prism spectrometer for RICH radiator refractometry

Abstract

A prism spectrometer has been developed to operate in the VUV wavelength range from 120 to 200 nm. It can be used as a pre-disperser in conjunction with a Fabry-Perot based gas refractometer. This instrument has also been used to measure the refractive index of the liquid radiator C₆F₁₄ in various spectral lines. This radiator is used in the RICH detectors of the DELPHI experiment and has been proposed for ALICE, and LHCb experiments. The spectral resolution of the system is improved as the wavelength decreases and the data are consistent with a wavelength accuracy about 0.4 nm at 140nm. The results for the dispersion curve of the above liquid are presented.     

Ridge waveguide laser in Nd:LiNbO3 by Zn-diffusion and femtosecond-laser structuring

Ridge waveguide lasers have been fabricated on Nd³⁺ doped LiNbO₃ crystals. The fs-laser writing technique was used to define ridge structures on a gradient-index planar waveguide fabricated by Zn-diffusion. This planar waveguide was formed in a z-cut LiNbO₃ substrate homogeneously doped with a 0.23% of Nd³⁺ ions. To obtain lateral light confinement, the surface was then micromachined using a multiplexed femtosecond laser writing beam, forming the ridge structures. By butting two mirrors at the channel waveguide end-facets, forming a waveguide laser cavity, TM-polarized laser action at 1085 nm was achieved by end-fire TM-pumping at 815 nm. The waveguide laser shows a threshold of 31 mW, with a 7% of slope efficiency.     

Elimination of Oxygen Inhibition in Photopolymer Systems Used as Holographic Recording Materials

Abstract

One of the problems involved in increasing photopolymer sensitivity, is oxygen inhibition. In this paper we present a photochemical process in which a two dye-sensitizer in the photoinitiator system leads to the suppression of the inhibiting effect of the oxygen. Additionally, the photochemical system presents the possibility of a panchromatic system with light sensitivity in the green and red lines. Diffraction efficiencies of 40% in the 514 and 633 nm lines are obtained with a sensitivity of less than 10 mJ cm^?2.     

Low-power two-wave mixing in dry methylene-bluesensitized gelatin films

Abstract

Experimental observations of the power transfer effect between two writing beams in dry methylene-blue-sensitized gelatin films are presented. A low-power He-Ne laser is used for the realization of two-wave mixing. The influences of the writing angles and the polarization of the writing beams on the holographic enhancement coefficient, which shows the magnitude of the power transfer, are demonstrated. The diffraction efficiency was 0.4% and the holographic enhancement coefficient 0.16.     

Fluorescence detection of superficial skin cancers

Abstract

We present a simple fluorescent diagnosis system for the demarcation of skin cancers. 5-aminolaevulinic acid (ALA) cream is applied to the suspicious area resulting in the accumulation of the photosensitizer Protoporphyrin IX (PpIX) in the cancerous tissue. When excited at 400 nm using a filtered mercury arc lamp, the PpIX fluoresces at 635 nm. Our imaging system acquires four spectrally specific images at 635, 540, 470 and 400 nm for the PpIX fluorescence, natural skin autofluorescence and illumination respectively. Subsequent image processing combines these images into a single display showing clearly the localization and extent of the tumour.     

Mode Interactions in Four-frequency Degeneracy-lifted Ring Lasers

Abstract

A non-planar ring cavity, with a non-reciprocal bias in the form of the longitudinal Zeeman effect on the gain medium, has been used to demonstrate four-mode bidirectional operation in a He-Ne ring laser at 632·8 nm. It has also been shown that this ring system is sensitive to rotation.     

Compact all-solid-state high repetition rate tunable ultraviolet source for airborne atmospheric gas sensing

Abstract

A diode-pumped, all-solid-state, high pulse repetition rate, tunable source of UV radiation is presented. The system is compact, has low power requirements and is suitable for the in situ monitoring of important atmospheric species from an airborne platform. At 3 kHz pulse repetition rate, up to 0.42 ± 0.02 mW of UV radiation was generated and the tuning range extended from ～282 nm to 292 nm. The bandwidth was ≤0.6nm and each pulse had a FWHM duration of 16.0 ± 1.4 ns. The stability of the output was 13% pulse-to-pulse and 4% for an average power measurement. The performance of the system is compared to theoretical expectations.     

Uncatalyzed synthesis of polypyrrole with viologen side groups and its chemical properties

A substituted polypyrrole (PPr) with viologen side groups (polymer-1) was obtained from the reaction of N-aminopyrrole with 1-hexyl-1′-(2,4-dinitrophenyl)-4,4′-bipyridinium dihalide (salt-1). A model compound (model-1) was synthesized by the reaction of N-aminopyrrole with N-(2,4-dinitrophenyl)-4-(4-pyridyl)pyridinium chloride (salt-2). UV–vis spectra revealed that polymer-1 had an expanded π-conjugation system along the polymer chain: the polymer showed an onset position of absorption at a wavelength approximately 200 nm longer than the corresponding wavelength of model-1. Polymer-1 received an electrochemical oxidation of the pyrrole ring and reduction of the viologen group within the polymer.     

High-energy sensitivity enhancement in panchromatic photopolymers for holography using a mixture of visiblelight photoinitiators

Abstract

A new aqueous photopolymer containing the monomers acrylamide, N,N'-methylenbisacrylamide and zinc acrylate, the initiators 4,5-diiodosuccinylfluorescein (2ISF) and methylene blue (MB), and the coinitiator sodium p-toluenesulphinate is described. This formulation exhibits a clear enhancement of the high-energy sensitivity upon irradiation with 514 or 633 nm light (Ar⁺ laser or He–Ne laser respectively), with respect to the same mixture but with only one of the two dyes, reaching maximum diffraction efficiencies of 15–20% with 15–60 mJ cm^?2. This enhancement is explained by the more efficient photogeneration of initiator radicals by the ground-state formation of an ion-pair complex between cationic (MB) and anionic (2ISF) chromophores. This must compensate the observed decrease in the absorbance of the mixture of the two dyes at 514 nm, with respect to the absorbance of the same medium but with only 2ISF (maximum absorption at 490 nm). A clear absorbance increase at 633 nm, with respect to the absorbance of this system with only MB (maximum absorption at 660 nm), must also favour photopolymerization.     

Optical effects caused by high laser powers in ion-implanted Bi4Ge3O12 waveguides

Abstract

The ‘optical writing’ of permanent channel waveguides in the surface of He⁺ ion-implanted bismuth germanate, Bi₄Ge₃O₁₂, has been studied, using end-coupled argon and Ti:sapphire CW laser beams. Comparisons of the self-focusing effect in waveguides and bulk samples have been made for both Nd³⁺ doped and undoped material. The permanent ‘writing’ effect has been compared with the structural change observed earlier in ion-implanted Bi₄Ge₃O₁₂ waveguides annealed at high temperature.     

Fabrication and characterization of UV-written channel waveguides in Bi2O3-based glass

Light-induced refractive index change was investigated in an erbium-doped Bi₂O₃-based bulk glass using direct UV laser writing at 244 nm. Single mode channel waveguides were obtained with a positive refractive index change of 4 × 10⁻⁴ at 633 nm. An upper limit for propagation losses at 980 nm was estimated at 4 dB/cm. A morphological study of the exposed regions revealed that strong interactions occurred between the UV beam and the glass sample, causing physical damage and stresses in the material. To our knowledge, this is the first paper reporting the use of 244 nm laser beam to produce waveguides in a Bi₂O₃-based glass.     

Synthesis and photovoltaic properties of conjugated copolymers bearing planar acenaphtho[1,2-b]quinoxaline moiety with deep HOMO level

A series of alternating copolymers (PC-AQx, PT-DTAQx, PC-DTAQx, PF-DTAQx, and PBDT-DTAQx) bearing novel planar acenaphtho[1,2-b]quinoxaline (AQx) or 8,11-di(thiophen- 2-yl)acenaphtho[1,2-b]quinoxaline (DTAQx)-acceptor cores have been synthesized via Suzuki or Stille coupling reactions. UV–vis absorption and GIXRD characterization results indicated that the presence of planar DTAQx unit is favorable for the promotion of well-ordered interchain packing in the solid state, and the incorporation of planar electron-donating benzo[1,2-b: 4,5-b′]-dithiophene (BDT) moiety would be propitious to the molecular self-organization. Electrochemical measurement results suggested that four copolymers possess deep HOMO energy level of −5.5 ~ −5.6 eV. The polymer solar cell with structure of ITO/PEDOT:PSS(30 nm)/polymer:PCBM (60 nm)/Bphen(10 nm)/Ag(100 nm) exhibited the highest V _oc of 0.84 V with PF-DTAQx as p-type polymer, while the best power conversion efficiency (PCE) of 0.9 % was obtained using a blend of PBDT-DTAQx and PCBM (1:4) as active layer.     

New approaches to tumor therapy with siRNA-decorated and chitosan-modified PLGA nanoparticles

Abstract

Objective: In this study, we aimed to develop a candidate modifited polymeric nanoparticle (NP) system that will kill cancer cells by facilitated to apoptosis and also reduce pain.

Significance: The primary goal of treatment, especially for metastatic cancers, is to control the growth of the cancer and to alleviate the symptoms. Pain is one of the commonest symptoms of cancer. In cancer treatment, directing cancer cells to death while simultaneously relieving pain will be a new approach.

Methods: Chitosan-modified PLGA NPs were prepared using an nanoprecipitation technique. The NPs were loaded with flurbiprofen and decorated with folic acid. STAT3-siRNA was adsorbed to these polymeric NPs using antisense technology.

Results: The NPs were small in size (176.9–220.3?nm) with positive zeta potential (+14.1?mV to +27.2?mV). They had high loading capacity and prolonged release properties over 144 hours. Cytotoxicity studies performed with siRNA showed effective electrostatic interaction due to the positively charged NPs. Folic acid facilitated entry into cancer cells and helped to kill them.

Conclusion: The formulation we developed is a potential carrier system for both treatment of cancer and prevention of pain, especially for metastatic cancers.     

Effect of the preparation route,PEG and annealing on the phase stability of Fe3O4 nanoparticles and their magnetic properties

Fe₃O₄ nanoparticles are synthesised via two different methods: (1) co-precipitation of Fe²⁺ and Fe³⁺ ions and (2) oxidative alkaline hydrolysis of Fe²⁺ ions under atmospheric pressure using different protective agents (PEG 200 and PEG 3000) and urea as a base. The preparation method and the polyethylene glycol (PEG) used are concurrently affecting the phase stability of the formation of the iron oxides: the co-precipitation method using PEG 200 (E4a) or PEG 3000 (E4b) leads to the formation of different ratios of Fe₂O₃ and Fe₃O₄, whereas the oxidative hydrolysis of Fe²⁺ using PEG 200 gives Fe₃O₄ (E2) powder as a major product. The average crystallites size of E4a and E4b is almost identical, i.e. around 19?nm but the saturation magnetisation of E4b is three times larger than that of E4a. The sample E2 shows the highest saturation magnetisation value 74?emu/g, with an average crystallites size of 71?nm. Transmission electron microscopy analysis confirmed that the E2 sample shows the presence of needles crystals with typical sizes around 10 and 50?nm and its selected area diffraction (SAD) shows a typical diffraction of the spinel structure of magnetite. On the other hand, E4b sample shows elongated nanoparticles with typical sizes around 24?nm and its SAD confirmed the presence of a mixture of Fe₂O₃ and Fe₃O₄ as many dispersed spots were obtained.     

Contemporary fiction and the history of photography

Abstract

Until recently, there has been a notable absence in fiction of photographers and photohistory.¹ So frequently do they appear now, however, that they comprise an intriguing sub-genre in contemporary writing. In the hands of talented writers, the character of the photographer has proved to be an interesting protagonist. Darkroom techniques are accurately recounted and even invested with a sense of romance. The nature of the medium itself, authors have found, suggests complex plots and startling revelations. The ‘vague pastness’ Susan Sontag observed in old photographs is being revisited and vivified with flights of intense imagination. More than a trendy direction in recent fiction, this new writing enhances our understanding and appreciation of the history of photography itself