Single‐mode distributed feedback laser operation with no dependence on the morphology of the gain medium

Organic distributed feedback (DFB) lasers can be useful photonic tools for biological applications where the roles of organic materials are important, because highly coherent single mode emission with broad tuning range can be obtained. However, the formulaic structures of organic lasers, and the uses of gain media as resonators themselves, are not suitable for inducing laser emission from irregular shaped gain media, such as dye‐staining cells and tissues. Here, we report a reusable photonic template comprising an exceedingly thin and discrete titanium dioxide (TiO₂) layer on a one‐dimensional (1D) quartz grating to induce single mode DFB lasing from a variety of states of optical gain media. Using the same template, the external gain media of optically thick and thin casted film, liquid, and a free‐standing thick film reveal single mode lasing with reliable performance. Numerical simulations demonstrate that the 25‐nm thick TiO₂ disconnected grating lines support a spatially confined DFB mode in the vertical direction, even under no index difference between superstrate and substrate. Additionally, not using the typical waveguide gain layer promises high sensitivity and detection limit in refractometric sensing. These results suggest that the photonic structure may serve as a versatile sensing platform for bioapplications.     

Advances in original fibres fabrication using innovative techniques

The development of original technologies to fabricate new kinds of fibres is presented here. First of all, the sol–gel process is developed to achieve fibres with original properties of waveguiding (new wavelength of emission of rare earths or transition metals, ultraviolet waveguiding): either a fibre composed of doped nanocrystals in a silica matrix or fibres composed of a one dimensional photonic bandgap structure. In this way, high refractive index dielectric oxides like ZrO₂, TiO₂ are studied. Secondly, the core suction technique associated with the stack and draw process is developed to fabricate fibres with various glasses and then, original profiles of refractive index to achieve multiwavelength lasers.     

Reverse-symmetry waveguides: theory and fabrication

We present an extensive theoretical analysis of reverse-symmetry waveguides with special focus on their potential application as sensor components in aqueous media and demonstrate a novel method for fabrication of such waveguides. The principle of reverse symmetry is based on making the refractive index of the waveguide substrate less than the refractive index of the medium covering the waveguiding film (n_water=1.33). This is opposed to the conventional waveguide geometry, where the substrate is usually glass or polymers with refractive indices of ≈1.5. The reverse configuration has the advantage of deeper penetration of the evanescent electromagnetic field into the cover medium, theoretically permitting higher sensitivity to analytes compared to traditional waveguide designs. We present calculated sensitivities and probing depths of conventional and reverse-symmetry waveguides and describe schemes for easy implementation of reverse symmetry. Polymer waveguides are demonstrated to be candidates for cheap, mass-producible reverse-symmetry sensor modules. The grating-coupled waveguiding films of controlled thickness are produced by soft lithography. The resulting films are combined with air-grooved polymer supports to form freestanding single-material polymer waveguides of reverse symmetry capable of guiding light. Received: 20 December 2001 / Published online: 14 March 2002     

A TiO2 Film/K+ Ion-Exchanged Glass Composite Optical Waveguide and Its Application to a Refractive Index Sensor

A composite optical waveguide for biological and chemical sensors was successfully developed by sputtering a thin TiO₂ film onto the surface of a low loss potassium ion-exchanged optical waveguide. The electric field of the evanescent wave at the film surface was made strong through adiabatic transition of the guided light. The attenuation of the guided light was sufficiently small in the air, and the guided light intensity was changed sensitively with the refractive index of the cladding layer (n_c). Thus, n_c can be sensitively monitored with a detecting sensitivity of about 10^-4.     

Photorefractive properties of ion-implanted waveguides in strontium barium niobate crystals

0.61 Ba_0.39Nb₂O₆, SBN61), either by proton or helium ion implantation. Proton-implanted samples show a large increase of dark conductivity that reduces or even prevents the recording of refractive index gratings. For waveguides formed by helium implantation this effect is absent, and they can be used for efficient holographic recording. Photorefractive properties of the waveguides are investigated by two-beam coupling. After implantation with 2.0 MeV He⁺ and doses of (0.5-5)×10¹⁵ cm^-2, the samples have to be polarized again, because heating or charge effects at the crystals surface during the implantation process decreases or even reverses the effective electrooptic coefficients in the waveguiding layer. For repoled samples, we find logarithmic gain coefficients of up to 45 cm^-1 with time constants for the build-up of the purely π/2-shifted refractive index grating of the order of 1 ms for the blue lines of an Ar⁺ laser. Photoconductivity depends nonlinearly on light intensity with an exponent x≈0.55. With increasing implanted helium dose, both electronic and nuclear damage of the waveguiding layer grows, and the photorefractive properties of the waveguides are considerably degraded. Received: 20 February 1997/Revised version: 1 May 1997     

Silver‐coated inverse opals formed from polystyrene spheres for surface‐enhanced Raman scattering

We have developed a new substrate for surface‐enhanced Raman scattering (SERS) measurements involving a thin silver layer deposited over an ion‐etched TiO₂ inverse opal. The latter is formed by chemically infiltrating a polystyrene opal array with TiO₂ followed by a thermal decomposition of the spheres. The SERS response of the these substrates is examined for several sphere sizes and lasers wavelengths; the results show that such substrates yield high enhance factors, comparable to substrates involving a silver layer deposited directly on a polystyrene opal array. Copyright © 2010 John Wiley & Sons, Ltd.     

Sensitivity enhancement for a multimode fiber sensor with an axisymmetric metal grating layer

This paper proposes a novel design for a surface plasmon resonance (SPR) fiber sensor with an axisymmetric sub-wavelength metal grating layer. The relationship between the sensor performance (the sensitivity S and the quality factor Q of the SPR dip) and the characteristic parameters are investigated. Numerical simulation results show that the proposed sensor can achieve a maximum sensitivity of 13,000 nm/RIU (refractive index unit) for a refractive index range from 1.3 to 1.4.     

Novel optical film sensor design based on p-polarized reflectance

A new scheme of optical film sensor is presented. The sensor is based on p-polarized reflectance, consisting of a sensing coated substrate, is easily optimized for maximum sensitivity in different applications. The resolutions of refractive index n_f, extinction coefficient k_f and thickness h_f of the sensitive films are predicted to be 10⁻⁷, 10⁻⁵ and 10⁻³ nm, respectively. Experimentally, we selected the sol–gel derived SnO₂ films as gas-sensitive films and conducted preliminary gas-sensing test. The results indicate that novel optical film sensor scheme has higher sensitivity, and the detection sensitivity is available to 10⁻¹ ppm on the condition of optimum optical parameters and incident angle.     

Study on the properties and charge generation in dye-sensitized n-TiO2|dye|p-CuI solid state photovoltaic solar cells

Transparent semiconducting copper iodide (CuI) films were prepared by XeCl Excimer laser and their characteristics are investigated. These films exhibited optical transmittance over 80% in the wavelength range from 400 to 900 nm and minimum resistivity of about 2 kΩ cm⁻¹. The optical absorption of the these films shows a remarkable blue shift compared to that of polycrystalline of CuI, which can be explained from the viewpoint formation of ultra fine of CuI grains. The titanium dioxide (TiO₂) films have been prepared by sol-gel method. The properties of pulsed laser deposited CuI and TiO₂ films in power output of n-TiO₂|dye|p-CuI cells is studied. An efficient charge generation is observed through the illumination of TiO₂ layer of the fabricated n-TiO₂|dye|p-CuI solid state photovoltaic solar cells. From the current-voltage characteristics, the fill factor and power conversion efficiency were about of 45 and 3%, respectively. The maximum photo-current of about 12.5 mA/cm² and photo-voltage of 475 mV under AM 1.5 conditions were obtained for the n-TiO₂|dye|p-CuI solid states photovoltaic solar cells with good reproducibility. Adsorbed dye molecules to the TiO₂ surface act as a relay, especially under illumination through TiO₂ layer in the wave range region of 300-400 nm.     

Characterization and sensor properties of sol-gel SiO2:TiO2 film/ion-exchange glass optical waveguides

The paper presents the characterization and sensor properties of composite sol-gel SiO₂:TiO₂ film/ion-exchange glass optical waveguides. In the theoretical part of the paper the analysis involving the influence of the parameters of a uniform waveguide film on homogeneous sensitivity has been presented. It has been shown that the highest sensitivity can be obtained making use of the interference of TM₀-TM₁ modes. In the experimental part of the work the manufacturing technology of composite sol-gel SiO₂:TiO₂ film/ion-exchange glass optical waveguides has been described and the analysis results of the produced structures have been presented. The paper presents the influence of the thickness of the uniform waveguide film and the influence of the refractive index of the cover on effective refractive indexes. The produced waveguide structures are characterized by high homogeneous sensitivity.     

Study on the photovoltaic property of Cu4SnS4 synthesized by mechanochemical process

Photovoltaic property of Cu₄SnS₄ (CTS) is studied by employing a superstrate solar cell structure of Mo/CTS/In₂S₃/TiO₂/fluorine-doped tin oxide (FTO) glass for the first time. The CTS absorber layer was prepared by a combination of mechanochemical and doctor blade processes. The annealing effects on the structural, optical and electronic properties of the CTS absorber layer were investigated. The novel CTS absorber layer shows conversion efficiency as high as 2.34% under the standard AM 1.5 condition.     

Design of a highly sensitive photonic crystal refractive index sensor incorporating ring-shaped GaAs cavity

Two highly sensitive optical sensor topologies are proposed and simulated in this paper. The proposed structures are optimized to provide better performance characteristics such as sensitivity, detection limit, and quality factor. They are based on two-dimensional photonic crystals consisting of rectangular arrays of GaAs rods in SiO₂ substrates. Such lattices have bandgaps for transverse magnetic modes. Two-dimensional finite difference time domain and plane wave expansion methods are used for the simulation and analysis of the refractive index sensors and particle swarm optimization method is used to optimize the structural parameters. The designed structures show a high sensitivity to refractive index variations. They are able to detect refractive indices from 1.33 to 1.5. An excellent figure of merit equal to 737 RIU^?1 is observed for the proposed structure and a significant improvement is observed compared to the structures reported in the literature.     

Method of Sellctive Intracavity Absorption Using Dye Lasers

The method of the selective intracavity absorption is based on the high sensitivity of the laser intensity on the absorption of the sample placed into the resonator. The method for Nd-glass laser with inhomogeneous broadening of the gain spectrum was grounded and experimentally checued by the authors of the papers^1,2. Later demands for the spectral region expanding of the method have led to application of dye lnsers for the intracavity measurements. The capability to broadrband emission in the different spectral regions makes of dye lasers the unique instruments for intracavity spectroscopy. A number of the successful experiments on the dye lasers applications to the sensitive detection of Na, da, Ho, Pr, I₂, Br₂ small concentrations was published until now^3–6 At the same time one cannot consider the theoretical analysis of the intracavity absorption characteristics of the dye lasers, i.e, the lasers with homogeneous spectral broadeninti of active media as sufficiently complete and reliable. The theoretical estimation of the sensitivity of the method for CW laser operation was given in papers^7,8. But analytical formulae for sensitivity in reference⁷ are valid only for near-threshold levels of pumping power which does not often correspond to experimental conditions and leaves open the question about potentialities of the method at the higher pumping power (two or more times exceeding the threshold). The authors of the reference⁸ consider the problem at any pumping levels, but their approach is based on the simplified model which seem to be doubtful and requires the special foundation. In particular they suppose the existence in the active media of the number of N_j particles, which interact only with one mode j and do not interact with other laser modes, and also of the number of N_o molecules (so called reservoir) equally interacting with every mode. In the case of the homogeneous spectral broadening such dividing the molecules into types does not seem to be rightful. In addition in the both the gain and loss of all modes (except one) are assumed to be of the same values so the total number of modes is given arbitrary and not connected with resonator, active media and pumping parameters. Because of this reason one cannot calculate correctly the value of the method sensitivity or analyze it's dependence on the parameters.     

He-Ne Laser for Intra-Cavity Enhanced Absorption Measurement

It has been shown recently that when a relatively weak absorber is placed within a laser cavity an enhacement of absorption occurs^1–3. This method has been succesfully used for trace analysis of Na^1,4, I₂ ^3,5, Sr and Ba^{+ 6}, Eu(NO₃)₃ ^2,8, Pr(NO₃)₃, NdCl₃ and HoCl₃.⁸ In these experiments the absorbing species were placed inside the cavity of: flashlamp-pumped dye lasers^1–4,6 continous wave dye lasers^3,5 and dye lasers pumped by a ruby laser⁸.     

Raman study of photo‐induced degradation of the Ru(II) complex adsorbed on nanocrystalline TiO2 films

Photo‐induced degradation of a monolayer of the Ru(II) complex adsorbed on anatase TiO₂ thin films was studied by using resonant micro‐Raman spectroscopy. We developed two contrastive experiments to analyze the degradation mechanism. An exponential decay law was found when the dye was irradiated in the absence of a reducing agent. While the sensitized TiO₂ thin film electrode was covered by the I^–/I₃^– redox couple, the dye degradation exhibited a slowed linear decay. The experimental result was compared and the degradation mechanism was analyzed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dye Laser for Absorption Trace Analysis of Sodium

The advent of organic dye lasers made possible many experiments in physics, chemistry and biology. Most of these applications are based on the possibility to tune dye laser emission almost at any wavelength between 3400 Å and 12000 Å.This quality combined with high light power made feasible multiphotone absorption, selective absorption and fluorescence experiments. Recently a group of authors published the papers: “Enhancement of absorption spectra by dye laser quenching I and II”^1,2 which start a new field of applications of dye lasers. Namely if a weak absorber is placed inside the broad band laser cavity the absorption is enhanced and this results in the same absorption bands or lines observable in the spectrally resolved laser output. This effect can be used for detection of the traces of elements as a complementary method to the classical absorption spectroscopy but with much higher sensitivity. Same method could be also very useful for detection of transient species with a very short lifetimes.     

基于开槽型DBR光纤激光器拍频的折射率探测

提出一种包层开槽型分布布拉格反射光纤激光器,用于光纤激光折射率探测.该激光器以单纵模双偏振分布布拉格反射光纤激光器为核心传感单元,以结构双折射产生的拍频信号为传感信号对折射率进行探测.实验中应用频率编码,灵敏度高且易于解调.利用二氧化碳激光器的热效应对分布布拉格反射光纤激光器的谐振腔加工开槽,通过控制其扫描范围和功率来制备不同宽度和深度的开槽结构,并研究了不同结构参量对灵敏度特性的影响.实验测得最优灵敏度达-240 MHz/RI-unit.此外,还对该结构的温度传感特性做了测试,相较于未刻槽结构,温度灵敏度提高了21%,达1.73 MHz/℃.该装置结构紧凑、稳定性高、制备简单且成本低廉,可用于进一步研究分布布拉格反射光纤激光器的传感和复用特性.     

Time-Resolved Fluorescent Imaging of Glucose

A method for the fluorescent imaging of glucose is described that is based on the detection of enzymatically produced hydrogen peroxide, using the europium(III) tetracycline complex as the fluorescent probe incorporated into a hydrophilic polymer layer. Coadsorption of glucose oxidase (GOx) makes these sensor layers respond to the hydrogen peroxide produced by the GOx-assisted oxidation of glucose. The hydrogel layers are integrated into a 96-microwell plate for a parallel and simultaneous detection of various samples. Glucose is visualized by means of time resolved luminescence lifetime imaging. Unlike in previous methods, the determination of H₂O₂ does not require the addition of peroxidase or a catalyst to form a fluorescent product. The lifetime-based images obtained are compared with conventional fluorescence intensity-based methods with respect to sensitivity and the dynamic range of the sensor layer. The main advantages provided by this sensing scheme for H₂O₂ include reversibility, applicability at neutral pH, and the straightforwardness of the transducer system and the imaging device.     

Sensor properties of planar waveguide structures with grating couplers

The paper presents the results of theoretical analysis as well as the results of experimental research involving planar sensor structures with input grating couplers of the period Λ = 800 nm. In the theoretical part of the paper we discussed the influence of the parameters of a sensor structure on it sensitivities. The experimental part of the work presents the results of experimental research involving the influence of refractive index of the cover on the coupling characteristics of sensor structures with grating couplers. The full widths at half maximum (FWHM) were from 0.023° to 0.029°. For the investigated structures we estimated detection thresholds for the changes of refractive index of the cover and the changes of sensitive film thickness. It has been demonstrated that by the application of the elaborated structures we can detect minimal changes of the refractive index (Δn_c)_min = 2.1×10⁻⁶ when the refractive index of the cover n_c = 1.333 and (Δn_c)_min = 1.0×10⁻⁶ when n_c = 1515. For sensitive films of the thickness w < 100 nm, by using the elaborated structures, we can detect mean changes of the thickness along the values lower than 10⁻³ nm.     

Investigation on optical properties of Bi<Subscript>2.85</Subscript>La<Subscript>0.15</Subscript>TiNbO<Subscript>9</Subscript> thin films by prism coupling technique

Layered-perovskite ferroelectric Bi_2.85La_0.15TiNbO₉ (LBTN) optical waveguiding thin films were grown on fused silica substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) revealed that the film is highly (00l) textured. We observed sharp and distinct transverse electric (TE) and transverse magnetic (TM) multimodes and measured the refractive indices of LBTN thin films at 632.8 nm. The ordinary and extraordinary refractive indices were calculated to be n _TE=2.358 and n _TM=2.464, respectively. The film homogeneity and the film-substrate interface were analyzed using an improved version of the inverse Wentzel–Kramer–Brillouin (iWKB) method. The refractive index of the film remains constant at n ₀ within the waveguiding layer. The average transmittance of the film is 70% in the wavelength range of 400–1400 nm and the optical waveguiding properties were evaluated by the optical prism coupling method. Our results showed that the LBTN films are very good electro-optical active material.