http://www.hindawi.comThe latest articles from Hindawi Publishing Corporation© 2012, Hindawi Publishing Corporation. All rights reserved.http://www.hindawi.com/journals/ijo/2012/537949/The excitation wavelength of conventional Tb3+-activated phosphor is near 270 nm. This study describes novel green-emitting Tb3+-activated amorphous calcium silicate by ultraviolet excitation at 378 nm. The Tb3+-activated amorphous calcium silicate was prepared by heating a sample of Tb3+-activated calcium silicate hydrate (CSH) at 900°C for 30 minutes. The emission wavelength of the resulting phosphor was 544 nm. The optimum excitation wavelength within the range 300–400 nm was 378 nm. The Tb3+-activated amorphous calcium silicate emitted green by ultraviolet irradiation. The optimum initial Tb/Ca atomic ratio of this phosphor was about 0.5. A mechanism for the action of the phosphor is proposed, in which Tb3+ ions existing in the layer of the CSH lead to loss of water molecules and OH groups.Yoshiyuki Kojima, Masaaki Numazawa, Shinnosuke Kamei, and Nobuyuki NishimiyaCopyright © 2012 Yoshiyuki Kojima et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/374581/We report the results of a numerical and experimental analysis of femtosecond pulse propagation in a highly nonlinear photonic crystal fibre (PCF). The propagation of ultrashort pulses in PCF is studied taking into account the effects of dispersion, self-phase modulation, stimulated Raman scattering, and self-steepening. The interaction between linear and nonlinear effects is investigated in order to better understand the interplay of these effects leading to the supercontinuum generation. The initial stage of an ultra-wide-band supercontinuum generation under femtosecond pumping is pointed out. Details about the expansion of the blue and red sides of the supercontinuum due to transfer of energy to dispersive waves are revealed and experimentally confirmed. Our experimental results are compared to numerical solution of the nonlinear Schrödinger equation. Good agreement is found.Rim Cherif and Mourad ZghalCopyright © 2011 Rim Cherif and Mourad Zghal. All rights reserved.http://www.hindawi.com/journals/ijo/2011/737684/Sum-frequency generation of UV pulses exceeding 25 mJ and with beam quality M2∼5 has been demonstrated by mixing the third harmonic pulses of a flash lamp pumped 1.06 μm Nd:YAG laser with 1.7 μm pulses from an optical parametric oscillator pumped by the same laser in a compact setup.Oystein Farsund and Gunnar RustadCopyright © 2011 Oystein Farsund and Gunnar Rustad. All rights reserved.http://www.hindawi.com/journals/ijo/2011/714265/Experimental investigation of a compact 40-kV diode-type photoelectric DC gun driven by 100 fs laser pulses revealed that the space-charge-limited current density could exceed 30 kA/cm2 and that the density increased linearly with the accelerating voltage. We explained these important properties by the balance between the cathode surface field and the field produced by sheet-like electron bunches near the cathode surface. Our simple physical model agreed well with the experimental results.Hiroko Yamamoto, Hiroaki Hamabe, Shintaro Sone, Soichiro Yamaguchi, and Makoto R. AsakawaCopyright © 2011 Hiroko Yamamoto et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/164023/This paper reviews and presents a coherent approach to the design of compact vertical coupler (VC) in InP-based compound semiconductor with variable polarization dependence. As a polarization-independent (PI) coupler, the VC is shown to transfer light with more than 90% efficiency for both transverse-electric (TE) and transverse-magnetic (TM) polarizations. As a polarization-mode splitter (PMS), the VC is shown to preferentially couple TE or TM modes with a contrast ratio of up to 20 dB. We further demonstrate the single-mesa VC, which simplifies the fabrication process and potentially could improve the process yield, and its integration with a multimode interferometer (MMI). The versatility makes the VC a compact and useful input-stage device that not only maximizes input/output coupling efficiency to small active devices but also provides a degree of polarization control before the actual device.Chee-Wei LeeCopyright © 2011 Chee-Wei Lee. All rights reserved.http://www.hindawi.com/journals/ijo/2011/263182/We present the first unique design of a polarization-independent dual-wavelength splitter for wavelengths around 1.3 μm and 1.55 μm that is potentially of great interest to passive optical network (PON) applications. The filter design is simple compared with the other architectures and is based on ridge-type lateral directional couplers that can be readily integrated with other planar waveguide devices. Two design examples, based on InP/InGaAsP and Si/SiGe waveguides, are given. This polarization-independent wavelength splitting is achieved by exploiting the polarization dependence of the waveguides to produce coupling lengths that are sensitive to polarization and wavelength. We show that, to split the wavelengths without splitting the polarizations, the coupling lengths must be sufficiently different for TE and TM and for the different wavelengths in order to give the correct required ratios between the TE and TM coupling lengths for the two wavelengths of interest. We also show that the same approach can be applied to the design of a polarization splitter. The crosstalk, optical bandwidth, and fabrication sensitivity for the wavelength filter are evaluated.Chee-Wei LeeCopyright © 2011 Chee-Wei Lee. All rights reserved.http://www.hindawi.com/journals/ijo/2011/629605/We investigate theoretically the generation of squeezed states in spontaneous and stimulated six-wave mixing process quantum mechanically. It has been found that squeezing occurs in field amplitude, amplitude-squared, amplitude-cubed, and fourth power of field amplitude of fundamental mode in the process. It is found to be dependent on coupling parameter “g” (characteristics of higher-order susceptibility tensor) and phase values of the field amplitude under short-time approximation. Six-wave mixing is a process which involves absorption of three pump photons and emission of two probe photons of the same frequency and a signal photon of different frequency. It is shown that squeezing is greater in a stimulated interaction than the corresponding squeezing in spontaneous process. The degree of squeezing depends upon the photon number in first and higher orders of field amplitude. We study the statistical behaviour of quantum field in the fundamental mode and found it to be sub-Poissonian in nature. The signal-to-noise ratio has been studied in different orders. It is found that signal-to-noise ratio is higher in lower orders. This study when supplemented with experimental observations offers possibility of improving performance of many optical devices and optical communication networks.Sunil Rani, Jawahar Lal, and Nafa SinghCopyright © 2011 Sunil Rani et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/683415/We report on a novel behavior of the electromagnetically induced absorption (EIA) resonance observed on the D2 line of Cs for atoms confined in cells with micrometric thickness. With the enhancement of light intensity, the EIA resonance amplitude suffers from fast reduction, and even at very low intensity (W < 1 mW/cm2), resonance sign reversal takes place and electromagnetically induced transparency (EIT) resonance is observed. Similar EIA resonance transformation to EIT one is not observed in conventional cm-size cells. A theoretical model is proposed to analyze the physical processes behind the EIA resonance sign reversal with light intensity. The model involves elastic interactions between Cs atoms as well as elastic interaction of atom micrometric-cell windows, both resulting in depolarization of excited state which can lead to the new observations. The effect of excited state depolarization is confirmed also by the fluorescence (absorption) spectra measurement in micrometric cells with different thicknesses.A. Krasteva, D. Slavov, and S. CartalevaCopyright © 2011 A. Krasteva et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/417075/The femtosecond nature of X-ray free electron laser (FEL) pulses opens up exciting research possibilities in time-resolved studies including femtosecond photoemission and diffraction. The recent developments of seeding X-ray FELs extend their capabilities by creating stable, temporally coherent, and repeatable pulses. This in turn opens the possibility of spectral engineering soft X-ray pulses to use as a probe for the control of quantum dynamics. We propose a method for extending coherent control pulse-shaping techniques to the soft X-ray spectral range by using a reflective geometry 4f pulse shaper. This method is based on recent developments in asymmetrically cut multilayer optic technology and piezoelectric substrates.A. Aquila, M. Drescher, T. Laarmann, M. Barthelmeß, H. N. Chapman, and S. BajtCopyright © 2011 A. Aquila et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/818302/We have developed an ultrasonic probe for fluorescence modulation to image fluorescence within biological tissues. The probe consists of a focused ultrasonic transducer mounted on actuators for mechanical fan scanning, which can be used in contact with the measuring object aiming for clinical application. The mechanical fan scanning employed in the probe has a beneficial feature of portability. As a result, fluorescent beads, which were localized with the diameter of 2 mm at 20 mm depth in a pork meat tissue, were detected with resolution of 3 mm. The system performance denotes the feasibility of development towards the final goal of ultrasonic fluorescence modulation tomography for clinical applications.Trinh Quang Duc, Shunsuke Kaneta, and Masaki KobayashiCopyright © 2011 Trinh Quang Duc et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/364594/We report the efficient enhancement of light emission from silicon crystal by covering the silicon surface with an ultrathin (several nm) SiO2 layer. The photoluminescence of Si band edge emission (1.14 μm band) at room temperature is enhanced by two orders of magnitude. Compared with a p-Si/n-Si diode, light emission from a p-Si/SiO2/n-Si diode by current injection via direct tunneling is enhanced by more than 3 orders of magnitude. The light-emission enhancement is attributed to the diminishment of nonradiation recombination at the surface/interface and to the space confinement of the carrier recombination. The simple structure and low operating bias (approximately 1 volt) of our light emitting diodes supply a new choice for realizing efficient current injection light source in silicon compatible with conventional ULSI technology.Shucheng Chu and Hirofumi KanCopyright © 2011 Shucheng Chu and Hirofumi Kan. All rights reserved.http://www.hindawi.com/journals/ijo/2011/581810/We theoretically study the problem of Raman gain maximization in uniform silicon photonic-crystal waveguides supporting slow optical modes. For the first time, an exact solution to this problem is obtained within the framework of the undepleted-pump approximation. Specifically, we derive analytical expressions for the maximum signal gain, optimal input pump power, and optimal length of a silicon Raman amplifier and demonstrate that the ultimate gain is achieved when the pump beam propagates at its maximum speed. If the signal’s group velocity can be reduced by a factor of 10 compared to its value in a bulk silicon, it may result in ultrahigh gains exceeding 100 dB. We also optimize the device parameters of a silicon Raman amplifier in the regime of strong pump depletion and come up with general design guidelines that can be used in practice.Ivan D. Rukhlenko, Malin Premaratne, and Govind P. AgrawalCopyright © 2011 Ivan D. Rukhlenko et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/825629/Early cancer detection has been playing an important role in reducing cancer mortality. Optical coherence tomography (OCT), due to its micron-scale resolution, has the ability to detect cancerous tissues at their early stages. For internal organs, endoscopic probes are needed as the penetration depth of OCT is about 1–3 mm. MEMS technology has the advantages of fast speed, small size, and low cost, and it has been widely used as the scanning engine in endoscopic OCT probes. Research results have shown great potential for OCT in endoscopic imaging by incorporating MEMS scanning mirrors. Various MEMS-OCT designs are introduced, and their imaging results are reviewed in the paper.Jingjing Sun and Huikai XieCopyright © 2011 Jingjing Sun and Huikai Xie. All rights reserved.http://www.hindawi.com/journals/ijo/2011/293684/The microcirculation is a complex system, and the visualization of microcirculation has great significance in improving our understanding of pathophysiological processes in various disease conditions, in both clinical and fundamental studies. A range of techniques are available or emerging for investigating different aspect of the microcirculation in animals and humans. This paper reviews the recent developments in the field of high-resolution and high-sensitive optical imaging of microcirculatory tissue beds, emphasizing technologies that utilize the endogenous contrast mechanism. Optical imaging techniques such as intravital microscopy, Capillaroscopy, laser Doppler perfusion imaging, laser speckle perfusion imaging, polarization spectroscopy, photo-acoustic tomography, and various implementations of optical coherence tomography based on Doppler and speckle contrast imaging are presented together with their prospectives and challenges.Hrebesh M. SubhashCopyright © 2011 Hrebesh M. Subhash. All rights reserved.http://www.hindawi.com/journals/ijo/2011/254154/Satellite quantum key distribution is a promising technique that overcomes the limited transmission distance in optical-fiber-based systems. The polarization tracking technique is one of the key techniques in the satellite quantum key distribution. With free-space quantum key distribution between an optical ground station and a satellite, the photon polarization state will be changed according to the satellite movement. To enable polarization based quantum key distribution between mobile terminals, we developed a polarization-basis tracking scheme allowing a common frame of reference to be shared. It is possible to orient two platforms along a common axis by detecting the reference optical signal only on the receiver side with no prior information about the transmitter's orientation. We developed a prototype system for free-space quantum key distribution with the polarization-basis tracking scheme. Polarization tracking performance was 0.092° by conducting quantum key distribution experiments over a 1 km free space between two buildings in a Tokyo suburb.Morio Toyoshima, Hideki Takenaka, Yozo Shoji, Yoshihisa Takayama, Masahiro Takeoka, Mikio Fujiwara, and Masahide SasakiCopyright © 2011 Morio Toyoshima et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/902158/Nanoradian Surface Profilers (NSPs) are required for state-of-the-art synchrotron radiation optics and high-precision optical measurements. Nano-radian accuracy must be maintained in the large-angle test range. However, the beams' notable lateral motions during tests of most operating profilers, combined with the insufficiencies of their optical components, generate significant errors of ∼1 μrad rms in the measurements. The solution to nano-radian accuracy for the new generation of surface profilers in this range is to apply a scanning optical head, combined with nontilted reference beam. I describe here my comparison of different scan modes and discuss some test results.Shinan QianCopyright © 2011 Shinan Qian. All rights reserved.http://www.hindawi.com/journals/ijo/2011/473910/Differences in the odd to even response for tin isotopes has been observed earlier in resonance ionization experiments, resulting in anomalous odd to even isotope ratios. I have used a theoretical approach known as the spectral simulation approach to understand the cause for such anomaly and the anomaly has also been experimentally verified and found to be in good agreement. The effects of laser parameters such as intensity, accuracy of the excitation laser wavelength and bandwidth on the determination of the tin isotope ratio have been analyzed theoretically and experimentally. The source for such anomalies was found to be the inaccuracy in the excitation laser wavelength. For the 5p2 (3P0) − 5p 6s (P310) (286.3317 nm) transition, an inaccuracy of the order of γL in the peak frequency of the excitation laser (γL=15 GHz) can cause anomalies as large as ∼31% (β=0.31). Use of a very large bandwidth laser (∼60 GHz) reduces the anomaly to as small as −0.003. Alternatively by employing a relatively narrow band laser (∼1.2 GHz), it has been observed that inaccuracy of the order of 3-4 γL in the laser peak frequency does not induce anomalies >0.05. The isotope ratio is sensitive to the inaccuracy in the excitation laser wavelength for an intermediate linewidth laser.Manda SankariCopyright © 2011 Manda Sankari. All rights reserved.http://www.hindawi.com/journals/ijo/2011/215404/A simple method for determining the stress fields in transparent glass artwork pieces is presented. The proposed method is based on the principles of photoelasticity combining imaging and quantitative measures with an image processing algorithm. The main goal was to develop a methodology to help glass artists to determine the ability of their work to resist the thermal shocks by giving them a hazard map based on the stress map of the glass piece.João M. P. Coelho, Catarina Silva, and Teresa AlmeidaCopyright © 2011 João M. P. Coelho et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/462505/Scattering optimization is a challenging engineering problem with an extensive list of potential applications. For some practical applications, the reflectance of the photonic cluster should be variable from minimal to maximal values. The theoretical investigation of such problem is extremely difficult due to multiple interactions between particles in the cluster. In this paper, the optimization of the light scattered by the photonic cluster made of small particles is studied with the help of the special optimization algorithm and the local perturbation method. It was shown that the photonic cluster can be transformed in such a way that its reflectance will be increased or decreased by several orders of magnitude for selected wavelength and direction.V. V. ProsentsovCopyright © 2011 V. V. Prosentsov. All rights reserved.http://www.hindawi.com/journals/ijo/2011/728350/We compare three attempts that have been made to decompose the angular momentum of the electromagnetic field into components of an “orbital” and “spin” nature. All three expressions are different, and there seems to be no reason to prefer one to another. It appears, on the basis of classical electrodynamics, that there is no unique way of decomposing the angular momentum of the electromagnetic field into orbital and spin components, even in a fixed inertial frame.A. M. StewartCopyright © 2011 A. M. Stewart. All rights reserved.http://www.hindawi.com/journals/ijo/2011/608139/Free surface fluctuations on a high-speed water jet were measured by a laser beam refraction technique. This method can be used to obtain quantitative time-series data on local surface fluctuations. The developed system employs two pulsed laser diodes, and it uses a high-speed optical sensor to detect the instantaneous positions of the laser beams that are refracted at the free surface. Fluctuations in the slope angle are measured at two locations separated by 1.27 mm. The wave speed of each free surface wave, which is determined by the zero-upcrossing method, is experimentally evaluated by the cross-correlation method. A two-dimensional waveform is obtained by integrating the slope angle data. The local mean wavelength and mean wave steepness are evaluated for average jet velocities up to U=10 m/s. Streamwise waveforms of the high-speed water jet at several locations exhibit appreciable asymmetry and have steep profiles.Kazuhiro Itoh, Yasuo Iwata, Hiroshige Kumamaru, and Yuji ShimogonyaCopyright © 2011 Kazuhiro Itoh et al. All rights reserved.http://www.hindawi.com/journals/ijo/2011/754610/Photonic crystal fiber (PCF) sensors based on surface-enhanced Raman scattering (SERS) have become increasingly attractive in chemical and biological detections due to the molecular specificity, high sensitivity, and flexibility. In this paper, we review the development of PCF SERS sensors with emphasis on our recent work on SERS sensors utilizing hollow-core photonic crystal fibers (HCPCFs). Specifically, we discuss and compare various HCPCF SERS sensors, including the liquid-filled HCPCF and liquid-core photonic crystal fibers (LCPCFs). We experimentally demonstrate and theoretically analyze the high sensitivity of the HCPCF SERS sensors. Various molecules including Rhodamine B, Rhodamine 6G, human insulin, and tryptophan have been tested to show the excellent performance of these fiber sensors.Xuan Yang, Chao Shi, Rebecca Newhouse, Jin Z. Zhang, and Claire GuCopyright © 2011 Xuan Yang et al. All rights reserved.http://www.hindawi.com/journals/ijo/2010/978763/The solubility of potassium dihydrogen phosphate (KDP) was determined for five different temperatures. Metastable zone width of aqueous solutions of KDP saturated at different temperatures was determined by nucleation method in pure form and adding a well-known chelating agent ethylenediaminetetraacetic acid (0.5 mol% EDTA). The induction period τ was measured, experiments were performed at selected degrees of supersaturation (C/C*), and the critical nucleation parameters like interfacial energy (σ) were calculated, based on the classical theory of nucleation. The structural and optical properties of the grown crystals have been investigated. FTIR analysis has been carried out to characterize the grown crystals. Band gap has been calculated from transmission spectra.Anisur Rahman and Jiban PodderCopyright © 2010 Anisur Rahman and Jiban Podder. All rights reserved.http://www.hindawi.com/journals/ijo/2010/601675/A novel study has been undertaken for the coupling efficiency enhancement on planar (refractive and diffractive) microlenses as coupling elements with various geometrical shapes in fibre optics communication. When the dispersion of the planar diffractive microlens is expressed in terms of an Abbe number, it is easy to solve for the refractive and diffractive power coupling efficiency that result in a single material. Coupling loss is reduced with microlens used as coupler even when increasing wavelength of the Gaussian beam. A slight difference of power coupling efficiency is observed with the different direction of electric polarisation for a Gaussian beam of large spot size at its waist due to coupling property has been discussed and shown to agree well with theoretical model.S. Selvanandan and P. M. AnbarasanCopyright © 2010 S. Selvanandan and P. M. Anbarasan. All rights reserved.http://www.hindawi.com/journals/ijo/2010/802385/Two patterns of functionally graded materials (FGMs) were successfully fabricated whose compositions gradually varied from 100% stainless steel 316L to 100% Inconel718 superalloy using laser engineered net shaping process. The microstructure characterization, composition analysis, and microhardness along the graded direction were investigated. The comparison revealed the distinctions in solidification behavior, microstructure evolution of two patterns. In the end, the abrasive wear resistance of the material was investigated.Dongjiang Wu, Xiaokang Liang, Qian Li, and Lijia JiangCopyright © 2010 Dongjiang Wu et al. All rights reserved.http://www.hindawi.com/journals/ijo/2010/137572/A rigorous and consistent approach is demonstrated to develop a model of the 4M structure (the four-media structure of a film on a substrate of finite thickness). The general equations obtained for the reflectance and transmittance spectra of the 4M structure are simplified by employing a procedure of the so-called device averaging to reduce them to a succinct form convenient for processing of experimental spectra for the structures with a thick substrate. The newly derived equations are applied to two special cases: (i) an arbitrary film on highly absorbing substrates and (ii) a slightly absorbing film on transparent substrates. The reflectance and transmittance spectra represented in the simplified (with the device averaging) form have a practical application for determining the film thickness and optical constants from experimental spectra by using the known techniques.Anatoly Barybin and Victor ShapovalovCopyright © 2010 Anatoly Barybin and Victor Shapovalov. All rights reserved.http://www.hindawi.com/journals/ijo/2010/261420/Ta and N co-doped SrTiO3 was synthesized by microwave-assisted solvothermal reaction using SrCl2⋅6H2O, Ti(OC3H7)4, TaCl5, and HMT (hexamethylenetetramine) in KOH and oleic acid mixed solutions. The products were characterized by XRD, TG-DTA, BET surface area measurement, TEM, EDX, DRS, PL-Spectra, and XPS. The nanoparticles of perovskite-type SrTi1-xTaxO3-yNy (x=0−0.1) were successfully synthesized. The photocatalytic activity of SrTiO3 for DeNOxability in the visible light region could be improved by the codoping of Ta5+ and N3-. The excellent visible light photocatalytic activity of this substance may be due to the generation of a new band gap by doping nitrogen that enables the absorption of visible light as well as a decrease in the incidence of the lattice defects, which acts as a recombination center of photo-induced electrons and holes by codoping with Ta5+.Uyi Sulaeman, Shu Yin, and Tsugio SatoCopyright © 2010 Uyi Sulaeman et al. All rights reserved.http://www.hindawi.com/journals/ijo/2010/201305/We present a remote, differential method for measuring the prescription of aspheric mirrors using null interferometry in the center-of-curvature configuration. The method requires no equipment beyond that used in a basic interferometery setup (i.e., there are no shearing elements or absolute distance meters). We chose this configuration because of its widespread use. However, the method is generalizable to other configurations with an adjustment of the governing equation. The method involves taking a series of interferograms before and after small, known misalignments are applied to the mirror in the interferometry setup and calculating the prescription (e.g., radius of curvature and conic constant) of the mirror, based on these differential measurements, using a nonlinear regression. We apply this method successfully to the testing of a Space Optics Research Lab off-axis parabola with a known focal length of 152.4 mm, a diameter of 76.2 mm, and an off-axis angle of 12°.Brian M. Robinson, Patrick J. Reardon, and Joseph M. GearyCopyright © 2010 Brian M. Robinson et al. All rights reserved.http://www.hindawi.com/journals/ijo/2010/897829/We present fiber-coupled dielectric-loaded plasmonic waveguides for 1.55 μm telecom wavelength fabricated by two-photon polymerization and nanoimprint lithography. The waveguide structures include 100-μm-long plasmonic waveguides connected on both ends to tapered dielectric waveguides used for end-fire coupling with optical fibers. The excitation of plasmonic waveguides is verified via polarization-resolved measurements of the overall transmission, demonstrating thereby that this technology is suitable in principle for the integration of plasmonic components into fiberoptics. Loss mechanisms are investigated and improvements suggested to reduce transmission losses and consequently increase the viability of practical application.Andreas Seidel, Jacek Gosciniak, Maria U. Gonzalez, Jan Renger, Carsten Reinhardt, Roman Kiyan, Romain Quidant, Sergey I. Bozhevolnyi, and Boris N. ChichkovCopyright © 2010 Andreas Seidel et al. All rights reserved.http://www.hindawi.com/journals/ijo/2010/275910/The exact solutions of the Schrödinger equation for quantum damped oscillator with modified Caldirola-Kanai Hamiltonian are evaluated. We also investigate the cases of under-, over-, and critical damping.Akpan N. Ikot, Louis E. Akpabio, Ita O. Akpan, Michael I. Umo, and Eno E. ItuenCopyright © 2010 Akpan N. Ikot et al. All rights reserved.