• Volume 82, Issue 1

January 2014,   pages  1-171

• Preface

• Imaging unsteady three-dimensional transport phenomena

Careful and continuous measurements of flow, heat and mass transfer are required in quite a few contexts. Using appropriate light sources, it is possible to map velocity, temperature, and species concentration over a cross-section and as a function of time. Image formation in optical measurements may rely on scattering of radiation from particles. Alternatively, if the region of interest is transparent, refractive index would be a field variable and beam bending effects can be used to extract information about temperature and concentration of solutes dissolved in liquids. Time-lapsed images of light intensity can be used to determine fluid velocity. Though used originally for flow visualization, optical imaging has now emerged as a powerful tool for quantitative measurements. Optical methods that utilize the dependence of refractive index on concentration and temperature can be configured in many different ways. Three available routes considered are interferometry, schlieren imaging, and shadowgraph. Images recorded in these configurations can be analysed to yield time sequences of three-dimensional distributions of the transported variables. Optical methods are non-intrusive, inertia-free and can image cross-sections of the experimental apparatus. The image data can be jointly analysed with the physical laws governing transport and principles of image formation. Hence, with the experiment suitably carried out, three-dimensional physical domains with unsteady processes can be accommodated. Optical methods promise to breach the holy grail of measurements by extracting unsteady three-dimensional data in applications related to transport phenomena.

• High-power Yb-doped continuous-wave and pulsed fibre lasers

High-power laser generation using Yb-doped double-clad fibres with conversion efficiencies in excess of 80% have attracted much attention during the last decade due to their inherent advantages in terms of very high efficiency, no misalignment due to in-built intracore fibre Bragg gratings, low thermal problems due to large surface to volume ratio, diffraction-limited beam quality, compactness, reliability and fibre-optic beam delivery. Yb-doped fibres can also provide a wide emission band from ∼1010 nm to ∼1170 nm, which makes it a versatile laser medium to realize continuous-wave (CW), Q-switched short pulse, and mode-locked ultrashort pulse generation for various applications. In this article, a review of Yb-doped CW and pulsed fibre lasers along with our study on self-pulsing dynamics in CW fibre lasers to find its role in high-power fibre laser development and the physical mechanisms involved in its generation has been described. A study on the generation of high-power CWfibre laser of 165Woutput power and generation of high peak power nanosecond pulses from acousto-optic Q-switched fibre laser has also been presented.

• Development of coherent tunable source in 2–16 𝜇m region using nonlinear frequency mixing processes

A very convenient way to obtain widely tunable source of coherent radiation in the infrared region is through nonlinear frequency mixing processes like second harmonic generation (SHG), difference-frequency mixing (DFM) or optical parametric oscillation (OPO). Using commonly available Nd:YAG laser and its harmonic pumped dye laser radiation as parent beams, we have been able to generate coherent tunable infrared radiation (IR) in 2–16 𝜇m region using different nonlinear crystals by DFM and OPO. We have also generated such IR source in the 4–5 𝜇m region through SHG of CO2 laser in different infrared crystals. In the process we have characterized a large number of nonlinear crystals like different borate group of crystals, KTP, KTA, LiIO3, MgO:LiNbO3, GaSe, AgGaSe2, ZnGeP2, AgGa$_{1−x}$In$_x$Se2, HgGa2S$_4$ etc. To improve the conversion efficiencies of such frequency conversion processes, we have developed some novel schemes, like multipass configuration (MC) and positive optical feedback (POF). The significance of the obtained results lies in the fact that to get the same conversion in SHG or DFM, one now requires fundamental input radiation with much lower intensity.

• Optical fibre probes in the measurement of scattered light: Application for sensing turbidity

Optical fibre probes or optrodes often form the heart of multimode fibre-based measurements and sensors. An optrode usually comprises a bundle of multimode fibres, out of which one or more fibres are used for irradiating the sample, and the remaining fibres are used to collect the light reflected/scattered/fluoresced from the sample containing the measurand(s). The so-collected light carries the characteristic signature of the measurand. Here we present our work on the design and realization of optrodes for the measurement of scattered light from liquid samples. Optical properties of a solution are usually characterized by the parameters absorption coefficient $\mu_a$, scattering coefficient $\mu_s$, and anisotropy factor 𝑔. We have developed a simple method to determine $\mu_a$, $\mu_s$, and 𝑔, of a turbid medium, and a Monte–Carlo model was used to simulate the light scattering from the turbid medium. As an application, we describe the development of a turbidity sensor that has been designed and realized by employing an optrode in conjunction with a concave mirror. The estimation of turbidity is done on the basis of total interaction, by considering scattering and absorption of light from the sample solution. Details of the experiments and results are presented here.

• High-pressure continuously tunable CO2 lasers and molecular laser isotope separation

The acronym MLIS (molecular laser isotope separation) defines the laser process whereby the isotopes of uranium can be separated by mid-infrared laser/s when the molecule employed is UF6. The theoretical and spectroscopical data to configure and enable experiments and demonstrations in the laboratory is adequate. However, the engineering and commercial aspects require innovative technology solutions that are not presently available in the literature on these topics. This paper is an overview of the most salient features of MLIS and its potential utility at an industrial level.

• Subwavelength propagation and localization of light using surface plasmons: A brief perspective

Surface plasmons at the metal–dielectric interface have emerged as an important candidate to propagate and localize light at subwavelength scales. By tailoring the geometry and arrangement of metallic nanoarchitectures, propagating and localized surface plasmons can be obtained. In this brief perspective, we discuss: (1) how surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) can be optically excited in metallic nanoarchitectures by employing a variety of optical microscopy methods; (2) how SPPs and LSPs in plasmonic nanowires can be utilized for subwavelength polarization optics and single-molecule surface-enhanced Raman scattering (SERS) on a photonic chip; and (3) how individual plasmonic nanowire can be optically manipulated using optical trapping methods.

• Compact, common path quantitative phase microscopic techniques for imaging cell dynamics

Microscopy using visible electromagnetic radiation can be used to investigate living cells in various environments. But bright field microscopy only provides two-dimensional (2D) intensity distribution at a single object plane. One of the ways to retrieve object height/thickness information is to employ quantitative phase microscopic (QPM) techniques. Interferometric QPM techniques are widely used for this. Digital holographic microscopy (DHM) is one of the stateof-the-art methods for quantitative three-dimensional (3D) imaging. Usually it is implemented in two-beam geometry, which is prone to mechanical vibrations. But to study dynamics of objects like red blood cells, one needs temporal stability much better than the fluctuations of the object, which the two-beam geometry fails to deliver. One way to overcome this hurdle is to use selfreferencing techniques, in which a portion of the object beam will act as the reference beam. Here the development of self-referencing QPM techniques is described along with the results.

• Attosecond-correlated dynamics of two electrons in argon

In this work we explored strong field-induced decay of doubly excited transient Coulomb complex Ar$^{\ast\ast} \to$Ar2++2𝑒. We measured the correlated two-electron emission as a function of carrier envelop phase (CEP) of 6 fs pulses in the non-sequential double ionization (NSDI) of argon. Classical model calculations suggest that the intermediate doubly excited Coulomb complex loses memory of its formation dynamics. We estimated the ionization time difference between the two electrons from NSDI of argon and it is 200 ± 100 as (N Camus et al, Phys. Rev. Lett. 108, 073003 (2012)).

• Progress in sub-femtosecond control of electron localization in molecules

Recent advances in controlled generation of intense, ultrashort laser pulses in the femtosecond and attosecond time-scales have pushed new avenues of research in the coherent control of ultrafast electron dynamics in atoms and molecules. We present a topical review on the phenomenon of control of electron localization in small dissociating molecules. By creating and controlling coherent superposition of the symmetric and antisymmetric electronic states, it becomes possible to confine the evolving electron cloud onto a preferred nucleus, thereby steering the molecule towards a desired dissociation route. We discuss the origin of the idea and various mechanisms to achieve electron localization in small molecules. To highlight recent experimental progress, we explain how one can employ few-cycle IR pulses and different attosecond extreme ultraviolet (EUV) pulses in various ways to successfully demonstrate the control of electronic dynamics. Future research opportunities and challenges on this topic are envisioned.

• Ultrashort laser pulse–matter interaction: Implications for high energy materials

The interaction of ultrashort [nanosecond (ns)/picosecond (ps)/femtosecond (fs)] pulses with materials is an exhaustive area of research with underlying, and often extremely rich, physics along with a plethora of applications evolving from it. High-energy materials (HEMs) are chemical compounds or mixture of compounds which, under suitable initiation, undergoes a very rapid exothermic and self-propagating decomposition. Herein, we describe the interaction of laser pulses with materials and its implications for studies on HEMs in four parts: (a) ns and fs laserinduced breakdown spectroscopic (LIBS) studies of HEMs towards understanding the molecular dynamics and discrimination, (b) ps/fs pulses interaction with metallic solids towards the production of nanoparticles, nanostructures and their utility in identifying explosive molecules using surface-enhanced Raman scattering studies, (c) interaction of laser pulses with the bulk and surface of glasses and polymers producing micro- and nanostructures for microfluidic/lab-on-a-chip applications, and (d) ultrafast spectroscopic studies for comprehending the excited state dynamics towards elucidation of vibrational dynamics in HEMs. Several applications resulting from these interactions will be discussed in detail.

• Probing strong field ionization of solids with a Thomson parabola spectrometer

Intense ultrashort laser pulses are known to generate high-density, high-temperature plasma from any substrate. Copious emission of hot electrons, from a solid substrate, results in strong electrostatic field that accelerates the ions with energies ranging from a few eV to MeV. Ion spectrometry from laser–plasma is convolved with multiple atomic systems, several charge states and a broad energy spread. Conventional mass spectrometric techniques have serious limitations to probe this ionization dynamics. We have developed an imaging ion spectrometer that measures charge/mass-resolved ion kinetic energies over the entire range. Microchannel plate (MCP) is used as the position-sensitive detector to perform online and single shot measurements. The wellresolved spectrum even for the low-energy ions, demonstrates that the spectral width is limited by the space-charge repulsion for the ions generated in the hot dense plasma.

• Feedback-controlled electro-kinetic traps for single-molecule spectroscopy

A principal limitation of single-molecule spectroscopy in solution is the diffusionlimited residence time of a given molecule within the detection volume. A common solution to this problem is to immobilize molecules of interest on a passivated glass surface for extending the observation time to obtain reliable data statistics. However, surface tethering of molecules often introduces artifacts, particularly when studying the structural dynamics of biomolecules. To circumvent this limitation, we investigated alternative ways to extend single-molecule observation times in solution without surface immobilization. Among various possibilities, the so-called anti-Brownian electro-kinetic trap (or ABEL trap) seems best suited to achieve this goal. The essential part of this trap is a feedback-controlled electro-kinetic steering of a molecule’s position in reaction to its diffusive Brownian motion which is monitored by fluorescence, thus keeping the molecule within a sub-micron sized detection volume. Fluorescence trace recordings of over thousands of milliseconds duration on individual dye molecules within an ABEL trap have been reported. In this short review, we shall briefly discuss the principle and some results of ABEL trapping of individual molecules with possible extensions to future works.

• Laser applications in nuclear power plants

This paper reports the state of the art of using a solid-state Nd:YAG laser for material processing applications such as cutting, welding and drilling of several components of operational nuclear reactors in radioactive environment. We have demonstrated several advantages of laserbased material processing over conventional methods, and these are discussed briefly. At NPCIL, we have used laser techniques to cut stainless steel sheets up to 14 mm thickness and stainless steel weld up to a depth of 3 mm. This remotely operable laser system has been engineered for its robustness with proper fixtures and tooling for various material processing operations on industrial scale.

• Erbium–ytterbium fibre laser emitting more than 13W of power in 1.55 𝜇m region

We report the work on erbium:ytterbium-doped double clad fibre laser (EYDFL), that is pumped at 976 nm. The maximum output power generated is 13.6Win 1550 nm region with a slope efficiency of about 21%. To the best of our knowledge, this is the highest power reported from an EYDFL, that uses commercially available off-the-shelf large mode area Er:Yb-doped double-clad fibre.

• Benefits of cryogenic cooling on the operation of a pulsed CO2 laser

The paper presents results of a theoretical model of a pulsed electron beam controlled CO2 laser (EBCL) to investigate the effect of cooling on the laser gas mixture. It is shown that cryogenic cooling can significantly improve the performance of the laser. The efficiency of an EBCL improved from 20% to 25.3% by cooling it to 200 K. The improvement is mainly due to the decrease of thermal population of the CO2 (0 1 0) vibration level.

• Enhanced performance of a repetitively pulsed 130 mJ KrF laser with improved pre-ionization parameters

Studies related to the effect of pre-ionizer on laser output energy of a repetitively pulsed KrF laser are presented. The dependence of laser output energy, spectral width and beam spot homogeneity on pre-ionization parameters, namely its current and voltage rise time are reported here. Here, effectiveness of pre-ionization is optimized by improving pre-ionization current and rise time of the pump pulse of the automatic UV pre-ionizer KrF laser. It is observed that by increasing pre-ionization current from 6 kA to 10.6 kA, the output energy increases by about 30% (from 100 to 130 mJ). It is also observed that the emission spectral width reduces by almost 60% by increasing the pre-ionization current. Regular homogeneous and well-developed beam spot (nearly Hat-Top profile) was achieved under these optimized conditions.

• Measurement of the figure of merit of indigenously developed Nd-doped phosphate laser glass rods for use in high power lasers

High energy, high power (HEHP) Nd:glass laser systems are used for inertial confinement fusion and equation of state (EOS) studies of materials at high temperature and pressure. A program has been undertaken for the indigenous development of Nd-doped phosphate laser glass rods and discs for HEHP lasers. In this paper, we report the characterization of the Nd-doped phosphate laser glass rods produced under this program and compare the indigenously developed laser glass to LHG-8 laser glass of M/s Hoya, Japan. We experimentally measured the values of the stimulated emission cross-section (𝜎) and coefficient of intensity-dependent refractive index ($n_2$) and hence the figure of merit $F = \sigma/n_2$ of the indigenous phosphate laser glass rods. This value of figure of merit is found comparable to the reported value of identically doped Nd:glass rods.

• Design and performance characteristics of a krypton chloride (𝜆 = 222 nm) excimer laser

Development of a discharge-pumped krypton chloride (KrCl) laser operating at 222 nm wavelength is demonstrated. In this paper the design, successful realization and operating characteristics of KrCl excimer laser are reported. The laser is driven by a simple and efficient excitation technique using automatic UV pre-ionization with discharge-pumped self-sustained capacitor– capacitor (C–C) energy transfer circuit. The experimental investigations including output laser energy, temporal pulse parameters, emission spectra and beam profile of the KrCl laser were recorded. For high repetition rate operation, in-built, compact gas circulation system using tangential blower was incorporated. The laser was operated at 25 kV discharge voltage, gas mixture of 5 mbar HCl, 160 mbar kypton and neon as balance with a total gas pressure of ∼2.5 bar. These experiments produced an efficient and reliable output energy of 25 mJ from an active volume of 60 cm3.

• # Pramana – Journal of Physics

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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019