Rik Chattopadhyay

Highly fluorescent silver nanoclusters in alumina-silica composite optical fiber

An efficient visible fluorescent optical fiber embedded with silver nanoclusters (Ag-NCs) having size ∼1 nm, uniformly distributed in alumina-silica composite core glass, is reported. Fibers are fabricated in a repetitive controlled way through modified chemical vapour deposition process associated with solution doping technique. Fibers are drawn from the transparent preforms by conventional fiber drawing process. Structural characteristics of the doped fibers are studied using transmission electron microscopy and electron probe micro analysis. The oxidation state of Ag within Ag-NCs is investigated by X-ray photo electron spectroscopy. The observed significant fluorescence of the metal clusters in fabricated fibers is correlated with electronic model. The experimentally observed size dependent absorption of the metal clusters in fabricated fibers is explained with the help of reported results calculated by ab-initio density functional theory. These optical fibers may open up an opportunity of realizing tunable wavelength fiber laser without the help of rare earth elements.

Plasmonic hollow-core photonic band gap fiber for efficient sensing of biofluids

We report a new family of hollow-core photonic crystal fibers with embedded metal wires for sensitive refractive index measurement of fluids. These fibers operate on the principle of resonant coupling of the guided core mode with the surface plasmon mode generated at the surface of metal wires embedded in the photonic crystal structure. A maximum sensitivity of 2151?nm RIU?1 (nanometer per refractive index unit) can be achieved even for refractive index values lower than 1.26. The fiber is simple in design and can be fabricated with a certain modification to the conventional hollow-core photonic band gap fiber manufacturing technique. The proposed design is useful for detecting a small change in refractive index of the sample and may be implemented to estimate the bulk permittivity of the core fluid.

Study of nonlinear dynamics in silver-nanoparticle-doped photonic crystal fiber

Linear and nonlinear properties of silver-nanoparticle-doped photonic crystal fibers (SNPCFs) are explored to obtain exciting nonlinear pulse dynamics. SNPCF offers additional control over the Kerr nonlinearity of the core glass. Unlike traditional PCFs, these composite fibers offer a significantly large negative nonlinearity at lower wavelengths. The interplay between large negative nonlinearity and dispersion leads to interesting dynamics of ultrashort pulse evolution where blueshifted Raman solitons are generated along with phase-matched radiations. Further, it is observed that the numeric sign of self-steepening coefficient provides an additional tool in harnessing the dispersive wave generation.

Stratified composite-loaded plasmonic waveguide for sensing biofluids

A new integrated plasmonic waveguide sensor is reported with high sensitivity (1600  nm/RIU). The integrated structure, loaded with stratified composite, makes this device robust and easy to fabricate on a chip to use as a sensor probe. The device works on the principle of resonant coupling between surface plasmon and fundamental TM mode. By selecting proper stratified structure (metal–dielectric) and core glass, one can tune the sensitivity and the range of operating wavelength.

Implications of a zero-nonlinearity wavelength in photonic crystal fibers doped with silver nanoparticles

Photonic crystal fibers doped with silver nanoparticles exhibit a Kerr nonlinearity that can be positive or negative depending on the input wavelength and which vanishes at a specific wavelength. The existence of negative nonlinearity allows soliton formation even in the normal-dispersion region of the fiber, and the zero-nonlinearity wavelength (ZNW) acts as a barrier for the Raman-induced redshift of solitons. We adopted the variational principle to understand the role of the zero-nonlinearity point on Raman redshift and verified its prediction numerically for fundamental and higher-order solitons. We show how the simultaneous presence of a ZNW and a zero-dispersion wavelength affects soliton evolution inside such fibers and find a number of unique features such as the position and the spectral bandwidth of the dispersive wave that change with the location of the ZNW.

Experimental and theoretical study of red-shifted solitonic resonant radiation in photonic crystal fibers and generation of radiation seeded Raman soliton

Redshifted solitonic resonant radiation (RR) is a fascinating phase-matching phenomenon that occurs when an optical pulse, launched in the normal dispersion regime of photonic crystal fiber, radiates across the zero-dispersion point. The formation of such phase-matched radiation is independent of the generation of any optical soliton and mainly governed by the leading edge of an input pump which forms a shock front. The radiation is generated at the anomalous dispersion regime and found to be confined both in the time and frequency domain. We experimentally investigate the formation of such radiation in fabricated photonic crystal fiber for two different pulse width regimes (femtosecond and picosecond) with detailed theoretical analyses. Theoretically predicted results corroborate well with experimental results and confirm the existence of such unique radiation which is robust in nature. Further, we extend the study to long-length fiber and investigate the interplay between redshifted solitonic RR and intrapulse Raman scattering (IPRS). The consequence of the formation of such solitonic RR in an anomalous dispersion domain is found to be very interesting where it seeds a series of Raman solitons and behaves like a secondary source. These Raman solitons are now continuously redshifted and open up the possibility of wideband supercontinuum generation even in normal dispersion pumping. We fabricate a suitable photonic crystal fiber and experimentally demonstrate the RR-seeded IPRS process.

Structural and light coupling characteristics of patterned silica–titania sol–gel thin films with/without nano gold coatings

High fidelity one-dimensional (1D) stripe/two-dimensional (2D) square pillar-like mesoscale patterned silica–titania sol–gel thin films with and without nano gold coatings on silica glass were successfully fabricated using a topographically stripe-like patterned (periodicity, 1.5 μm; peak height, 120 nm) polydimethylsiloxane stamp by a facile soft lithography technique. The material properties of the films were characterized for detailed analyses of their surface morphologies, microstructures, chemical bonding, etc. The films without Au coatings were amorphous in nature and highly transparent (∼80%) over the visible region. The visible absorption peak of nano gold in the Au-coated films confirmed the formation of surface plasmons (SPs) of cubic nano-Au. The films showed different water wetting behaviors as determined by static water contact angle measurements. This change in water wetting behavior can be explained on the basis of the root mean square surface roughnesses of the films. A very low optical loss (∼0.23 dB cm−1) was found for the silica–titania waveguide film at 632.8 nm. Moreover, the 1D/2D patterned films showed above 90% absolute diffraction efficiency with a 632.8 nm red laser source. Finite difference time domain software was used to observe the light confinement, coupling and coupled mode propagation of the fabricated patterned films. These fabricated patterned films could be used as efficient optical grating/SP resonance couplers for application in waveguide sensor devices.

Quantum sized Ag nanocluster assisted fluorescence enhancement in Tm3+-Yb3+ doped optical fiber beyond plasmonics

We report a process for enhancing fluorescence emission from conventional rare earth ions in optical fiber by metal nanocluster (MNC) in nonresonant indirect pumping. The process is completely different from formal metal enhanced fluorescence phenomenon as the MNCs are too small in size to support localized surface plasmon and the excitation wavelength is far from plasmon resonance frequency. We used an established theory of two coupled oscillators to explain the simultaneous enhancement of Ytterbium (Yb3+) and Thulium (Tm3+) emission by silver (Ag) NCs under nonresonant pumping in optical fiber. The fiber is pumped with a 980 nm fiber pigtailed laser diode with input power of 20–100 mW to excite the Yb3+. Four times enhancement of Yb3+ emission of 900–1100 nm and Tm3+ upconversion emission around 474 nm, 650 nm, and 790 nm is observed in the fiber with Ag NCs.

Light propagation at Dirac point in twisted photonic crystal

Dirac point supports two degenerate intrinsic PC modes. Vortex like PC mode at Dirac point can be excited and coupled in a hollow core PCF when it is helically twisted about the propagation axis.

Modelling of Palladium Gold alloy- dielectric stratified loaded plasmonic waveguide for hydrogen detection at room temperature

A plasmonic waveguide loaded with metal - dielectric stratified layer is reported for room temperature hydrogen sensing. Pd-Au alloy with atomic ratio 3:1 (Pd/Au) is used as the metal and air is used as dielectric layer in the sensor. When hydrogen is loaded in the waveguide the alloy absorbs hydrogen and the effective permittivity of the stratified medium changes. Density functional theory and Bruggemans effective medium theory are used to calculate the permittivity of bare and hydrogenated alloy. At a specific wavelength the change in the absorption loss of the device on hydrogen absorption is used as the measurement for hydrogen detection. At 633 nm the loss decreases almost linearly with increasing hydrogen concentration. For a device length of 10 μm the change of loss is achieved about 0.03 dB for 1% change of hydrogen concentration.