Pramitha Vayalamkuzhi

The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused Ga ion beam with an acceleration voltage of 30 keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.

The Hilbert transform (HT) method is a technique to perform optical phase retrieval from a single off-axis interference pattern. However, one of the significant issues with the HT method is the error generated due to noise in the recorded interference pattern, which can be avoided using a proper pre-filtering step. The importance of pre-filtering of the interference patterns is studied in this study. The observations are verified experimentally by performing quantitative phase imaging of a white blood cell (WBC) using a Mach-Zehnder interferometer configuration.

We investigate the evolution of light transmitted by a fork-shaped phase grating etched on the core of an optical fiber using a near-field scanning optical microscope. The optical near-field intensity, simulated based on the angular spectrum method of propagation matches well with the experimental measurements. We propose a scheme to estimate the complex optical field within nanometres of the device by applying an iterative phase retrieval algorithm based on the angular spectrum method of propagation. The recorded intensity scans at different heights from the device accurately match with that calculated from the estimated phase carried out using the propagation of the resulting complex field. The proposed method provides a first of its kind insight into the evolution of the intensity and phase characteristics of propagating near-field.

We demonstrate that crack propagation in uniaxially strained reduced graphene oxide (rGO) films is substantially dependent on the film thickness, for films in the sub-micron regime. rGO film on flexible polydimethylsiloxane (PDMS) substrate develop quasi-periodic cracks upon application of strain. The crack density and crack width follow contrasting trends as film thickness is increased and the results are described in terms of a sequential cracking model. Further, these cracks also have a tendency to relax when the strain is released. These features are also reflected in the strain-dependent electrical dc and ac conductivity studies. For an optimal thickness (3-coat), the films behave as strain-resistant, while for all other values it becomes strain-responsive, attributed to a favorable combination of crack density and width. This study of the film thickness dependent response and the crack propagation mechanism under strain is a significant step for rationalizing the application of layered graphene-like systems for flexible optoelectronic and strain sensing applications. When the thickness is tuned for enhanced extent of crack propagation, strain-sensors with gauge factor up to 470 are realized with the same material. When thickness is chosen to suppress the crack propagation, strain-resistive flexible TiO2-rGO UV photoconductor is realized.

Graphene: polymer composite based electrically conducting films are realized by a facile solution processable method. Ultraviolet Photoelectron Spectroscopy (UPS) measurements on the composite films, reveal a low work function of reduced graphene oxide (rGO) obtained from hydrazine hydrate reduction of graphene oxide (GO). We suggest that the low work function could potentially make rGO: PMMA composite suitable for electron conducting layer in perovskite solar cells in place of traditionally used expensive PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) layer. Further, we demonstrate from the gravimetric experiments conducted on rGO: PMMA films, that the same coating is also resistant to moisture permeation. This latter property can be used to realize a protective coating layer for perovskite films, which are prone to moisture induced degradation. Thus, dual functionality of rGO-PMMA films is demonstrated towards integration with perovskite solar cells. Architecture of perovskite solar cell based on these concepts is proposed.

Diffractive optics, a way of controlling or transforming light, exists because of our ability to create small features. In this talk, I will explore the possibilities when optics gets smaller, in terms of features and substrates.

Conventional graphene oxide (GO) is characterized by low sp2 content in a sp3 rich matrix, which is responsible both for electrical insulation and water super-permeation. Upon reduction, electrical conduction is achieved at the expense of water permeation ability. Here, we demonstrate that charge conduction and water permeation can be simultaneously restricted in a functionalized form of GO. Gravimetric studies reveal that diffusion of water vapor through a glassy polymer membrane is arrested by loading a hydrophobic form of GO (H-GO) in the polymer matrix, even as such, water inhibition cannot be realized by substantially increasing the thickness of the bare polymer. As an application, the ability of the coating to impede the degradation of methyl ammonium lead iodide films under high humidity conditions is demonstrated. At the same time the H-GO film has a resistance over 107 times higher when compared to thermally reduced GO of similar sp2 fraction. We attribute this unique behavior to the presence of a sub-micron matrix of GO with simultaneous presence of large (∼9.5 Å) and small (∼4.7 Å) interlayer spacing. This leads to disruption of the spatially distributed percolation pathways for electrical charge, and it also serves to block the nanocapillary networks for water molecules.

We investigate temperature-dependent charge transport in reduced graphene oxide (rGO) films coated on flexible polydimethylsiloxane (PDMS) substrates which are subject to uniaxial strain. Variable strain, up to 10%, results in an anisotropic morphology comprising of quasi-periodic linear array of deformations which are oriented perpendicular to the direction of strain. The anisotropy is reflected in the charge transport measurements, when conduction in the direction parallel and perpendicular to the applied strain are compared. Temperature dependence of resistance is measured for different values of strain in the temperature interval 80-300 K. While the resistance increases significantly upon application of strain, the temperature-dependent response shows anomalous decrease in resistance ratio R 80 K/R 300 K upon application of strain. This observation of favorable conduction processes under strain is further corroborated by reduced activation energy analysis of the temperature-dependent transport data. These anomalous transport features can be reconciled based on mutually competing effects of two processes: (i) thinning of graphene at the sites of periodic deformations, which tends to enhance the overall resistance by a purely geometrical effect, and (ii) locally enhanced inter-flake coupling in these same regions which contributes to improved temperature-dependent conduction.

The near-field amplitude and phase characteristics of fractional optical vortex beam from a spiral phase plate fabricated on the tip of an optical fiber is reported here. A near-field scanning optical microscope is used to measure the intensity and phase structure of the fractional optical vortex beam as a function of propagation distance, Z and fractionality, l. The measured intensity-phase structure of the fractional optical vortex beam as a function of Z, from near- to far-field and l are complex and dominated by diffraction-related smoothening of the structure. Theoretical simulations carried out using the angular spectrum method matches well with the experimentally measured behavior. The trajectory of singular point in the vortex beam, as its charge is changed from l = – 1 to +1 in fractional steps, is simulated and measured using an interferometer setup. Such studies enable us to understand the complex nature of the fractional optical vortex beam in the near-field, its modifications during propagation to far-field, and as a function of fractionality.

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.