Shuvan Prashant Turaga

Free-standing terahertz chiral meta-foils exhibiting strong optical activity and negative refractive index

A chiral meta-foil consisting of a self-supported square array of interconnected conjugated rosettes is demonstrated at terahertz frequencies. It exhibits strong optical activity and circular dichroism. Negative refractive index with a figure-of-merit as high as 4.2 is achieved, attributed to its free-standing nature. Experimental results are in good agreement with numerical simulation. Free-standing chiral meta-foils provide a unique approach to create a completely all-metal chiral metamaterial, which can be flexibly integrated into optical setups while eliminating dielectric insertion losses.

Magnetic annihilation of the dark mode in a strongly coupled bright–dark terahertz metamaterial

Dark mode in metamaterials has become a vital component in determining the merit of the Fano type of interference in the system. Its strength dictates the enhancement and suppression in the amplitude and Q-factors of resulting resonance features. In this work, we experimentally probe the effect of strong near-field coupling on the strength of the dark mode in a concentrically aligned bright resonator and a dark split ring resonator (SRR) system exhibiting the classical analog of the electromagnetically induced transparency effect. An enhanced strong magnetic field between the bright-dark resonators destructively interferes with the inherent magnetic field of the dark mode to completely annihilate its effect in the coupled system. Moreover, the observed annihilation effect in the dark mode has a direct consequence on the disappearance of the SRR effect in the proposed system, wherein under the strong magnetic interactions, the LC resonance feature of the split ring resonator becomes invisible to the incident terahertz wave.

Molecular Organization Induced Anisotropic Properties of Perylene – Silica Hybrid Nanoparticles

Optically active silica nanoparticles are interesting owing to high stability and easy accessibility. Unlike previous reports on dye loaded silica particles, here we address an important question on how optical properties are dependent on the aggregation-induced segregation of perylene molecules inside and outside the silica nanoparticles. Three differentially functionalized fluorescent perylene - silica hybrid nanoparticles are prepared from appropriate ratios of perylene derivatives and tetraethyl orthosilicate (TEOS) and investigated the structure property correlation (P-ST, P-NP and P-SF). The particles differ from each other on the distribution, organization and intermolecular interaction of perylene inside or outside the silica matrix. Structure and morphology of all hybrid nanoparticles were characterized using a range of techniques such as electron microscope, optical spectroscopic measurements and thermal analysis. The organizations of perylene in three different silica nanoparticles were explored using steady-state fluorescence, fluorescence anisotropy, lifetime measurements and solid state polarized spectroscopic studies. The interactions and changes in optical properties of the silica nanoparticles in presence of different amines were tested and quantified both in solution and in vapor phase using fluorescence quenching studies. The synthesized materials can be regenerated after washing with water and reused for sensing of amines.

Synthesis, characterization and application of luminescent silica nanomaterials

Luminescent inorganic nanoparticles are interesting owing to their high stability and photophysical characteristics. In this study, luminescent ZnO–SiO2 nanoparticles (ZnOSiO2-X NPs, X = 1, 2, 3, 4) were designed and synthesized using the reverse microemulsion method. Here ZnO quantum dots are encapsulated and homogeneously distributed inside the silica matrix of ZnOSiO2-X NPs. The structural and optical properties of the nanoparticles were fully established using a variety of techniques. High-resolution TEM micrographs and electron diffraction data confirmed the presence of ZnO quantum dots inside silica nanoparticles. The ZnOSiO2-X NPs gave intense blue-white luminescence and the intensity varied with changes in the composition of the particles. In addition to fluorescence emission, these particles showed phosphorescence emission in solution and in the solid state. As a proof of concept, UV active invisible printing was demonstrated by using a ZnOSiO2/PDMS mixture in inkjet printing and mechanical stamping. The prepared stable luminescent NPs can be used for applications such as bioimaging, ink-jet printing or stamping in nanoelectronics.

Optical microcavities fabricated using direct proton beam writing

Proton beam writing (PBW) is a high-resolution direct write lithographic technique suitable for the fabrication of micro/nano optical components with smooth vertical sidewalls. In the present work PBW was used to fabricate smooth micro cavities in negative tone photoresist SU-8 and Rhodamine B doped SU-8. Two different laser cavities based on whispering gallery mode resonators were fabricated using PBW. The laser cavities in Rhodamine B doped SU-8 resist were optically pumped with a pulsed frequency doubled Nd: YAG laser, and emits light in the chip plane at 643 nm. The presented laser cavities showed pump threshold as low as 3 μJ/mm2, which is the lowest threshold reported in planar cavities fabricated in Rhodamine B dye based polymer laser cavities.

Oriented perylene incorporated optically anisotropic 2D silica films

Controlling the assembly of molecules in thin films is essential for developing functional materials for sensitive and smart coatings. Highly oriented heterogeneous and optically anisotropic 2D silica films were prepared at the liquid–liquid interface using octadecylsilane and perylenesilane as starting materials. The film formation involved the organization of molecules assisted by van der Waals interactions and network formation from silica polymerization. All films were fully characterized using a wide range of instruments. Wetting behavior of the thin film was established using contact angle measurement. The octadecylsilica (O-Si) film showed a water contact angle of ∼107° on the hydrophobic side and ∼70° on the hydrophilic side. The films prepared from perylenesilane (P-Si) and mixtures of perylenesilane and octadecylsilane (POx-Si) through hydrolysis, were fully characterized and showed higher contact angle than the O-Si films. As expected, an increase in the concentration of octadecylsilane in POx-Si film led to the disruption of the π–π stacking of perylene groups, followed by changes in optical properties of the film, which were established using spectroscopic techniques. Such bifunctional anisotropic films can be used for creating interesting functional coatings on different substrates.

Tailoring fabrication parameters of silver helical metamaterials for enhanced polarization properties in terahertz regime

Metallic helices have been extensively researched and demonstrated for their application as broadband circular polarizers in different frequency regimes. For making such 3D helices, two photon lithography (TPL) has been employed in conjunction with electroplating of metals. Recently, our group has demonstrated selective silver electroless plating of two photon fabricated polymer (SU-8) structures on silicon substrate. This procedure allows us to make metal-coated polymer helices. In this work, we examine how these fabrication process parameters could be tailored to obtain higher extinction ratios for circular polarizers in THz regime (or MIR regime). We further analyze the role of aspect-ratio of helices in their polarizing action. We will present both simulation and experimental results to show the improved performance of the polarizers.

Pulsed and high-speed FTIR spectroscopy

Fourier transform interferometry is commonly performed by means of mechanically scanning interferometers such as a Michelson and characterized by one scanning mirror. This results in severe limitations of the capability of measuring fast signals. To overcome this drawback, we present a multi-channel FTIR spectrometer (MC-FTIR) that is capable of single-shot operation no matter how short the single pulse is, provided it delivers sufficient photons for the signal to exceed the noise. It can capture fast transient signals, limited by the signal-to-noise ratio and data transfer rate of the detector. Our device is based on a micro/nanomanufactured 3D multimirror array (MMA) which allows collecting a whole interferogram simultaneously. MMAs are manufactured by means of a patented multiple moving mask grey-level deep X-ray lithography process. Up to 640 mirror cells, generating optical path differences from 0 to about 1 mm, were achieved so far at optical quality. We have demonstrated sub-millisecond pulses and a theoretical spectral resolution of 10 cm-1 in the mid-IR. The optical system is similar to a Czerny-Turner mount with the MMA replacing the grating and an MCT focal plane array (FPA) capturing the interferogram. Our MC-FTIR enables extension of FTIR-based IR spectroscopy to arbitrarily short pulses and to fast transient signals. As the optical system is small and rugged, the instrument lends itself readily to field applications. Ongoing work is aimed at emerging applications including biomedical, laser-induced breakdown spectroscopy, and spectroscopy of synchrotron radiation.

A review: mid-infrared photonic crystals in silicon and porous silicon based on ion beam irradiation

Silicon and porous silicon based photonic crystals are key aspects of photonic circuits with good compatibility with integrated circuits. Here a brief review is carried out on the fabrication of mid infrared photonic crystals using experimental processes of combining ion beam irradiation and electrochemical anodisation of silicon. Experimental processes have been developed to fabricate high aspect ratio trenches in porous silicon, high aspect ratio silicon pillars, buried channels in porous silicon, and multilevel freestanding silicon wires. These structures have the potential to be used for photonic crystals. Several 2D, quasi-3D and 3D mid infrared photonic crystals in porous silicon and silicon have been designed and fabricated.

Conductively coupled resonator scheme for dispersive transparency in metamaterials

We present and demonstrate a new type of single resonator based planar metamaterial exhibiting electromagnetically induced transparency (EIT)-like transmission behavior. The novel design involves physically coupled split-ring resonator (SRR) and a dipolar ring as opposed to many inductively coupled resonators explored in the past. Both experiments and simulations reveal a dispersive transparency due to coupled resonances and the underlying mechanism. Further, the conductive and inductive coupling scenarios for this structure were compared where conductive coupling was found to coerce the direction of light induced currents and stronger in effect than inductive coupling. Resonance tuning is achieved by moving the bar coupling SRR and the ring. Hence, we show that conductive coupling has potential in tailoring coupled resonances of desired quality factor and fabricating metamaterials for enhanced sensing.