Sudhir Cherukulappurath

Nano-optical Trapping of Rayleigh Particles and Escherichia coli Bacteria with Resonant Optical Antennas

Immobilizing individual living microorganisms at designated positions in space is important to study their metabolism and to initiate an in situ scrutiny of the complexity of life at the nanoscale. While optical tweezers enable the trapping of large cells at the focus of a laser beam, they face difficulties in maintaining them steady and can become invasive and produce substantial damage that prevents preserving the organisms intact for sufficient time to be studied. Here we demonstrate a novel optical trapping scheme that allows us to hold living Escherichia coli bacteria for several hours using moderate light intensities. We pattern metallic nanoantennas on a glass substrate to produce strong light intensity gradients responsible for the trapping mechanism. Several individual bacteria are trapped simultaneously with their orientation fixed by the asymmetry of the antennas. This unprecedented immobilization of bacteria opens an avenue toward observing nanoscopic processes associated with cell metabolism, as well as the response of individual live microorganisms to external stimuli, much in the same way as pluricellular organisms are studied in biology.

Controlling the optical near field of nanoantennas with spatial phase-shaped beams.

We report on a novel approach, based on sub-wavelength spatial phase variations at the focus of high-order beams, to reconfigure the optical near field distribution near plasmonic nanostructures. We first show how the introduction of phase jumps in the incident field driving a gap nanoantenna strongly affects its near field response. Beyond, we demonstrate the feasibility of exploiting this approach to selectively switch on and off hot-spots sites within a complex antenna architecture.

Millimeter-Sized Suspended Plasmonic Nanohole Arrays for Surface-Tension-Driven Flow-Through SERS.

We present metallic nanohole arrays fabricated on suspended membranes as an optofluidic substrate. Millimeter-sized suspended nanohole arrays were fabricated using nanoimprint lithography. We demonstrate refractive-index-based tuning of the optical spectra using a sucrose solution for the optimization of SERS signal intensity, leading to a Raman enhancement factor of 107. Furthermore, compared to dead-ended nanohole arrays, suspended nanohole arrays capable of flow-through detection increased the measured SERS signal intensity by 50 times. For directed transport of analytes, we present a novel methodology utilizing surface tension to generate spontaneous flow through the nanoholes with flow rates of 1 μL/min, obviating the need for external pumps or microfluidic interconnects. Using this method for SERS, we obtained a 50 times higher signal as compared to diffusion-limited transport and could detect 100 pM 4-mercaptopyridine. The suspended nanohole substrates presented herein possess a uniform and reproducible geometry and show the potential for improved analyte transport and SERS detection.

Local observation of plasmon focusing in Talbot carpets

We present a detailed experimental and theoretical study of plasmon Talbot effect. A theoretical model based on simple scattering theory is developed to describe the Talbot self-imaging pattern generated by a linear arrangement of cylindrical nanostructures forming a periodic array. We first show the experimental observation of plasmon Talbot carpets created by propagating surface plasmon polaritons (SPP) interacting with cylindrical nanostructures positioned on a thin Au film using leakage radiation microscopy. Such images provide information on the distribution of the plasmon intensity close to the nanostructures. Next, heterodyne interferometer based near-field imaging is carried out to extract information on the plasmonic modes forming the Talbot carpet deployment. We report the experimental observation of Talbot focal spots with dimensions down to lambda/4.

Template-Stripped Tunable Plasmonic Devices on Stretchable and Rollable Substrates

We use template stripping to integrate metallic nanostructures onto flexible, stretchable, and rollable substrates. Using this approach, high-quality patterned metals that are replicated from reusable silicon templates can be directly transferred to polydimethylsiloxane (PDMS) substrates. First we produce stretchable gold nanohole arrays and show that their optical transmission spectra can be modulated by mechanical stretching. Next we fabricate stretchable arrays of gold pyramids and demonstrate a modulation of the wavelength of light resonantly scattered from the tip of the pyramid by stretching the underlying PDMS film. The use of a flexible transfer layer also enables template stripping using a cylindrical roller as a substrate. As an example, we demonstrate roller template stripping of metallic nanoholes, nanodisks, wires, and pyramids onto the cylindrical surface of a glass rod lens. These nonplanar metallic structures produced via template stripping with flexible and stretchable films can facilitate many applications in sensing, display, plasmonics, metasurfaces, and roll-to-roll fabrication.

Mode mapping of plasmonic stars using TPL microscopy

We investigate the far-field and near-field optical properties of a complex plasmonic system made of gold disks forming a star-like pattern. Scattering spectroscopy first enabled us to monitor the resonant behavior of the structures. Two-photon luminescence (TPL) microscopy was then used to probe the local fields under different orientations of the incident linear polarization. Unlike the scattering spectrum, the TPL distribution over the structure is found to depend drastically on the incident polarization state, in good agreement with a full 3D theoretical simulation of the electric near-field intensity.

Dielectrophoresis-Assisted Raman Spectroscopy of Intravesicular Analytes on Metallic Pyramids

Chemical analysis of membrane-bound containers such as secretory vesicles, organelles, and exosomes can provide insights into subcellular biology. These containers are loaded with a range of important biomolecules, which further underscores the need for sensitive and selective analysis methods. Here we present a metallic pyramid array for intravesicular analysis by combining site-selective dielectrophoresis (DEP) and Raman spectroscopy. Sharp pyramidal tips act as a gradient force generator to trap nanoparticles or vesicles from the solution, and the tips are illuminated by a monochromatic light source for concurrent spectroscopic detection of trapped analytes. The parameters suitable for DEP trapping were optimized by fluorescence microscopy, and the Raman spectroscopy setup was characterized by a nanoparticle based model system. Finally, vesicles loaded with 4-mercaptopyridine were concentrated at the tips and their Raman spectra were detected in real time. These pyramidal tips can perform large-area array-based trapping and spectroscopic analysis, opening up possibilities to detect molecules inside cells or cell-derived vesicles.

Spectroscopic TPL imaging of gold nano-antennas

Two-photon induced photoluminescence (TPL) microscopy has been used to probe the local field of nanoantennas. We demonstrate that TPL imaging is directly correlated to the antenna electromagnetic mode computed with a full 3D solver. Furthermore, spectroscopic mode mapping while scanning the incident wavelength enables near-field spectroscopy of specific areas of the antenna response, providing a deeper insight into its resonant properties.