Browsing by Author "Subramaniyam, Nagarajan"

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    Atomic/molecular layer deposition of europium-organic thin films on nanoplasmonic structures towards FRET-based applications
    (Royal Society of Chemistry, 2023-10-14) Ghazy, Amr; Ylönen, Jonas; Subramaniyam, Nagarajan; Karppinen, Maarit; Department of Chemistry and Materials Science; Inorganic Materials Chemistry; Xfold imaging Oy
    We present a novel atomic/molecular layer deposition (ALD/MLD) process for europium-organic thin films based on Eu(thd)3 and 2-hydroxyquinoline-4-carboxylic acid (HQA) precursors. The process yields with appreciably high growth rate luminescent Eu-HQA thin films in which the organic HQA component acts as a sensitizer for the red Eu3+ luminescence, extending the excitation wavelength range up to ca. 400 nm. We moreover deposit these films on nanoplasmonic structures to achieve a twentyfold enhanced emission intensity. Finally, we demonstrate the FRET-type energy transfer process for our Eu-HQA coated nanoplasmonic structures in combination with commercial Alexa647 fluorophor, underlining their potential towards novel bioimaging applications.
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    Fabrication of large area surface plasmonic nanogratings
    (2020-01-20) Västi, Juha; Subramaniyam, Nagarajan; Sähkötekniikan korkeakoulu; Sopanen, Markku
    Surface plasmon resonance is a phenomenon in which illuminated nanoscale particles exhibit strong near eld electric eld enhancement. This has many applications in the area of sensing, with spectroscopic measurements such as uorescence and Raman scattering having their signal to noise ratio enhanced by the strong electric near eld. For surface plasmons to be excited in bulk material, coupling structures need to be fabricated. One of these structures are nanogratings, and this thesis explores the fabrication of these nanogratings, with an emphasis on mass production. Two methods of fabrication were used. First a series of samples were patterned with a grating pattern using electron beam lithography (EBL). After lithography, the samples were etched using reactive ion etching (RIE). The etched samples were characterized using scanning electron (SEM) and atomic force microscopy (AFM). The EBL pattern quality was found to be adequate, and the process was optimized to maximize throughput. The RIE grating etch was found to be unsatisfactory, and a new, working etching process was devised and demonstrated. Re ectance spectroscopy measurements suggested plasmonic resonance for these samples. The second method used was nanoimprint lithography (NIL). For this, stamps were fabricated, starting by fabricating the gratings using EBL and in a similar way as before. After this, a process for etching the mesa of the stamp whilst conserving the grating was developed. The fabricated stamps were then used to pattern substrates. The rst experiments were failures, with the resist sticking to the stamp. From the failed experiment it became apparent an anti-sticking coating was needed. An anti-sticking layer was rst deposited unsuccessfully using a manual process, but the second method of deposition via atomic layer deposition (ALD) was successful. With the coated stamps more imprinting experiments were done. These experiments unfortunately failed, rst due to alignment problems and afterwards due to an unknown reason possibly related to the alignment.
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    Nonlinear plasmonic behavior of nanohole arrays in thin gold films for imaging lipids
    (2018-06-04) Subramaniyam, Nagarajan; Shah, Ali; Dreser, Christoph; Isomäki, Antti; Fleischer, Monika; Sopanen, Markku; Department of Electronics and Nanoengineering; Markku Sopanen Group; University of Tübingen; University of Helsinki
    We demonstrate linear and nonlinear plasmonic behaviors of periodic nanohole arrays in thin gold (Au) films with varying periodicities. As expected, the linear optical transmission spectra of the nanohole arrays show a red-shift of the resonance wavelength and Wood's anomaly with increasing hole spacing. The optical transmission and electric near-field intensity distribution of the nanohole arrays are simulated using the finite element method. The nonlinear plasmonic behavior of the nanohole arrays is studied by using picosecond pulsed excitation at near-infrared wavelengths. The characteristic nonlinear signals indicating two-photon excited luminescence (TPEL), sum frequency generation, second harmonic generation, and four-wave mixing (FWM) are observed. A maximum FWM/TPEL signal intensity ratio is achieved for nanohole arrays with a periodicity of 500 nm. Furthermore, the significant FWM signal intensity and contrast compared to the background were harnessed to demonstrate the ability of surface-enhanced coherent anti-Stokes Raman scattering to visualize low concentrations of lipids deposited on the nanohole array with a periodicity of 500 nm.
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    Optical amplification by surface-plasmon-resonant Au grating substrates: Monolayer MoS2 with 170-fold second harmonic generation and 3-fold (off-resonance) Raman scattering
    (Academic Press Inc., 2021-12) Holmi, Joonas T.; Raju, Ramesh; Ylönen, Jonas; Subramaniyam, Nagarajan; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Ilkka Tittonen Group; Department of Electronics and Nanoengineering
    Nanoplasmonics is a potential game-changer in the development of next-generation on-chip photonic devices and computers, owing to the geometrically controlled and amplified linear and nonlinear optical processes. For instance, it resolves the limited light-matter interaction of the unique two-dimensional (2D) crystalline materials like semiconducting monolayer molybdenum disulfide (1L-MoS2). Metal grating (MG) substrates excel at this because their surface plasmons (SPs) can lead to stark field confinement near the surface. This work studies optical amplification of 1L-MoS2 on the gold (Au) MG substrate, which was designed to operate in a glycerol environment with SP resonance (SPR) at 850 nm excitation wavelength. Its design was verified by simulated and experimental reflectances, and topographically inspected by atomic force microscopy (AFM). Two advanced imaging modalities, second harmonic generation (SHG) and confocal Raman microscopy (CRM) were used to evaluate its 170-fold SHG on- and 3-fold CRM off-resonance optical amplifications, respectively. Some MoS2-to-grating adhesion issues due to trapped liquid showed as image nonuniformities. Possible improvements to limitations like surface roughness were also discussed. These Au MG substrates can boost conventional linear and nonlinear backscattering microscopies because they are tunable in the visible and near-infrared range by selecting geometry, metal, and environment.
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    Quantitative imaging of anti-phase domains by polarity sensitive orientation mapping using electron backscatter diffraction
    (2017-12-01) Naresh-Kumar, Gunasekar; Vilalta-Clemente, A.; Jussila, H.; Winkelmann, A.; Nolze, G.; Vespucci, S.; Subramaniyam, Nagarajan; Wilkinson, A. J.; Trager-Cowan, Carol; Department of Electronics and Nanoengineering; University of Strathclyde; University of Oxford; Bruker Nano GmbH; Federal Institute for Materials Research and Testing
    Advanced structural characterisation techniques which are rapid to use, non-destructive and structurally definitive on the nanoscale are in demand, especially for a detailed understanding of extended-defects and their influence on the properties of materials. We have applied the electron backscatter diffraction (EBSD) technique in a scanning electron microscope to non-destructively characterise and quantify antiphase domains (APDs) in GaP thin films grown on different (001) Si substrates with different offcuts. We were able to image and quantify APDs by relating the asymmetrical intensity distributions observed in the EBSD patterns acquired experimentally and comparing the same with the dynamical electron diffraction simulations. Additionally mean angular error maps were also plotted using automated cross-correlation based approaches to image APDs. Samples grown on substrates with a 4° offcut from the [110] do not show any APDs, whereas samples grown on the exactly oriented substrates contain APDs. The procedures described in our work can be adopted for characterising a wide range of other material systems possessing non-centrosymmetric point groups.
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    Simulation of resonant nanostructures for electric near field enhancement
    (2018-02-12) Ylönen, Jonas; Subramaniyam, Nagarajan; Sähkötekniikan korkeakoulu; Sopanen, Markku
    Near field enhancement is beneficial in a wide range of applications in imaging, spectroscopy and amplification of nonlinear optical processes. These often rely on using nanostructures to focus and amplify the surrounding electric field. This enhanced field is localized around the emitting feature, and requires the object it interacts with to be placed in close proximity. This thesis investigates the structural geometry and optical phenomena of different types of resonant nanostructures designed for electric near field enhancement. The goal is to design a nanostructure that can provide a strong enhancement uniformly over its surface, making it usable as a substrate for imaging applications. The types of structures investigated are gratings, metal covered distributed Bragg reflectors (DBR) and optical cavities, which support surface plasmons, Tamm plasmons and cavity modes, respectively. The main method of study in this thesis is finite element method (FEM) simulation. The simulations are performed in 2D geometry using transverse magnetic (TM) and transverse electric (TE) radiation. The gratings, metal covered DBR and cavities are first studied separately, then structures combining the grating and optical cavity are investigated, in an attempt to benefit from coupling between the different resonances. Finally, dielectric gratings on plain metal surfaces are examined. In addition to the simulations, some of the structures are fabricated for reflectance measurements. The simulations managed to provide a lot of information about the resonant behaviour of the nanostructures. Structural parameters for optimal coupling of light at wavelengths close to 500 nm were discovered for many of the structures. The combinations of structures displayed evidence of coupling between the different resonant behaviours, but this coupling was not able to significantly improve the electric field enhancement. The dielectric gratings investigated at the end proved more promising for imaging applications.