Saulius Juodkazis

Statistically quantified measurement of an Alzheimer's marker by surface-enhanced Raman scattering

Fibrillar forms of the Amyloid-β (Aβ) protein have been implicated in the early stages of Alzheimers disease (AD), however there are no standardised assays for soluble Aβ oligomer biomarkers that provide the best indication of the disease progression [1,2]. As a step towards a fast and label-free method for testing different AD biomarkers, we have combined laser nano-textured substrates with a SERS mapping technique and validated it using soluble Aβ-40 oligomers [3-5]. The nano-textured SERS substrates provide fast (&5 min), label-free spectra associated with soluble Aβ-40 oligomers down to a concentration of 10 nM. Statistical analysis of the spectral intensities mapped over the substrate surface shows a quantitative correlation with the oligomer concentration. Schematics of experiments: SERS mapping of Aβ-40 (left figure: measured SERS intensity overlayed with an SEM image of ripples) was carried out on the laser nano-textured (ripple) surface of sapphire and statistical analysis of the SERS intensity was carried out for qualitative (a high SERS intensity at low probability) and quantitative (a moderate SERS intenisty at the highest probability) measures. Quantitative statistical analysis of SERS mapping data can be performed off line for cross correlations with other known SERS signatures.

Simulation and Measurement of Solar Harvesting Enhancement of Silver Plasmonic Nanoparticles on GaSb Nanodots

The performance of a plasmonic antireflection layer which can be utilized for deep-space radiationresistant GaSb solar cells is investigated numerically and experimentally. The layer consists of nanodots made by plasma etching of a GaSb substrate and subsequent physical vapor deposition of Ag nanoparticles on the nanodot tips, in a partially ordered configuration determined by the plasma energy level. This technique is readily applicable to patterning of silicon. We measure the substrate reflectivity and model the reflection and absorption of the substrates using the 3D finite difference time domain (FDTD) method, which are realistically imported as 3D layers from the scanning electron microscopy (SEM) images. The variation of the height of the Ag nanoparticles on top of the GaSb pillars shows that the plasmonic effect remarkably enhances the absorption. The presence of GaSb pillars enhances absorption and tunes the maximum absorption wavelength peak.

Development of an optical fiber SERS microprobe for minimally invasive sensing applications

Numerous potential biomedical sensing applications of surface-enhanced Raman scattering (SERS) have been reported, but its practical use has been limited by the lack of a robust sensing platform. Optical fiber SERS probes show great promise, but are limited by the prominent silica Raman background, which requires the use of bulky optics for filtering the signal collection and excitation delivery paths. In the present study, a SERS microprobe has been designed and developed to eliminate the bottlenecks outlined above. For efficient excitation and delivery of the SERS signal, both hollow core photonic crystal fiber and double clad fiber have been investigated. While the hollow core fiber was still found to have excessive silica background, the double clad fiber allows efficient signal collection via the multi-mode inner cladding. A micro filtering mechanism has been designed, which can be integrated into the tip of the optical fiber SERS probe, providing filtering to suppress silica Raman background and thus avoiding the need for bulky optics. The design also assists in the efficient collection of SERS signal from the sample by rejecting Rayleigh scattered light from the sample. Optical fiber cleaving using ultra-short laser pulses was tested for improved control of the fiber tip geometry. With this miniaturized and integrated filtering mechanism, it is expected that the developed probe will promote the use of SERS for minimally invasive biomedical monitoring and sensing applications in future. The probe could potentially be placed inside a small gauge hypodermic needle and would be compatible with handheld portable spectrometers.

Far-side geometrical enhancement in surface-enhanced Raman scattering with Ag plasmonic films

Surface-enhanced Raman scattering (SERS) is a surface sensitive technique where the large increase in scattering has primarily been attributed to electromagnetic and chemical enhancements. While smaller geometrical enhancements due to thin film interference and cavity resonances have also been reported, an additional enhancement in the SERS signal, referred to as the ‘far-side geometrical enhancement’, occurs when the SERS substrate is excited through an underlying transparent dielectric substrate. Here the far-side geometrically-enhanced SERS signal has been explored experimentally in more detail. Thermally evaporated Ag plasmonic films functionalised with thiophenol were used to study the dependence of the geometrically-enhanced SERS signal on the excitation wavelength, supporting substrate material and excitation angle of incidence. The results were interpreted using a ‘geometrical enhancement factor’ (GEF), defined as the ratio of far-side to near-side SERS signal intensity. The experimental results confirmed that the highest GEFs of 3.2-3.5× are seen closer to the localized surface plasmon resonance peak of the Ag metallic nanostructures. Interestingly, the GEF for Ag plasmonic films deposited on glass and sapphire were the same within the measurement errors, whereas increasing angle of incidence showed a decrease in the GEF. Given this improved understanding of the far-side geometrical SERS enhancement, the potential for further signal amplification and optimisation for practical sensing applications can now be considered, especially for SERS detection modes at the farend of optical fibre probes and through process windows.

Graphene bolometer for vis-IR spectral range made on nano-SiN membrane

A sensitive bolometric detector for visible and infrared wavelengths based on a novel assembly principle of a graphene monolayer on a nano/micro SiN membrane is realised. The basic operating principle of the optical detector relies on the absorption of electromagnetic radiation in the graphene and creation of a strong thermal gradient, rT, which is detected via the Seebeck effect: Voltage = S x ∇rT, where S is the Seebeck coefficient of graphene. A simple lithography-free deposition of two metal contacts with different electron work functions: Pd (by sputtering) and Ag (by jet printing and annealing) was used. Sensitivity of the bolometer was the same ~1:1 mV/mW at 1030 and 515 nm wavelengths.

Polarization effects in 3D femtosecond direct laser writing nanolithography

In this work we reveal an influence of polarization of the laser beam on polymerization in direct laser writing. It was experimentally found that the width of suspended lines fabricated in SZ2080, OrmoComp and PETA (pentaerythritol triacrylate) pre-polymers directly depends on the incident polarization and is largest when the angle between the electric field vector and the sample translation direction is α = 90° and the smallest when α = 0°. The size of polymerized structures is consistent with theoretical simulations based on vectorial Debye theory. Experiments were performed by using average laser power corresponding to the middle value of the fabrication window. Polarization was found to be affecting feature sizes while structuring various widespread photoresists, the observed variation was material dependent and measured from 5 to 22% in the line-width. The performed study proves that polarization can be used as a variable parameter for fine tuning of the voxels aspect ratio.

3D Micro-Optics Via Ultrafast Laser Writing: Miniaturization, Integration, and Multifunctionalities

Advances of the growing field of micro-optical elements and components created via direct laser writing (DLW) are overviewed. The basic principles and the most recent developments in three-dimensional (3D) DLW in polymers enable creation of miniaturized and integrated optical devices. The employed technique based on femtosecond laser pulses provides efficient and reliable structuring with up to ∼100 nm feature definition as well as less than 10 nm surface roughness required for optical/photonic applications. Research into the structuring by ultrashort laser pulses has seen immense growth over the past decade due to its unique material processing features involving linear and nonlinear photophysical and photochemical mechanisms. This enables nanostructuring of pure organic, hybrid organic–inorganic, as well as hydrogel or elastomer materials via avalanche-induced cross-linking, thus without the usage of any photosensitizers. This approach empowers manufacturing of the low optical losses and high damage threshold, as well as biocompatible elements. Examples of various micro-optical elements are provided with focus on refractive index measurement and engineering for a particular optical function along with unique performance of the miniature devices.