Adaptive optics approach to surface-enhanced Raman scattering.

Surface-enhanced Raman scattering (SERS) spectroscopy is a popular technique for detecting chemicals in small quantities. Rough metallic surfaces with nanofeatures are some of the most widespread and commercially successful substrates for efficient SERS measurements. A rough metallic surface creates a high-density random distribution of so-called "hot spots" with local optical field enhancement causing Raman signal to increase. In this Letter, we revisit the classic SERS experiment [Surf. Sci.158, 229 (1985)SUSCAS0039-602810.1016/0039-6028(85)90297-3] with rough metallic surfaces covered by a thin layer of copper phthalocyanine molecules. As a modification to the classic configuration, we apply an adaptive wavefront correction of a laser beam profile. As a result, we demonstrate an increase in brightness of local SERS hot spots and redistribution of Raman signal over the substrate area. We hypothesize that the improvement is due to optimal coupling of the shaped laser beam to the random plasmonic nanoantenna configurations. We show that the proposed adaptive-SERS modification is independent of the exact structure of the surface roughness and topography, works with many rough surfaces, and gives brighter Raman hot spots in comparison with conventional SERS measurements. We prove that the adaptive SERS is a powerful instrument for improving SERS sensitivity.

Quantum plasmonic control of trions in a picocavity with monolayer WS2.

Monitoring and controlling the neutral and charged excitons (trions) in two-dimensional (2D) materials are essential for the development of high-performance devices. However, nanoscale control is challenging because of diffraction-limited spatial resolution of conventional far-field techniques. Here, we extend the classical tip-enhanced photoluminescence based on tip-substrate nanocavity to quantum regime and demonstrate controlled nano-optical imaging, namely, tip-enhanced quantum plasmonics. In addition to improving the spatial resolution, we use the scanning probe to control the optoelectronic response of monolayer WS2 by varying the neutral/charged exciton ratio via charge tunneling in Au-Ag picocavity. We observe trion "hot spots" generated by varying the picometer-scale probe-sample distance and show the effects of weak and strong coupling, which depend on the spatial location. Our experimental results are in agreement with simulations and open an unprecedented view of a new range of quantum plasmonic phenomena with 2D materials that will help to design new quantum optoelectronic devices.

Fluorescence imaging of stained red blood cells with simultaneous resonance Raman photostability analysis.

Optical spectroscopic imaging of biological systems has important applications in medical diagnosis, biochemistry, and image-guided surgery. Vibrational spectroscopy, such as Raman scattering, provides high chemical selectivity but is limited by weak signals and a large fluorescence background. Fluorescence imaging is often used by introducing specific dyes in biological systems to label different system parts and to increase the image contrast. However, the extrinsic fluorescence of the staining molecules often masks the intrinsic vibrational signals of biomolecules, which could also be simultaneously detected using the same excitation laser source. Therefore, fluorescence staining is often accompanied by the loss of other important complimentary information. For example, the high laser power often used for the rapid, high-quality imaging could lead to photo-induced suppression or bleaching of the fluorescence and Raman signals resulting in sample photodamage. Therefore, simultaneous imaging and photodamage analysis need to be performed in a controlled bioimaging experiment. Here we perform simultaneous spectroscopic bioimaging and photostability analysis of rhodamine 6G (R6G) stained red blood cells (RBCs) using both fluorescence and resonance Raman imaging in a single 532 nm laser excitation experiment. We develop a corresponding data processing algorithm which allows separation of the two spectroscopic signals. We control the relative intensity of the R6G and RBC signals by varying the excitation laser power and simultaneously monitor the photostability of RBCs. We observe no significant photodamage of RBCs through the absence of changes in the relative Raman peak intensities. Conversely, the R6G molecules show bleaching with the suppression of both the fluorescence and resonance Raman signals. Our approach may be generalized to other types of stained cells with the appropriate selection of fluorescent dyes and excitation sources.

Tip-Enhanced Raman Imaging of Single-Stranded DNA with Single Base Resolution.

Tip-enhanced Raman scattering (TERS) is a promising optical and analytical technique for chemical imaging and sensing at single molecule resolution. In particular, TERS signals generated by a gap-mode configuration where a silver tip is coupled with a gold substrate can resolve a single-stranded DNA (ssDNA) molecule with a spatial resolution below 1 nm. To demonstrate the proof of subnanometer resolution, we show direct nucleic acid sequencing using TERS of a phage ssDNA (M13mp18). M13mp18 provides a known sequence and, through our deposition strategy, can be stretched (uncoiled) and attached to the substrate by its phosphate groups, while exposing its nucleobases to the tip. After deposition, we scan the silver tip along the ssDNA and collect TERS signals with a step of 0.5 nm, comparable to the bond length between two adjacent DNA bases. By demonstrating the real-time profiling of a ssDNA configuration and furthermore, with unique TERS signals of monomeric units of other biopolymers, we anticipate that this technique can be extended to the high-resolution imaging of various nanostructures as well as the direct sequencing of other important biopolymers including RNA, polysaccharides, and polypeptides.

Morphology dependent catalysis and surface enhanced Raman scattering (SERS) studies using Pd nanostructures in DNA, CTAB and PVA scaffolds.

Palladium nanoparticles (Pd NPs) of three different morphologies viz., nanocubes with cetyltrimethylammonium bromide (CTAB), nanowires with polyvinyl alcohol (PVA) and Pd NPs with deoxyribonucleic acid (DNA) scaffolds were synthesized by UV-irradiation. Catalysis and surface enhanced Raman scattering (SERS) studies were done with the synthesized morphologically distinct Pd nanostructures for the very first time. The catalytic rate was extremely high with Pd nanowires templated with PVA and the order of the catalytic rate was Pd nanowires in PVA > Pd nanocubes in CTAB > DNA-Pd wire-like assemblies. The highest catalytic rate was observed for PVA capped Pd nanowires which is a few hundred fold higher than other metal NP catalysts. Methylene blue (MB) was used as a Raman analyte for the SERS study and the largest EF of 1.9 × 105 at a peak position of 1391 cm-1 was observed with Pd nanowires in the DNA scaffold as a SERS substrate. The order of the SERS EF values was DNA-Pd wire-like assemblies > Pd nanocubes in CTAB > Pd nanowires in PVA. Beyond everything, the present synthesis route is easy, faster, candid, highly reproducible and cost-effective. In the near future, the same protocol could be applied to synthesize other materials for various applications.

Shape-selective catalysis and surface enhanced Raman scattering studies using Ag nanocubes, nanospheres and aggregated anisotropic nanostructures.

Three morphologies of silver nanoparticles (Ag NPs) such as nanocubes, aggregated anisotropic Ag NPs, and nanospheres were prepared using polystyrene sulfonate (PSS) and citrate as stabilizing agents utilizing a simple wet-chemical and microwave heating route respectively. Ag nanocubes were prepared within one min through microwave heating whereas anisotropic Ag NPs and spherical Ag NPs via 5 and 30min of normal stirring at room temperature (RT) respectively. The shape effect of three different morphologies of Ag NPs were examined in catalysis reaction and in surface enhanced Raman scattering (SERS) studies. For catalysis experiments, reduction of various nitroaromatics was done taking excess NaBH4 in presence of those morphologically different Ag NPs as catalyst and the corresponding catalytic activity is ordered as: Ag nanospheres>aggregated anisotropic Ag NPs>Ag nanocubes. The highest catalytic rate of ∼1.34×10-1min-1 was observed with citrate capped Ag nanospheres. SERS study was done taking methylene blue (MB) as the Raman probe where a highest enhancement factor (EF) of ∼1.05×107 was observed with Ag nanospheres and the order of EF values is as follows: Ag nanospheres>Ag nanocubes>aggregated anisotropic Ag NPs. The highest catalytic and SERS activity of citrate stabilized spherical Ag NPs are attributed due to the fast electron transfer in catalysis and creation of more number of surface active 'hot spots' in SERS studies. In future, the overall process we highlighted here might found potential application for the preparation of other varieties of nanomaterials applicable to catalysis reaction and in SERS-based trace analysis of various biologically important molecules and fine chemicals.

Improving resolution in quantum subnanometre-gap tip-enhanced Raman nanoimaging.

Two-dimensional (2D) materials beyond graphene such as transition metal dichalcogenides (TMDs) have unique mechanical, optical and electronic properties with promising applications in flexible devices, catalysis and sensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopy can reveal the effects of structure, strain, doping, edge states, and surface functionalization from materials to bioscience. However, Raman signals are inherently weak and so far have been limited in spatial resolution in TMDs to a few hundred nanometres which is much larger than the intrinsic scale of these effects. Here we overcome the diffraction limit by using resonant tip-enhanced Raman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical images with ~20 nm spatial resolution. This becomes possible due to electric field enhancement in an optimized subnanometre-gap resonant tip-substrate configuration. We investigate the limits of signal enhancement by varying the tip-sample gap with sub-Angstrom precision and observe a quantum quenching behavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, which enable surface mapping based on this new contrast mechanism. This quantum regime of plasmonic gap-mode enhancement with a few nanometre thick MoS2 junction may be used for designing new quantum optoelectronic devices and sensors with a wide range of applications.

Time-resolved surface-enhanced coherent sensing of nanoscale molecular complexes.

Nanoscale real-time molecular sensing requires large signal enhancement, small background, short detection time and high spectral resolution. We demonstrate a new vibrational spectroscopic technique which satisfies all of these conditions. This time-resolved surface-enhanced coherent anti-Stokes Raman scattering (tr-SECARS) spectroscopy is used to detect hydrogen-bonded molecular complexes of pyridine with water in the near field of gold nanoparticles with large signal enhancement and a fraction of a second collection time. Optimal spectral width and time delays of ultrashort laser pulses suppress the surface-enhanced non-resonant background. Time-resolved signals increase the spectral resolution which is limited by the width of the probe pulse and allow measuring nanoscale vibrational dephasing dynamics. This technique combined with quantum chemistry simulations may be used for the investigation of complex mixtures at the nanoscale and surface environment of artificial nanostructures and biological systems.

Rapid-phase modulation of terahertz radiation for high-speed terahertz imaging and spectroscopy.

Rapid voltage-controlled phase modulation of cw terahertz (THz) radiation is demonstrated. By transmitting an infrared beam through a lithium niobate phase modulator the phase of the THz radiation, which is generated by the photomixing of two infrared beams, can be directly modulated through a 2pi phase shift. The 100 kHz modulation rate that is demonstrated with this technique is approximately 3 orders of magnitude faster than what can be achieved by mechanical scanning.

Generation and detection of terahertz radiation with multilayered electro-optic polymer films.

We use optical rectification and electro-optic sensing to generate and detect terahertz radiation, using individual and multilayered stacks of electro-optic polymer films. For comparable thickness, the polymer films are more efficient generators and detectors of terahertz radiation than ZnTe crystals.