Surface-enhanced Coherent anti-Stokes Raman Scattering Research Articles

Ultracompact Lab-on-a-Chip Device for Surface-Enhanced Coherent Anti-Stokes Raman Scattering.

This paper introduces an ultracompact lab-on-a-chip device with a size of [Formula: see text] for surface-enhanced coherent anti-Stokes Raman scattering. This device comprises of a unique hybrid plasmonic-photonic vertical coupler, for light-coupling between the device and a light source, and a heptamer plasmonic nanotweezer for trapping and manipulation of nanoparticles. The coupler with its nanoscale size of [Formula: see text] offers maximum coupling efficiency and directivity of -4.2 dB and 17.8 dB with a 3 dB bandwidth across the wavelength range of [Formula: see text]. Based on a finite element method, it is theoretically shown that the tear-drop based nanotweezer can empower the device to yield a very high Raman gain of ≥ 1017 , making the detection at the single-molecule level possible. The proposed device can exhibit low damage to bioparticles through the use of near infrared wavelengths range with lower energy photons. Due to its short optical path, this device is expected to improve the Raman signal to noise ratio by reducing the level of background in-coherent Raman signals. Finally, thanks to its configuration, this device can enable the creation of parallel measurement channels, paving the way toward the development of high-throughput and mass-produced biosensors.

A Fano-resonance plasmonic assembly for broadband-enhanced coherent anti-Stokes Raman scattering

Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) technique has triggered huge interests due to the significant signal enhancement for high-sensitivity detection. Previous SECARS work has tended to focus only on the enhancement effect at a certain combination of frequencies, more suitable for single-frequency CARS. In this work, based on the enhancement factor for broadband SECARS excitation process, a novel Fano resonance plasmonic nanostructure for SECARS is studied. In addition to the 12 orders of magnitude enhancement effect that can be realized under single-frequency CARS, this structure also shows huge enhancement under broadband CARS in a wide wavenumber region, covering most of the fingerprint region. This geometrically-tunable Fano plasmonic nanostructure provides a way to realize broadband-enhanced CARS, with potentials in single-molecular monitoring and high-selectivity biochemical detection.

Enhancement of Molecular Coherent Anti-Stokes Raman Scattering with Silicon Nanoantennas.

Surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) takes advantage of surface plasmon resonances supported on metallic nanostructures to amplify the coherent Raman response of target molecules. While these metallic antennas have found significant success in SE-CARS studies, photoinduced morphological changes to the nanoantenna under ultrafast excitation introduce significant hurdles in terms of stability and reproducilibty. These hurdles need to be overcome in order to establish SE-CARS as a reliable tool for rapid biomolecular sensing. Here, we address this challenge by performing molecular CARS measurements enhanced by nanoantennas made from high-index dielectric particles with more favorable thermal properties. We present the first experimental demonstration of enhanced molecular CARS signals observed at Si nanoantennas, which offer much improved thermal stability compared to their metallic counterparts.

Theoretical Study on the Ultrafast Selective Excitation of Surface-Enhanced Coherent Anti-Stokes Raman Scattering Based on Fano Resonance of Disk-Ring Nanostructures by Shaped Femtosecond Laser Pulses

The enhancement and selective excitation of coherent anti-Stokes Raman scattering (CARS) and the suppression of background noise are very important problems for real-time detection at the single-molecule level. Optimizing the plasmonic substrate to ensure that all the hot spots of the pump, probe, Stokes, and anti-Stokes light are at the same position is the key to increasing the CARS signal to reach the level of single-molecule detection. The selective excitation of the target CARS peak and the suppression of the other peaks are the key to improving the signal-to-noise ratio. In this paper, we present a theoretical study to control the selective excitation and enhancement of any one of the three CARS peaks using the Fano resonance of a disk-ring structure. By optimizing the modulation of the pump, Stokes, and probe pulse, one CARS peak is maximized, while the other two are suppressed to zero. Fano resonance is applied to simultaneously enhance the four surface plasmon modes of the pump, probe, Stokes, and anti-Stokes light and to ensure that all the hot spots are located at the same position by adjusting the size of the disk-ring structure. The hot spots of the four pulses are concentrated in the disk-ring gap with a deviation distance of less than 2 nm, and the intensity of the CARS is enhanced by 1.43 × 1012 times, which is much higher than the requirement of single-molecule detection. The time, frequency, and phase distribution of the input and the response of the four pulses are studied in detail. It was found that the selective excitation and the spectra of CARS are both well preserved.

Surface-Enhanced Coherent Anti-Stokes Raman Scattering of Molecules near Metal–Dielectric Nanojunctions

Surface-Enhanced Coherent Anti-Stokes Raman Scattering of Molecules near Metal–Dielectric Nanojunctions

Numerical simulation study of surface enhancement coherent anti-Stokes Raman scattering reinforced substrate

Plasma nanostructures are of particular significance for serving as a substrate for spectroscopic detection and identification of individual molecules. By combining the excitation wavelength of the molecule with the resonance wavelength of the nanostructure, the sensitive single-molecule Raman detection can be achieved. A high and stable plasma substrate for coherent anti-Stokes Raman scattering(CARS) is very useful for developing the surface-enhanced coherent anti-Stokes Raman scattering (SECARS). In the plasma nanostructures, the strong coupling of plasmonic nanoparticles with an inter-particle gap smaller than the diameter of the individual nanoparticles results in the hybridization of the optical properties of these individual nanoparticles. There are also the charge transfer plasmons(CTP) appearing in conductive bridging nanoparticles. Their unique properties make linked nanosystems a suitable candidate for building artificial molecules, nanomotors, sensors, and other optoelectronic devices. In this work, we, starting from reality, theoretically design a new linked nanosystem SECARS substrate where Fano resonance can be generated by the plasmon hybridization (PH) model resonance and the charge transfer plasmon resonance. The introduction of charge transfer plasma improves the tunability of structural resonance. By adjusting the conductivity of the conductive junction, the wavelength of the charge transfer plasma resonance can be easily adjusted to change the wavelength position of the Fano resonance. The data obtained by numerical simulation of the Raman mode at 1557 cm<sup>–1</sup> of L-tryptophan when a 1064 nm light source is used as the pump light show that this spatially symmetrical structure can generate multiple high-enhancement hot spots that do not depend on the polarization direction of the incident light. Ordinary CARS signal can generally be enhanced by 10<sup>12</sup>, and its maximum can reach 10<sup>14</sup>. Due to the ultrastrong field enhancement and insensitive-to-polarization, this method of using charge transfer plasma to design a substrate can be used in the practical substrate of SECARS and provide new ideas for designing other nonlinear optical processes such as four wave mixing and stimulated Raman scattering.

Spatially Resolving the Enhancement Effect in Surface-Enhanced Coherent Anti-Stokes Raman Scattering by Plasmonic Doppler Gratings.

Well-designed plasmonic nanostructures can mediate far and near optical fields and thereby enhance light–matter interactions. To obtain the best overall enhancement, structural parameters need to be carefully tuned to obtain the largest enhancement at the input and output frequencies. This is, however, challenging for nonlinear light–matter interactions involving multiple frequencies because obtaining the full picture of structure-dependent enhancement at individual frequencies is not easy. In this work, we introduce the platform of plasmonic Doppler grating (PDG) to experimentally investigate the enhancement effect of plasmonic gratings in the input and output beams of nonlinear surface-enhanced coherent anti-Stokes Raman scattering (SECARS). PDGs are designable azimuthally chirped gratings that provide broadband and spatially dispersed plasmonic enhancement. Therefore, they offer the opportunity to observe and compare the overall enhancement from different combinations of enhancement in individual input and output beams. We first confirm PDG’s capability of spatially separating the input and output enhancement in linear surface-enhanced fluorescence and Raman scattering. We then investigate spatially resolved enhancement in nonlinear SECARS, where coherent interaction of the pump, Stokes, and anti-Stokes beams is enhanced by the plasmonic gratings. By mapping the SECARS signal and analyzing the azimuthal angle-dependent intensity, we characterize the enhancement at individual frequencies. Together with theoretical analysis, we show that while simultaneous enhancement in the input and output beams is important for SECARS, the enhancement in the pump and anti-Stokes beams plays a more critical role in the overall enhancement than that in the Stokes beam. This work provides an insight into the enhancement mechanism of plasmon-enhanced spectroscopy, which is important for the design and optimization of plasmonic gratings. The PDG platform may also be applied to study enhancement mechanisms in other nonlinear light–matter interactions or the impact of plasmonic gratings on the fluorescence lifetime.

Giant Chemical Surface Enhancement of Coherent Raman Scattering on MoS2

Raman spectroscopy is a powerful tool for molecular chemical analysis and bioimaging, which shows an astonishing sensitivity when combined with a huge enhancement by the coherence and surface effects. Noble metal nanoparticles have been commonly used for the spontaneous surface-enhanced Raman scattering (SERS) and for the surface-enhanced coherent anti-Stokes Raman scattering (SECARS) spectroscopies, as they provide large enhancement factors via the electromagnetic and chemical mechanisms. Recently, two-dimensional (2D) semiconductors, such as monolayer molybdenum disulfide (MoS2), were used for potential SERS applications as cheaper substrates compared to noble metal nanoparticles. However, the coherent enhancement of SECARS on 2D materials has not been previously explored. Here we present the experimental SECARS measurements of pyridine–ethanol solutions containing 2D MoS2 nanocrystals with the giant chemical enhancement factor of 109 over coherent anti-Stokes Raman scattering (CARS), which is attribute...

Surface-enhanced FAST CARS: en route to quantum nano-biophotonics

AbstractQuantum nano-biophotonics as the science of nanoscale light-matter interactions in biological systems requires developing new spectroscopic tools for addressing the challenges of detecting and disentangling weak congested optical signals. Nanoscale bio-imaging addresses the challenge of the detection of weak resonant signals from a few target biomolecules in the presence of the nonresonant background from many undesired molecules. In addition, the imaging must be performed rapidly to capture the dynamics of biological processes in living cells and tissues. Label-free non-invasive spectroscopic techniques are required to minimize the external perturbation effects on biological systems. Various approaches were developed to satisfy these requirements by increasing the selectivity and sensitivity of biomolecular detection. Coherent anti-Stokes Raman scattering (CARS) and surface-enhanced Raman scattering (SERS) spectroscopies provide many orders of magnitude enhancement of chemically specific Raman signals. Femtosecond adaptive spectroscopic techniques for CARS (FAST CARS) were developed to suppress the nonresonant background and optimize the efficiency of the coherent optical signals. This perspective focuses on the application of these techniques to nanoscale bio-imaging, discussing their advantages and limitations as well as the promising opportunities and challenges of the combined coherence and surface enhancements in surface-enhanced coherent anti-Stokes Raman scattering (SECARS) and tip-enhanced coherent anti-Stokes Raman scattering (TECARS) and the corresponding surface-enhanced FAST CARS techniques. Laser pulse shaping of near-field excitations plays an important role in achieving these goals and increasing the signal enhancement.

Imaging properties of surface-enhanced coherent anti-Stokes Raman scattering microscopy on thin gold films

In a wide-field surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS) microscope, the sample is driven by surface plasmon polaritons supported on a thin gold film. Subsequent radiation at the anti-Stokes frequency is coupled through the gold layer onto a far-field camera, enabling the recording of surface-enhanced CARS images of structures in close proximity to the gold surface. The effective enhancement of the CARS signal can be as high as seven orders of magnitude, allowing CARS imaging at extraordinarily low light levels. In this work, we analyze the imaging properties of the SE-CARS microscope in detail, which are markedly different from the imaging properties of a point-scanning CARS microscope. Using a dipole model to describe the sample, we show that the strength of the signal and the appearance of coherent artifacts depend strongly on the geometry of the sample. We explain the observed radiation profile in the back focal plane and discuss representative imaging examples.

Surface-enhanced coherent anti-Stokes Raman imaging of lipids.

This work describes in detail a wide-field surface-enhanced coherent anti-Stokes Raman scattering (CARS) microscope, which enables enhanced detection of sample structures in close proximity (∼100 nm) of the substrate interface. Unlike conventional CARS microscopy, where the sample is illuminated with freely propagating light, the current implementation uses evanescent fields to drive Raman coherences across the entire object plane. By coupling the pump and Stokes excitation beams to the surface plasmon-polariton mode at the interface of a 30 nm thick gold film, we obtained strong CARS signals from cholesteryl oleate droplets adhered to the surface. The surface-enhanced CARS imaging system visualizes lipid structures with vibrational selectivity using illumination doses per unit area that are more than four orders of magnitude lower than in point-scanning CARS microscopy.

Ultrafast Coherent Raman Scattering at Plasmonic Nanojunctions

Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) measurements carried out on individual nanosphere dimer nantennas are presented. The ν-domain and t-domain CARS measurements in the few-molecule limit are contrasted as vibrational autocorrelation and cross-correlation, respectively. We show that in coherent Raman spectroscopies carried out with ultrashort pulses, the effect of surface enhancement is to saturate stimulated steps at very low incident intensities (100 fJ in 100 fs pulses), and the principal consideration in sensitivity is the effective quadratic enhancement of spontaneous emission cross sections, σ* = (EL/Eo)2σ. Through multicolor femtosecond SECARS measurements we show that beside enhancement factors, an effective plasmon mode matching consideration controls the interplay between coherent electronic Raman scattering on the nantenna and vibrational Raman scattering on its molecular load. Through extensive measurements on individual nantennas, we establish the tolerable average ...

Near-field engineering of Fano resonances in a plasmonic assembly for maximizing CARS enhancements.

Surface enhanced coherent anti-Stokes Raman scattering (SECARS) is a sensitive tool and promising for single molecular detection and chemical selective imaging. However, the enhancement factors (EF) were only 10~100 for colloidal silver and gold nanoparticles usually used as SECARS substrates. In this paper, we present a design of SECARS substrate consisting of three asymmetric gold disks and strategies for maximizing the EF by engineering near-field properties of the plasmonic Fano nanoassembly. It is found that the E-field “hot spots” corresponding to three different frequencies involved in SECARS process can be brought to the same spatial locations by tuning incident orientations, giving rise to highly confined SECARS “hot spots” with the EF reaching single-molecule sensitivity. Besides, an even higher EF of SECARS is achieved by introducing double Fano resonances in this plasmonic nanoassembly via further enlarging the sizes of the constituent disks. These findings put an important step forward to the plasmonic substrate design for SECARS as well as for other nonlinear optical processes.

Localized surface plasmon resonances in nanostructures to enhance nonlinear vibrational spectroscopies: towards an astonishing molecular sensitivity.

Vibrational transitions contain some of the richest fingerprints of molecules and materials, providing considerable physicochemical information. Vibrational transitions can be characterized by different spectroscopies, and alternatively by several imaging techniques enabling to reach sub-microscopic spatial resolution. In a quest to always push forward the detection limit and to lower the number of needed vibrational oscillators to get a reliable signal or imaging contrast, surface plasmon resonances (SPR) are extensively used to increase the local field close to the oscillators. Another approach is based on maximizing the collective response of the excited vibrational oscillators through molecular coherence. Both features are often naturally combined in vibrational nonlinear optical techniques. In this frame, this paper reviews the main achievements of the two most common vibrational nonlinear optical spectroscopies, namely surface-enhanced sum-frequency generation (SE-SFG) and surface-enhanced coherent anti-Stokes Raman scattering (SE-CARS). They can be considered as the nonlinear counterpart and/or combination of the linear surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman scattering (SERS) techniques, respectively, which are themselves a branching of the conventional IR and spontaneous Raman spectroscopies. Compared to their linear equivalent, those nonlinear vibrational spectroscopies have proved to reach higher sensitivity down to the single molecule level, opening the way to astonishing perspectives for molecular analysis.

Measurement of the Surface-Enhanced Coherent Anti-Stokes Raman Scattering (SECARS) Due to the 1574 cm−1 Surface-Enhanced Raman Scattering (SERS) Mode of Benzenethiol Using Low-Power (<20 mW) CW Diode Lasers

The surface-enhanced coherent anti-Stokes Raman scattering (SECARS) from a self-assembled monolayer (SAM) of benzenethiol on a silver-coated surface-enhanced Raman scattering (SERS) substrate has been measured for the 1574 cm(-1) SERS mode. A value of 9.6 ± 1.7×10(-14) W was determined for the resonant component of the SECARS signal using 17.8 mW of 784.9 nm pump laser power and 7.1 mW of 895.5 nm Stokes laser power; the pump and Stokes lasers were polarized parallel to each other but perpendicular to the grooves of the diffraction grating in the spectrometer. The measured value of resonant component of the SECARS signal is in agreement with the calculated value of 9.3×10(-14) W using the measured value of 8.7 ± 0.5 cm(-1) for the SERS linewidth Γ (full width at half-maximum) and the value of 5.7 ± 1.4×10(-7) for the product of the Raman cross section σSERS and the surface concentration Ns of the benzenethiol SAM. The xxxx component of the resonant part of the third-order nonlinear optical susceptibility |3 χxxxx((3)R)| for the 1574 cm(-1) SERS mode has been determined to be 4.3 ± 1.1×10(-5) cm·g(-1)·s(2). The SERS enhancement factor for the 1574 cm(-1) mode was determined to be 3.6 ± 0.9×10(7) using the value of 1.8×10(15) molecules/cm(2) for Ns.

Molecular Imaging of Brain Lipid Environment of Lymphocytes in Amyotrophic Lateral Sclerosis Using Magnetic Resonance Imaging and SECARS Microscopy

This paper highlights some of the key technologies of using two innovative molecular imaging modalites, magnetic resonance imaging (MRI) and nonlinear optical microscopy, for imaging intravenously injected ultra small paramagnetic iron oxide nanoparticles cross linked with antibodies (CLUSPIO) in the amyotrophic lateral sclerosis (ALS) experimental model in vivo or ex vivo, respectively. Intensive efforts have been made in investigating the causes of abnormalities in lipid metabolism, monitored in some neurodegenerative disorders systems. It has been shown that an abnormal accumulation of some common lipids in motor nerve cells may play a critical role in the development of amyotrophic lateral sclerosis. The presented experiments were performed on brain specimens from the transgenic rat model expressing multiple copies of mutated (G93A) human SOD-1 gene, after CD4+ lymphocytes were magnetically labeled with i.v.i. CLUSPIO antibodies. In vivo MRI revealed marked signal intensity enhancements in specific pathological regions of the ALS rat brain as compared to the wild type. Surface-enhanced coherent anti-Stokes Raman scattering (SECARS) microscopy indicated cellular interactions based on lipids association to anti-CD4 CLUSPIO.

Immuno-Surface-Enhanced Coherent Anti-Stokes Raman Scattering Microscopy: Immunohistochemistry with Target-Specific Metallic Nanoprobes and Nonlinear Raman Microscopy

Immunohistochemistry (IHC) is one of the most widely used staining techniques for diagnostic purposes. The selective localization of target proteins in tissue specimens by conventional IHC is achieved with dye- or enzyme-labeled antibodies in combination with light microscopy. In this contribution, we demonstrate the proof-of-principle for IHC based on surface-enhanced coherent Raman scattering for contrast generation. Specifically, antibody-labeled metallic nanoshells in conjunction with surface-enhanced coherent anti-Stokes Raman scattering (SECARS) microscopy are employed for the selective, sensitive, and rapid localization of the basal cell protein p63 in normal prostate tissue. Negative control experiments were performed in order to confirm the selective binding of the target-specific metal nanoprobes and to disentangle the role of plasmonic (metal) and molecular (Raman reporter) resonances in this plasmon-assisted four-wave mixing technique.

Molecular Imaging Cellular SPIO Uptake with Nonlinear Optical Microscopy

We report a novel application to demonstrate and visualize the selective binding of lipids in cells of the reticuloendothelial system to super paramagnetic iron oxide (SPIO) nanoparticles. Ten New Zealand White rabbits that were experimentally injected intravenously with SPIO and five controls were investigated with vibrational microspectroscopy based on surface-enhanced coherent anti-Stokes Raman scattering (SECARS) microscopy. Marked cellular intensity enhancements in hepatic Kupffer cells and melanomacrophages of spleen have been observed in the range of 2850–2875 cm −1 in SPIO-injected animals but not in controls. The enhancements are related to the selective association of lipid molecules in cells of the reticuloendothelial system to uptaken SPIO, which can uniquely be visualized with SECARS microscopy.

Monitoring Lipids Uptake in Neurodegenerative Disorders By SECARS Microscopy

In this study, we investigated a novel application of Surface-Enhanced Coherent anti-Stokes Raman Scattering (SECARS) microscopy for imaging intravenously injected ultra small paramagnetic iron oxide nanoparticles (USPION) in Amyotrophic Lateral Sclerosis (ALS) experimental model. Experiments were performed on transgenic rat model, expressing multiple copies of mutant (G93A) human SOD-1 gene (hSOD-1G93A), treated with antibodies against CD4+ T cells magnetically labelled with USPION. Marked intensity enhancements have been observed in specific pathological regions of the ALS brain as compared to the wild-type model. The results obtained correlated SECARS enhancements to selective association of lipids to up-taken USPION which shows high accumulation in the brainstem and midbrain region. The work presented shows the promising potential of SECARS microscopy in investigating neurodegenerative disorders in experimental systems using USPION as molecular nanoprobes. [DOI: 10.1380/ejssnt.2010.362]