Raman Spectroscopy is a type of scattering spectrum representing the characteristic vibrational modes of molecules and chemical bonds. With its advantages of a unique representation of different species, non-invasiveness and pollution-free in the sample measuring process, Raman Spectroscopy boasts a wide range of applications in material science, biology, chemistry, physics and medical science. It is a powerful tool in both daily life such as monitoring the water quality, food safety and detergent compositions, and scientific research such as the study of single molecule structures and the break and form of its respective chemical bonds.
The shift of the scattered light frequency after a beam of light hits the sample is known as Raman scattering. This process is so weak that for 10^10 incident photons, only about 1 photon will be Raman scattered. Therefore, essential enhancement techniques shall be applied to increase the detectability of Raman scattering which has been a hot topic in recent several decades among researchers in related fields of optics, physics and chemistry. Surface-enhanced Raman Scattering (SERS), benefiting from a combination of physical and chemical enhancements, has made possible the detection of low concentration of molecules.
From the perspective of physical (electromagnetic, EM) enhancement, plasmonic nanoantenna has found its extensive applications in the research field of SERS. With its outstanding performance in confining incident light into nanometer-sized volume and enhancing the local electromagnetic (EM) field to a great extent, the SERS signals are accordingly-enhanced.
Single molecule Surface-enhanced Raman Scattering (SM-SERS) has been the “holy grail” in the research field of SERS. In order to achieve this goal, many studies have been carried out whose nano-fabrication methods can be classified into two categories: bottom-up and top-down. The former is capable of obtaining single-molecule detectability but with randomly distributed SERS hot spots; the latter can achieve reproducible SERS hot spots but with moderate enhance factors (EF) due to limited fabrication resolution that renders small non-resonant molecules detection difficult.
To address the unsettled challenge of achieving ultra-high EM enhancement in a well-controlled and well-repeated manner, Tian’s research group has studied the concept of nanosphere-plane antenna structure and its peculiar excitation scheme under radially polarized laser excitation, and has in experiment obtained reproducible ultrasensitive single-molecule detectability, which provides an EMEF of 10^9~10 at the gap of each individual nanosphere-plane antenna and a root-mean-square error down to 10^0.08 between them. The EMEF is 1~2 orders of magnitude larger than the results of previous research with other nanostructures and excitation schemes, which is highly sensitive and repeatable for small non-resonant molecules detections. This research has paved a way for the analysis of single-molecule structures as well as single-molecule dynamics.
(a) Illustration of the gold nanosphere-plane antenna under RP excitation. The mirror image of the nanosphere is also plotted. (b) The LSPR spectrum of an antenna, by measuring its scattering outside of the NA of the illuminating objective. The nanosphere’s diameter is 60 nm. The laser wavelength and the three strongest Raman bands in the SERS experiment are labeled as red and green lines, respectively. (c) FDTD simulation of |Ez^2| in the antenna’s junction gap, normalized by the |Ez^2| of an incident p-wave at its resonance wavelength 691 nm. (d) The SERS spectrum of an antenna (lifted up by 10000 counts) and that of a monolayer of MGITC on a bare gold plane (background subtracted by 20000 counts). The laser power at the sample is 300 nW for the antenna and 1.5 mW for the bare plane. The integration time is 4 s for the antenna and 10 s for the bare plane. (e) SERS EMEFs of twenty antennas for three Raman bands at 1180 cm^(−1), 1370 cm^(−1) and 1618 cm^(−1). Each three dots with the same color come from one same antenna. The dashed lines are the average EMEFs for each band.
This work is supported by the National Science Foundation of China under grant #11204177 and #11574207, the Fundamental Research Program of Science and Technology Commission of Shanghai Municipality under grant #14JC1491700, the Research Fund for the Doctoral Program of Higher Education of China under grant #20120073110050.
Background information
Dr. Tian Yang joined JI as an associate professor and doctoral adviser in 2009. His current research work is focused on the following subjects:
nano-lasers and photonic integrated circuits, quantum and nonlinear effects in nano-plasmonic structures, detection of single molecules and single chemical events by Raman spectroscopy, fiber-optics integrated biomolecular sensing and acoustic detection, nano-photonic technologies in translational medicine.
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