Paper on tip-enhanced nonlinear spectroscopy selected as a featured article in Journal of Chemical Physics
A paper titled "Tip-enhanced sum frequency generation spectroscopy using temporally asymmetric pulse for detecting weak vibrational signals," published on February 19, 2026 by a research team from the Institute for Molecular Science (Atsunori Sakurai, Shota Takahashi, Tatsuto Mochizuki, and Toshiki Sugimoto) and Tohoku University (Tomonori Hirano and Akihiro Morita), has been selected as a "Featured Article" in The Journal of Chemical Physics, published by the American Institute of Physics (AIP), in recognition of its particularly noteworthy research.
The paper is available at the following URL: https://pubs.aip.org/aip/jcp/article/164/7/074202/3380428/Tip-enhanced-sum-frequency-generation-spectroscopy
Outline of this paper summary Vibrational sum frequency generation (SFG) spectroscopy is widely used to probe molecular structures and orientations at surfaces, but its spatial resolution is limited by optical diffraction. To overcome this constraint, tip-enhanced SFG (TE-SFG) based on a scanning tunneling microscope was developed. By introducing a temporally asymmetric second pulse and a controlled delay between the first and the second laser pulses, the strong non-resonant background originating from metallic substrates was effectively suppressed, thereby enhancing the contrast of weak vibrational signals. In addition, absolute molecular orientations were determined. Simultaneous detection of forward- and backward-scattered signals confirmed their near-field origin. The signal enhancement factor was estimated to be on the order of 107, demonstrating nanoscale vibrational spectroscopy beyond the diffraction limit.
The structures and orientations of molecules at material surfaces are key factors that determine catalytic activity and material functionality. Vibrational sum frequency generation (SFG) spectroscopy is a powerful technique for probing molecular vibrations at surfaces; however, its spatial resolution is limited to the micrometer scale by the optical diffraction limit. To acquire detailed information on surface molecules at the nanoscale, the research team developed tip-enhanced SFG (TE-SFG) spectroscopy, which combines SFG spectroscopy using ultrafast laser pulses with the localized near field at the apex of a sharp metallic tip in a scanning tunneling microscope (STM). This approach has enabled the detection of molecular vibrations with nanoscale spatial resolution.
However, when measurements are performed on metallic substrates, a strong background signal arising from the free-electron response of the metal is generated, which can distort or obscure weak molecular vibrational signals. To address this issue, we shaped the near-infrared laser pulse into a temporally asymmetric waveform--characterized by a steep rise and a gradual decay--and introduced a controlled time delay between the near-infrared and infrared pulses. While the metal-derived background decays almost instantaneously, molecular vibrations persist for a longer time. By exploiting this difference in temporal dynamics, we effectively suppressed the background component and successfully enabled the detection of weak molecular vibrational signals.
As a result, we achieved the detection of weak vibrational modes originating from aromatic rings that had not been observed in our previous research. Furthermore, by simultaneously detecting forward- and backward-scattered signals, we clearly demonstrated that the observed signals originated from tip enhancement within the nanoscale gap between the tip and the substrate rather than from far-field contributions. The signal enhancement factor is estimated to be on the order of 107, as determined from the experimental data. In addition, by analyzing the interference pattern between the vibrational signal and the background, we showed that the absolute molecular orientation--namely, whether molecules are oriented "upward" or "downward" relative to the surface--can be determined with nanoscale precision.
This work advances TE-SFG spectroscopy by enabling the reliable extraction of weak molecular signals that were previously buried in the background, and by establishing a technique for determining absolute molecular orientations at the nanoscale. These achievements provide important insights into the molecular-level understanding of catalytic reactions and the design of molecular devices. In the future, by sweeping the interpulse delay, we plan to extend this approach to time-resolved measurements, ultimately enabling highly sensitive tracking of ultrafast molecular dynamics and chemical reaction processes on ultrashort timescales.
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The paper is available at the following URL: https://pubs.aip.org/aip/jcp/article/164/7/074202/3380428/Tip-enhanced-sum-frequency-generation-spectroscopy
Outline of this paper summary Vibrational sum frequency generation (SFG) spectroscopy is widely used to probe molecular structures and orientations at surfaces, but its spatial resolution is limited by optical diffraction. To overcome this constraint, tip-enhanced SFG (TE-SFG) based on a scanning tunneling microscope was developed. By introducing a temporally asymmetric second pulse and a controlled delay between the first and the second laser pulses, the strong non-resonant background originating from metallic substrates was effectively suppressed, thereby enhancing the contrast of weak vibrational signals. In addition, absolute molecular orientations were determined. Simultaneous detection of forward- and backward-scattered signals confirmed their near-field origin. The signal enhancement factor was estimated to be on the order of 107, demonstrating nanoscale vibrational spectroscopy beyond the diffraction limit.
The structures and orientations of molecules at material surfaces are key factors that determine catalytic activity and material functionality. Vibrational sum frequency generation (SFG) spectroscopy is a powerful technique for probing molecular vibrations at surfaces; however, its spatial resolution is limited to the micrometer scale by the optical diffraction limit. To acquire detailed information on surface molecules at the nanoscale, the research team developed tip-enhanced SFG (TE-SFG) spectroscopy, which combines SFG spectroscopy using ultrafast laser pulses with the localized near field at the apex of a sharp metallic tip in a scanning tunneling microscope (STM). This approach has enabled the detection of molecular vibrations with nanoscale spatial resolution.
However, when measurements are performed on metallic substrates, a strong background signal arising from the free-electron response of the metal is generated, which can distort or obscure weak molecular vibrational signals. To address this issue, we shaped the near-infrared laser pulse into a temporally asymmetric waveform--characterized by a steep rise and a gradual decay--and introduced a controlled time delay between the near-infrared and infrared pulses. While the metal-derived background decays almost instantaneously, molecular vibrations persist for a longer time. By exploiting this difference in temporal dynamics, we effectively suppressed the background component and successfully enabled the detection of weak molecular vibrational signals.
As a result, we achieved the detection of weak vibrational modes originating from aromatic rings that had not been observed in our previous research. Furthermore, by simultaneously detecting forward- and backward-scattered signals, we clearly demonstrated that the observed signals originated from tip enhancement within the nanoscale gap between the tip and the substrate rather than from far-field contributions. The signal enhancement factor is estimated to be on the order of 107, as determined from the experimental data. In addition, by analyzing the interference pattern between the vibrational signal and the background, we showed that the absolute molecular orientation--namely, whether molecules are oriented "upward" or "downward" relative to the surface--can be determined with nanoscale precision.
This work advances TE-SFG spectroscopy by enabling the reliable extraction of weak molecular signals that were previously buried in the background, and by establishing a technique for determining absolute molecular orientations at the nanoscale. These achievements provide important insights into the molecular-level understanding of catalytic reactions and the design of molecular devices. In the future, by sweeping the interpulse delay, we plan to extend this approach to time-resolved measurements, ultimately enabling highly sensitive tracking of ultrafast molecular dynamics and chemical reaction processes on ultrashort timescales.
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