Idus 416

All SERS spectra were obtained using a confocal micro Raman system (XperRF, Nanobase) equipped with an optical microscope (BX41M-LED; Olympus, Tokyo, Japan). The signal was detected using a thermoelectrically cooled (− 60 °C) charge-coupled device detector (Idus 416, Andor Technology). The 532-, 660-, and 785-nm photoexcitation lasers were focused, and the Raman signals except the SERS enhancement factor (EF) calculation data were collected using a × 10 objective lens (0.25 NA, Olympus).

The ultrafast laser system used in the current work is a Ti:Sapphire oscillator (Element™ 2, Newport Spectra-Physics) which produces laser pulses of ~6 fs duration with a bandwidth spanning from 650 nm to 1050 nm at a repetition rate of ~80 MHz. Probe pulses centred at ~728 nm with a duration of ~500 fs were generated by narrowband filtering (Ultra Narrow Bandpass Filter 728.1/1.5, AHF) of the broadband ~6 fs long laser pulses. Pump pulses (~750–805 nm) and Stokes pulses (~805–920 nm) were also generated by bandpass filtering of the broadband laser pulses. Two precise (resolution ~0.1 μm) delay stages were used to control the delay time τ₁₂ between the pump and Stokes pulses and τ₂₃ between the Stokes and probe pulses. An achromatic lens (diameter: 50 mm; focusing length: 75 mm) was mounted inside the UHV chamber to focus the laser beams onto the apex of the Au tip. The TERS signal was collected through the same achromatic lens and then focused onto the entrance slit of a spectrometer (Kymera 328i, ANDOR) and detected by a thermoelectrically cooled charge coupled device (iDus 416, ANDOR). TERS experiments were also performed with CW excitation by using a Helium-Neon (He-Ne) CW laser (HNL150L, Thorlabs) centred at ~633 nm.