Fds1010

Our SRS imaging system consists of a dual output femtosecond (80 MHz repetition rate) laser (InSight DS+, MKS) with tunable (680 to 1300 nm) and fixed (1040 nm) outputs, a delay line to attain and adjust beam overlap, and a custom-built laser scanning microscope.²⁶ (link) Spectral focusing is achieved using compact adjustable-dispersion TIH53 glass blocks²⁷ (link) to chirp the femtosecond pulses into the picosecond domain, at 2.06 ps and 1.1 ps for pump and Stokes, respectively, and adjusting pulse overlap with the delay line, allowing for an $\sim 28{\mathrm{cm}}^{-1}$ spectral resolution. We measured the stimulated Raman loss (SRL) signal using lock-in detection with the Stokes beam modulated by an acousto-optic modulator (1205C-2, Isomet) at 4.9 MHz. The pump beam intensity was detected by a $1\times 1\mathrm{cm}$ photodiode (FDS1010, Thorlabs) biased to 60 V, filtered through a 2 to 7 MHz electronic bandpass filter, and delivered to a lock-in amplifier (MFLI, Zurich instruments) to extract the SRL signal. A pixel dwell time of $20\mu \mathrm{s}$ was used for the SRS image. ScanImage²⁸ (link) (Version 5.6, Vidrio Technologies) was used for control of the laser scanning and image acquisition.

All measurements were performed under inert atmosphere. DC I–V curves were measured using a Keithley 2636a dual source-meter. For photocurrent measurements, a xenon lamp combined with a monochromator was used to generate monochromatic visible/near-infrared light, which was collimated and directed towards the sample. Light intensity was controlled by neutral density filters and measured using a commercial Si photodiode (FDS1010 from Thorlabs). Photocurrent was determined from the DC current under illumination after subtracting the dark DC current. Spectral responsivity measurements (Fig. 4e) were performed under a modulation of 75 Hz for different photon wavelengths.
Time-resolved photocurrent measurements (Fig. 6) were performed by generating light pulses from an LED (M660L3 from Thorlabs) using an Arduino electronics board. Short-time transient current (microseconds to milliseconds) was measured with a Femto low-noise current amplifier (DLPCA-200) and a digital oscilloscope (TDS 640 from Tektronix), while long-time current traces (milliseconds to tens of seconds) were measured using the Keithley source-meters. Gate pulses were generated and synchronized with the light pulses using the Arduino board.

Cross-sections of 50 μm thickness were cut at internodes IN5, IN9, IN15 and IN20 from miR408_OX poplars, using a razor blade. The SRS imaging microscope using a mode-locked Nd: YVO4 laser (High Q Laser) was used to generate a 7 ps, 76 MHz pulse train of both 1,064 nm (1 W average power) and 532 nm (5 W average power) laser beams. The 1,064 nm output was used as the Stokes light. The 532 nm beam was 50/50 split to pump two optical parametric oscillators (Levante Emerald, A · P · E Angewandte Physik und Elektronik GmbH). The output wavelength of the optical parametric oscillators was at 909 nm to use as pump beam to induce the Stimulated Raman signal for the 1,600 cm⁻¹ lignin aromatic ring vibration. All pump and Stokes beams were directed into an Olympus laser scanning microscope scanning unit (BX62WI/FV300; Olympus) and focused by a high numerical aperture water-immersion objective (UPLSApo 60×1.20 NA W; Olympus). The light transmitted through the sample was collected by an oil-immersion condenser (1.45 NAO; Nikon). The stimulated Raman loss signals were detected by silicon PIN photodiodes (FDS1010; Thorlabs) and a lock-in amplifier (SR844; Stanford Research Systems)^{54 (link)}. For each type of plant sample, at least three images were selected for intensity analysis. About 50 cells were selected and the average intensity for selected cells was calculated.