# Ultrafast Optical Heating Induced Polarization-Dependent Optical Switching in Gold Nanowires
## 3 Ultrafast Optical Switching Performance  
### Measurement Results of the Optical Switching Processes  
The optical switching effect of the gold nanowires shown in Figure 2a was investigated using femtosecond pump-probe measurements. A Ti: sapphire amplifier Legend Elite from Coherent  
Figure 1: (**a**) SEM image of the gold nanowires; (**b,c**) optical extinction spectra measured at different angles of incidence for transverse electric (TE) and transverse magnetic (TM) polarizations, respectively. provides 150 fs pulses at 800 nm with a repetition rate of 1 kHz and a maximum pulse energy of 1 mJ. These 800-nm pulses were used as the pump after some part of the pulse energy was sent to a 1-cm cell containing D\({}_{2}\)O for supercontinuum generation. The supercontinuum pulse was used as the probe, which extends from sub-300-nm to longer than 1200 nm in spectrum. The delay between the pump and probe pulses was adjusted by a linear stage in the beam path of the pump pulse. The transient absorption (TA) spectrum was collected by the fiber spectrometer using CCD as the detection unit as a function of time delay. According to Figure 1b,c, the optical extinction at normal incidence is about 0.22 and 0.08 OD at 800 nm for TM and TE polarization, respectively. Therefore, stronger interaction between the gold nanowires and the femtosecond pump laser pulses may be achieved for TM than for TE polarization. Based on this consideration, TM polarization was employed for the 800-nm pump pulses. Thus, we present experimental results for two pump-probe schemes, where the pump pulses were TM polarized and the probe were TM and TE polarized, respectively. A pump fluence of 17.8 \(\upmu\)/cm\({}^{2}\) was employed in the following demonstrated experimental results. Transient absorption was evaluated by \(\Delta\)A = \(-\)log\({}_{10}\)(T\({}_{\text{on}}\)/T\({}_{\text{off}}\)), where T\({}_{\text{on}}\) and T\({}_{\text{off}}\) are the transmission rate of the probe pulse through the sample with the pump pulse switched on and off, respectively. Figure 2a shows the pump-probe dynamics of the optical switching performance measured at two typical wavelengths of 667 (black curves) and 678 nm (red curves), where the open circles show TM-pump/TE-probe and the solid ones show TM-pump/TM-probe measurement results. The pump beam was sent to the sample at an angle of 2 degrees, and the probe beam was incident normal to the sample surface. For the TM-pump/TM-probe scheme, a much smaller optical switching signal with positive \(\Delta\)A values both at 667 and 678 nm were observed, as compared with that for TM-pump/TE probe. However, a strong negative signal within the first 1 ps of the time delay after the onset of the pump pulse was measured at 667 nm, and a strong positive signal was measured at 678 nm for the TM-pump/TE-probe scheme. Furthermore, in TM-pump/TE-probe, a much faster optical switching process than in the TM-pump/TM-probe scheme can be observed within the first 500 fs after the finish of the pumping process, as can be measured by evaluating the falling and rising edges of the red and black open circles, respectively, which is followed by a much slower dynamics. This faster process can be attributed to plasmonic electron-dominated scattering process, and the slower tail can be identified as a phonon-dominated process. Looking back at the two curves by solid circles, we can find that both of the optical switching signals at 667 and 678 nm are dominated by the slow dynamics. Comparing the optical switching dynamics of the two pump-probe schemes by the amplitude, speed, and sign of the \(\Delta\)A values, we can conclude that they correspond to different photophysical processes. For TE-polarized probe, we need to base our analysis on the black spectrum  
Figure 2: (**a**) Optical switching dynamics of the gold nanowires at two wavelengths (667 nm and 678 nm) for two schemes of TM-pump/TM-probe and TM-pump/TE-probe; (**b**) 3D plot of the optical switching signal as a function of wavelength and time delay.