Fig. 3: Fabry-Pérot cavity and speed control with a local constriction.

a The ratio between the frequency quantisation of the Fabry-Pérot cavity, f_FP, and the bandwidth, Δf for the plasmon wavepackets. The dashed line indicates f_FP/Δf = 1. f_FP(=v_p/2L_FP) as a function of V_QPC1 is calculated from v_p in Fig. 2c. Δf( = 1/t_FWHM) is calculated from t_FWHM in Fig. 1b. Normalised amplitude of fast Fourier transform (FFT) of the voltage pulse in Fig. 1b for 52 ps-long voltage pulse (b) and 500 ps-long voltage pulse (c). The bandwidth value, Δf, is indicated by a black solid line. In addition, f_FP at V_QPC1 = −0.2 V and its multiples are indicated by the red dashed lines. d Time-resolved measurement of a plasmon wavepacket excited by 500 ps pulse for different gate voltages applied to QPC1 in the 100 μm-long electronic waveguide while it is connected to the Ohmic contact O_r. The amplitude is normalised to one. Each curve is offset vertically for clarity. The gate voltage V_QPC1 was stepped from  −0.2 V (bottom) to  −1.4 V (top). The peak position is indicated by the black dots. e Comparison of plasmon speed as a function of V_QPC1 with and without the FP cavity for the wavepackets excited by 500 ps pulse. The data without the FP cavity (new data) and the data with the FP cavity (from Fig. 2c) is provided for direct comparison. Here, the number N on top of the grey shaded gate voltage indicates the number of transmitting electron channels across QPC1, which is determined by the observation of the quantised conductance.