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(a) Schematic diagram of the G-line waveguide, which was fabricated on a quartz substrate with integrated photoconductive switch regions. In transmission mode, voltages control the input pulse generated by photoexcitation of switch gaps 1 and 2, respectively, while detection is achieved by simultaneous measurement of the current developed in photoconductive detectors and , as a function of the time delay applied to the laser pulse used to excite switch gaps 3 and 4. The radial mode is launched by a transition from a coplanar waveguide (dimensions and ). A stub filter element (shown), length and width , was added for later experiments. (b) The instantaneous electric field intensity, calculated using HFSS, and plotted as a vector field around sections of (i) the launch coplanar waveguide, and (ii) the center of the G-line section, in both cases for a frequency of 260 GHz. Grey areas represent waveguide metallization. The quadrupole coplanar excitation mode is converted to a radial mode in the G-line by the smoothly tapered ground plane sections.
(a) Main figure: Signal detected by photoconductive detectors located either side of the G-line waveguide, showing symmetry of the recorded signal for a range of biases (where ), applied simultaneously to the two input switches [switch gaps 1 and 2 in Fig. 1(a)]. (Data for detector 1 are offset by 2500 pA for clarity.) Inset: Peak photocurrent generated in both detectors for varying input bias applied to both excitation photoconductive switches; squares for detector 1 and circles for detector 2. Lines are linear fits as guides to the eye. (b) Fourier transforms of pulses recorded by both photoconductive detectors measured simultaneously, taken for several time windows from . Data for 100 and 50 ps time windows are offset by factors of 2× and 4×, respectively, on the logarithmic scale for clarity. Note that the secondary pulse at was included in each Fourier transform. The similarity demonstrated between the two detectors implies a symmetric electric field mode propagating around the G-line.
Fourier transform of the transmitted current pulses measured through a G-line device with an integrated bandstop stub filter, taken over a time window of 50 ps. Arrows indicate the bandstop response centered at , along with a third harmonic bandstop response near . Inset: time domain response of the bandstop filter.
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