(a) Schematic of the THz-PP-TDS system. The 10 fs, 800 nm laser pulse train is split into three pulse trains: An optical probe (20 mW), an optical pump 1(60 mW, generating THz pump pulse) and an optical pump 2 (30 mW, generating THz probe pulse). The THz-PP-TDS system consists of four 90° off-axis gold-coated parabolic mirrors, two PC THz emitters, and an electro-optic sampling setup. The generated THz radiation pump pulse and TH radiation probe pulse were collected and co-linearly focused along with the optical probe pulse onto a 500 μm thick ⟨111⟩ ZnSe electro-optic crystal. The THz radiation-induced polarization modulation of the optical probe was extracted via a balanced detection arrangement, and lock-in detection was carried out at the bias voltage modulation frequency. (b) Schematic of the THz emitters. The two PC THz emitters were placed near each at a separation of 200 μm. The bias voltages of the PC THz emitters 1 and 2 were modulated at frequencies f 1 and f 2, respectively. Through phase-locking at the modulation frequency of the biasing voltage, this configuration allows locking at and detecting the THz radiation due to the THz pump signal, or the THz probe signal, or both.
Electric fields of the THz radiation pump and probe pulses as generated and detected by the abovementioned THz-PP-TDS system. The THz probe pulse is delayed by 5.5 ps with respect to the THz pump pulse. The modulating frequencies (f 1 and f 2) of bias voltages applied to both PC THz emitters were set to 81 KHz. The ratio of the THz pump to THz probe was set to 3:1 in this experiment.
The spot sizes of both the THz radiation pump and probe pulses were mapped out using the knife edge technique in the: (a) x-direction and (b) y-direction. The FWHM spatial extent of the THz radiation focal region was found to be 510 μm and 740 μm, along the x- and y-directions, respectively. In the x-direction, both the THz radiation pump pulse and the THz radiation pulse probe overlapped and along the y-direction, the centers of the focal spots were displaced by 200 μm with 90% overlap. The x- and y-directions are depicted in the inset of (b).
Electric fields of the THz radiation pump and probe pulses where the polarity of the THz probe pulse is reversed by 180° with respect to the THz pump pulse. The THz probe pulse is delayed by 5.5 ps with respect to the THz pump pulse and the modulating frequencies (f 1 and f 2) of bias voltages applied to both PC THz emitters were set to 81 KHz. The ratio of the THz pump to THz probe was set to 3:1 in this experiment.
The THz-TDS probe signal pulse acquired at 40.5 kHz. Here, the PC THz emitter generating the THz radiation pump pulse was modulated at f 1 = 81 kHz while the PC THz emitter generating the THz radiation probe pulse was modulated at f 2 = 40.5 kHz.
(a) Time-domain THz probe signals transmitted through Cu/CuxO particle sample at various delay times (△t), and (b) their corresponding spectral power plots.
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