INTENSE TERAHERTZ GENERATION VIA TWO-COLOR LASER FILAMENTATION
Oh, Taek il
Milchberg, Howard M
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The main focus of this dissertation is intense terahertz (THz) generation using two-color laser mixing in air plasma. In this scheme, the fundamental and second harmonic of an ultrashort pulsed laser are combined and focused into air to create a gaseous plasma, which produces an intense THz pulse in the far field. To understand the generation process, we have developed a two-dimensional (2-D) plasma current model. Using this model, we have simulated the conditions for optimal THz generation and verified them experimentally. A full control of THz output is demonstrated by varying the phases and polarization states of the input laser pulses. We have studied how the generated THz energy scales with various focal lengths and input laser energies up to 60 mJ. For high enough energy inputs, the resulting THz saturates. This arises from inefficient laser energy coupling into the plasma, which results from plasma-induced laser defocusing in filamentation. We have overcome the saturation effect by elongating the filaments and achieved 7 µJ of THz energy with 60 mJ laser energy. This provides a conversion efficiency of 10-4 from optical to THz energy. In addition, we have investigated high-power THz generation in two-color laser filamentation with terawatt (TW) lasers including a 0.5 TW, 1 kHz repetition (rep) rate system, as well as, 2 TW and 30 TW systems, both operating at 10 Hz rep rate. In particular, our 1 kHz rep rate THz source can provide high-energy (>1 µJ), high-average power (>1 mW), intense (>1 MV/cm), and broadband (0.01~60 THz) THz radiation via two-color filamentation in air. Based on our observed scaling law, a ~30 TW laser can produce >0.1 mJ of THz radiation with multi-gigawatt (GW) peak power in ~1.5 m long filamentation. We have also studied various THz detection methods covering a broad range of THz frequency bands. We observe our THz source produces extremely broad electromagnetic (EM) radiation ranging from radio-micro waves to infrared frequencies, confirmed by our complementary THz detection methods. This source could be a useful tool for broadband linear and nonlinear THz spectroscopy.