Ultrabroadband infrared pulse ranging from terahertz region to near infrared using air for both generation and detection

Recently, coherent terahertz pulses have been generated through plasma induced by focusing intense two-color ultrashort laser pulses in air. Using hollow-fiber compressed sub-20-fs pulses as the light source, the infrared wave generation and coherent detection up to 70 THz were achieved. We extended the high frequency limit up to 200 THz by using hollow-fiber compressed 10-fs pulses, however, the frequency range of the coherent detection using electro-optic (EO) sampling with a crystal was only to 100 THz. For further extension of the detection range, we focused our attention on the probe-pulse energy dependence of the signal intensity. When we use photoconductive antennas and EO sampling, the signal sensitivity is proportional to the probe-pulse energy, while using air for detection, the signal intensity shows the quadratic dependence on the probe-pulse energy. From this the probe pulse duration can be effectively shortened by the factor of 1.41 using air for detection so that higher frequency detection can be expected than other cases. Thus in the present study, we adopted the field-induced SH generation in air, called terahertz air breakdown coherent detection (THz-ABCD), and extended the high frequency limit of the coherent detection up to 150 THz.