On the Development of Intense THz sources for nonlinear optics and spectroscopy

Abstract

In last couple of decades, the THz science and technology has made huge leap of progress and THz sources and detectors in limited field strength and frequency bandwidth are being made available commercially for applications in spectroscopy, sensing, imaging and medical diagnostics. This is primarily because of the mature NIR femtosecond laser technology and the coherent nature of the THz radiation allowing researchers to create simple table top experimental tools for their THz studies. However, there is a growing demand for intense THz sources which can allow perform nonlinear optics and spectroscopy and develop new methodologies. For nonlinear THz radiation and matter interaction, one needs typically THz-fields comparable to the atomic fields $(\sim 10\wedge 6\mathrm{V}/$cm) such as those provided by lasers in other regions of the electromagnetic spectrum. Photoconductive antennas based on narrow band-gap materials such as InGaAs have seen a great improvement in their performances in terms of optical-to-THz efficiencies and maximum output pulsed THz radiation powers at frequencies above 1 THz due to the availability of high power laser sources operating at the optical telecommunication wavelengths. Even more powerful THz emitters have generally been developed by using nonlinear optical crystals where optical rectification of intense near infrared femtosecond laser pulses produces broadband THz radiation of field strengths reaching few hundreds of kV/cm to even few thousands kV/cm. Zinc telluride and lithium niobate crystals are very popular in this case [1]. Another popular mechanism for generating strong THz fields is by creating plasma in air and phase mismatch between the fundamental and the second harmonic fields of NIR femtosecond pulses [2]. Hetero-structures of layers of ferromagnetic materials and nonferromagnetic films have recently been used to produce THz radiation by excitation with femtosecond laser pulses [3].