Jianming Dai

Coherent heterodyne time-domain spectrometry covering the entire “terahertz gap”

Terahertz waves, electromagnetic radiation in the spectral region commonly defined between 0.3 and 10THz, allow innovative sensing and imaging techniques that can provide spectroscopic information unavailable at other wavelengths. However, simultaneously intense, broadband, and coherent spectroscopic measurement remains challenging. We report spectrometry using gases ionized by femtosecond pulses to generate and sense broadband terahertz pulses. Using a coherent heterodyne technique, the measurements span the “terahertz gap” with ⩾10% of the maximum signal from 0.3to10THz. This spectrometer, using a recycled optical probe beam and coherent detection, offers a high field strength and time-resolved measurement.

Terahertz Wave Air Photonics: Terahertz Wave Generation and Detection With Laser-Induced Gas Plasma

Ultrabroadband, intense terahertz (THz) waves can be generated and detected using gaseous media as both the THz wave emitter and sensor, with a bandwidth covering the entire “THz gap” and well beyond. We review our most recent experimental and theoretical progress in THz generation, detection, and manipulation in laser-induced gas plasma. In the generation part, emphasis will be on the case with two-color excitation of gaseous media. Potential applications of these novel THz generation and detection techniques will also be discussed and proof-of-principle experiments are demonstrated.

Enhancement of terahertz wave generation from laser induced plasma

It is well known that air plasma induced by ultrashort laser pulses emits broadband terahertz waves. The authors report the study of terahertz wave generation from the laser induced plasma where there is a preexisting plasma background. When a laser beam from a Ti:sapphire amplifier is used to generate a terahertz wave, enhancement of the generation is observed if there is another laser beam creating a plasma background. The enhancement of the terahertz wave amplitude lasts hundreds of picoseconds after the preionized background is created, with a maximum enhancement up to 250% observed.

Terahertz wave generation from gas plasma using a phase compensator with attosecond phase-control accuracy

We report the use of a precise phase compensator for the generation of intense terahertz waves from laser-induced gas plasma excited by a femtosecond pulse (ω) and its second harmonic (2ω) at both close contact and standoff distances. The attosecond accuracy phase-control capability of the device enables further optimization of the terahertz emission from gas plasma and elimination of the temporal walkoff between the ω and 2ω pulses traveling in dispersive media, resulting in intense terahertz generation at a distance of over 100 m by sending the optical beams far away and focusing them locally.

Recent Progresses in Terahertz Wave Air Photonics

Terahertz (THz) wave generation and detection using gaseous medium as both the THz wave emitter and sensor, also termed “THz wave air photonics”, has attracted much scientific attention. We report the most recent progresses in THz generation and detection in laser-induced gas plasma. The overall emphasis of this paper is on THz wave detection with gaseous medium, while in the generation part, emphasis will be on the case with two-color laser excitation.

Laser air photonics: beyond the terahertz gap

Through the ionization process, the very air that we breath is capable of generating terahertz (THz) electromagnetic field strengths greater than 1 MV/cm, useful bandwidths of over 100 THz, and highly directional emission patterns. Following the ionization of air, the emitted air-plasma fluorescence or acoustics can serve as an omnidirectional, broadband, THz wave sensor. Here we review significant advances in laser air photonics that help to close the “THz gap,” enabling ultra-broadband THz wave generation and detection, for applications including materials characterization and non-destructive evaluation. The feasibility for remote sensing, as well as the remaining challenges and future opportunities are also discussed.

Terahertz wave amplification in gases with the excitation of femtosecond laser pulses

We report the observation of terahertz wave amplification in gases excited by femtosecond laser pulses. When a seed terahertz wave is sent into the ∼5mm long plasma created by focusing a 800nm beam and its second harmonic into nitrogen gas, a peak amplification factor of about 65% is measured with total optical excitation intensity of 8×1014W∕cm2. The amplification effect occurs within a time scale of less than 400fs of the onset of ionization processes due to the optical excitation pulse duration. Four-wave-mixing parametric processes during the plasma formation are proposed to be responsible for the amplification effect.

Optical property of beta barium borate in terahertz region

The optical property of beta barium borate (β−BBO) in the terahertz region (0.2–12THz) was studied using a broadband air photonic terahertz time-domain spectrometer. The crystal orientation and frequency dependence of β−BBO refractive index and absorption coefficient were experimentally investigated over the temperature range of 10–293K. Four TO phonon modes were observed below 3.5THz. The behavior of these phonon modes at different temperatures has been characterized.

Terahertz wave generation from thin metal films excited by asymmetrical optical fields

We experimentally demonstrated terahertz (THz) wave emission from thin metal (gold) films excited by asymmetrical optical fields synthesized using an in-line phase compensator. By driving the electrons in thin metal films asymmetrically, THz wave emission is observed at normal incidence of two-color pump beams. Coherent control of THz wave emission from metal films suggests that a mechanism similar to that of the air-plasma THz source excited by two-color laser fields can be used to describe the generation processes.

AMBIENT AIR USED AS THE NONLINEAR MEDIA FOR THZ WAVE GENERATION

We report the first systematic study of broadband THz wave generation with ambient air as the nonlinear media. Generation of pulsed THz waves with mixing the fundamental and the second harmonic beams in air has been previously demonstrated by Cook et al. and Hartmut et al. while different groups obtained different results. To verify the proposed mechanism of strong THz wave generation, we measured dependence of generated THz field on the polarization, amplitude and phase of the individually controlled two beams. Our results confirm that four-wave-mixing rectification is the major mechanism this phenomenon, and the amplitude and polarity of generated THz wave can be controlled by the relative phase of the beams. This work is significant by providing the feasibility of THz wave generation and detection with a standoff distance greater than 50 meters.