1 Introduction
2 Terahertz air photonics
2.1 Generation and detection of terahertz waves in air
Fig.1 (a) Schematic diagram of a THz-ABCD spectroscopic system in both transmission and reflection mode. The system can be converted from transmission to reflection mode by taking out the mirrors indicated with an enclosing dashed box. PMT: photomultiplier tube; BS: beamsplitter; β-BBO: beta-barium borate; (b) photograph of laser-induced air-plasma created after focusing the optical beam from left to right through a lens (left) and mounted nonlinear crystal (center) used for second harmonic generation. The bright horizontal line emits an intense, highly directional terahertz field to the right |
Fig.2 (a) Dependence of terahertz field on fundamental (ω) pulse energy, with fixed second-harmonic (2ω) pulse energy; (b) dependence of terahertz field on second-harmonic pulse energy, with fixed fundamental pulse energy. The solid line and curve are the linear and square-root fits, respectively. Reprinted figure with permission from Ref. [4] |
Fig.3 (a) Time-resolved terahertz signals generated and detected using dry nitrogen gas as compared to conventional electro-optic (EO) crystal detection in ZnTe. The probe beam for air detection has energy of 85 μJ and pulse duration of 32 fs; (b) corresponding spectra after Fourier transformation |
Fig.4 (a) Basic concept of THz-ABCD: electrodes are placed at the geometric focus of collinearly focused terahertz and optical probe beams with a variable time delay. Second harmonic light is induced from the terahertz field and the local bias field . Modulating allows for heterodyne detection for enhanced sensitivity; (b) measured second harmonic intensity vs. third order nonlinear susceptibility χ(3). All χ(3) are normalized with respect to nitrogen. Data in (b) courtesy of Dr. Xiaofei Lu |
2.2 Terahertz air photonic system electronics
2.2.1 Square wave high voltage modulator
2.2.1.1 High voltage switching
2.2.1.2 Operational testing
Fig.8 Square wave high voltage modulator operating at maximum output voltage as determined by the DC-DC converter. The laser TTL output is sent to the modulator and a voltage divider is used to monitor the CH1 output. When operating with electrodes, the total output potential is CH1−CH2, twice that shown |
2.2.2 Pulsed high voltage modulator
2.2.2.1 Digital circuitry
2.2.2.2 Analog circuitry
2.2.3 Summary
2.3 Terahertz detection through air-plasma fluorescence
Fig.16 (a) Experimental geometry for THz-REEF from air-plasma using a single-color laser pulse excitation; (b) electron acceleration in the terahertz field and collision with neighboring molecules; (c) THz-enhanced fluorescence spectra of nitrogen gas-plasma under influence of 100 kV/cm peak field. © IEEE, reprinted, with permission, from Ref. [24] |
Fig.17 (a) Time-resolved air-plasma fluorescence enhancement from terahertz wave interaction with antiparallel, symmetric, and parallel electron drift velocities with respect to the laser field, controlled by changing the relative phase between the ω and 2ω optical pulses; (b) subtracting the parallel curve from the antiparallel curve removes the incoherent energy transfer by electrons after inelastic collisions and scattering in random directions. This reveals the terahertz waveform in the form of fluorescence modulation. The optical pulse leads the terahertz pulse in time for delay td<0 |
2.4 All air-plasma terahertz spectroscopy
2.4.1 Prior efforts for remote terahertz wave sensing
2.4.2 Experimental setup
Fig.18 “All air-plasma” terahertz spectroscopy system. Air-plasma filaments are used for both generation and detection of the terahertz electromagnetic radiation. (Light blue: terahertz), (Red: 800 nm pulse), (Blue: 400 nm pulse), (Purple: nitrogen fluorescence). Nitrogen fluorescence emitted from the probe plasma carries the encoded terahertz pulse information |
2.4.3 Experimental results and discussion
Fig.20 (a) Terahertz waveforms for pellet samples NG, 2,4-DNT, and HMX containing 20% chemical mixed with polyethylene obtained using electro-optic sampling; (b) absorbance signatures corresponding to samples in (a); (c) identical samples and corresponding waveforms obtained using radiation enhanced emission of fluorescence (REEF) encoding; (d) absorbance signatures corresponding to samples in (c). All curves are offset for clarity |
3 Terahertz enhanced acoustics
3.1 Theoretical model
3.2 Experiments and discussion
3.2.1 Incoherent detection
Fig.21 (a) Experimental setup for performing terahertz enhanced acoustics using single-color femtosecond laser excitation; (b) single photoacoustic waveforms measured at 5 mm distance with (red-dashed) and without (black-solid) a 100 kV/cm terahertz field. The insert shows the acoustic spectra in linear scale. Amp.: amplitude; Acous. Freq.: acoustic frequency |
Fig.23 Normalized pressure enhancement signal at 100 kHz as a function of time delay td. Region I: terahertz pulse leads the optical pulse in time; region II: terahertz pulse trails the optical pulse in time. The dashed line is the calculated signal. Inset shows the acoustic signal at 100 kHz for different terahertz intensities incident on single-color laser-induced plasma and a linear fit |
3.2.2 Coherent detection
Fig.25 Dependence of terahertz wave generation from plasma on the relative phase delay between 800 nm pulse and 400 nm pulse. The arrow refers to the electron drift direction. At the maxima of the terahertz emission, the electron drift velocity is highly asymmetric, while at the minima of the terahertz emission, the electron drift velocity is nearly symmetric |
Fig.26 (a) Acoustic pressure enhancement as a function of time delay td at relative phase delay of π/2 (solid line) and –π/2 (dashed line). Inset, the experimental schematic of interaction of the terahertz pulse and two-color laser plasma; (b) comparison between the terahertz time-domain waveforms measured by terahertz-wave-enhanced acoustic emission and electro-optic sampling respectively in ambient air; (c) corresponding spectral comparison of the waveforms in (b). TEA; terahertz enhanced acoustics; EO: electro-optic |
3.3 Remote sensing with acoustics
Fig.27 Temporal and spectral characteristics of acoustic pulses collected using a broadband microphone mounted at the 3 inch focus of a 12 inch diameter parabolic reflector. (a) Normalized temporal pressure transients at 0.5 and 11 meters from the plasma source; (b) normalized spectral comparison of 0.5 and 11 meter acoustic pulse propagation |
Fig.28 (a) Acoustic pulses collected with and without direct line of sight to the plasma acoustic source from several meters; (b) terahertz enhanced acoustic signal collected at standoff distance of 1 and 3 meters from the plasma source using a 12" diameter parabolic reflector with the microphone positioned at the 3" focus |