Generation and detection of THz radiation by air

by Yee Wei Kheoh

b) ω0  focuses through BBO crystal to generate 2ω0. THz pulse is attributed to four-wave-mixing(FWM) and transient current inside the gas plasma. 

a) Ponderomotive force in laser-induced plasma generates THz waves.

c) Dichroic mirror(DM) is used to combine ω0 and 2ω0, and control their relative phase difference.

Jianming Dai, Jingle Liu, and X.-C. Zhang "Terahertz Wave Air photonics: Terahertz wave generation and detection with laser-induced gas plasma"   

•In-line phase compensator can be used to control the relative delay (i.e. similar to the conventional mechanical ‘optical delay line’ but has finer steps in attosecond) of ω0 and 2ω0, and also their polarization to be the in the same direction as shown in the diagram on the left.

•HWP is used to change the polarization of the 2ω0 beam.

•Dichroic mirror is used to split and combine ω0 and 2ω0

• Once the combined pulse energy of ω0 and 2ω0 beams exceeds the air ionization threshold(~150µJ), the THz amplitude follows the equation:

 

• Evidence that plasma generation is essential for THz emission from the focal region.

To verify the phase dependence of maximal THz amplitude, one can change the relative phase φ between the ω0 and  2ω0 waves by varying distance d between BBO crystal and focus to produce a phase shift. 

Markus Kress, Torsten Löffler, Susanne Eden, Mark Thomson, and Hartmut G. Roskos, Opt. Lett. 29, 1120-1122 (2004)

• The optimal efficiency of the THz field generation is achieved when all the waves (w, 2w, and THz waves) possess the same polarization in the FWM process.

    x - p polarized     y - s polarized

• Increasing directionality of the radiation profile is correlated with increasing length of the laser-induced plasma.

• Effective source for terahertz frequency imaging and spectroscopy applications at standoff distance.

• Attosecond phase compensator(PC) is used to control the relative phase between ω0 and 2ω0 to achieve optimal THz emission efficiency.

• Focusing of optical pulses ω0 and 2ω0 remotely at a standoff distance of 17 meters and 116 meters to create plasma as the THz emitter have been demonstrated.

• Utilizing the lower atmospheric attenuation at the visible range (<0.01dB/km) from air.

J. Dai and X.-C. Zhang,Appl. Phys. Lett. 94, 021117(2009)   J. Dai and X. Zhang, "Demonstration of 17 Meter Standoff THz Wave Generation," 

• Resolved time-domain THz waveforms that are generated at different distances.

• Higher-power laser

can be used to further extend standoff distance(>100m) at which THz wave can be generated.

• Another way to generate THz wave at standoff distance; based on Cherenkov-like radiation from laser-induced plasma filaments.

• Two orders of magnitude lower than two-color generated THz electric field.

A Houard, Y Liu, B Prade, VT Tikhonchukb, Phys. Rev. Lett., vol.  100, pp.255006-1-255006-4, 2008

• THz waveform information is encoded into a change of fluorescence emission at a different time delay td.

• 'all-optical' technique of THz wave detection.• Omni-directional fluorescence emission. Circumvent the limitation of

forward directional detection requirement.

THz Radiation-Enhanced-Emission-of-Fluorescence (THz-REEF)

J. Liu, J.Dai, S.L. Chin & X.-C. Zhang, Nature Photonics 4, 627 - 631 (2010) 

J. Liu, J.Dai, S.L. Chin & X.-C. Zhang, Nature Photonics 4, 627 - 631 (2010) 

THz Radiation-Enhanced-Emission-of-Fluorescence (THz-REEF)

•Relative phase, Δ Φω, 2ω, determines asymmetry or symmetry ionization. The latter produces relatively lower photocurrent than the first. •Different relative phase produces different initial electron drift velocity.

•Upon interaction with incident THz pulses, electrons inside the gas plasma are accelerated and deaccelerated.

•Fluorescence emission occurs by electron-impact excitation of the ionized gas.

•Different REEF emission pattern is observed at different relative phase due to phase dependence of initial electron drift velocity.

•In b above, we can see that the blue(Δ Φω, 2ω= π/2) and red(Δ Φω, 2ω= -π/2) patterns are symmetrical with the black(Δ Φω, 2ω= 0).

•The differential of the blue and red patterns produces time-resolved THz wave which is in good agreement if it was measured with EO sampling.

• Parabolic mirror focuses the collimated THz beam through laser ω-induced plasma.

• Four-wave mixing of ω probe beam and THz field produces 2ω signal.

• Coherent detection of the THz waves is obtained when probe intensity reaches above 3.3 X 1014 W/cm2.

• The third term of the equation below should be dominant in order to have a coherent detection.

, unipolar waveform; incoherent detection

, bipolar waveform; coherent detection

J. Dai, X. Xie, and X.-C. Zhang, Phys. Rev. Lett., vol. 97, pp. 103903-1-103903-4, 2006

•Introduces an ac external bias to the optical focus to generate bias field induced second harmonic pulse, with field amplitude denoted by 

N. Karpowicz, J. Dai, X. Lu, Y. Chen, M. Yamaguchi, H. Zhao, X.-C. Zhang, L. Zhang, C. Zhang, M. Price-Gallagher, and C. Fletcher, Coherent heterodyne time-domian spectrometry covering the entire terahertz gap, Appl. Phys. Lett.  vol. 92, pp. 011131-1–011131-3, 2008

     Advantage:• Reduces probe pulse energy that was

previously required to breakdown air for coherent THz detection. 

• Heterodyne detection of lock-in amplifier that is referenced to known LO modulation frequency can increase signal-to-noise ratio.

OverviewJianming Dai, Xiaoyu Guo, and X.-C. Zhang,"Terahertz Air Photonics for Standoff Explosive Detection"

Issues: • Bandwidth of semiconducting photoconductive dipole antennas is limited

by carrier lifetime and phonon absorption. And dispersion of electro-optic crystals in THz range is problematic.

• Limitation by ambient moisture absorption in the THz range.

Conclusion:

• THz air photonics makes broadband standoff terahertz spectroscopy, imaging, remote sensing and identification feasible.

• 'all-optical' technique of THz wave generation and detection circumvents limitation of moisture ambient absorption in the THz range.

• THz wave generation with ultra-broad bandwidth: 0.3 - 35 THz; the useable bandwidth of THz air photonics is laser pulse duration limited.

• Due to ultra-broad bandwidth, broad spectral information of a target material can be obtained and reduces false alarm rates.