Terahertz Radiation
THz defined
THz is Far-Infrared radiation located between microwaves and infrared in the electromagnetic spectrum. It is an non-ionizing radiation which can penetrate many non-polar, non-conducting materials such as clothing, paper, masonry or plastic. Critical uses of THz include;
- Medical imaging - can penetrate several millimeters into tissues with low water content, or deep into teeth, without damage.
- Security screening- it can penetrate clothing and reveal hidden weapons of specific spectral signatures of substances of interest.
- Spectroscopy- THz time domain spectroscopy can perform measures on substances that are opaque in the visible and near-infrared regions of the spectrum.
- Manufacturing- The contents of paper packages can be easily visualized for quality control
THz generation
THz radiation is typically generated via optical rectification (OR) in a nonlinear optical (NLO) material or impulsively from photoconductive dipole antennae. Finite carrier lifetime limits the bandwidth available from photoconductive materials to less than that acheivable via difference frequency mixing (i.e. OR) in NLO materials.
In a non linear material the polarization will not reverse its sign at the same time as the driving field. If the field is applied with a femptosecond laser with a large spectral bandwidth, the mixing of different frequencies results in electromagnetic waves in the terahertz region. This depends on the second order susceptibility of the medium.
See Sum and Difference Frequency Generation
EO Polymers for THz
EO poled polymers have the potential to acheive orders of magnitude higher optical nonlinearities than crystalline materials. Poled polymers have acheived electro-optic coefficients >400 pm/V, which is two orders of magnitude larger than the inorganic crystal ZnTe, which is a standard material for THz generation.
Phase matching (i.e. velocity matching) is necessary for efficient THz generation. Due to phonon lattice resonances in the THz regime, NLO crystals can be quite dispersive. Many crystals are also disperive in the visible and near infrared (NIR). Polymers have very low dispersion. Their NIR and THz indeces of refraction are nearly the same, yielding very good phase matching.
See Sinyukov, Hayden [1]
See Hayden 2003 [2]
Spectroscopy Applications
When THz radiation passes through an object its dielectric properties can be revealed.
THz Time Domain Spectroscopy
The electric field of the THz pulse is sampled via the electrooptic effect in a second order nonlinear material.
The field transmitted through the sample is compared to that transmitted through its substrate (or through free space for free standing samples) and through fourier analysis the frequency dependent index of refraction and absorption coefficients are extracted.
Optical Pump THz Probe Spectroscopy
The THz waveform transmitted through the excited sample is compared to that transmitted through the unexcited sample and through fourier analysis the full frequency dependent complex conductivity can be obtained. Modeling on this photo-induced conductivity yields the mobility and carrier density, also giving the photon-to-carrier yield. Control of the delay between the excitation (the "pump") and the THz pulse (the "probe") provides the time evolution of the photoexcited state.
To extract the complex conductivity we use the following approximate analytic relationship following the work of Sundstrum
- <math>\tilde{\sigma} (\omega) \approx - \frac {(n_{THz} +1 )}{Z_0d}\frac {\Delta \tilde{E}(\omega)}{\tilde{E}_0(\omega)}\,\!</math>
where
- <math>d\,\!</math> is the absorption depth at λexc
Advantages
- Non-contact low energy (meV) non-perturbing optical probe with sub-ps resolution
- Coherent detection provides magnitude and phase of THz waveform, i.e. direct access to the complex dielectric properties of materials under study
- THz radiation is sensitive to charge carriers as carrier scattering times are fs-ps placing the scattering (damping) rate in the THz regime
Carrier Dynamics
This technique has be used to examine carrier dynamics, Drude-like and deviations from the Drude model in popular semiconductors such as semi-insulating (SI-) GaAs
Carrier trapping
It can also be used to identify carrier trapping. The rapid decrease in THz absorption (below) in photoexcited LT-GaAs is due to carrier trapping by As clusters.
We have applied this technique to study carrier dynamics in RR-P3HT and ultrafast charge transfer in the bulk heterojunction P3HT/PCBM 1:1 excited with either 400 nm (3.1 eV) and 800 nm (1.44 eV) light, the results of which are published in J Phys Chem C.
At low temperature there is an increase in the photoconductivity (mobility) of P3HT due to decreased torsoinal disorder which increases the effective conjugation length. The faster recombination dynamics are associated with inhibited interchain hopping at low temperature.
The extracted complex frequency-dependent conductivty of P3HT exibits characteristics of strong (Anderson) carrier localization and inhibited long range trasport due to disorder (below). Fitting of the conductivity to the Drude-Smith model provides photon-to-carrier yields of < 1.5% and a hole mobility of ~35 cm^2/Vs. The THz regime mobility agrees well with published DFT calculations of the predicted intrinsic hole mobility in well ordered P3HT.
The high frequency THz radiation probes carriers over short (nanoscale) distances, which corresponds to highly ordered domains in this system. Through this experiment we are able to experimentally determine the upper limit of the carrier mobility that would be seen in a device with perfect ordering, instead of the disorder limited mobility typically seen in devices.
External Links
See Wikipedia Terahertz Radiation
See Wikipedia Terahertz time-domain spectroscopy
See Wapedia Terahertz
References
- ↑ 2003 Efficient Electrooptic Polymers for THz Applications J. Phys. Chem. B 2004, 108, 8515-8522
- ↑ L.M. Hayden, A. Sinyukov, M. Leahy, P. Lindahl, J. French, W. Herman, M. He, R. Twieg, "New Materials for Optical Rectification and Electro-optic Sampling of Ultra-short Pulses in the THz Regime," J. Polymer Sci. B. Polymer Phys., Vol 41, 2492 -2500 (2003)