Terahertz Radiation

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THz defined

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1 THz ~ 100-300 µm wavelength ~33 wave numbers ~ 4 meV- Courtesy of www.SURA.org

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

Experimental setup- source. Polymer Sci. B. Polymer Phys- will redraw

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.

In a typical experimental setup a 180 fs pump laser is directed on an EO emitter which generates THz by optical rectification. The radiation induces a change in the EO material in the sensor via the electrooptic effect. A probe beam is passed through the sensor and then compared to a paired reference beam using a photodetector.

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.

Phase (velocity) matching by comparing the THz (a) and the visible (b) indices of refraction of EO poled polymers. The Lorentz Oscillator model of the THz index of ZnTe is included in the inset of (a) for reference.

See Sinyukov, Hayden [1]

See Hayden 2003 [2]


Spectroscopy Applications

Time domain spectroscopy and Optical pump THz Probe compared

When THz radiation passes through an object its dielectric properties can be revealed.

THz Time Domain Spectroscopy

In THz Time Domain Spectroscopy, a ~1ps single-cycle THz pulse is propagated through a sample.

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.

The index of refraction of amorphous polycarbonate from 0-7THz.


Optical Pump THz Probe Spectroscopy

In Optical-Pump THz-Probe Spectroscopy, a THz pulse is propagated through a photo-excited sample.

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.

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(left) Photoconductivity dynamics described by pump delay dependent changes in THz transmission through the photoexcited sample. (Right) The change in the transmitted THz waveform is recorded at various delays to allow for the extraction of the evolving frequency-dependent dielectric properties.


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

Excitation at 800nm vs 400nm. (Left) A band diagram of GaAs shows that the excess 1.49eV provided by the 400nm excitation allows carriers to scatter to higher valleys. (Right) The longer rise time of the 400nm photo-induced conductivity signal corresponds to the time required for carriers to return to the high mobility gamma valley.

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


File:GaAs 800sigma.png
The frequency-dependent photoconductivity of GaAs shows deviations from the simple Drude model.


Carrier trapping

comparison of the photo- conductivity of low-temperature grown (LT-) GaAs with SI-GaAs

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.

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Inclusion of a liquid He cryostat allowed for temperature dependent studies
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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.

There is only very weak absorption at 800 nm.
File:P3HT pump.png
Both energy excitations yield similar dynamics on subpicosecond time scales.
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Ultrafast electron transfer from P3HT to PCBM is marked by an increase in the long lived photoconductive state in the bulk heterojunction compared to neat films of P3HT.
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Bulk heterojunction compared to P3HT



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.

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The complex conductivity of P3HT measured from 200 GHz - 2.1 THz extracted at 1, 1.5 and 6.2 ps after photoexcitation. The solid lines are fits to the Drude-Smith model.
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The mobility in perfectly ordered P3HT far exceeds that typically measured in devices due to the effect of disorder and defects.


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

See THz Group Wiki- password required THz Polymers Wiki


References

  1. 2003 Efficient Electrooptic Polymers for THz Applications J. Phys. Chem. B 2004, 108, 8515-8522
  2. 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)
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