Applications and Requirements of ultrafast THz detection

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Applications and Requirements ofultrafast THz
detection

• Steven JamisonSchool of Computing and Advanced
  Technologies, University of Abertay Dundee
• and
• Department of Physics, University of Strathclyde

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Electro-optic detection of Terahertz
(THz pump optical probe)

• sampled THz electric field causes retardation in
  optical pulse
• Infer THz field from intensity of optical pulse
  (after polarisers)
• map THz field by scanning relative timing of
  optical and THz pulse

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Single-shot detection also possible
Propagating electric field
polariser
chirped probe beam
EO crystal
Optical detection in spectrometer or
cross-correlator gt need 1D image
detection
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Photo-conductive antenna THz detection
THz pulse
Optical pulse
Antenna is gated by 50fs optical pulsefocused
onto 5?m gap
5 mm
Works well, but has limited capability for very
fast detection (i.e. gt200fs)
100 mm
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Electro-optic detection setup
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Electro-optic detectionwhat do we get?
Able to detect over 0.1 -- 40 THz spectral region
(1 -- 1300cm-1)
More commonly used in 1-200 cm-1 region
measured
Inferred spectrum (also have phase information)
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THz spectroscopy.
Humid air
Dry air
F.T.
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THz spectroscopy of flames.
Measuring transitions linewidths of 2GHz
(0.1cm-1)
Cheville et al J. Opt. Soc. Am B, 16 317 (1999)
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Plasma diagnostics.
Spectra of phase shifts and absorption
Measured THz transmission through plasma
(time after discharge start)
Can determine plasma density and collision
properties
Jamison et al. J. Appl. Phys. 93 4334 (2003)
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THz generation in semiconductors
THz emission initiated by ultrafast laser.
Intensity depended provides evidence of
spectral-holeburing during excitation process.
Jamison et al. Current Applied Physics, 4 217
(2004)
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Ultrafast carrier dynamics(THz probe optical
pump)
Measure carrier properties with THz transmission,
for varying delays with respect to optical
excitation of semiconductor
Leitenstorfer et al., Physics B, 314 248 (2002)
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Transient effects in two levels
systems(fundamental QM effects with
single-cycle THz pulses)
Transient enhancement in excited state
population, as inferred from the THz
transmission (and consistent with 2-level
calculations)
Kersting et al. Optics Letters 25 272 (2000)
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THz coherent synchrotron radiation (CSR)...
e-
THz
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THz
Probe laser
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Synchrotron radiation beam profile exit window
Use pyro-electric array to measure THz beam
profile, and THz beam alignment. (1D array,
scanned across beam)
THz pulse energy
0.3 ?J/electron bunch (from observed 1.8
mJ/macrobunch)
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Synchrotron radiation beam profile at focus
THz pulse energy
45 nJ/electron bunch (from observed
280?J/macrobunch)
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Electro-optic detection of synchrotron radiation..
Jamison et al. Energy Recover Linac workshop
2005(to be published in NIM-A)
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Electro-optic THz spectra of proteins..
THz spectra of bovine serum albumin, and DNA
THz spectra may enable identification of protein
folding processes. Interested in picosecond to
millisecond time scales
Markelz et al., Chem Phys. Lett. 320 42 (2000)
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THz spectroscopy.100?m of water
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Detector requirements maximum bandwidth
Larger bandwidth enables broader spectral range
in spectroscopy measurements
Larger bandwidth enables higher time resolution
200fs
100fs
1ps
30fs
EO detection efficiency
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Detector requirements calibrated response

• phase matching
• crystal absorption
• ?(2)(?)

• carrier dynamics
• antenna coupling

Calibration NOT necessary for spectral
applications
Detector response incorporated in reference
spectra
Calibration essential for (most) temporal
investigations
Detector response (phase and amplitude) needed to
reconstruct true temporal structureCurrently no
complete measurements of existing detectors
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Detector requirements enhanced EO materials
Alternative electro-optic materials
Crystal absorption leads to missing spectral data
cant be calibrated
Some alternatives already used, e..g GaP, GaSe,
DAST?
Better quality in existing materials
Uniform, stress and defect free large crystals
for imaging. Consistency in supplied
electro-optic crystals.
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Steve Jamison Allan MacLeod Allan
Gillespie Dino Jaroszynski Giel Berden
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Detector requirements best time-resolution
Spectral dependence of material properties Limits
ability to recover true electric field. .

• phase matching, crystal absorption
• ?(2)(?) coefficient frequency dependence

200fs
100fs
1ps
30fs
EO detection efficiency
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CCD images of cross-correlator second-harmonic
FEL on
reference
difference
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e-beam as sub-cycle radiation source ?
Lorentz contracted magnetic field Field
strengths gt 1Tesla at SLAC,
2ps unipolar pulse
Tudosa et al., Nature 428 p831 (2004)
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Calculated Absorption and Dispersion
7.5 torr CH3F
FFT
Reference
FFT-1
Sixth Echo
First Echo
Data from H. Harde, R. A. Cheville, D.
Grischkowsky, J. Phys. Chem. A, vol. 101, p.
3646, 1997.