THzTDS

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THz-TDS

• Terahertz Time-Domain Spectroscopy

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1 Introduction

• The term THz-TDS
• Topics covered

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Topics

• Introduction
• Aim (of BSc.Thesis)
• Principles (Theory)
• Data Analysis
• Software
• Experiments
• Conclusions
• Questions

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Aim

• Obtaining dielectric function of a slab of
  material in the THz frequency range
• Writing software that computes said dielectric
  function from measurements
• Writing software to fit measurements to
  theoretical models
• Increasing the frequency resolution of the
  resulting data

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Etalons

• Using the etalon to increase frequency
  resolution too difficult
• Time signal will have to be cut off at the etalon

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Principles

• Optical Rectification
• P is proportional to E2
• The carrier-frequency-independent term is thus
  defined by the modulation of the carrier, giving
  shape to the THz pulse

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Principles

• THz pulse detection (FEOS)

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Principles

• Spectroscopy make two measurements a reference
  measurement, and an actual measurement.
• The difference between those two describes
  whatever it is we change between the reference
  and actual measurement.

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Principles

• Unexcited (spatially homogeneous) sample

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Principles (unexcited)
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Principles (unexcited)

• Assuming the sample is low-absorbing, we get the
  following expressions for the samples index of
  refraction and extinction

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Principles (excited)

• Optically excited (spatially inhomogeneous) sample

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Principles (excited)
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Principles (excited)

• tm and fm derived from
• n and a Numerically solve, using
  Newton-Raphson, from tm and fm.
• Starting values for n and a perform the same
  computations using the front interface only.

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Topics

• Introduction
• Aim (of BSc.Thesis)
• Principles (Theory)
• Data Analysis
• Software
• Experiments
• Conclusions
• Questions

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Data Analysis

• We want frequency-dependent n() and a.
• We have two time signals.
• Solution Fourier-Transform the time signals
  yields frequency-dependent attenuation and phase
  retardation.
• We want to fit the results of the analysis to
  theoretical models (Drude, Lorentz oscillator)

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Data Analysis (phase unwrapping)

• Fourier-Transform yields phases between 0 and
  2p.
• Phase retardation is generally greater then one
  phase.
• We need phase unwrapping to correct this.

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Data Analysis (phase unwrapping)

• The input data may be too noisy to reconstruct f
  (especially at low frequencies).
• Manual fixing may be needed.

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Data Analysis (Theoretical models)

• Drude
• Lorentz oscillators

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Software

• Load the time signals
• Performs analysis (producing values for n and a)
• Fixing phase unwrapping errors
• Exporting results

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Software

• Fixing phase unwrapping by clicking bad points
• Easy to use

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Software

• Manual fitting of results to theoretical models
• Hard to get right

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Experiments unexcited ZnTe
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Experiments optically excited GaAs
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Conclusions

• Software is functional and easy to use
• Two types of measurements can be processed
• Fitting is tedious, but a black-box fitting
  routine is hard to implement.
• Using the etalons to improve frequency resolution
  proved to be difficult

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Questions?
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End