Measures of Central Tendency: Mean, Median and Mode
Terahertz_Applications
1. Terahertz time domain
Spectroscopy
Prof.V.Krishnakumar
Professor and Head
Department of Physics
Periyar University
Salem, India.
2. Aim of the talk
Introduction to spectroscopy
Need of terahertz frequency in
spectroscopy
Terahertz Spectroscopy-
Instrumentation
THz Generation
THz Detection
End note
3. • Spectroscopy is the study of quantized
interaction of energy (typically
electromagnetic energy) with matter.
• Infrared (IR) electromagnetic radiation causes
vibrations in molecules.
• Mid IR: 4000 to 600 cm-1
(2.5 – 15 μm)
• Near IR: 12500 to 4000 cm-1(0.8 to 2.5 μm)
• Far IR: 200 to 12.5 cm-1 (50 to 800 μm).
4.
5. Vibrational spectroscopy
Is a valuable tool for elucidation of
molecular structure.
Can be utilized to identify the functional
groups present in the molecule.
6.
7. STRETCHING
A change in the length of a bond, such as C-H or C-C
BENDING
A change in the angle between two bonds
ROCKING
A change in angle between a group of atoms
WAGGING
A change in angle between the plane of a group of
atoms
TWISTING
A change in the angle between the planes of two
groups of atoms
8. Any limitation in FTIR spectroscopy?
Many complex molecules give lattice
vibration in the far IR region. But it gives
less intense in the spectra and it’s not
enough to elucidating molecular spectra
9. Terahertz frequency - what can it do?
This energies are much
less than the electronic
state transition of
atoms and molecules
for laser source
1012 Hz = 300 μm = 33.3 cm-1 = 4 meV = 50 Kelvin
The ability to penetrate packing materials, such as paper,
cardboards, plastic, glass and clothing and its non ionizing
character makes THz radiation very attractive for different
imaging and spectroscopic applications.
Terahertz (THz) spectroscopy has been employed to
investigate a variety of materials from solids to gases.
10. Methods to generate & detect T-rays:
1. Optical rectification
2. Photoconduction .
terahertz time-domain spectroscopy (THz-TDS)
It is a spectroscopic technique in which the properties of a material
are probed with short pulses of terahertz radiation.
The generation and detection scheme is sensitive to the sample
material's effect on both the amplitude and the phase of the terahertz
radiation.
In this respect, the technique can provide more information than
conventional Fourier-transform spectroscopy, which is only sensitive
to the amplitude.
It is the study of dynamic processes in materials or chemical
compounds by means of spectroscopic techniques.
12. Generation and detection of terahertz pulses
The THz-TDS is based on a pump-probe optical setup.
Pulses generatation - By means of optical rectification in non-linear
crystals. When an intense ultrashort laser pulse hits such a crystal, its
rapidly oscillating electromagnetic field is rectified, giving rise to a
terahertz emission --1 mm thick crystal (ZnTe) is irradiated with 800
nm laser pulses of 100 fs duration, it emits broadband terahertz pulses
covering a spectrum from 0 up to ~2.5 THz.
Pulse detection
The detection of THz pulses is usually done by free space electro optic
sampling (FSEOS). Electric field of the THz pulse induces birefringence
in a detector crystal. If prope beam travels through the detector crystal
at the same time as the THz pulse, its polarization is rotated. This
rotation of the polarization is proportional to the magnitude of the THz
electric field.
The probe beam turns to circular polarization and its 2 perpendicular
components were equaly seperated by the wollastom prism into
balance photodiode. One can record the differece signal to describe the
THz waveform.
13. Photoconductive antenna (PCA) for
terahertz (THz) waves consists of a highly
resistive direct semiconductor thin film with
two electric contact pads.
A photoconductive antenna (PCA) for
terahertz (THz) waves consists of a highly
resistive direct semiconductor thin film with
two electric contact pads. It will produce
transient current the material.
A short laser puls with puls width < 1 ps is focused between the electric
contacts of the PCA. The photons of the laser pulse have a photon energy
E = h× n larger than the energy gap Eg and are absorbed in the film. Each
absorbed photon creates a free electron in the conduction band and a
hole in the valence band of the film and makes them for a short time
electrical conducting until the carriers are recombined.
15. What it do with in the molecules?
Gasses with a permanent dipole moment show very sharp
absorption line in the microwave and THz spectral region. These
absorptions result from the interaction of the radiation field with the
rotation of the molecules.
In the crystalline state, the constituents (atom or molecules)
are held close to their equilibrium locations by a balance or attractive
or repulsive force. This leads to collective vibrations of the crystals at
certain frequencies. This energy of vibration is quantized and the
corresponding energy levels described as ‘phonon’. Due to the energy
and momentum conservation, the selection rule Δk=0 applies for
infrared absorption in crystals.
Semiconductor and ionic crystals exhibit strong absorption due
to phonons.
Many biologically relevant molecules like glucose, saccharose,
lactose etc., show sharp resonance in the THz range connected to the
strong hydrogen bond network in the crystals.
16. For gas-phase molecules, rotational and vibrational
states typically occupy the THz region. Molecules which are polar,
such as water vapor, exhibit many spectral lines due to their strong
interaction with the THz electric field. Non-polar molecules interact
very little and are therefore transparent.
For liquids, the rotational and vibrational excitations are
strongly damped by the proximity of neighboring molecules. They
are highly absorbing over a broad range of THz frequencies, thus
generally yielding broad and continuum THz spectra. The THz
absorption spectra of crystalline solids can result from both intra-molecular
vibrations as well as from large scale intermolecular
vibrational motion of the crystal structure.
17. Non polar liquids = Transmission geometry.
Polar liquids = Reflection mode.
The visualization and verification of
liquid explosives and flammable liquids.
Femto second pulse duration at near infrared frequencies.
quarter-wave plate creates a quarter-wavelength phase shift.
The pump beam induces material changes after it hits in the target material, and the probe beam monitors this transformation at different time delays. The short pulse laser, which has up to several tens of femtoseconds time resolution, is used because of higher time resolution.
Wollaston prism. It separates randomly polarized or unpolarized light into two orthogonal, linearly polarized outgoing beams. At this point, the phase modulation of the probe beam is converted to an intensity modulation of the orthogonal polarization of the probe beam, which are steered into a pair of photodiodes.
