Terahertz time domain Spectroscopy

Prof.V.KrishnakumarProfessor and HeadDepartment of PhysicsPeriyar UniversitySalem, India.

Aim of the talkIntroduction to spectroscopy

Need of terahertz frequency in spectroscopy

Terahertz Spectroscopy- Instrumentation THz Generation THz DetectionEnd note

• 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).

Vibrational spectroscopy

Is a valuable tool for elucidation of molecular structure.

Can be utilized to identify the functional groups present in the molecule.

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

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

1012 Hz = 300 m = 33.3 cmμ -1 = 4 meV = 50 Kelvin

This energies are much less than the electronic

state transition of atoms and molecules

for laser source

Terahertz frequency - what can it do?

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.

Methods to generate & detect T-rays: 1. Optical rectification2. 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.

Ti : sapphire laser

τ

BS

M2 M5

M1

M3 M4

Emitter

Electro-Optic Sampling

SZnTe

λ/4

WP

Balanced photodiodes

Probe beam

Pump beam

THz pulse

THz pulse

Δτ

120 fs; 2 W; 810 nm; 82 MHz

Sub pico second THz

Time-resolved spectroscopy

Generation and detection of terahertz pulsesThe 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.

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.

Mechanism of THz emission from photoconductive switch.

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.

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.

Non polar liquids = Transmission geometry.

Polar liquids = Reflection mode.

The visualization and verification of

liquid explosives and flammable liquids.

THz absorption spectra of the explosives RDX, HMX, PETN, and TNT.