09-07-2012, 04:13 PM
TERAHERTZ IMAGING
[attachment=27296]
Introduction
In spite of their considerable success, X-rays, magnetic resonance imaging and ultrasound all have shortcomings.
Safer and more cost-effective imaging techniques are necessary
Quality of an image to improve rapidly with increase in wavelength but this will limit the spatial resolution of the objects
The wavelength has to be sufficiently small to provide good resolution, yet large enough to prevent serious losses by scattering
Physicists looked to the so-called terahertz gap in the electromagnetic spectrum — the region between 300 GHz and 20 THz (i.e. 15 um—1 mm in wavelength)
Generation Of THz
In1980s David Auston and co-workers at Columbia University in NY demonstrated that "photoconductive emitters" could be used to generate coherent picosecond (10—12 s) pulses at terahertz frequencies
When a photo-conductive emitter is illuminated with a subpicosecond pulse of visible or near-infrared light, electron—hole pairs are created in a semiconducting layer within the device
These charge carriers are then accelerated by a bias voltage
The resulting transient photocurrent is proportional to this acceleration and radiates at terahertz frequencies.
Detection of THz
It is the inverse of the generation mechanism
Photoconductive can be readily paired to "coherent" detectors
Coherent nature of these detectors means that they provide both phase and amplitude information about the pulse, and can reject noise due to background radiation
Also by measuring the time it takes a terahertz pulse to travel through a medium, we can determine both its thickness and refractive index
IMAGING WITH TERAHERTZ PULSES
The spectral response of many organic and inorganic materials to low-frequency terahertz light is dominated by the dielectric response of the materials.
At high frequencies, however, the response is dominated by specific intra- or inter-molecular vibrations and rotations
In 1995 Martin Nuss and co-workers at AT&T in the US became the first to demonstrate terahertz-pulse imaging.
To produce a terahertz pulse image, an object is translated through the beam or by scanning the beam across it
One of the main advantages of terahertz light is that a variety of common materials are transparent or semi-transparent in this frequency range and each has its own unique THz image
[attachment=27296]
Introduction
In spite of their considerable success, X-rays, magnetic resonance imaging and ultrasound all have shortcomings.
Safer and more cost-effective imaging techniques are necessary
Quality of an image to improve rapidly with increase in wavelength but this will limit the spatial resolution of the objects
The wavelength has to be sufficiently small to provide good resolution, yet large enough to prevent serious losses by scattering
Physicists looked to the so-called terahertz gap in the electromagnetic spectrum — the region between 300 GHz and 20 THz (i.e. 15 um—1 mm in wavelength)
Generation Of THz
In1980s David Auston and co-workers at Columbia University in NY demonstrated that "photoconductive emitters" could be used to generate coherent picosecond (10—12 s) pulses at terahertz frequencies
When a photo-conductive emitter is illuminated with a subpicosecond pulse of visible or near-infrared light, electron—hole pairs are created in a semiconducting layer within the device
These charge carriers are then accelerated by a bias voltage
The resulting transient photocurrent is proportional to this acceleration and radiates at terahertz frequencies.
Detection of THz
It is the inverse of the generation mechanism
Photoconductive can be readily paired to "coherent" detectors
Coherent nature of these detectors means that they provide both phase and amplitude information about the pulse, and can reject noise due to background radiation
Also by measuring the time it takes a terahertz pulse to travel through a medium, we can determine both its thickness and refractive index
IMAGING WITH TERAHERTZ PULSES
The spectral response of many organic and inorganic materials to low-frequency terahertz light is dominated by the dielectric response of the materials.
At high frequencies, however, the response is dominated by specific intra- or inter-molecular vibrations and rotations
In 1995 Martin Nuss and co-workers at AT&T in the US became the first to demonstrate terahertz-pulse imaging.
To produce a terahertz pulse image, an object is translated through the beam or by scanning the beam across it
One of the main advantages of terahertz light is that a variety of common materials are transparent or semi-transparent in this frequency range and each has its own unique THz image