TERAHERTZ IMAGING By, Nizamudeen E.A

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

THE CHARACTERISTICS OF THz WAVE Wave-particle Duality: has particle nature and wave nature, such as interference and diffraction. Penetrability : on a lot of dielectric material and non-polar liquids. Can be supplement of X-ray imaging and ultrasound imaging Security : the energy of THz radiation is only mill-electron volt, lower than the energy of different types of chemical bond. So will not cause harmful ionizing reaction The Resolving Power of Spectrum : the THz spectroscopy of material contains a wealth of physical and chemical information, which making them the unique characteristics, like fingerprints Image quality : Images has a higher spatial resolution, and therefore the image has more depth of field while maintaining the same spatial resolution.

THE REAL-TIME IMAGING SYSTEM

Working Figure illustrates a real-time imaging system. A femtosecond laser pulse is divided into pump and probe beams Average power is 800 mW for the pump beam and 1.4 mW for the probe beam. The pump beam goes through a delay line, and drives the large aperture photoconductive antenna, which emits THz radiation. The antenna consists of a semi-insulating GaAs wafer between gold electrodes. A bias voltage of 5 kV is applied between the electrodes. THz radiation passes through the sample and is focused onto an Electro-Optic crystal by two polyethylene lenses to form the image for the sample. Probe beam is expanded and is linearly polarized

Working (contd.) The probe beam is then led into the same optical axis as the THz radiation by beam splitter. When the probe beam passes through the EO crystal, the polarization is changed by the birefringence that is induced by the THz radiation An analysing polarizer, with the transmission axis crossed with respect to the first polarizer, is placed after the EO crystal; Only the intensity change in the crossed polarization components can be detected by the CCD camera In this way, the camera can capture THz images

THz APPLICATION OF SCIENCE AND TECHNOLOGY Application in Biomedicine Safety Monitoring and Quality Control Nondestructive Testing Astronomy and Atmospheric Research Military Applications Chemical and Biological Agent Detection

Biomedicine, Safety Monitoring and Quality Control Applications Organisms has a unique response to THz wave therefore, can be used for disease diagnosis, organisms detection and imaging THz wave can also be applied to computer- assisted tomography THz radiation can also be used for detecting pollutants, biological and chemical detection, and therefore can be used to monitor the process of food preservation and food processing. The characteristics of THz wave, penetrating objects and security, can be used for non-contact, non-injury to detect specific substances, such as hidden explosives, drugs, weapons,

Nondestructive Testing & Military Applications The safety and penetrable properties of THz wave can be used for nondestructive testing of building. THz imaging has been selected as the one of four type of future technology of NASA to detect the defects in space shuttle THz technology has applications in the military field: ◦ THz non­destructive detection ◦ Anti-stealth THz ultra-wideband radar..

THz radar imaging of military targets.