Terahertz radiation
Terahertz radiation, also known as submillimeter lies in the electromagnetic spectrum between infrared and microwave radiation. It is sometimes also attributed to the far infrared. At a wavelength of less than 1 mm ( = 1000 microns ) and greater than 100 microns, the frequency range is 300 GHz (3 x 1011 Hz ) to 3 THz ( 3 × 1012 Hz). It is located in the border area, the RF heterodyne receiver almost no more, but not yet cover optical sensors.
- 3.1 spectroscopy
- 3.2 Non-destructive testing of materials
- 3.3 Communication
- 3.4 Security Technology
- 3.5 Biology and Medicine
- 3.6 Astronomy
- 3.7 Time-resolved measurements
Properties
Since the terahertz radiation was not long or very limited use, they also spoke of the terahertz gap in the electromagnetic spectrum. This band gap is located between the frequency range that was developed by the classical microwave technology, and the infrared frequency range. The main problem of the use of the terahertz frequency range is the production of transmitters and receivers. Compact and cost- stations with sufficient output power are not yet available today. The receiver technology needs further development in order with more sensitive receivers to be able to still detect weaker signals. With a Golay cell, one can detect terahertz radiation.
Terahertz radiation penetrates many materials such as paper or plastic or organic tissue, but acts due to the low photon energy - in the range of a few milli - electron volts - not ionizing. In this energy range there are many molecular rotations, which makes terahertz radiation for spectroscopy very interesting to identify specific substances. Water, other polar substances and metals absorb the rays and can heat up as a result. Applications especially in the fields of medicine and biology are set by the strong water absorption limits, even high humidity represents a challenge for some applications
Technology
Continuous terahertz radiation
Every body emits thermal radiation, including in the terahertz range. Since this radiation is incoherent, such a station must be considered as a noise source. To the very low noise powers that emit body according to Planck's radiation law, detect, highly sensitive radiometric measuring devices are used. Radiometer can be (usually at 4 K ) constructed here both uncooled and cooled. When cooled radiometers is usually resorted to superconducting mixer elements such as bolometers or SIS mixer. Uncooled radiometers also GaAs Schottky diodes may be used.
In the production of coherent THz radiation different transmitters are used. In addition to generating terahertz power by frequency multiplication (usually with the help of GaAs Schottky diodes) or difference frequency generation of two laser signals ( for example, distributed feedback lasers ) to nonlinear devices exist quantum cascade lasers, molecular gas lasers, free electron lasers, optical parametric oscillators and backward wave oscillators. If a high -frequency tuning range needed frequently photomixer (Low -Temperature - Grown GaAs, Uni- traveling- carrier photodiode, ni - pn -ip superlattice photodiodes ) are used, which convert the difference frequency of two lasers in alternating current which eventually radiated by a suitable antenna.
Pulsed terahertz radiation
Ultrashort laser pulses with a duration of a few femtoseconds ( 1 fs = 10-15 s ) can produce semiconductors or non-linear optical materials terahertz pulses in the picosecond ( 1 ps = 10-12 s ). This terahertz pulses consist of only one or two cycles of the electromagnetic wave. By electro- optical methods, they can also coherently, that is a time-resolved, are measured.
Applications
Spectroscopy
Terahertz spectroscopy substances with weak bonds, such as hydrogen bonds, or bonds with heavy binding partners, such as collective excitation of atomic associations, which are phonons in crystals.
Non-destructive testing of materials
Since many everyday materials such as paper, plastics, or ceramics are permeable to terahertz radiation, others such as metals or water but not complement terahertz pictures other methods such as optical or X-ray images. Moreover, it is possible to obtain spatially resolved spectroscopic information. This makes it possible to make defects in the interior of a body visible and measure without having to destroy these, also called non-invasive (mostly in the medical field ) or antidestruktive method.
Communication
Wireless communication plays today in many areas of life a major role (see radio network ) and typically operates at carrier frequencies in the microwave range. WLANs or cellular (LTE -Advanced ) to achieve transfer rates of several 100 Mbit / s - in principle about 10 Gbit / s are possible. The frequency spectrum up to 275 GHz is heavily regulated and offers too little unused bandwidth to meet the increasing demand ( doubling every 18 months ) to meet in the future.
The THz radiation offers itself, because frequencies between 300 GHz and 1 THz is still no regulation and higher carrier frequencies with large bandwidths ( 10-100 GHz) data rates with more than 100 Gbit / s allow. In scientific experiments, data rates of 24 Gbit / s at 300 GHz and 100 Gbit / s have already been shown ( 4 channels ) at 237.5 GHz. The heterodyning technique allows the use of different carrier frequencies below 1 THz and could be the medium of interest for commercial microwave links. These systems are too large and too expensive for private use.
The water vapor in the atmosphere absorbs THz radiation and limit its spread. Below 1 THz, however, there are three frequency windows with an attenuation of less than 60 dB / km, which can be used for telecommunication. Beyond 1 THz absorption ( water vapor and other atmospheric gases ) in the atmosphere rises too strong to implement systems with high data rates. This restriction defines the possible application areas: Attenuation in the atmosphere plays in data communication indoors no major role and the need for higher bandwidth increases (including HD videos, streaming) constantly. Outside the connect homes to the Internet ( last mile ) or backhaul links in the mobile sector are conceivable. A further possibility is the inter-satellite communication or a satellite-based internet for aircraft. The limited range and low penetration of receivers could make the technology in respect of interest on interceptibility for military purposes.
In addition to the availability of compact, powerful and inexpensive sources and receivers, the special properties of terahertz radiation must be examined more closely. In buildings reflections from surfaces and multilayer systems and scattering processes play a major role. In the strong directional dependence - especially with optimized antennas - vary the submillimeter by the microwaves.
Safety Technology
When searching for explosives or drugs unknown substances in the body or containers could be identified, since they have in excess of 500 GHz characteristic absorption spectra. The measurements were hitherto often performed under laboratory conditions, ie under idealized conditions: absorption measurements in transmission ( good signal -to-noise ratio), sheer fabric samples or at low temperatures ( sharper spectra). The challenges of a possible implementation are as follows: From 500 GHz, the atmosphere absorbs much more strongly, clothing is largely transparent, but at the interfaces reflections occur in the materials it comes to scattering processes. With several layers of clothing, the signal is very weak. In mixtures overlap the absorption spectra and the identification is difficult. The surface structure also affects the reflection behavior. Therefore, many scientists express extremely critical to a simple implementation.
In addition to body scanners, there are other applications in the security industry, the implementation is simpler. Mail could be scanned for dangerous or banned substances out additives in explosives could provide and help you determine the origin of conclusions about the manufacturing process. Drugs could be checked for authenticity, or whether the drugs have changed during storage ( through the package ).
The biggest obstacle is currently the lack of inexpensive, compact and tunable THz sources.
Biology and Medicine
In biology and medicine is used that terahertz radiation the water content of a sample maps to distinguish, for example, tumors from healthy tissue. In this way, also the amount of combustion disease can be determined, for which previously there were only very limited methods for combustion diagnosis. The coherent measurement of the terahertz pulses, the thickness of a sample can be determined by measuring the time delay is measured that has experienced when passing through the sample of the pulse. This mapping methods are for the most part only laboratory applications. The first commercial devices are indeed available, but have not yet become established in practice.
Astronomy
In astronomy, the terahertz radiation opens up new possibilities. Thus, for example, measures the ESA in this way, the surface temperature of the earth. Also, detection of simple chemical compounds such as carbon monoxide, water, hydrogen cyanide and other is possible by measurement of the emissions resulting from rotational transitions of the molecules in the terahertz range. Such instruments ( for example, German Receiver for Astronomy at Terahertz Frequencies, Great ) to be incorporated into the SOFIA airborne telescope. Also the Herschel space telescope is equipped with appropriate instruments.
Thermal radiation of a body with earth temperature of T = 287 K
1.5 THz broadband spiral antenna for astronomy
Time-resolved measurements
Terahertz pulses often have a duration of less than one picosecond and therefore suitable for the measurement of physical or chemical processes on this time scale. To the material to be investigated is excited by a short laser pulse as well. The change of the transmission of the THz pulse is measured as a function of the time which has elapsed since excitation. An important example of these so-called pump-probe measurements to study the dynamics of charge carriers in semiconductors.