THz-Imaging on its way to industrial application

Transcription

1 THz-Imaging on its way to industrial application T. Pfeifer Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen niversity Manfred-Weck Building, Steinbachstraße 19, D Aachen, Germany Tel.: + 49 [241] Fax: +49 [241] t.pfeifer@wzl.rwth-aachen.de Abstract Terahertz radiation, which fills the gap between 100 GHz and 10 THz ( = 30 µm 3 mm) in the electro-magnetic spectrum, has seldom been used outside of astronomy and other scientific research. However, in recent years there has been a significant interest in investigating THz radiation for different new applications. Especially the ability of terahertz radiation to penetrate deep into many organic materials without the damage associated with ionizing radiation such as X-rays lead to recent interests chiefly in the fields of security technology and biomedical imaging. The attribute of many different materials to be transparent for terahertz radiation, was also the reason for many difficulties in practical applications outside of research. sing radiation that can pass through so many materials so well makes detection difficult. In addition, sources to generate light at terahertz frequencies have suffered from low output intensity and other problems. Since the 1990s, technical breakthroughs in sources and detectors have brought terahertz technology within striking distance of significant commercial markets [1]. The pressure to develop new terahertz sources arose from two dramatically different groups - ultrafast timedomain spectroscopists who wanted to work with longer wavelengths, and long wavelength radio astronomers who wanted to work with shorter wavelengths. Today there are continuous-wave (CW) sources available as well as pulsed sources [2]. The aim of this paper is to provide an overview of key scientific developments which currently represent the basics of the mentioned THz technology. Beginning with the working principle of opto-electronic THz sources and detectors, the paper explains different setups for transmitting and using THz radiation. Furthermore it shows different applications of different business branches and gives an outlook for industrial application in the fields of metrology and quality control. Keywords: THz-Radiation, THz-Imaging, THz-Applications 1. Introduction & motivation Terahertz radiation, which fills the gap between 100 GHz and 10 THz ( = 30 µm 3 mm) in the electro-magnetic spectrum, has seldom been used outside of astronomy and other scientific research. However, in recent years there has been a significant interest in investigating THz radiation for different new applications

2 Introduction Cosmological radiation [Hz] Aachen, 17. Juli Although the THz-Radiation was already known as part of the electromagnetic spectrum the radiation couldn t be used for technical application because of missing technical THz- Emitters. Basic principles The spectrum of electromagnetic radiation? radio waves microwaves THz infrared light V X-rays Frequency [Hz] = 3m = 300mm = 300µm = 300nm Terahertz-range: f = 0,1 THz 10 THz = 30 µm 3 mm Aachen, 17. Juli Basic principles The basic principle of emitting THz-radiation is based on the Auston switch - which is a semiconductor with conductive micrometersized metal lines lithographically defined on the top surface. A laser pulse incident on an absorbing semiconductor creates charge carriers in the conducting band. The acceleration of these carriers in an electrical bias field gives rise to a transient photocurrent that radiates electromagnetic waves in the THz-Range

3 hole e - Basic principles Technical generation of THz-radiation Photoconductive switch ~50 m low temperature grown LT-GaAs ~ 50 V E ~ 10 KV/cm laserintensity Laserintensität I(t) I(t) Stromdichte current density j(t) j(t) el. field E THz (t) Zeit time Aachen, 17. Juli Another possibility to emit continuous THz-Waves could be realized by mixing two laser sources in order to generate a beat carrier in the THz-Range. Basic principles : Generation and detection of THz-radiation Generation of continuous THz-radiation (cw) Photon mixing frequency 1 THz-wave (cw) + E THz frequency 2 beat GaAs Aachen, 17. Juli Imaging THz-systems As shown in the figure below the THz-Radiation could be used for Time domain Spectroscopy. Spectroscopic Analysis could be done by transforming the THz-Time-Signal into fourier space

4 THz-systems Time-domain-spectroscopy THz-spectrometer emitter A detector I (t) I (t + t) specimen E(t) t Aachen, 17. Juli Another interesting THz-System could be designed as shown in the figure below. In the same way as optical coherence tomography (OCT) is used to generate tomographic images for transparent materials T-Rays could be used for non transparent materials. THz-systems THz-tomography Measurement of layer thickness detector I (t + t) Zoom emitter I (t) A x y Aachen, 17. Juli Applications Much of the recent interest in terahertz radiation stems from its ability to penetrate deep into many organic materials without the damage associated with ionizing radiation such as X- rays. This property directly leads to applications in the field of safety engineering or biomedical diagnostics. The rising demand on wireless high speed data networks enables a totals different marked for THz-Technologies

5 Applications Markets for THz-technologies Markets for THz-technologies Notebook Base Mobile PDA Notebook safety engineering medical and biotechnologies wireless data transmission quality control Aachen, 17. Juli In aerospace industries THz-Imaging is already used for nondestructive testing and quality control. Applications Quality control in aerospace Example: fuel tank isolation segment THz-image defect Aachen, 17. Juli References 1. Terahertz Systems: Technology & Emerging Markets. A Thintri Market Study. Thintri Inc., Terahertz Radiation: Applications and Sources. Eric R. Mueller. The Industrial Physicist