Di Bartolo / Forte Advances in Spectroscopy for Lasers and Sensing

2006
ISBN: 978-1-4020-4789-3
Verlag: Springer Netherland
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, Band 231, 588 Seiten, eBook

Reihe: NATO Science Series II: Mathematics, Physics and Chemistry

This volume presents the Proceedings of the Institute “New Development in Optics and Related Fields,” held in Erice, Sicily, Italy, from the 6th to the 21st of June, 2005. This meeting was organized by the International School of Atomic and Molecular Spectroscopy of the “Ettore Majorana” Center for Scientific Culture. The purpose of this Institute was to provide a comprehensive and coherent treatment of the new techniques and contemporary developments in optics and related fields. Several lectures of the course addressed directly the technologies required for the detection and identification of chemical and biological threats; other lectures considered the possible applications of new techniques and materials to the detection and identification of such threats. Each lecturer developed a coherent section of the program starting at a somewhat fundamental level and ultimately reaching the frontier of knowledge in the field in a systematic and didactic fashion. The formal lectures were complemented and illustrated by additional seminars and discussions. The course was addressed to workers in spectrosco- related fields from universities, laboratories and industries. Senior scientists were encouraged to participate. The Institute provided the participants with an opportunity to present their research work in the form of short seminars or posters. The secretary of the course was Ottavio Forte.

Zielgruppe

Research

Autoren/Hrsg.

Di Bartolo, Baldassare

Forte, Ottavio

Inhaltsverzeichnis

Lectures.- Spectroscopy of photonic atoms: a means for ultra-sensitive specific sensing of bio-molecules.- Laser sources for high resolution sensing.- Relaxation and decoherence: what's new?.- Challenges for current spectroscopy: detection of security threats.- Sources for threat detection.- Yb3+-doped CaF2 fluoride as an example of our research approach in solid-state laser-type crystals.- Terahertz sensing and measuring systems.- Ultrafast spectroscopy of biological systems.- Luminescence spectroscopy of solids: localized systems.- Laser spectrometers for atmospheric analysis.- Sensitive detection techniques in laser spectroscopy.- Luminescence spectroscopy of solids: delocalized systems.- Laser induced breakdown spectroscopy (LIBS).- Optical spectroscopy of semiconductor quantum structures.- ZnO rediscovered – once again!?.- Coherent spectroscopy of semiconductor nanostructures.- Dynamics of upconversion in laser crystals.- Waveguide fabrication methods in dielectric solids.- Spectral properties of films.- Basic physics of semiconductor laser.- Judd-Ofelt theory: principles and practices.- Periodic dielectric and metallic photonic structures.- Cooling and trapping of atoms.- Non-linear propagation of femtosecond terawatt laser pulses in air and applications.- Spectroscopy of the gap states in Ge based on its neutron transmutation doping kinetics.- Interdisciplinary lectures.- The status of unified theories of fundamental interactions.- Angular momentum of the human body.- Climate change and global security.- Seminars.- Seminars.- Posters.- Posters.

Leseproben

TERAHERTZ SENSING AND MEASURING SYSTEMS (p. 103)

JOHN W. BOWEN

Cybernetics
The University of Reading
Whiteknights, Reading
RG6 4EU, United Kingdom
[e-mail],

1. Introduction

The terahertz (THz) region of the electromagnetic spectrum extends from 100 GHz to 10 THz, a wavelength range of 3 mm to 30 µm. While it is the last part of the electromagnetic spectrum to be fully explored, systems operating in this frequency range have a multitude of applications, in areas ranging from astronomy and atmospheric sciences to medical imaging and DNA sequencing.

As the terahertz range lies between the microwave and infrared parts of the spectrum, techniques from both of these neighbouring regions may be extended, specially adapted and at these frequencies has traditionally been difficult and expensive because of the low power available from sources and the precision machining required in their fabrication. Recently, new techniques based on the generation and detection of terahertz radiation using ultra-fast pulsed lasers have been developed and these have led to exciting advances in terahertz imaging and spectroscopy. The output frequencies of quantum cascade lasers have also been steadily moving down into the terahertz range and may lead to compact solid-state terahertz sources in the near future.

This paper will cover techniques for the generation, detection and analysis of terahertz radiation. After a discussion of quasi-optical techniques for the design of terahertz systems, the operation of three exemplar terahertz systems will be explained. Following a summary of terahertz sources and detectors, an overview of the operation of terahertz systems for a wide range of sensing and measuring applications will be given.

2. Waveguides versus Quasi-optics

At microwave frequencies, systems are often based around hollow metal pipe waveguides, typically of rectangular or circular cross-section, which provide a means of controlled propagation of the radiation. This approach may be carried over into the terahertz region, although, School of Systems Engineering

hybridised for the generation, detection and analysis of terahertz radiation. However, operation as the frequency increases, the skin depth in metals decreases and obtaining a surface finish on the inside walls of the waveguide sufficient to keep propagation loss at an acceptable level becomes increasingly difficult. Exacerbating this is the fact that the cross-sectional dimensions of a single-moded (i.e. dispersionless) waveguide must be reduced as the frequency is increased. This makes hollow metal waveguide difficult and expensive to manufacture for the terahertz region using conventional machining techniques. Terahertz hollow metal waveguide structures fabricated using thick resist photolithographic micro-machining [1] have been demonstrated to frequencies as high as 1.6 THz, but the fabrication technique is still in its infancy. The use of other types of transmission line have been explored, for example dielectric waveguide and planar transmission lines such as microstrip, but losses confine their use to the lower frequency end of the terahertz range, most having an upper usable frequency of about 150 GHz.

As an alternative, many systems operating at terahertz frequencies make use of optical components, such as lenses and reflectors, to control and manipulate beams travelling through free-space. The advantages of this approach in comparison to waveguide-based systems include: lower loss, wider (multi-octave) bandwidth, easier fabrication and, therefore, lower cost. However, unlike optical systems in the visible part of the spectrum, where optical components typically have lateral dimensions which are tens of thousands times the wavelength, practical size constraints limit the size of terahertz optical components to only a few tens of wavelengths. Therefore, diffraction becomes a significant aspect of propagation and must be taken into account in the design of optical systems, particularly as optical components often have to be used in the transition region between the near- and far-fields. Optical systems operating in this regime, where geometrical optics ceases to hold, are termed quasi-optical. Some of the sources and detectors used at terahertz frequencies are small compared to the wavelength and so, in order to efficiently couple power between them and a well directed freespace beam, it is necessary to use an antenna. Often, the best performance is achieved when the source or detector is mounted inside a hollow metal waveguide and a horn antenna is used to launch a beam through a quasi-optical system. Therefore, while quasi-optical propagation is to be preferred over long runs of waveguide, it is quite common for systems to include some short lengths of waveguide as well as quasi-optics.