Advances in Spectroscopy for Lasers and Sensing (eBook)

CONTENTS 7
PREFACE 20
LIST OF PAST INSTITUTES 23
SPECTROSCOPY OF PHOTONIC ATOMS: A MEANS FOR ULTRA-SENSITIVE SPECIFIC SENSING OF BIO-MOLECULES 26
LASER SOURCES FOR HIGH RESOLUTION SENSING 44
RELAXATION AND DECOHERENCE: WHAT’S NEW? 58
CHALLENGES FOR CURRENT SPECTROSCOPY: DETECTION OF SECURITY THREATS 88
SOURCES FOR THREAT DETECTION 98
Yb3+ 2 APPROACH IN SOLID-STATE LASER-TYPE CRYSTALS-DOPED CaF FLUORIDE AS AN EXAMPLE OF OUR RESEARCH 108
TERAHERTZ SENSING AND MEASURING SYSTEMS 128
ULTRAFAST SPECTROSCOPY OF BIOLOGICAL SYSTEMS 144
LUMINESCENCE SPECTROSCOPY OF SOLIDS: LOCALIZED SYSTEMS 154
LASER SPECTROMETERS FOR ATMOSPHERIC ANALYSIS 182
SENSITIVE DETECTION TECHNIQUES IN LASER SPECTROSCOPY 212
LUMINESCENCE SPECTROSCOPY OF SOLIDS: DELOCALIZED SYSTEMS 232
LASER INDUCED BREAKDOWN SPECTROSCOPY (LIBS) 254
OPTICAL SPECTROSCOPY OF SEMICONDUCTOR QUANTUM STRUCTURES 280
ZnO REDISCOVERED – ONCE AGAIN!? 302
COHERENT SPECTROSCOPY OF SEMICONDUCTOR NANOSTRUCTURES 320
DYNAMICS OF UPCONVERSION IN LASER CRYSTALS 358
WAVEGUIDE FABRICATION METHODS IN DIELECTRIC SOLIDS 360
SPECTRAL PROPERTIES OF FILMS 376
BASIC PHYSICS OF SEMICONDUCTOR LASERS 390
JUDD-OFELT THEORY: PRINCIPLES AND PRACTICES 428
PERIODIC DIELECTRIC AND METALLIC PHOTONIC STRUCTURES 460
COOLING AND TRAPPING OF ATOMS 484
NON-LINEAR PROPAGATION OF FEMTOSECOND TERAWATT LASER PULSES IN AIR AND APPLICATIONS 486
SPECTROSCOPY OF THE GAP STATES IN Ge BASED ON ITS NEUTRON e TRANSMUTATION DOPING KINETICS 508
INTERDISCIPLINARY LECTURES THE STATUS OF UNIFIED THEORIES OF FUNDAMENTAL INTERACTIONS 524
ANGULAR MOMENTUM OF THE HUMAN BODY 538
CLIMATE CHANGE AND GLOBAL SECURITY 540
SEMINARS 564
UV-LASER IRRADIATION OF AMORPHOUS SIO2: GENERATION AND CONVERSION OF POINT DEFECTS AND POST-IRRADIATION PROCESSES 564
MAGNETO-OPTICAL SPECTROSCOPY AND SPIN INJECTION 566
NEW CRYSTALS OF SARCOSINE COMPLEXES: STRUCTURE, VIBRATIONAL SPECTRA AND PHASE TRANSITIONS 566
SCATTERING STATE SPECTROSCOPY AS A PROBE OF CHEMICAL REACTION DYNAMICS AND NON-RADIATIVE ENERGY TRANSFER: LI(NP) + M SYSTEM 567
OPTICAL AND ELECTRICAL INVESTIGATION OF THE PHOTOREFRACTIVE PROPERTIES OF HAFNIUM-DOPED CONGRUENT LITHIUM NIOBATE SINGLE CRYSTALS. 568
EFFECT OF THE COMPOSITION ON SPECTROSCOPIC AND STRUCTURE OF TM3+: TEO2-PBF2 GLASSES 568
FOURIER TRANSFORM INFRARED SPECTROSCOPY ON ZN1-XMNXSE EPILAYERS 569
FEMTOSECOND LASER DISSECTION OF NEURONS IN C. ELEGANS 570
ULTRAFAST DYNAMICS OF METALLIC THIN FILMS 570
POSTERS 571
RAMAN SCATTERING OF MG2SI PRODUCED BY IBS 571
OPTICAL PROPERTIES OF FEW AND SINGLE ZNO NANOWIRES 571
INVESTIGATIONS OF A FADOF SYSTEM AS AN EDGE-FILTER RECEIVER FOR A BRILLOUIN-LIDAR FOR REMOTE SENSING OF THE OCEAN TEMPERATURE 571
RAMAN STUDY OF THE EFFECT OF THE ANNEALING ON ION-BEAM 
572 
AVOIDING THERMAL EFFECTS IN Z-SCAN MEASUREMENT BY HIGH REPETITION RATE LASERS 572
TOWARDS CONTROL OF PHOTODYNAMICS BY FEMTOSECOND POLARISATION SHAPING AND COINCIDENCE VELOCITY MAP IMAGING 573
LABORATORY FREQUENCY METROLOGY AND THE SEARCH FOR A TEMPORAL 
 
574 
DIPOLE STRENGTHS OF THE PHOTOSYNTHETIC PIGMENTS AND CAROTENOIDES FROM BACTERIA PROSTECOCHLORIS AESTUARI 574
PERSISTENT LUMINESCENCE OF LU2O3 :TB CERAMICS 575
SPECTROSCOPIC MONITORING OF THE FORMATION OF DISPERSED MANGANESE OXIDE ON ALUMINA SURFACE 575
TRANSIENT LOCAL STRUCTURE OF SOLVATED TRANSITION METAL COMPLEXES PROBED BY PICOSECOND TIME-RESOLVED XAFS 576
STRUCTURAL AND LUMINESCENCE PROPERTIES OF EUROPIUM DOPED BA1-XSRX R TIO3 NANOCRYSTALLITES PREPARED BY DIFFERENT METHODS 577
WAVEGUIDE STRUCTURES WRITTEN WITH FS-LASER PULSES ABOVE THE CRITICAL SELF-FOCUSING THRESHOLD IN SIO2-PBO BASED GLASSES 578
THE LATTICE RESPONSE OF QUANTUM SOLIDS TO AN IMPULSIVE LOCAL PERTURBATION 578
APPLICATION OF IN-PLANE TRANSPORT IN GAAS-BASED NANOSTRUCTURES FOR BROAD BAND AND SELECTIVE THZ SENSING 579
MICROSTRUCTURE AND PROPERTIES OF PERIODIC MULTILAYER THIN-FILM STRUCTURES WITH NANOCRYSTALS 580
STRUCTURAL AND OPTICAL STUDIES OF ERBIUM DOPED NANOCRYSTALLINE SILICON THIN FILMS PRODUCED BY R.F. SPUTTERING 580
THIN FILMS PRODUCED BY R.F. SPUTTERING MICROSTRUCTURE AND THERMAL FEATURES OF A-SI:H AND NC-SI:H 581
CONFOCAL MICROSCOPY FOR INVESTIGATIONS ON LITHIUM FLUORIDE COLOUR CENTERS 582
CAVITY RING-DOWN SPECTROSCOPY AS A DETECTOR IN LIQUID CHROMATOGRAPHY 582
OPTOELECTRONIC DEVICES USING FEMTOSECOND LASER MICROSTRUCTURED SILICON 583
INDEX 585

TERAHERTZ SENSING AND MEASURING SYSTEMS (p. 103)

JOHN W. BOWEN

Cybernetics
The University of Reading
Whiteknights, Reading
RG6 4EU, United Kingdom
cybjb@cyber.reading.ac.uk,

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.