Principles of terahertz science and technology

346 Pages·2009·9.712 MB·English

by  Yun-Shik Lee

Yun-Shik Lee

Principles of Terahertz Science
and Technology
Yun-Shik Lee
Physics Department
Oregon State University
Corvallis, Oregon 97331
USA

ISBN 978-0-387-09539-4 e-ISBN 978-0-387-09540-0

Library of Congress Control Number: 2008935382

 2009 Springer Science+Business Media, LLC
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To my parents Su-Ho Lee and Soon-Im Shin
Preface
Over the last two decades, THz technology has ripened enough that a thor-
ough summary and review of the relevant topics is in order. Many different
disciplines such as ultrafast spectroscopy, semiconductor device fabrication,
and bio-medical imaging involve the recent development of THz technology.
Itisanimportanttasktolaydownacommongroundamongtheresearchers,
so that they can communicate smoothly with one another. Besides, the THz
communityisgrowingfastandtheTHztechnologyisinatransitionalperiod.
The THz research activities have mainly focused on generation and detection
until lately, but the focal point has shifted to the practical applications such
as high-speed communication, molecular spectroscopy, security imaging, and
medical diagnosis, among many others.
Thisbookcoversabroadrangeoftopicsandfundamentalissues.Individu-
alsfromdistinctdisciplineshavehelpeddevelopingnewTHztechnologies,and
in order to reach the next level, i.e., practical applications, the technology re-
quiresits researchersto understand and communicatewith one another. This
bookservesthisgeneralpurposebyprovidingtheresearcherswithacommon
reference, thus bridging “the THz gap”.
I have tried to elucidate the fundamentals of THz technology and science
for their potential users. This book surveys major techniques of generating,
detecting,andmanipulatingTHzwaves.Italsodiscussesanumberofessential
processes where THz waves interact with physical, chemical, and biological
systems. Scientists and engineers of various disciplines realize that the THz
gap in the electromagnetic spectrum is now accessible thanks to the recent
advances in THz source and detection technologies. Many are seeking ways
by which they can incorporate the new technologies into their expertise and
research agenda. Younger researchers, who wish or are to join THz research
groups, would also find this new field challenging due to many barriers, the
lackofcomprehensiveintroductionand/orinstructionamongthem.Potential
users of THz technology should be prepared in the essential concepts and
techniquesofTHzscienceandtechnology;Ihopethisbookbeanintroductory
guide for the new comers.
viii Preface
Duringtheprocessofwritingthisbook,manycolleagues,friends,andstu-
dentsgavemeworthycriticismandintroduction.Althoughitisimpossibleto
acknowledge all scientific contributions, I am deeply grateful of those whose
works I use in this book. I am much obliged to Joe Tomaino, Andy Jameson,
and Jeremy Danielson for their invaluable advice. I am indebted to the Na-
tional Science Foundation and the Alexander von Humboldt-Foundation for
their generous support. Finally, I thank my wife, JungHwa, for her support
in every possible ways.
Corvallis
September 2008 Yun-Shik Lee
Contents
1 Introduction............................................... 1
1.1 Terahertz Band ......................................... 1
1.2 Terahertz Generation and Detection ....................... 3
1.2.1 Terahertz Sources ................................. 3
1.2.2 Terahertz Detectors................................ 5
1.3 Terahertz Applications ................................... 7
2 Basic Theories of Terahertz Interaction with Matter....... 11
2.1 Electromagnetic Waves in Matter.......................... 11
2.1.1 The Wave Equation ............................... 12
2.1.2 Reflection and Transmission ........................ 14
2.1.3 Coherent Transmission Spectroscopy................. 17
2.1.4 Absorption and Dispersion ......................... 19
2.1.5 Plasma Frequency ................................. 21
2.1.6 Electric Dipole Radiation........................... 21
2.1.7 Quasi-Optical Propagation in Free Space ............. 24
2.2 Terahertz Radiation and Elementary Excitations ............ 28
2.2.1 Quantum Theory of Electric Dipole Interaction ....... 28
2.2.2 Energy Levels of Hydrogen-like Atoms ............... 34
2.2.3 Rotational and Vibrational Modes of Molecules ....... 36
2.2.4 Lattice Vibrations ................................. 43
2.3 Laser Basics ............................................ 47
3 Generation and Detection of Broadband Terahertz Pulses . 51
3.1 Ultrafast Optics......................................... 51
3.1.1 Optical Pulse Propagation in Linear and Dispersive
Media............................................ 51
3.1.2 Femtosecond Lasers................................ 54
3.1.3 Time-resolved Pump-Probe Technique ............... 58
3.1.4 Terahertz Time-Domain Spectroscopy................ 59
x Contents
3.2 Terahertz Emitters and Detectors Based on Photoconductive
Antennas............................................... 59
3.2.1 Photoconductive Antenna .......................... 59
3.2.2 Generation of Terahertz Pulses from Biased
Photoconductive Antennas ......................... 61
3.2.3 Substrate Lenses: Collimating Lens and Hyper-
Hemispherical Lens ................................ 67
3.2.4 Terahertz Radiation from Large-Aperture
Photoconductive Emitters .......................... 70
3.2.5 Time-Resolved Terahertz Field Measurements with
Photoconductive Antennas ......................... 74
3.3 Optical Rectification..................................... 76
3.3.1 Nonlinear Optical Interactions with
Noncentrosymmetric Media......................... 77
3.3.2 Second-Order Nonlinear Polarization and
Susceptibility Tensor............................... 80
3.3.3 Wave Equation for Optical Rectification.............. 84
3.3.4 Dispersion at Optical and Terahertz Frequencies ...... 87
3.3.5 Absorption of Electro-Optic Crystals at the Terahertz
Frequencies....................................... 90
3.4 Free-Space Electro-Optic Sampling ........................ 92
3.5 Ultrabroadband Terahertz Pulses.......................... 98
3.5.1 Optical Rectification and Electro-Optic Sampling...... 98
3.5.2 Photoconductive Antennas .........................101
3.6 Terahertz Radiation from Electron Accelerators .............103
3.7 Novel Techniques for Generating Terahertz Pulses ...........106
3.7.1 Phase-MatchingwithTiltedOpticalPulsesinLithium
Niobate ..........................................106
3.7.2 Terahertz Generation in Air ........................108
3.7.3 Narrowband Terahertz Generation in Quasi-Phase-
Matching Crystals.................................109
3.7.4 Terahertz Pulse Shaping ...........................112
4 Continuous-Wave Terahertz Sources and Detectors ........117
4.1 Photomixing............................................117
4.2 Difference Frequency Generation and Parametric Amplification122
4.2.1 Principles of Difference Frequency Generation.........123
4.2.2 Difference Frequency Generation with Two Pump
Beams ...........................................125
4.2.3 Optical Parametric Amplification....................129
4.3 Far-Infrared Gas Lasers ..................................132
4.4 P-Type Germanium Lasers ...............................133
4.5 Frequency Multiplication of Microwaves ....................136
4.6 Quantum Cascade Lasers.................................138
4.6.1 Lasing and Cascading..............................139
Contents xi
4.6.2 Prospective Development...........................140
4.7 Backward Wave Oscillators ...............................141
4.8 Free-Electron Lasers .....................................144
4.8.1 Operational Principles .............................144
4.8.2 Free Electron Laser Facilities .......................146
4.9 Thermal Detectors.......................................146
4.9.1 Bolometers .......................................147
4.9.2 Pyroelectric Detectors .............................151
4.9.3 Golay Cells.......................................154
4.10 Heterodyne Receivers ....................................155
5 Terahertz Optics...........................................159
5.1 Dielectric Properties of Solids in the Terahertz Region .......159
5.2 Materials for Terahertz Optics ............................161
5.2.1 Polymers.........................................162
5.2.2 Dielectrics and Semiconductors......................164
5.2.3 Conductors .......................................168
5.3 Optical Components .....................................170
5.3.1 Focusing Elements.................................170
5.3.2 Antireflection Coatings.............................171
5.3.3 Bandpass Filters ..................................172
5.3.4 Polarizers ........................................174
5.3.5 Wave Plates ......................................175
5.4 Terahertz Waveguides....................................177
5.4.1 Theory of Rectangular Waveguides ..................177
5.4.2 Hollow Metallic Tubes .............................179
5.4.3 Dielectric Fibers ..................................181
5.4.4 Parallel Metal Plates ..............................183
5.4.5 Metal Wires ......................................184
5.5 Artificial Materials at Terahertz Frequencies ................189
5.5.1 Metamaterials ....................................189
5.5.2 Photonic Crystals .................................194
5.5.3 Plasmonics .......................................202
5.6 Terahertz Phonon-Polaritons..............................211
6 Terahertz Spectroscopy of Atoms and Molecules...........215
6.1 Manipulation of Rydberg Atoms...........................215
6.2 Rotational Spectroscopy..................................220
6.2.1 Basics of Rotational Transitions.....................220
6.2.2 High-Resolution Spectroscopy.......................222
6.2.3 Atmospheric and Astronomical Spectroscopy..........224
6.3 Biological Molecules .....................................232
6.3.1 Liquid Water .....................................233
6.3.2 Normal Modes of Small Biomolecules ................236
6.3.3 Dynamics of Large Molecules .......................248
xii Contents
7 T-Ray Imaging ............................................259
7.1 Introduction ............................................259
7.2 Imaging with Broadband THz Pulses ......................261
7.2.1 Amplitude and Phase Imaging ......................261
7.2.2 Real-Time 2-D Imaging ............................264
7.2.3 T-Ray Tomography................................266
7.3 Imaging with Continuous-Wave THz Radiation..............273
7.3.1 Raster-Scan Imaging...............................274
7.3.2 Real-Time Imaging with a Microbolometer Camera ....278
7.4 Millimeter-Wave Imaging for Security ......................281
7.4.1 Active Imaging....................................282
7.4.2 Passive Imaging...................................284
7.5 Medical Applications of T-Ray Imaging ....................288
7.5.1 Optical Properties of Human Tissue .................288
7.5.2 Cancer Diagnostics ................................289
7.5.3 Reflective Imaging of Skin Burns ....................292
7.5.4 Detection of Dental Caries..........................294
8 Terahertz Spectroscopy of Condensed Matter..............295
8.1 Intraband Transitions in Semiconductors ...................295
8.1.1 Band Structure of Intrinsic Semiconductors...........296
8.1.2 Photocarrier Dynamics.............................297
8.1.3 Impurity States ...................................299
8.1.4 Semiconductor Nanostructures: Quantum Wells,
Quantum Wires, and Quantum Dots.................302
8.2 Strongly Correlated Electron Systems ......................311
8.2.1 Quasiparticle Dynamics in Conventional
Superconductors ..................................312
8.2.2 Low Energy Excitations in High Temperature
Superconductors ..................................317
References.....................................................327
Index..........................................................337
1
Introduction
Terahertz (THz) radiation is electromagnetic radiation whose frequency lies
between the microwave and infrared regions of the spectrum. We cannot see
THz radiation, but we can feel its warmth as it shares its spectrum with
far-infrared radiation. Naturally occurring THz radiation fills up the space
of our everyday life, yet this part of the electromagnetic spectrum remains
the least explored region mainly due to the technical difficulties involved in
makingefficientandcompactTHzsourcesanddetectors.Thelackofsuitable
technologies led to the THz band being called the “THz gap”. This techno-
logical gap has been rapidly diminishing for the last two decades. Optical
technologies have made tremendous advances from the high frequency side,
while microwave technologies encroach up from the low frequency side. This
chaptergivesabriefperspectiveonthebasicpropertiesofTHzradiationand
its interaction with materials, which lays down the foundation to discuss the
progress of THz science and technology in subsequent chapters.
1.1 Terahertz Band
“Terahertzradiation”isthemostcommontermusedtorefertothisfrequency
band, analogous to microwaves, infrared radiation, and x-rays. It is rather
awkwardtouseafrequencyunitfornamingaspectralband.Nevertheless,as
“terahertz” has become a symbolic word designating the entire field, we will
universally use this term throughout this book. An alternative and seemingly
betterterminologyis“T-rays”,where“T”standsforterahertz.Itwasinitially
coined for an imaging technique, and will be used in Chapter 7, which is
dedicated to T-ray imaging technologies and applications.
Until quite recently, THz technologies had been independently developed
by researchers from several different disciplines. In practice, different com-
munities use different units to describe the spectrum of THz radiation. We
will use THz (1012 Hz) as the universal unit in this book, but other units