Sensing with terahertz radiation

One aspect of the field of THz radiation is the marriage of microwave and optical techniques. By its very nature, THz radiation bridges the gap be tween the microwave and optical regimes. The former can be characterized by the fact that most devices are comparable in size to the wavelength of the radiation. As a result, the propagation of energy in these devices is generally in the form of single-mode or low-order-mode guided waves. In contrast, the optical and infrared ranges are generally characterized by beams containing many modes, with dimensions much larger than the wavelength. Of course, there are exceptions to these rules, notably the single-mode propagation of optical radiation in fibers. Nonetheless, the general description holds true. Because of these fundamental differences, it is natural that the techniques used in their implementation are quite distinct. Much of the research in the THz field has been based on the melding of these disparate ideas.

Daniel R. Grischkowsky and Daniel MittlemanFrank C. De LuciaDaniel MittlemanZhiping Jiang and Xi-Cheng ZhangSean M. Duffy and Simon Verghese and K. Alex McIntoshR. Alan Cheville and Matthew T. Reiten and Roger McGowan and Daniel R. GrischkowskyMartin KochDaniel W. van der Weide

Introduction	p. 1
Spectroscopy in the Terahertz Spectral Region	p. 39
1 Introduction	p. 39
1.1 Radiation and Matter	p. 40
1.2 What Phenomena Fall in this Energy Range?	p. 40
1.3 Gases	p. 41
1.4 Liquids and Solids	p. 43
1.5 Applications and Impact of Terahertz Spectroscopy	p. 43
2 Theoretical Underpinnings	p. 45
2.1 Absorption Strengths	p. 45
2.2 Energy Levels and Transitions Frequencies	p. 47
2.3 The Character of Rotational Spectra	p. 47
2.4 Rotation-Vibration Spectra	p. 49
3 Spectroscopic Techniques and Results	p. 51
3.1 Harmonic Generation	p. 53
3.2 Sources Based on Mixing of Optical Sources	p. 61
3.3 Tunable Sideband Sources	p. 65
3.4 Electron Beam Sources	p. 68
3.5 Femtosecond Sources	p. 73
4 Applications	p. 76
4.1 Atmospheric Spectroscopy	p. 76
4.1.1 Microwave-Like Instruments	p. 78
4.1.2 Infrared-Like Instruments	p. 85
4.2 Astronomical Spectroscopy	p. 87
4.2.1 Some Telescope Facilities	p. 90
4.2.2 Two Representative THz Telescopes	p. 90
4.2.3 Examples of Other Results	p. 99
Acknowledgments	p. 107
References	p. 107
Terahertz Imaging	p. 117
1 Introduction	p. 117
2 Key Components of a THz Imaging System	p. 118
2.1 The Femtosecond Laser Source	p. 119
2.2 Optical Delay Line	p. 119
2.3 Terahertz Optoelectronic Switches	p. 120
2.4 Terahertz Beam Optics	p. 123
2.5 Polarization of the THz Beam	p. 128
2.6 Signal Acquisition	p. 129
2.7 Data Processing	p. 130
3 Imaging with THz-TDS	p. 131
3.1 Amplitude and Phase Imaging	p. 132
3.2 Terahertz Imaging of Liquid Water	p. 137
3.3 Processing for Amplitude and Phase Imaging	p. 139
3.4 Reflection Imaging	p. 139
3.5 Burn Diagnostics	p. 141
3.6 Terahertz Tomography: The Third Dimension	p. 143
3.7 Interferometric Tomography	p. 145
4 Future Prospects	p. 149
References	p. 149
Free-Space Electro-Optic Techniques	p. 155
1 Introduction	p. 155
2 Generation	p. 155
3 Detection	p. 159
3.1 Measurement Principle	p. 161
3.2 Measurement of Coherent Mid-Infrared Fields	p. 165
3.3 Parallel Measurement: Chirped-Pulse Measurement	p. 165
3.4 Parallel Measurement: Terahertz Streak Camera	p. 168
3.5 Parallel Measurement: 2D Imaging	p. 171
3.6 Near-Field Terahertz Imaging	p. 174
3.7 Detection Geometry and Working Conditions	p. 175
3.8 Comparison Between Photoconductive Sampling and EO Sampling	p. 177
3.9 EO Sampling for Continuous-Wave Terahertz Beams	p. 178
4 Applications	p. 178
4.1 Dynamics of Interaction of Lattice with Infrared Photons	p. 178
4.2 Spatiotemporal Coupling of Few-Cycle Pulses	p. 180
4.3 Point Scanning Terahertz Imaging	p. 182
4.4 Electro-Optic Terahertz Transceiver	p. 185
4.5 Compact System	p. 186
References	p. 187
Photomixers for Continuous-Wave Terahertz Radiation	p. 193
1 Introduction	p. 193
1.1 Demonstrated CW Technology for Terahertz Generation	p. 194
2 Photomixers: Principle of Operation	p. 194
2.1 Overview of Operation	p. 195
2.2 Lifetime of Carriers	p. 198
2.2.1 Lifetime Modified by Electric Field Profile	p. 201
2.3 External Quantum Efficiency	p. 202
2.4 Thermal Limits	p. 205
2.4.1 Improved Thermal Designs: Thin LTG GaAs on AlAs	p. 207
2.4.2 Improved Thermal Designs: Optically Resonant Cavity (DBR)	p. 208
2.5 Trade-offs for Enhanced Output Power	p. 209
2.6 Proven High-Power Methods	p. 211
3 Planar Antennas and Circuitry	p. 213
3.1 Electrode Capacitance	p. 213
3.2 Log-Spiral	p. 215
3.3 Single Full-Wave Dipoles	p. 215
3.4 Dual-Antenna Elements	p. 216
3.4.1 Dual-Dipole Elements	p. 216
3.4.2 Graphical Design Procedure	p. 218
3.4.3 Dual-Slot Elements	p. 220
3.5 Distributed Photomixers	p. 221
4 Hyperhemisphere Lens	p. 222
5 Photomixer Design and Examples	p. 224
5.1 Dual-Dipole Results	p. 224
5.2 Practical Measurement Issues and Power Calibration Difficulties	p. 226
5.3 Maximum Power Limitations	p. 227
6 Applications	p. 230
6.1 Local Oscillators	p. 230
6.2 Transceivers	p. 230
Acknowledgments	p. 233
References	p. 233
Applications of Optically Generated Terahertz Pulses to Time Domain Ranging and Scattering	p. 237
1 Introduction	p. 237
1.1 Perspective	p. 237
1.2 Theory	p. 238
1.3 Measurements	p. 241
1.4 Outline	p. 244
2 Experiment	p. 244
2.1 Overview of Experimental Configurations	p. 244
2.2 Generation and Detection of Terahertz Electromagnetic Transients	p. 246
2.3 Terahertz Beam Optics for Target Illumination	p. 251
2.4 Targets	p. 254
2.5 Scattered Radiation	p. 255
2.6 Bistatic Range	p. 256
3 Theory	p. 257
3.1 Scaling of Maxwell's Equations in Terahertz Impulse Ranging	p. 258
3.2 Transfer Function Description of Terahertz Impulse Ranging System	p. 259
3.3 Calculation in Time and Frequency	p. 262
3.4 Calculation of Scattering Coefficients	p. 264
3.4.1 Cylinders	p. 264
3.4.2 Spheres	p. 265
4 Measurements	p. 267
4.1 Conducting and Dielectric Cylinders	p. 267
4.2 Dielectric Spheres	p. 272
4.3 Data Analysis and the Geometrical Optics Model	p. 273
4.4 Angularly Resolved Scattering Measurements	p. 281
4.5 Gouy Phase Shift	p. 285
4.6 Realistic Targets	p. 287
5 Summary and Future Directions	p. 289
References	p. 290
Bio-medical Applications of THz Imaging	p. 295
1 Introduction	p. 295
1.1 Some General Remarks Regarding Biomedical Imaging	p. 295
1.2 The Closure of the Terahertz Gap	p. 297
2 Description of the Terahertz Imaging Setup	p. 298
3 Dendrochronology: Density Mapping of Wood	p. 300
4 Plant Physiology: Monitoring the Water Flow in Plants	p. 303
4.1 Clematis	p. 305
4.2 Mimosa	p. 307
5 Medical Imaging on Histopathological Samples	p. 308
5.1 Time-Domain Imaging	p. 309
5.2 Continuous-Wave Imaging	p. 311
6 Conclusion	p. 312
Acknowledgments	p. 312
References	p. 313
Electronic Sources and Detectors for Wideband Sensing in the Terahertz Regime	p. 317
1 Introduction	p. 317
2 Dual-Source Interferometer	p. 320
3 Description of DSI	p. 321
4 Analysis of Dual-Source Interferometer	p. 322
5 DSI Results from Dual-Source Interferometer	p. 324
6 Reflection Spectroscopy	p. 325
7 Coherent Signal Generation with Scanned Delay Lines	p. 328
8 Conclusions	p. 331
Acknowledgments	p. 331
References	p. 331
Index	p. 335

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