Broadband On-chip Terahertz Spectroscopy Christopher Russell, BEng Submitted in accordance with the requirements for the degree of Doctor of Philosophy University of Leeds School of Electronic and Electrical Engineering August 2013 The candidate confirms that the work submitted is his own, except where work which has formed part of jointly authored publications has been included. The candidate confirms that the appropriate credit has been given within the thesis where reference has been made to the work of others. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement. © 2013 The University of Leeds and Christopher Russell i Abstract Abstract An enhancement of on-chip terahertz time domain spectroscopy (THz-TDS) systems for the analysis of polycrystalline materials has been made. An in depth review of planar Goubau lines is presented for which there are no analytically defined terms for transmission line parameters such as characteristic impedance and effective permittivity. Using a simulation package, Ansoft HFSS, the transmission line is optimised for spectroscopy applications and the bandwidth enhanced using a variety of methods. The theory for calculating the effective permittivity of the transmission line is derived based on a two dimensional interpretation of electromagnetic field patterns and cross-section transmission line geometry. The resulting formulae have a significant impact on both bandwidth and resonant filter designs. The excitation of the planar Goubau line’s quasi transverse magnetic mode, which is typically excited using coplanar to planar Goubau line transition, has been modified in favour of a novel all-planar Goubau line on-chip spectroscopy system utilising photoconductive generation and detection methodologies. In doing so, the frequency resolution of the system is heightened enabling a narrow line width system to be resolved. The planar Goubau lines are fabricated on a quartz substrate with epitaxial transferred low- temperature-grown GaAs based photoconductive switches for both THz generation and detection. The bandwidth of the planar Goubau line is enhanced using a substrate thinning methodology to 2 THz for a 1-mm-long planar Goubau line. Using the enhanced bandwidth, THz-TDS spectroscopy using a planar Goubau line is demonstrated for the first time, where spectra of polycrystalline lactose monohydrate is obtained with a 3.75 GHz frequency- resolution over variable temperature range (4 – 298 K). The THz-TDS spectra are compared with spectra found using alternative THz spectroscopy systems to highlight the improved benefits of using this device. The theoretical development of a narrow bandstop filter design is presented, with the analytical terms defined. This novel filter enables the dielectric sensing of overlaid materials at a multiple of predefined frequencies to be pushed away from the transmission lines, which would otherwise reduce the bandwidth of the system. Results presented in this thesis present a strong candidacy for planar Goubau lines to be utilised in a broad range of applications which hold information in the THz regime. ii Contents Contents Abstract ................................................................................................................................ i Contents .............................................................................................................................. ii Acknowledgements ............................................................................................................ vi List of Figures ................................................................................................................... vii List of tables .................................................................................................................... xxii Chapter 1. The Latest Developments in THz Spectroscopy ............................................ 1 1.1. Introduction to THz Radiation .............................................................................. 2 1.2. Free Space Spectroscopy Systems ........................................................................ 3 1.2.1. Fourier Transform Infrared Spectrometer ...................................................... 4 1.2.2. Free space THz-TDS .................................................................................... 7 1.3. Free Space Coupled Waveguides ........................................................................ 11 1.3.1. Attenuated Total Reflection ........................................................................ 11 1.3.2. Parallel Plate............................................................................................... 12 1.3.3. Sommerfeld, Goubau and Zenneck waveguides .......................................... 14 1.4. On-chip THz-TDS Systems ................................................................................ 16 1.4.1. Microstrip Lines ......................................................................................... 16 1.4.2. Coplanar Transmission Lines ...................................................................... 19 1.4.3. Planar Goubau Lines................................................................................... 24 1.5. On-chip Generation and Detection of Picosecond Pulses..................................... 28 1.5.1. Electro Optic Detection............................................................................... 28 1.5.2. Electro Absorption Detection ...................................................................... 34 1.5.3. Electro Optic Generation ............................................................................ 36 1.5.4. Photoconductive Generation and Detection ................................................. 38 1.5.4.1. Material Considerations .......................................................................... 41 1.5.4.2. Switch Geometries .................................................................................. 43 iii Contents 1.5.4.3. Different Illumination and Bias Considerations for PC Switches ............. 47 1.5.4.4. External PC Detection of On-chip Systems ............................................. 49 1.6. Conclusion ......................................................................................................... 50 Chapter 2. Transmission Line Theory and Simulation Models ...................................... 53 2.1. Lumped Element Method ................................................................................... 54 2.1.1. Lumped element method for PGLs.............................................................. 59 2.2. General Solutions from Maxwell’s Equations ..................................................... 59 2.2.1. Solving Maxwell’s Equations for Radial Fields ........................................... 61 2.3. Introduction to Ansoft HFSS .............................................................................. 63 2.4. PGL Simulation Results ..................................................................................... 66 2.4.1. PGL Field Patterns...................................................................................... 67 2.4.2. Substrate Permittivity ................................................................................. 69 2.4.3. PGL Transmission Line Width .................................................................... 70 2.4.4. PGL Length ................................................................................................ 74 2.5. Experimental Comparisons with HFSS Results ................................................... 74 2.6. Conclusion ......................................................................................................... 75 Chapter 3. The Fabrication and Measurements of THz On-chip Systems and the Bandwidth Enhancement of Planar Goubau Lines .............................................................. 76 3.1. Design of Second Generation PGLs .................................................................... 77 3.2. Fabrication of THz On-chip Device on Quartz .................................................... 78 3.2.1. The Growth and Annealing of LT-GaAs ..................................................... 79 3.2.2. Van der Waals Bonding of LT-GaAs to Quartz and etching of LT-GaAs to define photoconductive switches ................................................................................ 80 3.2.3. Lithography and Metallization .................................................................... 83 3.2.4. Mounting of On-chip Systems to PCBs ....................................................... 86 3.3. System Setup and Measurement Scheme ............................................................ 88 3.4. Initial Testing of On-chip Systems ...................................................................... 93 3.4.1. Initial PGL Device Characterization............................................................ 93 iv Contents 3.4.2. Electrical Chopping of the PC Switch ....................................................... 103 3.4.3. Annealing Measurements .......................................................................... 106 3.5. Wafer Thinning Theory .................................................................................... 109 3.6. Chemical and Mechanical Thinning of Quartz Substrates.................................. 112 3.6.1. Hand Lapping Process .............................................................................