Enhanced Polarisation Control and Extreme Electric Fields: Advances in Terahertz Spectroscopy Applied to Anisotropic Materials and Magnetic Phase Transitions (Springer Theses)

This thesis reports advances in terahertz time-domain spectroscopy, relating to the development of new techniques and components that enhance the experimentalist's control over the terahertz polarisation state produced by photoconductive emitters. Two methods of controlling the terahertz polarisation state are reported: The first method is based upon mechanical rotation of an interdigitated photoconductive emitter, and is implemented in a rotatable-polarisation terahertz time-domain spectrometer; the calibration of which is demonstrated to produce a highly uniform polarisation state at all angles. This method is then demonstrated experimentally to identify the orientations of the normal modes of propagation in the plane of birefringent samples, to extract the full complex refractive index along these directions, and to investigate the optical selection rules of an absorbing material. The second method presents a new photoconductive emitter design, based upon separate interdigitated pixel elements for the generation of the horizontally and vertically polarised components of the terahertz pulses, that permits rotation of the polarisation state solely by electrical control. The design, fabrication and experimental verification of the device is reported, demonstrating polarisation control on timescales orders of magnitude faster than those achievable in mechanical rotation methods. Additionally, preliminary investigations into the properties of materials exposed to extreme terahertz optical electric fields are performed. Nonlinear terahertz transmission in single-walled carbon nanotube films and evidence for nonlinear behaviour of electromagnons in CuO are observed. Further to these advances, included in the original thesis (but not reproduced in the Springer version), a method of using the electromagnon response in Cu 1-x Zn x O alloys as a sensitive probe of a phase transition is also demonstrated. Using this method, the phase transition is observed to broaden upon the introduction of spindisorder when alloying with non-magnetic zinc ions, and the first-order nature of the transition is confirmed by the observation of thermal hysteresis. Supervisor’s Foreword 6 Abstract 7 Publications 8 Acknowledgements 10 Contents 11 1 Introduction 14 1.1 Crystal Optics 15 1.1.1 The Dielectric Tensor 15 1.1.2 Principal Axes and the Index Ellipsoid 16 1.1.3 Plane Wave Propagation Through an Anisotropic Medium 19 1.1.4 Effects of an Anisotropic Medium on the Polarisation State of Light 21 1.2 Describing the Polarisation State of Electromagnetic Waves 22 1.2.1 Ellipticity and Orientation Angle 22 1.2.2 Alternative Descriptions of the Polarisation State 23 1.3 Electromagnons in Improper Ferroelectrics 25 1.3.1 Magnons 26 1.3.2 Electromagnons 28 1.3.3 Improper Ferroelectricity and Electromagnons in CuO 30 1.4 Structure of This Thesis 34 References 35 2 Terahertz Time-Domain Spectroscopy 37 2.1 Generation of Broadband Terahertz Radiation 37 2.1.1 Photoconductive Emitters 38 2.1.2 Optical Rectification 40 2.2 Electro-optic Sampling 41 2.2.1 Polarisation-Resolved Electro-optic Sampling 42 2.3 THz-TDS Experimental Setup 44 2.3.1 Performing a THz-TDS Experiment 45 2.4 Extracting Sample Properties and Polarisation Information ... 47 2.4.1 Complex Refractive Index 48 2.4.2 Ellipticity and Orientation Angle 49 2.5 Summary 50 References 50 3 Rotatable-Polarisation Terahertz Time-Domain Spectroscopy of Anisotropic Media 52 3.1 Investigating Anisotropy at Terahertz Frequencies 52 3.1.1 Disambiguating Spectral Features Using Polarisation-Resolved Detection Methods 53 3.1.2 Terahertz Polarimetry and Ellipsometry 54 3.1.3 Methods of Terahertz Polarisation Rotation 56 3.2 Rotatable-Polarisation Terahertz Time-Domain Spectrometer 58 3.2.1 Rotating the Terahertz Polarisation State 58 3.2.2 Polarisation-Resolved Detection Method and Alignment 59 3.2.3 Calibration of the Terahertz Emission Strength 60 3.2.4 Polarisation State of the Rotated Terahertz Pulses 62 3.3 Comparison of Rotatable Polarisation to Projection via Wire-Grid Polarisers 63 3.4 Experimental Implementation of RP-THz-TDS 66 3.4.1 Sample Details 67 3.4.2 Mapping Birefringence and Identifying Polarisation Eigenvectors Using RP-THz-TDS 68 3.4.3 Extracting the Full Complex Refractive Index Using RP-THz-TDS 71 3.4.4 Anisotropic Absorption and Chromatic Dispersion in CuO 74 3.5 Summary 77 References 78 4 Scalable Interdigitated Photoconductive Emitters for the Electrical Modulation of Terahertz Beams with Arbitrary Linear Polarisation 80 4.1 Photoconductive Emitter Geometry and Terahertz Polarisation State 81 4.1.1 Electric Dipole Radiation from Photoconductive Emitters 81 4.1.2 Controlling the Terahertz Polarisation State with the Emitter Geometry 82 4.2 Multi-Pixel Interdigitated Photoconductive Emitters 83 4.2.1 Emitter Concept and Design 83 4.2.2 Device Fabrication 85 4.2.3 Simulated Device Performance 86 4.3 Experimental Device Performance 88 4.3.1 Initial Electrical Biasing Tests 88 4.3.2 Generating Arbitrary Linear polarisation States via Electrical Control 90 4.4 Rapid Modulation of Circular Polarisation States for Circular ... 93 4.4.1 Converting from Linear to Circular Polarisation via a Prism 93 4.4.2 Stokes Parameters 94 4.4.3 Experimental Setup 95 4.4.4 Experimental Results 97 4.5 Summary 98 References 99 5 High-Field Terahertz Time-Domain Spectroscopy of Single-Walled Carbon Nanotubes and CuO 100 5.1 Terahertz Spectroscopy Using Extreme Electric Fields 100 5.1.1 Generating High-Field Terahertz Radiation 101 5.1.2 Choosing the Right High-Field THz Source for You 102 5.2 High-Field Terahertz Time-Domain Spectrometer 103 5.2.1 Tilted Pulse-Front Pumping in LiNbO3 103 5.2.2 Experimental Setup 107 5.2.3 Controlling the Electric Field Strength of Terahertz Pulses 109 5.3 A Test Case of Nonlinear THz Transmission: Indium Antimonide 111 5.3.1 Experimental Results 112 5.4 Nonlinear THz Transmission in Single-Walled Carbon Nanotube Films 113 5.4.1 Experimental Results 114 5.5 Electric Field-Dependent THz Transmission of CuO 116 5.5.1 Electric Field-Dependence of the Electromagnon Response in CuO 117 5.6 Summary 119 References 120 6 Conclusions 122 6.1 Outlook and Future Work 123 Appendix Tracking Disorder Broadening and Hysteresis in First-Order Phase Transitions via the Electromagnon Response in Improper Ferroelectrics 125 This thesis reports advances in terahertz time-domain spectroscopy, relating to the development of new techniques and components that enhance the experimentalist’s control over the terahertz polarisation state produced by photoconductive emitters. It describes how utilising the dynamic magnetoelectric response at THz frequencies, in the form of electromagnons, can probe material properties at a transition between two magnetically ordered phases. Additionally, preliminary investigations into the properties of materials exposed to extreme terahertz optical electric fields are reported. The work presented in this thesis may have immediate impacts on the study of anisotropic media at THz frequencies, with photoconductive emitters and detectors being the most commonly used components for commercially available terahertz spectroscopy and imaging systems, and by providing a new way to study the nature of magnetic phase transitions in multiferroics. In the longer term the increased understanding of multiferroics yielded by ultrafast spectroscopic methods, including terahertz time-domain spectroscopy, may help develop new magnetoelectric and multiferroic materials for applications such as spintronics.