Intro; Supervisor's Foreword; Abstract; Acknowledgements; Contents; 1 Introduction; 1.1 Rydberg Atoms; 1.2 THz Technology; 1.2.1 Terahertz Detectors; 1.3 Terahertz Applications for Rydberg Atoms; 1.4 Thesis Overview; References; 2 Atomic Structure and Atom-Light Interactions; 2.1 Atomic Structure; 2.1.1 Gross Level Structure; 2.1.2 Fine Structure; 2.1.3 Hyperfine Structure; 2.1.4 Caesium Atomic Structure; 2.2 Atom Light Interaction; 2.2.1 Dipole Matrix Elements; 2.2.2 Master Equation; 2.2.3 Spontaneous Decay; 2.3 Conclusion; References; 3 Experimental Methods; 3.1 Caesium Vapour

3.2 Laser Systems3.2.1 Laser Frequency Stabilisation; 3.2.2 Rydberg Laser; 3.2.3 Terahertz Source; 3.2.4 Bench Layout; 3.3 Experiment Read-Out; 3.4 Conclusion; References; 4 Probing an Excited State Transition Using Quantum Beats; 4.1 Introduction; 4.1.1 Principle of Quantum Beats; 4.2 Caesium Hyperfine Quantum Beats; 4.3 Quantum Beats Modified by Driving an Excited-State Transition; 4.3.1 Experiment Results; 4.3.2 Computer Simulation; 4.3.3 Analysis; 4.4 Conclusion; References; 5 Intrinsic Rydberg Optical Bistability; 5.1 Introduction; 5.1.1 Rydberg Level Shift; 5.1.2 Chapter Outline

5.2 Optical Bistability Example5.3 Phenomenological Modeling; 5.3.1 Steady State Phase Map; 5.3.2 Critical Slowing Down; 5.4 Phase Boundary; 5.4.1 Phase Boundary Model; 5.4.2 Connection to Spectral Lineshape; 5.5 Conclusion; References; 6 Terahertz Electrometry with Rydberg EIT; 6.1 Introduction; 6.2 Principle of Rydberg Microwave Electrometry; 6.3 Computer Modelling; 6.4 Electric Field Amplitude Measurement; 6.4.1 Terahertz Attenuator Calibration; 6.5 Polarisation Effects; 6.6 Conclusion; References; 7 Real-Time Near-Field Terahertz Field Imaging; 7.1 Introduction; 7.2 Fluorescence Signal

7.2.1 Fluorescence Spectrum7.2.2 Image Sensitivity Calibration; 7.2.3 Sensitivity Bandwidth; 7.3 Standing Wave Image; 7.3.1 Autler-Townes Line Shape Model; 7.3.2 Cell Wall Interaction; 7.3.3 Motional Blurring; 7.4 Real-Time Imaging; 7.5 Conclusion; References; 8 Terahertz-Driven Phase Transition in a Hot Rydberg Vapour; 8.1 Introduction; 8.2 Spectral Line Modification; 8.2.1 Linear Shift of Hysteresis Laser Detuning; 8.3 Static Response to Terahertz Intensity; 8.3.1 Phase Domain Hopping; 8.3.2 Latching Detector; 8.4 Conclusion; References; 9 Summary and Outlook