📡Terahertz Engineering Unit 6 – Terahertz Devices and Components

Fundamentals of Terahertz Radiation

• Terahertz (THz) radiation occupies the electromagnetic spectrum between microwave and infrared regions (0.1 THz to 10 THz)
• Corresponds to wavelengths ranging from 3 mm to 30 μm
• Exhibits unique properties such as penetration through non-conducting materials (plastics, ceramics) and sensitivity to molecular vibrations
• Terahertz photon energy is relatively low (4.1 meV at 1 THz) compared to other regions of the electromagnetic spectrum
  • Enables non-ionizing and non-destructive interaction with matter
• Terahertz waves undergo strong absorption by water and atmospheric gases (water vapor), limiting their propagation distance in air
• Offers high spatial resolution due to short wavelengths, enabling imaging with sub-millimeter resolution
• Provides spectroscopic information about chemical composition and molecular structure of materials

Terahertz Sources and Emitters

• Electronic sources generate terahertz radiation through oscillating currents or voltages
  • Examples include resonant tunneling diodes (RTDs), Gunn diodes, and high-electron-mobility transistors (HEMTs)
  • Limited by the transit time of charge carriers and parasitic capacitances
• Optical sources rely on nonlinear optical processes or ultrafast laser pulses to generate terahertz waves
  • Photoconductive antennas (PCAs) use ultrafast laser pulses to excite photocarriers in a semiconductor substrate, generating terahertz pulses
  • Difference frequency generation (DFG) mixes two laser beams with slightly different frequencies in a nonlinear crystal to produce terahertz radiation
• Quantum cascade lasers (QCLs) are compact and coherent terahertz sources based on intersubband transitions in semiconductor heterostructures
  • Offer high output power and narrow linewidth but require cryogenic cooling for operation
• Backward wave oscillators (BWOs) and free-electron lasers (FELs) can generate high-power terahertz radiation but are bulky and expensive
• Terahertz parametric oscillators (TPOs) utilize nonlinear optical crystals to convert near-infrared laser light into tunable terahertz waves

Terahertz Detectors and Sensors

• Terahertz detectors convert incoming terahertz radiation into measurable electrical signals
• Thermal detectors, such as bolometers and pyroelectric detectors, respond to the heating effect caused by absorbed terahertz radiation
  • Offer broadband detection but have slower response times and lower sensitivity compared to coherent detectors
• Coherent detectors, such as Schottky diodes and field-effect transistors (FETs), directly rectify the terahertz electric field into a DC signal
  • Provide fast response times and high sensitivity but require careful impedance matching and calibration
• Electro-optic sampling (EOS) uses the Pockels effect in nonlinear crystals (ZnTe, GaP) to detect terahertz electric fields with ultrafast laser pulses
  • Enables time-domain measurements with sub-picosecond resolution
• Photoconductive antennas can also be used as terahertz detectors by sampling the terahertz-induced photocurrent with ultrafast laser pulses
• Microbolometers and antenna-coupled microbolometers offer room-temperature operation and can be integrated into focal plane arrays for terahertz imaging

Waveguides and Transmission Lines

• Terahertz waveguides and transmission lines efficiently guide and propagate terahertz waves while minimizing losses
• Metallic waveguides, such as rectangular and circular waveguides, confine terahertz waves using conducting boundaries
  • Suffer from high ohmic losses due to skin effect at terahertz frequencies
• Dielectric waveguides, such as plastic fibers and sapphire fibers, guide terahertz waves through total internal reflection
  • Offer lower losses compared to metallic waveguides but may have limited bandwidth and mode confinement
• Photonic crystal fibers (PCFs) use periodic dielectric structures to create a bandgap that confines terahertz waves in a hollow core
  • Enable low-loss propagation and dispersion control but require precise fabrication techniques
• Planar transmission lines, such as microstrip lines and coplanar waveguides (CPWs), are compatible with integrated circuit technology
  • Provide easy integration with active devices and antennas but suffer from higher losses compared to waveguides
• Parallel plate waveguides and slot waveguides offer low-loss propagation and can be coupled to free-space terahertz beams using horn antennas

Terahertz Antennas and Arrays

• Terahertz antennas efficiently couple free-space terahertz radiation to guided modes in waveguides or transmission lines
• Dipole antennas and bow-tie antennas are simple and broadband but have limited directivity and gain
• Horn antennas, such as pyramidal and conical horns, provide high gain and directivity but are bulky and require careful design for impedance matching
• Lens antennas, such as silicon lenses and plastic lenses, focus terahertz radiation into a small spot size, improving the coupling efficiency to detectors or waveguides
• Reflectarray antennas consist of an array of reflective elements (patches, dipoles) that introduce a phase shift to the reflected terahertz wave, enabling beam steering and focusing
• Phased array antennas electronically steer the terahertz beam by controlling the phase and amplitude of each antenna element
  • Offer fast beam steering and adaptive beamforming but require complex feeding networks and phase shifters
• Leaky-wave antennas, such as slotted waveguide antennas and periodic surface antennas, radiate terahertz waves along their structure, enabling frequency-dependent beam steering
• Antenna arrays can be used to increase the overall gain and directivity of terahertz systems, enhancing the signal-to-noise ratio and range

Passive Components and Optical Elements

• Terahertz lenses, such as silicon lenses and polymer lenses (TPX, HRFZ-Si), focus or collimate terahertz beams
  • Aspherical lenses minimize aberrations and improve the focusing performance
• Terahertz mirrors, such as metal mirrors and dielectric mirrors, reflect and steer terahertz beams
  • Off-axis parabolic mirrors minimize astigmatism and provide diffraction-limited focusing
• Terahertz beam splitters, such as silicon wafers and mylar films, divide terahertz beams into transmitted and reflected components
  • Wire grid polarizers act as polarizing beam splitters, separating terahertz waves into orthogonal polarization states
• Terahertz filters, such as metal mesh filters and frequency selective surfaces (FSS), control the spectral content of terahertz beams
  • Bandpass filters, low-pass filters, and high-pass filters can be realized using periodic metallic structures
• Terahertz waveplates, such as birefringent crystals (quartz, sapphire) and metamaterials, manipulate the polarization state of terahertz waves
  • Quarter-wave plates convert linear polarization to circular polarization, while half-wave plates rotate the polarization angle
• Terahertz attenuators, such as metallic screens and dielectric absorbers, control the intensity of terahertz beams without altering their spectral or spatial properties
• Terahertz modulators, such as graphene-based modulators and liquid crystal modulators, dynamically control the amplitude, phase, or polarization of terahertz waves

Terahertz Imaging and Spectroscopy Systems

• Terahertz time-domain spectroscopy (THz-TDS) systems measure the temporal profile of terahertz pulses transmitted through or reflected from a sample
  • Provide amplitude and phase information, enabling the extraction of complex dielectric properties and thickness of materials
• Terahertz frequency-domain spectroscopy (THz-FDS) systems measure the spectral content of terahertz radiation using coherent or incoherent detection techniques
  • Fourier-transform infrared spectroscopy (FTIR) and terahertz photomixing are common THz-FDS techniques
• Terahertz imaging systems map the spatial distribution of terahertz radiation transmitted through or reflected from an object
  • Pulsed terahertz imaging uses time-domain techniques to acquire depth-resolved images, while continuous-wave terahertz imaging provides high-speed and real-time imaging capabilities
• Terahertz computed tomography (THz-CT) reconstructs 3D images of objects by measuring terahertz projections at different angles and applying tomographic reconstruction algorithms
• Terahertz near-field imaging and spectroscopy systems use subwavelength apertures or tips to confine terahertz fields, enabling nanoscale spatial resolution
  • Scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM) coupled with terahertz detectors are common near-field techniques
• Terahertz non-destructive testing (THz-NDT) systems inspect the internal structure and defects of materials without causing damage
  • Applications include quality control, fault detection, and structural health monitoring in industries such as aerospace, automotive, and electronics

Applications and Future Trends

• Terahertz communication systems exploit the large bandwidth and high carrier frequencies of terahertz waves for high-speed wireless data transmission
  • Potential applications include wireless backhaul, data center interconnects, and satellite communications
• Terahertz sensing and imaging for security applications, such as concealed weapon detection, explosives identification, and illicit drug screening
  • Terahertz waves can penetrate clothing and packaging materials while providing spectroscopic signatures of hidden objects
• Biomedical applications of terahertz technology, such as cancer diagnosis, tissue imaging, and drug discovery
  • Terahertz spectroscopy can identify cancerous tissues based on their unique absorption spectra, while terahertz imaging can provide high-contrast images of biological samples
• Industrial process control and quality monitoring using terahertz sensors and imagers
  • Examples include moisture content analysis, thickness measurements, and defect detection in manufacturing processes
• Astronomical and space applications, such as Earth observation, planetary exploration, and interstellar medium studies
  • Terahertz spectroscopy can detect and identify complex molecules and ions in the universe, providing insights into the chemistry of space
• Future trends in terahertz technology include the development of compact and efficient terahertz sources and detectors, the integration of terahertz components with electronic and photonic systems, and the exploration of new materials and metamaterials for terahertz applications
  • Quantum technologies, such as superconducting devices and quantum cascade lasers, are expected to play a significant role in the advancement of terahertz technology