Fundamentals of Terahertz Devices and Applications

THz sources and receivers benefit from the availability of technologies relying on ultrafast photoconductors and PIN‐based photodiodes and operate at frequencies that can exceed 300 GHz. These are analyzed and compared in detail in Chapter 3by considering the associated optical and transport physics, but also practical effects such as contact effects, thermal stress, and circuit limits. A variety of THz PC sources are studied including PC‐switches, photomixers, p‐i‐n photodiodes, and metal‐semiconductor‐metal (MSM) bulk photoconductors. The fundamental principles of THz antenna coupling are discussed and the input impedance, as well as the increase in the equivalent isotropic radiated power (EIRP) of the transmitting antenna, are reviewed for planar antennas on dielectric substrates. Resonant antennas and self‐complementary antennas are also studied. Good understanding of material growth is necessary for ultrafast photoconductors and low‐temperature GaAs, as well as InGaAs are considered for this purpose. To characterize with high precision THz components and in particular their power properties, a new, traceable thin‐film pyroelectric detector technology is discussed. Wireless communications and spectroscopy are two major applications of THz technology. These are extensively discussed together with device as well as signal processing considerations for their better understanding.

Further information on the generation of THz continuous waves based on the optical heterodyne approach is provided in Chapter 4by employing two‐slightly detuned infrared lasers. The ultrafast photoconductors necessary for this purpose are based on sub‐picosecond carrier lifetime semiconductors such as low‐temperature grown GaAs and InGaAs:Fe and uni‐travelling‐carrier (UTC) InP/InGaAs photodiodes. Electrical models are extracted for the photoconductors, PIN photodiodes, and UTCs, and their efficiency and maximum power achieved are examined. Attention is paid on the characteristics of backside illuminated and waveguide‐fed UTC photodiodes. Planar and micromachined antennas are being considered for photomixing systems and attention is paid on their on wafer as well as free‐space characterization.

Plasmonics based approaches can be used to enhance the performance of PC antennas and are discussed in Chapter 5. Good understanding of the photoconductor physics is necessary for this purpose together with consideration of the impact of the PC antenna and its operation as emitter and detector of pulsed and continuous‐wave (CW) THz radiation. The fundamentals of plasmonics are analyzed for better performance optimization of THz devices and design considerations are made for plasmonic nanostructures. Studies are also performed on PC THz devices with plasmonic contact electrodes, large area plasmonic PC nanoantenna arrays, and plasmonic PC THz devices with optical nanocavities.

QCLs are promising devices for THz signal generation. Chapter 6discusses their design, state‐of‐the‐art performance and limitations, and potential for improvement based on novel materials systems. Intersubband (ISB) transitions in quantum wells (QWs) allow laser emission at THz frequencies and provide a solution to the difficulty encountered due to the lack of materials with sufficiently small bandgap energies. The basic physics involved in them is reviewed including optical absorption and emission processes and phonon‐assisted nonradiative transitions. Considerations are made for the design of the QC gain medium and optical cavity, as well as the use of plasmonic waveguides to achieve strong optical confinement. Other QCL properties of interest are spectral coverage, output power, and temperature characteristics. The limitations imposed in the use of GaAs/AlGaAs QWs due to, the presence of THz‐range optical phonons and thus ability to cover the entire THz spectrum emit without cryogenic cooling can be overcome through the use of GaN/AlGaN QWs, where the optical phonon frequencies are above THz range, and SiGe which has significantly weaker electron–phonon and photon–phonon interactions compared to III–V compound semiconductors.

2D layer technology can be used for various devices including those operating at THz as described in Chapter 7. Of interest is their very strong tunable electromagnetic response at THz, which can be utilized for realizing active devices such as amplitude and phase modulators as well as active filters. Beam shaping and real‐time terahertz imaging can be achieved using metamaterial structures as well as large arrays. Graphene and graphene‐based, as well as transition‐metal dichalcogenides, offer the possibility of realizing terahertz devices. Their modeling is discussed and system applications of them are considered using modulator arrays in terahertz imaging.

To respond to the needs of THz sensing, imaging, and communication technology for detectors with high responsivity, selectivity, and large bandwidth plasma wave electronics are explored in Chapter 8. Different material systems can be investigated for this purpose and responsivities up to tens of kV/W and noise equivalent power (NEP) down to the sub‐pW/Hz1/2 range have been achieved. Very high‐speed communications can take advantage of their bias dependent tuning and possibility of very high modulation frequency up to 200 GHz. Devices studied for this purpose include field‐effect transistors with resonant and broadband detection characteristics. Silicon and graphene materials are used for such devices Graphene and 2D‐layered materials (Black Phosphorous), as well as diamond, are other possible candidates.

Details on multipliers fundamentals and their space applications are provided in Chapter 9. Their basic properties are analyzed together with a consideration of their noise characteristics. A practical approach is presented for the design of frequency multipliers and the evolution of THz frequency multiplier technology is discussed with an emphasis on the building of local oscillators. The design and fabrication of modern terahertz frequency multipliers are discussed and the case study of 2.7 THz balanced triplers is analyzed. Power combining together with integration considerations are also made. A new generation of room‐temperature terahertz Schottky diode‐based frequency multiplier sources presents 1 mW of output power at 1.6 THz and measured conversion efficiencies follow the theoretical limit predicted by physics‐based numerical models. These yield a very significant increase in performance above 1 THz in both conversion efficiency and generated output power.

Frequency multipliers using diodes have offered the possibility of generating up to THz signals using initially hybrid approaches and later on planar and integrated design. These are discussed in Chapter 10with main emphasis on GaN‐based approaches which offer the possibility of handling the high‐power levels currently possible at millimeter‐wave frequencies, enabling compact size signal generation at THz. Theoretical considerations of GaN Schottky diodes using analytical and numerical approaches allow a better understanding of their non‐linear properties and the way they can be best optimized. Parameters of interest to be studied are the device structure (materials, composition, geometry), breakdown voltage, I–V characteristics, as well as parameters the series resistance and C–V characteristics. They can be correlated to performance properties such as power handling capability, losses, and nonlinearity. Optical and E‐Beam lithography may be used for diode fabrication. The latter opens the possibility for sub‐micron anode realization, meeting the requirements of THz applications. Small but also large‐signal characterization allows to extract their properties and derive models for their circuit applications. They can also assist in explaining difficulties arising in performance optimization from periphery effects, dislocation assisted reverse current. The large‐signal network analyzer (LSNA) method can provide rapid evaluation of diodes which is important for rapid device development and multipliers. various multiplier device types, designs, and fabrication approaches are being considered for frequency multipliers. This includes GaN‐based vertical device and heterojunction designs, i.e. InN/GaN, transistors.