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 10 with 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.
RTDs is a good candidate for THz oscillators at room temperature and are discussed in Chapter 11. Promising results with oscillation frequency up to 650 GHz have been reported in the 90s for RTDs with planar antennas. Improvement in the RTD structure for short electron delay time and the antennas for low conduction loss allowed more recently demonstration of operation reaching ~2 THz and both low, i.e. GaAs and InP, and large bandgap materials, i.e. GaN have been used for their fabrication. Progress on structure optimization for high‐frequency and high‐output power operation, resonator and radiator type, frequency‐tunable RTD oscillators, and compact THz sources allow their consideration for applications such as wireless data transmission, spectroscopy, and imaging.
Wireless communication systems at THz are described in Chapter 12. Since the electromagnetic spectrum is saturated on most already allocated frequencies, systems operating above 100 GHz, i.e., in the 200–320 GHz range draw considerable interest for very high‐speed wireless transmissions. Electronic and photonic building blocks are of interest for this purpose. THz transmitters, receivers, and the basic architecture of transmission systems are discussed together with various devices suitable for T‐ray communication such as photomixers and approaches suitable for the generation of modulated THz signals. Integration approaches, ways of interconnection, and antennas are key components to be investigated for the realization of THz communication systems. Communication links using both electronic‐ and photonic based approaches are also described.
The interest into solar system objects and the interstellar medium has led in space instrument investments and consideration of THz technologies that allow insight into solar system objects and the interstellar medium. The technology and engineering aspects of the heterodyne receiver which is the system of choice for conducting high‐resolution spectroscopy for space applications I described in Chapter 13. Its critical components such as mixers (Schottky diode, SIS Mixer, and Hot‐Electron Bolometric Mixers) and local oscillators (frequency multiplied chains) are also analyzed together with three distinct space science applications for THz instruments and how these applications are currently driving technology development. These include planetary science and miniaturization, astrophysics, and THz array receivers, as well as, earth science: and active THz systems.
2 Integrated Silicon Lens Antennas at Submillimeter‐wave Frequencies
Maria Alonso-delPino1,2, Darwin Blanco2 and Nuria Llombart Juan2
1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
2 Department of Microelectronics, Technical University of Delft, Delft, The Netherlands
2.1 Introduction
Most of submillimeter‐wave instruments, especially for space applications, make use of very high gain reflector‐based antennas to fulfill the desired resolution or sensitivity requirements. This reflector based quasi‐optical systems are illuminated by antenna feeds integrated with the transceiver/receiver front‐end. At submillimeter‐wave frequencies, these antenna feeds are mostly based on horns or silicon lens antennas.
For single‐pixel heterodyne split block waveguide‐based instruments such as [1], horn antennas are typically preferred due to their straightforward connection to a waveguide‐based front‐end architecture, their manufacturability using metal machining processes, and their good radiation properties. For example, the diagonal horn and Pickett‐Potter horn achieve a relatively good performance that is compatible with a simple split‐block fabrication process [2, 3]. For better performance, electroforming is a viable option to fabricate corrugated horns, which are commercially available for frequencies of 1.47 THz [4].
On the other hand, lenses are widely used for coupling into a planar antenna architecture that is often integrated with a bolometer detector or silicon‐based front‐ends. They have been widely used to couple to direct detectors instruments as in [5, 6]. There are also many examples of superconducting‐insulator‐superconducting (SIS) and hot‐electron bolometric (HEB) mixers based on planar antenna architectures for heterodyne instrumentation [7, 8]. Moreover, they are also commonly used for standalone photoconductive systems using broadband antennas such as bow‐ties, logarithmic spirals as in [9–11]. These hybrid antennas have multiple advantages compared to waveguide type of antennas: low loss, easy integration with receiver, and low cost of manufacture. The antenna and detector are processed using photolithographic processes on a wafer and the lenses can be fabricated using milling techniques. These lenses are usually made of silicon, which is set to match the permittivity of the substrate, are comparatively inexpensive and easy to assemble, and are air‐coated with a quarter wavelength Parylene layer. They operate over large bandwidths providing good performances.
Multiple applications in the submillimeter‐wave band require the use of multi‐pixel systems in order to maximize the data output or reduce the image acquisition time of an imaging system. The development of antenna focal plane arrays has been challenging due to the packaging and fabrication limitations of these antennas, especially above 0.5 THz. The recent advances in photolithographic, laser micro‐machining,
