Fundamentals of Terahertz Devices and Applications

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.

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

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, and metal computer numerical control (CNC) machining fabrication have enabled the development and growth of new terahertz antenna arrays, which are based on horns or lenses.

There are few examples at THz frequencies of horn arrays above 300 GHz due to the complexity of fabrication and integration. Initially, the approach was to individually packed horn antennas, as the horn array used in [12]. However, this approach limits the inter‐pixel spacing on the focal plane array and consequently the sampling of the observation image. In order to reduce the distance between the elements, the full array needs to be fabricated on the same metal block, which requires milling or drilling techniques. Milling techniques have been used in W‐band such as the 31‐pixel array at 100 GHz shown in [13]. For higher frequencies, drilling techniques with custom drill bits have been successfully employed allowing the fabrication of multi‐flare angle horn arrays as in [14]. Other less conventional techniques are laser silicon micromachining, which can be employed for the milling of corrugated horns as demonstrated at 2 THz in [15], and photolithographic processes, which has been demonstrated in [16]. Both methods rely on stacking together a number of thin gold plated silicon wafers with tapered holes etched at 90°.

Transmission lines at terahertz frequencies suffer from high losses due to the metal losses, which make the development of phased arrays impracticable. However, it is not the case if the microstrip lines are made of a superconducting material. This approach was used for the BICEP2 instrument, where an array of 10 × 10 different phased arrays coupled to an array of horns was developed [17]. Each phased array was composed of 100 pixels, a transition edge sensor (TES) detector and a horizontal and vertical slot, for horizontal and vertical polarization detector. Nevertheless, superconductor materials require cryogenic cooling to operate, which makes this solution not viable for many applications.

Even though integrated lens antennas are the most suited for focusing on a planar antenna, there a very few systems implemented with large lens arrays. There have been some examples of lens arrays fabricated and assembled individually as in [18]. However, it has not been until the last years that a great development on integrated lens arrays has been made at terahertz frequencies. Advances on silicon micromachining have enabled the fabrication of large arrays of lenses on a single block piece. An array of 989 silicon pixels integrated with Kinetic Inductance Detectors (KIDs) has been developed at 1.4 and 2.8 THz [19]. The silicon lenses were fabricated on a single silicon block using laser micromachining. Another example has been the use of a photolithographic process based on deep reactive ion etching (DRIE) to fabricate shallow lenses as in [20].