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, 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].
In terms of performance, silicon lens antennas have been able to reach a high quasi‐optical coupling efficiency for both single‐pixel and focal plane array architectures. It was traditionally achieved using resonant double slot antennas as in [21] and [22], and now broadband using a leaky‐wave slot as in [23]. New analysis methods and optimization techniques have been developed to improve the coupling between the lens and antenna mechanisms [24]. While books on horn and phased array design are extensive, the techniques to analyze integrated lenses antennas, especially for high directive feed antennas, i.e. leaky‐wave antennas, are not that common. This chapter explains how integrated lens antennas work and can be analyzed.
Recently, a new lens feed concept has been developed in [25, 26] coupling the shallow photo‐lithographic silicon lenses to a waveguide feed system with extremely low loss and high efficiency. The concept, which uses a novel leaky‐wave/Fabry–Perot resonator also formed monolithically, has negligible Ohmic losses at THz frequencies while producing a very directive field inside the lens; hence producing a highly isolated and directed output beam. These monolithic leaky‐wave feeds have now demonstrated the highest directivity‐to‐loss ratio ever reported in the THz band. Moreover, whereas traditional Fabry–Perot resonator‐based antennas typically have very narrow bandwidths, this leaky‐wave feed has yielded bandwidths in excess of 15%, well matched to most implemented or proposed THz heterodyne systems to date. This new lens feed concept unveils a wide range of possibilities for direct detection and heterodyne instruments, thanks to the high performance achieved for its use on highly packed focal plane arrays. This chapter will also address the design of these new lens feeds.
This book chapter is organized as follows. First, we cover the design and the analysis of elliptical lens antennas, providing analytical formulation to synthesize the lens and compute the radiation of the lens excited by a feed. Second, the semi‐hemispherical lens antenna is described, from its synthesis to its radiation properties while comparing it with the elliptical lens. Third, we explain the excitation of shallow lenses by leaky‐wave/Fabry–Perot feeds: starting from the analysis of the leaky‐wave effect and computing its propagation constant, then analyzing its radiation into an infinite medium (primary field), and finishing up with the optimization of the shallow‐lens geometry. Last, we explain how to develop a fly‐eye antenna array by describing the fabrication using silicon micromachining of the full lens antenna with DRIE techniques, evaluating the surface accuracy of the lens, and providing some examples of fabricated antennas. In addition, the chapter concludes with some worked examples to make the reader consolidate and reflect on the lessons learned.
2.2 Elliptical Lens Antennas
Integrated silicon lens antennas are well suited for submillimeter‐wave