# Microwave SINIS Detectors
## 1 Introduction  
Normal metal-insulator-superconductors (NISs), as well as SIS tunnel junctions, are the main building blocks in superconducting electronics. SINIS is one of the generic elements of a wide family of cryogenic devices [1, 2, 3], such as Andreev bolometers (AB) [4, 5], normal metal hot-electron bolometers (NHEB) [6], cold-electron bolometers (CEB) [7, 8], SINIS bolometers [9, 10, 11], SINIS detectors [12, 13, 14], NIS array thermometers [15, 16, 17], electron coolers [18, 19, 20, 21], and photon counters. Over the 30 years in which they have been in development, detectors with NIS tunnel junctions have been referred to as ANHEB--a normal metal hot-electron bolometer with Andreev reflection (SNS + NIS), actually operating at frequencies below 70 GHz; CCNHEB [22]--capactive coupled normal metal hot-electron bolometers (SINIS); CEB--cold-electron bolometers (SINIS), which are the same as CCNHEB, but operate with electron cooling; SINIS detectors--an SINIS structure in the operation mode of quantum response, at frequencies above 70 GHz; an NISIN detector with a structure similar to SINIS, or even NININ or MIMIM detectors with metal-insulator-metal tunnel junctions [23, 24]. A hot-electron bolometer with Andreev contacts between the N-absorber and S-electrodes, and with an NIS thermometer for temperature response measurement, was proposed in a publication [5]--see the schematic view in Figure 1. Such a bolometer demonstrated, under dc heating, a responsivity of up to \(10^{9}\) V/W and a noise equivalent power (NEP) = \(3\times 10^{-19}\) W/Hz\({}^{1/2}\). One of the first attempts to study ANHEB bolometers (Figure 2) under microwaves was made in [4], in which, under dc heating, \(\mathrm{NEP_{dc}}\) = \(5\times 10^{-18}\) W/Hz\({}^{1/2}\) was obtained and at 300 GHz irradiation, only \(\mathrm{NEP_{microwave}}\) = \(10^{-14}\) W/Hz\({}^{1/2}\) and \(dV/dP\) = \(10^{6}\) V/W were reached, whose values are much lower compared to the response at dc or low frequencies. The reason for this contradiction is that Andreev contacts are transparent to electrons with energy above the superconducting aluminum gap, which is about 70 GHz. This became clear in 2001 after theoretical analyses performed by [PERSON]. A solution to increasing the signal frequency was found by [PERSON], who proposed replacing the Andreev contacts with tunnel NIS junctions. The first practical results of an SINIS microwave detector were obtained in 2001 at Chalmers University of Technology [22], and these are described in detail in [25]. In the same year, pioneering work on electron cooling using NIS junctions appeared [26; 27]. As [PERSON] noted, since the NIS junction provides electron cooling, such cooling can also occur in the bolometer, so under certain modes of operation of the SINIS structure, it can switch from a hot-electron bolometer (HEB) operation mode to a cold-electron bolometer (CEB) mode, as was first proposed in [28]. In such a bolometer, at a dc bias close to the energy gap, it is possible to obtain electron cooling from the phonon temperature of 300 mK  
Figure 1: Basic schematics of ANHEB with superconducting antenna directly connected via Andreev junctions to normal metal absorber (black), and NIS junction thermometer connected to current source I.  
Figure 2: SEM view of ANHEB integrated in log-periodic mode (**a**) and a microbolometer (**b**) [4]. down to the electron temperature of about 100 mK, thus theoretically reducing the dc NEP to \(10^{-18}\) W/Hz\({}^{1/2}\). Depending on the frequency range and the required bandwidth, the designs of bolometers with different planar antennas were fabricated and experimentally studied. Such integrated antenna-SINIS structures were assessed in quasioptical lens systems, and optical response values up to 0.8 \(\times\) 10\({}^{9}\) V/W and NEP = 2 \(\times\) 10\({}^{-17}\) W/Hz\({}^{1/2}\) were obtained at frequencies around 350 GHz.