# Improvement of an InfraRed Pyroelectric Detector Performances in THz Range Using the Terajet Effect
## 2 Experimental Setup and Methods  
The basic scheme of the experimental setup is shown in Figure 1a, where a terahertz source based on a backward-wave oscillator [47] (BWO) \"OV-1\" from Istok Co. was use to excite the detector. It was chosen because of its special features such as stability, relative high output power (6-15 mW), good wave-front quality and wavelength tunability between 177 and 260 GHz. A certified attenuator was coupled to the BWO and used to control and measure the emitted power. The horn antenna of BWO was placed at a distance of approximately 210 mm from the detector's window (which corresponds to the far-field condition) to form a quasi-plane wavefront. A commercial Infrared PD (MG-33) was used for the detection of THz radiations (Figure 1b,c). The PD-MG33 sensor has a standard KT-3 package [36], which contains a primary amplifier located on the sensor chip, as shown in Figure 1. The THz beam was modulated by a chopper at 20 Hz rate, and the photoresponse was measured by a micro-voltmeter as well as an oscilloscope. A dielectric wavelength-scaled particle in the form of a cube was glued and placed directly on the PDwindow made of germanium and with a thickness around 70 \(\upmu\)m. To increase the coupling efficiency of the PD with THz radiation source at \(\lambda\)= 1.5 mm (frequency of about 0.2 THz), we use a terajet formed by a polytetrafluoroethylene (PTFE) cube with a dimension equal to 2\(\lambda\). The cubic shape of the dielectric particle was chosen because of the manufacturing simplicity and its attachment to the flat surface of the PD filter window. The refractive index of PTFE is n = 1.41 [48; 49; 50], and the beam waist of the terajet (in the unit of full width at half maximum) for the dimension of the PTFE cube is about 0.5 \(\lambda\)[41; 42; 43; 44]. The maximum field intensity enhancement was observed at the distance of about 0.7 mm from the shadow surface of the cuboid [41; 42; 43; 44]--see Figure 1c, taking into account the features of the constructive assembly of the PD shown in Figure 1a (minimal distance between window's shadow surface and sensitive element is not equal to zero and has a fixed value (\(d\)), fixed by the manufacturer technology).  
Full-wave electromagnetic simulations were performed using the software CST where the Maxwell equations were solved. It is a well known solver that is used to analyze electrically large structures where the physical size is much larger than the wavelength. The incident beam was considered as a plane propagating along the \(z\) direction (Figure 2a). Since the PTFE is a commercial one, the refractive index could vary depending on the density of the material, and two values of the refractive index (1.41 and 2.1) were simulated. Figure 2b shows the intensity of the electric field along the \(z\) direction for two values of the refractive index. A maximum was observed around \(z_{max}=1.7\) mm and \(z_{max}=0.5\) mm for \(n=1.41\) and 2.1, respectively. Those values are in agreement with the one found experimentally (0.7 mm). Figure 2c shows the intensity of the electric field fin and the \(xz\) plane for the cube with a length equal to 2\(\lambda\) and \(n=1.41\). As can be seen in the figure, the introduction of a cube induces the formation of an area in which the electric field is concentrated as a result of the terajet beam effect. The obtained gain from the simulation was around 9.5 and 12 dB for \(n=1.41\) and 2.1, respectively. The expected beam size was 1.44 \(\times\) 1.38 \(\times\) 6.15 mm\({}^{3}\) and 1.22 \(\times\) 0.76 \(\times\) 1.84 mm\({}^{3}\) in the \(xyz\) directions and for \(n=1.41\) and 2.1, respectively.  
Figure 1: Schematic description of the terahertz measurement setup where a commercial PD MG-33 combined with the dielectric particle (not in scale) was used as a detector, (**a**); MG-33 detector at the standard KT-3 package (**b**) and the top view of the PD with standard-diameter of window of \(\O=5\) mm, (**c**).