Improvement of an InfraRed Pyroelectric Detector Performances in THz Range Using the Terajet Effect

: An infrared (IR) pyroelectric detector was investigated for terahertz (THz) detection using the principle of the terajet effect, which focuses the beam beyond the diffraction limit. The terahertz beam was coupled to the detector’s optical window through a two-wavelength-dimension dielectric cubic particle-lens based on the terajet effect. We experimentally demonstrate an enhancement of about 6 dB in the sensitivity under excitation of 0.2 THz without degradation of the noise equivalent power value. The results show that the proposed method could be applied to increase the sensitivity of various commercial IR sensors for THz applications that do not require modiﬁcation of the internal structure, and it may apply also to acoustics and plasmonic detectors.


Introduction
Terahertz (THz) waves have recently received unprecedented interest in product quality control, medicine, biology, 5G and 6G communication, nondestructive testing, and homeland security applications, to name a few [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], that cannot be achieved at optical or infrared (IR) electromagnetic bands. THz detectors play a key role in terahertz technologies. The Golay cell [18] is a "photo-acoustic" device commonly used in the THz range. Bolometers, where the conductivity of the material changes with temperature induced by terahertz radiation, are cooled detectors and highly sensitive. The pyroelectric detector (PD), in which the output current is proportional to the changing rate of temperature of film material [19], has some important advantages such as room temperature operation and low cost. Various types of special THz PDs were developed in the last decade: a PD based on lithium tantalate crystal and film was considered in [20], a tetraaminediphenyl-based PD was investigated in [21], and a PD for THz time-domain spectroscopy (TDS) systems was developed in [22,23]. A PD consists of a pyroelectric polyvinylidene fluoride (PVDF) film coated with a metal oxide layers. It has been shown that to decrease the noise equivalent power value (NEP), the thickness of pyroelectric film should be reduced. However, all these and other similar developments require either the development of a new detector design or replacement of the material in the existing commercial detector. Moreover, the present PD, designed especially for THz range, is expensive and lacks reliability. Presently, PDs are one of the most used devices in uncooled IR detectors, providing a high efficiency of detection of the illuminated radiation, relative short response time, high signal-to-noise ratio, low cost, and high reliability [24][25][26][27].
Usually, the responsivity of PDs in the THz range is improved by: (i) a traditional gate structure with the grating-gate structure [28]; (ii) implementation of a silicon lens [29], diffractive grating [30], or wavelength-scaled dielectric cube [31,32]; and (iii) internal integration of an antenna structure [33,34]. All these methods, except for [31,32], require a change in the internal structure of the THz detector and its design and are intended for special operation in the THz range. In our work, we consider the use of an IR detector for operation in a non-standard THz range not intended by the detector manufacturer and show for the first time that without changing its internal design, it is possible to increase its sensitivity not in the IR but in the THz range. A THz detector based on an IR-PD (LHI778, Perkinelmer), initially designed for a wavelength range of 7-14 µm and used in THz imaging applications, was studied in [35]. A special amplification circuit to amplify and capture the signal from the sensor was designed. A responsivity of about 1.2 kV/W at 1.89 THz was achieved. However, direct application of conventional IR-PD to the THz band faces a problem of decreasing the detector's sensitivity.
Commercial IR-PD (MG-33) is a good substitute for special THz detector. Though it is designed for IR, it retains sensitivity at THz frequencies. Single-element IR-PD has the operation wavelength range between 3 and 20 µm [36] and is based on a free organic films of the PVDF type [23]. An active area of 1 × 1 mm, ensure an NEP sensitivity threshold in the IR range lower than 5 × 10 −10 W·Hz 0.5 and a sensitivity greater than 10 5 V/W [37]. To enhance the PD sensitivity in the THz band [38,39], a combination of a MG-33 detector and a metasurface-enabled absorber was considered in [40]. It was demonstrated that the efficient frequency-and polarization-selective detection in the range of 100-180 GHz with a degradation of NEP by a factor 2. However, this required a change in both the design of the detector itself (including the diameter of the optical window being specially increased from 3.5 mm to 5 mm in [40]) and the development of a new absorber. In addition, due to the nature of the metasurface, the detector has a pronounced resonance character.
The aim of this work is to demonstrate the possibility of the effective use of commercially IR-PD in the THz range without changing their internal design. One of the new approaches to increase the sensitivity of THz detector is based on the so-called terajet effect [41][42][43][44]. This effect is based on a reduction of the focused beam size caustic up to the sub-diffraction value at a distance close to the shadow surface of the wavelength-scaled particle-lens made from a conventional dielectric material [45]. The physics of the terajet effect is a complex interference and scattering phenomenon. The constructive interference between the incident field and diffracted and scattered fields by the wavelength-scaled dielectric particle leads to such a localization of electromagnetic wave in the form of terajet with a beam waist up to sub-diffraction limit. Like photonic jets in optics, it localizes intense electromagnetic beams formed near the shadow surface of a dielectric particle with a size greater or even equal to the illumination wavelength and propagating into the environment. It has recently been demonstrated that placing a dielectric cube in front of a sensitive area of strained silicon field effect transistor [32,46] and point contact detector [31] allows the sensitivity of THz detectors to be increased due to the localization of the incident radiation with the slight decreasing of NEP value. Here, we demonstrate that this method allows one to increase the sensitivity of IR commercial detectors in the THz range with room temperature operation.

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 PD window made of germanium and with a thickness around 70 µm. To increase the coupling efficiency of the PD with THz radiation source at λ= 1.5 mm (frequency of about 0.2 THz), we use a terajet formed by a polytetrafluoroethylene (PTFE) cube with a dimension equal to 2λ. 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 λ [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 2λ 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 × 1.38 × 6.15 mm 3 and 1.22 × 0.76 × 1.84 mm 3 in the xyz directions and for n = 1.41 and 2.1, respectively.

Results and Discussions
The detection measurements were performed with and without the cube particle with different emitted power. The measurements ( Figure 3) show a linear dependence of the voltage (U) with the power at λ = 1.5 mm (this value gives the maximum output power from the BWO). The method for measuring the NEP was similar to that described in Reference [40]. The idea of this configuration is to exploit a dielectric PTFE particle that has a wavelength-scale size to concentrate the radiating THz waves into the sensitive area of PD with a diffraction-limited value. This allows increasing the intensity of signal to be measured, such that the signal-to-noise ratio (SNR) can be improved.  Figure 4 shows the measured photoresponse without the cube (black) and with the cube (red). To demonstrate the sensitivity enhancement, the detector was excited by the source at its lowest power idem when the signal from the receiver (without a particle) was at the noise level. Then, we placed the particle and looked at the increase in the signal and the noise level. The measured amplitude with the cube was 6.1 ± 0.06 mV, which was 5.6 dB higher than that without the enhancer. The standard error was calculated from eight distinct measurements and a Student's (t) coefficient [51] of 2.132, corresponding to a 90% two-sided confidence interval. The standard deviations of the amplitude were measured for the cases without and with the PTFE particle, respectively. These results indicate that the proposed technique can enhance the detected THz intensity of 5.6 dB at 0.2 THz for MG-33 PD. Similar results were obtained for the MG-30 sensor, which has the same internal design as the MG-33 but differs only in standard 1203.15-1 package. The obtained gain is below the theoretical one because of the uncertainties in the setup such the dimension of the active area of the PD, and the distance from the optical window to the active element of the PD was fixed by the manufacturer and unknown. Furthermore, we did not verify the refractive index of particle material experimentally. From a practical point of view, it is quite difficult to perform a precise relative positioning between the cuboid and detector to ensure that the detector is located just at the point of maximum electric field generated by the cuboid. However, knowing the gain obtained experimentally, we can obtain an estimate of the real distance between the cuboid and the detector by looking at Figure 2b.

Conclusions
Commercial IR-PD performance was investigated in the THz range using the terajet effect. A gain enhancement of about 6 dB was experimentally obtained under excitation of 0.2 THz using the commercial PD without degradation of its NEP value. The research results confirm the prospects of pushing the operation frequency of the PD to the THz range where no design modification was required. The entire setup offers a cost-effective solution for sensitivity enhancement of IR-PDs in THz, which can be applied in many research fields, including imaging systems, THz wireless communication, etc. The method of increasing the sensitivity of commercial sensors can also be applied to both acoustics and plasmonics devices.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations
The following abbreviations are used in this manuscript: