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Coherent Detection of Terahertz Radiation with Graphene Petr A. Obraztsov,*,†,‡ Pavel A. Chizhov,† Tommi Kaplas,‡,§ Vladimir V. Bukin,† Martti Silvennoinen,∥ Cho-Fan Hsieh,⊥ Kuniaki Konishi,# Natsuki Nemoto,¶ and Makoto Kuwata-Gonokami¶ †

A.M. Prokhorov General Physics Institute, Russian Academy of Sciences, Moscow, 119991, Russia Institute of Photonics, University of Eastern Finland, Joensuu 80101, Finland § Department of Optoelectronics, Center for Physical Sciences and Technology, Vilnius 10257, Lithuania ∥ Simitec Ltd, Joensuu 80160, Finland ⊥ Industrial Technology Research Institute, Chutung, Hsinchu, Taiwan 31057, R.O.C # Institute for Photon Science and Technology, The University of Tokyo, Tokyo 113-0033, Japan ¶ Department of Physics, The University of Tokyo, Tokyo 113-0033, Japan

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S Supporting Information *

ABSTRACT: Despite that significant efforts have been made in the development of time-integrated graphene-based detectors operating in vis/IR/THz ranges, little is known about coherent detection of THz pulses with graphene. To date only a few timeresolved studies with on-chip detection schemes, which significantly limit the spectral range naturally provided by the gapless band structure of graphene, are known. Here we demonstrate free-space room-temperature detection of THz radiation in a wide spectral range with optically gated graphene. The detection principle is based on registration of the time-domain waveform of the THz field by measuring the hot-carrier photocurrent under THz pulse exposure in optically excited graphene using a pump−probe scheme. The applied method is simple and robust, while the sensitivity and working range of the developed graphene-based detector are comparable and in some aspects outperform materials conventionally used for terahertz timedomain spectroscopy based on electro-optic sampling and photoconductive antennas. In addition, we demonstrate that efficient coherent detection of terahertz radiation in a wide range to above 2 THz does not require highly crystalline, single-layer graphene but can be also realized with ultrathin graphite film, which is synthesized directly on an arbitrary dielectric substrate. Employment of such a material for fabrication of ultrafast terahertz detectors creates a versatile platform for the scalable production of wide-aperture photoconductive detectors applicable in spatially resolved time-domain terahertz spectrometers and visualizers. KEYWORDS: graphene, terahertz detector, time-domain terahertz spectroscopy, photoconductivity, hot carriers

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femtosecond laser systems was demonstrated by use of photoconductive antennas,1,4,5 electro-optical sampling,6,7 and second-harmonic generation in optical breakdown plasma.8 Though these methods are widely used, they are usually implemented as a one-point detector. There are also some techniques that provide a route to perform time-domain

erahertz radiation has wide perspectives of use from security to medicine. Development of femtosecond lasers and demonstration of the time-domain spectroscopy (TDS) technique induced scientific interest in the THz range.1−3 In contrast to power-sensitive detectors such as bolometers or photothermoelectric detectors, this technique enables a direct sampling of the electric field transient of a THz radiation pulse. From this transient waveform, the spectral content and phase information can be extracted via Fourier transform. The pump−probe detection of few-cycle THz pulses induced by © 2019 American Chemical Society

Received: April 9, 2019 Published: May 22, 2019 1780

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Figure 1. Scheme of the time-resolved THz detection experiment with a graphene detector and the operation principle of the device. The setup consists of two optical arms for THz generation and for an optical gating pulse with delay line. The THz signal is generated either in air-breakdown (plasma source) or in a lithium niobate crystal (LiNbO3 source).

Figure 2. Typical waveforms of photocurrent pulses induced by only optical J800nm and only THz JTHz are shown with dashed red and blue lines, correspondingly. The solid green and black lines represent the waveforms of the photoresponse obtained when the graphene detector is excited simultaneously by optical and THz radiation at random and 0 delay time between the pulses. Inset: Dependence of J800nm on the polarization angle of laser light.

Figure 3. Principal of time-domain detection. At negative delays (when the optical pulse reaches the graphene surface earlier then the THz pulse) there is only a minor negative Joffset signal. At zero delay time a small positive Jcoherent signal appears. By stepwise changing the delay time between pulses with a step of 100 fs the amplitude and polarity of Jcoherent strongly depends on the delay line position. Within the time window of a few picoseconds Jcoherent twice changes its sign (signal polarity) following the electric field temporal profile of the incident terahertz pulse. By plotting the photocurrent signal amplitude absolute value as a function of delay time we are able to retrieve the time-domain profile of Jcoherent.

measurements with wide-aperture electro-optic crystals.9−11 However, these techniques are strictly limited by size of the detection crystal, whereas fabrication of a large-aperture crystal is difficult and expensive. These obstacles have driven researchers to seek pathways to develop technology for inexpensive and scalable production of arrays of wide-aperture coherent detectors.12−14 For example, recently the combination of optically gated black phosphorus with a log-periodic antenna was proposed as a potential way to fabricate large-area time-resolved THz photoconductive detectors and emitters.15 An efficient THz photoconductive detection requires availability of certain parameters for the active medium of the sensor including a combination of appropriate absorptivity with ultrashort lifetime and high mobility of the photoexcited charge carriers. Graphene perfectly corresponds to these demands due to its intrinsically ballistic charge carriers, reaching a mobility of up to 180 000 cm2·V−1·s−1 at room temperature,16,17 and ultrafast relaxation time of excited carriers.18 Moreover, due to the gapless Dirac-cone electron band structure providing 2.3%19 optical absorption in a very

broad spectral range in graphene, charge carriers can be excited even with low-energy photons (i.e., in the infrared range). These properties make graphene competitive with materials commonly used for THz photoconductive antennas/switches such as low-temperature-grown or Cr-doped GaAs.20 There are various approaches allowing measurement of THz radiation power with graphene by bolometric response,21 photovoltaic,22 and photothermoelectric effect23−25 and by means of graphene-based field effect transistors (FETs).26−28 However, so far only on-chip coherent pulse detection and generation has been demonstrated.29,30 Furthermore, despite that graphene shows tremendous potential for THz detection, there are yet at least two limitations hampering the use of graphene for this purpose: (i) low responsivity due to the limited total absorption in atomically thin layers and (ii) 1781

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Figure 4. Normalized transients of THz pulses measured with ZnTe electro-optic sampling, a GaAs photoconductive detector, and the developed graphene detector and THz spectra found via the Fourier transformation of the corresponding waveforms are shown in (a, b, c) and (d, e, f). Temporal profiles registered with the graphene detector reproduce with a good accuracy those obtained via EO sampling and demonstrate linear feedback of the photocurrent Jcoherent induced in graphene on an applied AC THz field.

Figure 5. Image of the fabricated array of 15 directly deposited graphite THz detectors and microscopic image of a single detector.

moderate lateral dimensions of graphene, limiting scalability of the device fabrication. Various methodologies have been proposed to improve sensitivity of graphene-based photodetection via enhancement of the light−matter interaction of the single atomic layer. It includes, for instance, enhancing an optical field by a resonant31 or a waveguide structure.32 Another approach is based on the use of plasmon resonance33,34 or charge transfer in multilayered graphene/2D heterostructures.35,36 However, these techniques crucially depend on the quality of the material and require complex production technologies. In recent years a lot of work has been dedicated to implement graphene into industrial-scale processes. However, scaling up graphene production has shown to be a rather challenging task. Conventionally graphene is grown on metal foils by chemical vapor deposition (CVD) and thereafter transferred on a dielectric substrate.37 The transfer process is time-consuming and usually done manually, which is not appealing for industrial-scale processing. In contrast, direct, transfer-free synthesis provides a scalable technology for graphene production, and thus these techniques are eagerly awaited. Such techniques would allow one to avoid the complex and expensive substrate-to-substrate transfer step and

Figure 6. Normalized differential transmission for single-layer graphene (blue color) and ultrathin graphite (red color) measured under similar experimental conditions: pump wavelength 775 nm, probe wavelength 1300 nm. Biexponential fitting is shown with dashed lines of corresponding color. 1782

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Figure 7. Normalized waveforms of THz pulses generated by (a) LiNbO3 pumped at 775 nm, (b) optical breakdown in air, (c) LiNbO3 pumped at 1028 nm sources measured with fabricated graphitic photoconductive detectors and corresponding frequency spectra in comparison with the reference data obtained using the conventional EO sampling method. The data obtained with the GRF detector are shown with red color, and reference data are shown with blue color. The dashed lines in (d)−(f) define the frequency at which the measured signal is equal to the noise.

In order to study the dynamic THz response and to characterize the performance of the fabricated graphene device as a coherent detector, we performed femtosecond laser-driven pump−probe measurements traditionally used for THz timedomain spectroscopy. As a source of THz pulses we employed a well-known scheme for generation of high-power coherent terahertz radiation by optical rectification of tilted-intensityfront femtosecond pulses in a LiNbO3 nonlinear crystal.41 In the experiment the output beam of the Ti:sapphire regenerative amplifier (Coherent Legend Elite, 775 nm fundamental wavelength, 2.6 mJ pulse energy, 150 fs pulse duration, 1 kHz repetition rate) is split into two arms with a 90/10 beam splitter. A major portion of the laser beam is used to pump the LiNbO3 THz source.41 The output laser-driven THz beam is collimated and focused with two polytetrafluoroethylene (PTFE) lenses on the central part of graphene detector between the electrodes with focal lengths of the lenses of f = 60 mm and f = 100 mm and with a radiation spot size of ∼1 mm. A minor portion of the laser beam is focused on the graphene device with an intensity spot

should promote better compatibility with industrial-scale implementation and modern optoelectronics.38 In this work, we report on time-domain detection of THz radiation by optically gated graphene. We fabricated and evaluated a set of THz photoconductive detectors using graphene and ribbons of ultrathin graphitic film (GRF) synthesized directly on a silica substrate by the CVD method. Direct comparison of coherent photocurrent response of optically gated devices based on single-layer CVD graphene and GRF ribbons shows that both materials can be used for THz time-domain detection, while the sensitivity and applicability of directly deposited GRF outperforms singlelayer graphene. We start our study from the proof of concept experiment demonstrating the possibility of coherent detection of THz pulses propagating in free-space with optically gated graphene. For this study we transferred a single-layer CVD graphene (size of about 1 × 1 cm2)37,39,40 onto a silica substrate terminated with a two parallel conductive stripe-line electrodes (see Figure 1). 1783

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Figure 8. Peak amplitude of the Jcoherent signal as a function of the applied optical gating (a) and THz pulse power (b). The dashed lines on both graphs represent the offset level associated with JTHz and J800nm, correspondingly.

size diameter of ∼500 μm to optically excite the device. This weaker beam serves as an optical gating for the time-resolved detection. The optical and THz beams are spatially overlapped on the detector surface. The time delay between the optical and THz pulses at the detector position is controlled by a motorized delay line in the probe optical arm. The detector is mounted on a rotation optical stage, providing the possibility to vary the incidence angle of optical and THz radiation and orientation of electrodes without introducing an additional time delay or changing the detector position. The conductive electrodes of the fabricated graphene device are connected through a coaxial cable to a 600 MHz digital oscilloscope (LeCroy 62Xi) with a 50 Ω input impedance under shortcircuit conditions. The experimental scheme and the device are sketched in Figure 1. An average power and temporal (spectral) characteristics of the THz pulses produced by the used THz source were characterized correspondingly with a Golay-cell detector (Tydex GC-1P) and a standard time-domain electro-optic sampling system consisting of a 0.5 mm thick ZnTe crystal (cut ⟨110⟩), a quarter-waveplate, a Wollaston prism, and a pair of balanced Si photodiodes, placed instead of a graphene detector. The tilted-intensity-front THz source provides generation of single-cycle THz pulses with an average frequency of 0.3 THz and energy of about ∼7 nJ (∼7 μW average power on the Golay detector), which corresponds to about ∼15 kV/cm THz peak field at the detector. To get additional reference data, we also measured the time-domain waveform of the emitted THz pulses with a homemade largearea GaAs photoconductive switch. It has been already demonstrated that excitation of unbiased graphene with femtosecond optical pulses induces ultrafast photocurrent response assigned to a photon-drag effect also leading to THz generation.42−46 These second-order nonlinear photocurrents strongly depend on polarization, incidence angle, and intensity of the incident laser beam, and in centrosymmetric graphene (in a general case) they can be obtained only under oblique incidence. Similar direct-current (DC) photocurrent response under excitation with CW THz/ FIR free-electron lasers has been demonstrated and interpreted in terms of photon-drag and photogalvanic effects, arising due

to substrate- and edges-induced symmetry reduction of graphene.47,48 It is worth noting that the later effects demonstrate very similar dependence on the polarization state of light, and distinguishing between two effects is not trivial. In the current study we are not discussing the microscopic mechanisms underlying the polarization-sensitive nonlinear photocurrents but make use of them as a calibration signal to align our setup. We first installed our device at a 45° angle to both optical and THz beams to obtain maximum amplitude of the photocurrent signal. By irradiating the unbiased graphene device alternately with a femtosecond laser or THz pulses we were able to detect the corresponding photocurrent response as a voltage drop, U ∝ J, across a 50 Ω load resistor of a storage oscilloscope. Fine adjustments of the optical and THz beams’ spot sizes on the graphene surface and orientation of the detector allowed us to optimize amplitudes of both signals: the stronger signal of negative polarity, associated with photocurrent response on optical excitation (J800nm), and a weaker positive polarity signal, associated with THz-induced photocurrent (JTHz). The pronounced dependence of these photocurrent signals’ amplitude and polarity on polarization and incidence angle of incoming radiation confirms their either photon-drag or photogalvanic origin (see inset of Figure 2). The typical waveforms of the photocurrent pulses induced by only optical J800nm and only THz JTHz excitation are shown in Figure 2 with dashed red and blue lines, correspondingly. It is important to note here that the temporal profile of the obtained photocurrent waveforms is determined by the registration system bandwidth rather than by the properties of the graphene detector or actual temporal characteristics of the induced photocurrent, while its magnitude represents the time-integrated response. The solid green and black lines in Figure 2 represent the waveforms of the photoresponse obtained when the graphene detector is excited simultaneously by optical and THz radiation at the random (no temporal overlapping of THz and laser pulses) and zero delay time (temporal overlapping of THz and laser pulses) between the pulses. As one can clearly see from Figure 2 the joint J800nm+THz signal of negative polarity measured at a random delay time is 1784

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width (tc.l. ≪ tTHz) J(t) ∼ E(t), and in the reversed situation (tc.l. ≫ tTHz) J(t) ∼ ∫ t∞E(t) dt.29 Consequently the shorter the lifetime of excited carriers in the detector material, the more a precise temporal profile and/or broader spectrum of the THz pulse can be detected. On the other hand the photocurrent induced under simultaneous excitation of a photoconductive detector with optical and THz pulses J(t) ∼ NeμE(τ) is a function of the number of excited carriers N and their mobility μ. Therefore, enhancement of the number of highly mobile excited carriers would increase the response amplitude of the photoconductive device. Recently it has been shown that in graphene besides the photoconduction effect other mechanisms such as the hotcarrier bolometric effect could also lead to ultrafast photodetection.50−52 However, since both effects occur at the same time and rely on enhanced carrier density, it is not trivial to distinguish between them.53,54 In contrast to the photoconduction effect, the characteristic time of conductivity change in the hot-carrier bolometric effect is linked to the relaxation of electronic temperature (intraband carrier cooling time)22 rather than to the carrier lifetime. The creation and subsequent ultrafast relaxation of photocarriers in graphene have been extensively studied both theoretically and experimentally and are governed by the interplay of efficient carrier−carrier and carrier−optical−phonon scattering. In general carrier dynamics can be considered as the following process: A linearly polarized ultrafast optical pump pulse gives rise to an initially anisotropic distribution of carriers at higher energies. Efficient carrier−carrier and carrier−phonon interactions quickly relax the hot carriers to an isotropic thermal distribution, which is then followed by carrier cooling as energy is transferred from the carrier population to the lattice. The whole relaxation process lasts only a few picoseconds after initial ultrafast pulse excitation.18,55,50,56 Therefore, regardless of the specific microscopic mechanism responsible for the change of conductivity (intraband carrier cooling or interband relaxation), the change of conductivity is ultrafast and occurs within a few picoseconds. Moreover, this time is an intrinsic characteristic of graphitic material and weakly depends on the macroscopic quality of the sample.57 After successfully demonstrating the proof-of-concept with a monolayer graphene detector we carried out measurements with directly deposited GRF microribbons. For this purpose, we fabricated an array of 15 THz detectors with Ni−Au contacts. It is worth noticing that the synthesis of GRF ribbons is a wafer-scale process and the number of THz detectors was limited by the size of the Ni−Au electrodes. A detailed description of the transfer-free synthesis and characterization of the ultrathin graphitic material is presented in the Supporting Information and in refs 58 and 59. The image of 15 detectors on a 0.5 mm thick fused silica substrate is shown in Figure 5. Each detector consists of an 80 nm thick and 250 μm wide ribbon of graphitic material contacted by gold electrodes. The contact resistance of the graphitic ribbons and gold electrodes was reduced by depositing a 10 nm thin layer of Ni in between ribbons and contacts. The impedance between the contacts in different devices varied from 100 to 300 Ω. We perform a comparative study of ultrafast carrier dynamics in the fabricated directly deposited graphite ribbons and graphene. To reveal the real relaxation rates of photoexcited carriers in particular graphene and directly deposited graphitic ribbons to be used in THz detection

the sum of J800nm and JTHz signals. However, the amplitude of the waveform measured at zero delay differs significantly from the sum of signals. The change of amplitude obtained at zero delay time cannot be explained by any type of interference or interaction of polarization-sensitive photocurrents and therefore points out that the response obtained under excitation by temporally overlapped THz and optical pulses has a different origin. We will further distinguish between the “timeintegrated” offset photocurent Joffset independent of the delay time between THz and optical pulses and coherent response Jcoherent extremely sensitive to the delay time between pump and probe pulses. To additionally exclude the photon-drag or photogalvanic contribution to the generated coherent signal, the position of the graphene detector was changed to provide near normal incidence angle for both THz and 800 nm beams. Indeed, at normal incidence the amplitude of Joffset vanishes (in accordance with symmetry43) while the amplitude of the Jcoherent remains constant and does not depend on the polarization of the optical pump. We further measured the photocurrent signal at different time delays between optical and THz pulses. The principal of this measurement is depicted in Figure 3. Figure 3 shows that at negative delays (when the optical pulse reaches the graphene surface earlier then the THz pulse) there is only a minor negative Joffset signal. At zero delay time a small positive Jcoherent signal appears. By stepwise changing the delay time between pulses with a step of 100 fs we found that the amplitude and polarity of Jcoherent strongly depend on the delay line position. Within the time window of a few picoseconds Jcoherent twice changes its sign (signal polarity) following the electric field temporal profile of the incident terahertz pulse. By plotting the Jcoherent amplitude absolute value as a function of delay time we are able to retrieve the time-domain profile of the THz pulse as shown in Figure 3. The normalized transients of THz pulses measured with ZnTe electro-optic sampling, a GaAs photoconductive detector, and the manufactured graphene detector and THz spectra (determined via Fourier transformation of the corresponding waveforms) are shown in Figure 4(a,b,c) and (d,e,f), respectively. As one can see in Figure 4 temporal profiles registered with the graphene detector reproduce with a good accuracy those obtained via electro-optics (EO) sampling and demonstrate linear feedback of the photocurrent Jcoherent induced in graphene on an applied AC THz field. It allows us to conclude that the photocurrent response of the graphene detector is due to the instantaneous ultrafast response of hot carriers excited with femtosecond pulses (optically gated graphene) driven by a quasi-constant (in comparison with the pulse duration) THz pulse electric field. We now move to a discussion on the operation principles of the demonstrated coherent detection with optically gated graphene. In conventional unbiased photoconductive THz antenna detectors the incident terahertz radiation induces an electric field in the device active region. Under action of this field the carriers generated by the ultrafast laser pulse drift to the antenna electrodes so that a photocurrent proportional to the strength of the incident terahertz field at the arrival time of the optical pulse is induced.49 Therefore, the induced photocurrent as a function of delay time between optical and THz pulses can be explained as a convolution of the THz electric field and the change of conductivity due to photoexcitation of carriers: J(t) ∼ ∫ E(τ)THz σ(t−τ) dτ. In the case of short carrier lifetime as compared to the THz pulse 1785

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low dark resistivity of graphene and relatively large distance between conductive electrodes in the developed devices, we expect that application of a very large voltage will be required to achieve THz peak amplitudes (∼kV/cm) that are detectable employing the kHz repetition rate laser systems. Applying such a large voltage could lead to the flow of large currents and a short-circuit/damage of graphene even in the absence of an optical gate. The development of such a graphene-based emitter will require fabrication of devices with a much smaller interelectrode distance and/or application of external voltage in a pulsed regime. This will require further studies and will be reported elsewhere.

experiments, we employed a multicolor transient absorption pump−probe setup based on the same Ti:sapphire 150 fs laser as used in our THz experiments. A detailed description of the method and setup can be found elsewhere.55 The normalized differential transmissions of both samples measured at 1300 nm under excitation with a 775 nm pump are presented in Figure 6. The revealed ultrafast dynamics accords with the carrier relaxation scenario discussed above. The biexponential fitting of the induced absorption transients reveals two characteristic time constants: for ultrathin graphite τ1 = 340 ± 20 fs and τ2 = 1250 ± 280 fs and τ1 = 220 ± 10 fs and τ2 = 910 ± 35 fs for graphene, respectively. We evaluate the performance of graphitic photoconductive detectors fabricated by direct deposition using the LiNbO3 THz source pumped at 775 nm (described above) and two additional coherent sources: (i) a LiNbO3 THz source pumped with a Yb:KGW-femtosecond system operating at a 1028 nm central wavelength, generating single-cycle THz pulses with an average frequency of 0.6 THz, and (ii) a wideband THz source based on the optical breakdown (plasma) in air induced by two-color femtosecond laser pulses providing pulses with an average frequency of 0.5 THz but an almost 2 orders lower pulse energy of about 0.25 nJ.11 The former setup provides the possibility to use fundamental (1028 nm) and second-harmonics (514 nm) radiation as an optical gate for coherent photoconductive detection. As a reference detector for the source pumped by a 1028 nm laser we employed EO sampling with a 1 mm thick GaP crystal (cut ⟨110⟩). The normalized waveforms of THz pulses generated by all three sources measured with fabricated graphitic photoconductive detectors and corresponding frequency spectra in comparison with the reference data obtained using the EO sampling method are shown in Figure 7. Similar to the graphene detector (see Figure 4) the waveforms obtained with the graphite detector (fabricated using the direct deposition) are in a good agreement with reference data. The minor discrepancy in the shape of the measured waveforms is due to slightly different positioning of the reference electro-optic and graphite detectors. To confirm the linear operation of the detector, the peak amplitude of the Jcoherent-induced waveform was measured as a function of the optical gating and THz pulse power. As shown in Figure 8, the amplitude of the induced signal scales as linear and square root functions of the input optical and THz power, correspondingly. Square root dependence on the THz power corresponds to the linear dependence on the THz field amplitude. It is worth noticing that the sensitivity of the single-layer graphene photodetector was insufficient to detect the lowpulse-energy THz pulses from the plasma source. However, the detection was done successfully with the GRF detector. The dashed lines in Figure 7(d−f) define the frequency at which the measured signal is equal to the noise. Since the shape and the noise level of the THz pulses detected by both graphene and directly deposited GRF are in a good agreement with the EO-sampling reference data, we conclude that these materials can be used for efficient coherent detection of terahertz radiation. Furthermore, the demonstrated efficient photoconductive detection opens up the possibility of utilizing graphene and GRF as a photoconductive THz emitter by applying high bias voltage. However, taking into account the

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CONCLUSIONS We report a simple and robust approach for room-temperature time-domain detection of a terahertz field generated with femtosecond pulses. The approach is based on the use of ultrafast hot-carrier dynamics in photoexcited graphene instead of conventional electro-optics sampling to detect the generated THz waveform. The sensitivity of our approach is comparable to traditional nonlinear crystals and photoconductive antennas, while it does not require balanced detection and is applicable at an extraordinary wide range of pump photon energies available due to the zero bandgap of graphene, which is hard to reach in semiconductor-based devices. Moreover, we demonstrate that efficient coherent detection of terahertz radiation in the range exceeding 2 THz can be realized with directly grown, ultrathin graphite films. Employing such a material for fabrication of ultrafast terahertz detectors creates a versatile platform for scalable production of wide-aperture photoconductive detectors applicable in spatially resolved timedomain terahertz spectrometers and visualizers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.9b00536. Direct deposition of ultrathin graphitic material; characterization of ultrathin graphitic material; fabrication of THz detectors (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Petr A. Obraztsov: 0000-0002-4696-8143 Tommi Kaplas: 0000-0003-1688-3453 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Russian Science Foundation (grant #17-72-10303), Basic Research Program of the Presidium of Russian Aacademy of Sciences (#6), European Regional Development Fund under Measure No. 01.2.2-LMTK-718 (grant # DOTSUT-247), Academy of Finland (grants: #318596; #287886; #298298), JSPS international joint research program and the Photon Frontier Network Program funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. 1786

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