{'fraction_non_alphanumeric': 0.024809533111935924, 'fraction_numerical': 0.010353584684508694, 'mean_word_length': 5.228102189781022, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 3, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Video-speed Graphene Modulator Arrays for Terahertz Imaging Applications

Electrically tuneable high mobility charges on graphene yield an efficient electro-optical platform to control and manipulate terahertz (THz) waves. Real-world applications require a multiplex THz device with efficient modulation over a large active area. The trade-off between the efficient gating and switching speed, however, hinders the realization of these applications. Here, we demonstrate a large-format 256-pixel THz modulator which provides high-frame-rate reconfigurable transmission patterns. The time-domain and frequency-domain THz characterizations of graphene devices reveal the relaxation pathways of gate-induced charges and ion packing at graphene-electrolyte interface. The fundamental understanding of these limiting factors enables us to break the trade-off permitting switching frequencies up to 1 kHz. To show the promises of these devices, we demonstrate a single-pixel THz camera which allows spatial and spectroscopic imaging of large-area objects without any moving components. These results provide a significant advancement towards the achievement of non-invasive THz imaging systems using graphene-based platforms.

THz radiation promises a wide range of applications 1 including wireless communication, 2,3 non-invasive spectroscopic imaging, 4 biomedical diagnostics, 5 and product quality control 6 owing to the non-ionizing nature and submillimetre-wavelength. Being between optics and electronics, however, makes the THz waves challenging to access.
Approaching the THz spectrum from the electronic side is limited by the switching speed of transistors. On the other hand, optical methods face the fundamental limitation due to thermal energy (1 THz ~ 4 meV, the thermal energy at room temperature ~ 26 meV). More importantly, the lack of an efficient electro-optic material for THz light has been hindering the development of THz optoelectronic components.
Over the last decade, various materials systems have been explored to control THz radiation; three approaches have outperformed the rest. The first approach relies on tuneable metamaterials where the loss is controlled by depletion of substrate charge carriers. 7 An electrode array using interconnected split-ring resonators fabricated on a doped epitaxial GaAs enables both intensity and phase modulation. Following this seminal work, metamaterial-based spatial light modulators (SLM) and imaging systems have been demonstrated. 8 The requirement of an epitaxial wafer, the complex microfabrication process and the narrow spectral response due the metamaterial resonance are the limitations of this method. The second approach uses photo-induced carriers generated on a semiconductor surface to control the transmission of THz light. 9 Illumination of silicon wafers with high power visible or nearinfrared light photoexcites free charge carriers, thus alters the THz transmission. Structuring the illumination using micromirror arrays has enabled high-resolution THz imaging systems.
The downside of this method is the need for high optical power to pump carriers in the Si wafer.
The third approach relies on the electrostatic tuning of high mobility electrons on graphene, which yields Drude-like metallic behaviour at THz frequencies. 10,11 Altering the carrier concentration via electrostatic gating permits electrical control of reflection and transmission of THz radiation due to the tuneable optical conductivity. 12 The high carrier mobilities for both electrons and holes enable fast optical response covering the entire THz region. This method was first reported by Sensale-Rodriguez et al. using a back-gated graphene transistor 10 and attracted widespread attention. In their device, they used a graphene-dielectric-Si capacitor structure with the Si layer acting as a back-gate. Subsequently, this idea was extended to SLMs by combining back-gated devices into arrays. 13 However, the electrical breakdown of the dielectric layer limits the maximum charge density and consequently, the performance of these devices. Using plasmonic effects on patterned graphene or integrating metallic metamaterials improves the efficiency of the modulators. 11 Very recently, Chen et al. reported THz modulation by operating similar devices around the Brewster angle. 14 Steep variation of the reflectivity and phase around the Brewster angle enables significantly larger modulation of THz intensity and phase. Fabrication of these devices over a large active area is still challenging due to the requirement of an ultrathin large-area dielectric layer.
Electrolyte gating (EG) has been proposed as an alternative scheme to overcome the limitation of dielectric-based devices. [15][16][17] EG can provide an order of magnitude higher charge densities which can significantly alter the optical properties of graphene even in the visible wavelengths. 17,18 The critical enabling element of EG is the self-forming ultrathin electrical double layer (EDL) at the graphene-electrolyte interface that generates large electric fields (~10 9 V/m) in nanometre length scales without an electrical breakdown. The spontaneous formation of the EDL also eliminates the tedious thickness control, and yields charge accumulation over a large area without electrical short circuit. To exploit these superior features, we have been investigating electrolyte-based THz devices. [19][20][21] A trade-off between efficient gating and switching speed has always been a significant concern for practical applications. In this work, we break the trade-off by understanding the dynamics of the ion packing at the graphene electrolyte interface, which enables us to drive the modulators up to 1 kHz frequencies. We address the critical challenges associated with the electrolyte gating and device fabrication and demonstrate a large-format 256-pixel device with video-speed modulation. The results highlight the key capabilities and the scalability of our graphene devices without using any clean room processes or complex material preparation.  To reveal the high-performance capabilities of the device, we characterized a singlepixel device using time-domain THz spectrometer (Toptica TeraFlash). This system contains two fibre-coupled InGaAs antennas that can generate and detect THz pulses with > 4 THz bandwidth and over 90 dB dynamic range (Figure 2a). The device shows an efficient modulation (>70%) for the entire working range of the spectrometer. Fabry-Perot interference through the device yields variation of the THz modulation. Figure 2b shows the change of the normalized transmittance at 1 THz as the voltage bias varies between -4 to 4 V. The THz modulation is due to the Drude response of high mobility electrons and holes which yields frequency ( ) dependent optical conductivity as where is the low frequency electrical conductivity,  is the electron scattering time which decays from 100 fs down to 50 fs as the Fermi energy changes from 0.1 to 1.1 eV. 22 The device shows the maximum THz transmittance at the Dirac point (~ 0.1 V). We observed 80% modulation for 1 THz at a bias voltage of 4 V. It is worth mentioning that, bias voltages > 3 V generate significant hysteresis and a slow response limiting the operation frequency of the device. To provide more insight into the ionic gating, we measured the Fermi energy as a function of bias voltage using optical absorption measurement (see supplementary Figure S4)   reconfigurable THz masks were constructed using tuneable metamaterials 8 and structured illumination of silicon wafers. 9 In contrast, our THz modulator is a broadband electro-optic solution for THz imaging applications. Figure 5a illustrates the experimental setup. THz pulses are generated by the spectrometer, and collimated with a 3″ parabolic mirror. The THz modulator array is placed on the collimated optical path. To minimize diffraction, we place the object very close to the modulator and focus the transmitted pulses on the THz detector. Our time-domain spectroscopy (TDS) system provides fast acquisition of THz pulses (up to 50 pulses/s). We record the transmitted pulses while reconfiguring the mask. Figure 5b and 5c show three sample THz pulses and their spectra. The resolution of the spectrometer is sufficient to resolve the sharp absorption lines of water in the air. Figure 5d shows the variation of the pulse intensity when only a single pixel is modulated. The sequence of intensity measurements with known transmittance patterns enables us to reconstruct the image of an object. Figure 5e shows the reconstructed images of two large-area objects (5×5 cm 2 ) with cross and double-line patterns (photographs are shown in the insets) using the intensity measurement of 256 sequential masks. The passive matrix addressing of pixels limits the performance of the proofof-concept THz imaging system. Higher performance for practical applications is expected with active-matrix addressing of an individual pixels that requires integration of backplane electronics. In summary, we demonstrated a graphene-based SLM that can generate reconfigurable  Additional information: Authors declare no competing financial interests.

Materials and Methods
Terahertz measurements: To investigate the frequency response of single-pixel devices we used Frequency-Domain Terahertz spectrometer (TeraScan from Toptica). It generates CW THz radiation by mixing light from two DFB diode lasers having precisely tuneable slightly different wavelengths near 1550 nm in an InGaAs photo mixer (Fig. 4a). A square wave voltage was applied to the modulator. The cut-off frequency of a modulator was determined by the LCR parameters of the circuit. The capacitance of single-pixel devices has been studied with Rohde&Schwarz Programmable LCR Bridge (Fig. 4e).
The experimental setup for the single-pixel imaging is shown in Fig. 6a. It is based on a timedomain THz spectrometer (TeraFlash from Toptica). THz radiation generated by the emitter passes through the object (metal mask), propagates through the SLM holding a certain transparency pattern, and detected by the detector. The pattern on the SLM was configured using National Instruments DAQ electronics.