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Excitation of Dirac plasmon polaritons (DPPs) in bi-dimensional materials have attracted considerable interest in recent years, both from perspectives of understanding their physics and exploring their transformative potential for nanophotonic devices, including ultra-sensitive plasmonic sensors, ultrafast saturable absorbers, modulators, and switches. Topological insulators (TIs) represent an ideal technological platform in this respect because they can support plasmon polaritons formed by Dirac carriers in the topological surface states. Tracing propagation of DPPs is a very challenging task, particularly at terahertz (THz) frequencies, where the DPP wavelength becomes over one order of magnitude shorter than the free space photon wavelength. Furthermore, severe attenuation hinders the comprehensive analysis of their characteristics. Here, the properties of DPPs in real TI-based devices are revealed. Bi2Se3 rectangular antennas can efficiently confine the propagation of DPPs to a single dimension and, as a result, enhance the DPPs visibility despite the strong intrinsic attenuation. The plasmon dispersion and loss properties from plasmon profiles are experimentally determined, along the antennas, obtained using holographic near-field nano-imaging in a wide range of THz frequencies, from 2.05 to 4.3 THz. The detailed investigation of the unveiled DPP properties can guide the design of novel topological quantum devices exploiting their directional propagation.
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Low-dimensional nanosystems are promising candidates for manipulating, controlling, and capturing photons with large sensitivities and low noise. If quantum engineered to tailor the energy of the localized electrons across the desired frequency range, they can allow devising of efficient quantum sensors across any frequency domain. Here, we exploit the rich few-electron physics to develop millimeter-wave nanodetectors employing as a sensing element an InAs/InAs0.3P0.7 quantum-dot nanowire, embedded in a single-electron transistor. Once irradiated with light, the deeply localized quantum element exhibits an extra electromotive force driven by the photothermoelectric effect, which is exploited to efficiently sense radiation at 0.6 THz with a noise equivalent power <8 pWHz-1/2 and almost zero dark current. The achieved results open intriguing perspectives for quantum key distributions, quantum communications, and quantum cryptography at terahertz frequencies.
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Uncooled terahertz photodetectors (PDs) showing fast (ps) response and high sensitivity (noise equivalent power (NEP) < nW/Hz1/2) over a broad (0.5-10 THz) frequency range are needed for applications in high-resolution spectroscopy (relative accuracy â¼10-11), metrology, quantum information, security, imaging, optical communications. However, present terahertz receivers cannot provide the required balance between sensitivity, speed, operation temperature, and frequency range. Here, we demonstrate uncooled terahertz PDs combining the low (â¼2000 kB µm-2) electronic specific heat of high mobility (>50â¯000 cm2 V-1 s-1) hexagonal boron nitride-encapsulated graphene, with asymmetric field enhancement produced by a bow-tie antenna, resonating at 3 THz. This produces a strong photo-thermoelectric conversion, which simultaneously leads to a combination of high sensitivity (NEP ≤ 160 pW Hz-1/2), fast response time (≤3.3 ns), and a 4 orders of magnitude dynamic range, making our devices the fastest, broad-band, low-noise, room-temperature terahertz PD, to date.
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Although the detection of light at terahertz (THz) frequencies is important for a large range of applications, current detectors typically have several disadvantages in terms of sensitivity, speed, operating temperature, and spectral range. Here, we use graphene as a photoactive material to overcome all of these limitations in one device. We introduce a novel detector for terahertz radiation that exploits the photothermoelectric (PTE) effect, based on a design that employs a dual-gated, dipolar antenna with a gap of â¼100 nm. This narrow-gap antenna simultaneously creates a pn junction in a graphene channel located above the antenna and strongly concentrates the incoming radiation at this pn junction, where the photoresponse is created. We demonstrate that this novel detector has an excellent sensitivity, with a noise-equivalent power of 80 pW/[Formula: see text] at room temperature, a response time below 30 ns (setup-limited), a high dynamic range (linear power dependence over more than 3 orders of magnitude) and broadband operation (measured range 1.8-4.2 THz, antenna-limited), which fulfills a combination that is currently missing in the state-of-the-art detectors. Importantly, on the basis of the agreement we obtained between experiment, analytical model, and numerical simulations, we have reached a solid understanding of how the PTE effect gives rise to a THz-induced photoresponse, which is very valuable for further detector optimization.
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At terahertz (THz) frequencies, scattering-type scanning near-field optical microscopy (s-SNOM) based on continuous wave sources mostly relies on cryogenic and bulky detectors, which represents a major constraint for its practical application. Here, we devise a THz s-SNOM system that provides both amplitude and phase contrast and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a quantum cascade laser that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique. We demonstrate its performance by probing a phonon-polariton-resonant CsBr crystal and doped black phosphorus flakes.
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Topological insulators (TIs) represent a novel quantum state of matter, characterized by edge or surface-states, showing up on the topological character of the bulk wave functions. Allowing electrons to move along their surface, but not through their inside, they emerged as an intriguing material platform for the exploration of exotic physical phenomena, somehow resembling the graphene Dirac-cone physics, as well as for exciting applications in optoelectronics, spintronics, nanoscience, low-power electronics, and quantum computing. Investigation of topological surface states (TSS) is conventionally hindered by the fact that in most of experimental conditions the TSS properties are mixed up with those of bulk-states. Here, we activate, probe, and exploit the collective electronic excitation of TSS in the Dirac cone. By engineering Bi2Te(3-x)Sex stoichiometry, and by gating the surface of nanoscale field-effect-transistors, exploiting thin flakes of Bi2Te2.2Se0.8 or Bi2Se3, we provide the first demonstration of room-temperature terahertz (THz) detection mediated by overdamped plasma-wave oscillations on the "activated" TSS of a Bi2Te2.2Se0.8 flake. The reported detection performances allow a realistic exploitation of TSS for large-area, fast imaging, promising superb impacts on THz photonics.
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We report on the development of an innovative class of nanowire-based Terahertz (THz) detectors in which the metamaterial properties of an antenna have been imported in the detection scheme of an overdamped plasma-wave field-effect transistor making its response resonant to THz radiation. Responsivities of ~105 V/W at 0.3 THz, with noise equivalent power levels ≈ 10(-10) W/âHz, detectivities ~2 · 10(8) cmâHz/W and quantum efficiencies ~1.2 · 10(-5) are reached at room-temperature. The resonant nature of the detection scheme provided by the four-leaf-clover-shaped geometry and the possibility to extend this technology to large multi-pixel arrays opens the path to demanding applications for ultra-sensitive metrology, spectroscopy and biomedicine.
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Polaritons in two-dimensional layered crystals offer an effective solution to confine, enhance and manipulate terahertz (THz) frequency electromagnetic waves at the nanoscale. Recently, strong THz field confinement has been achieved in a graphene-insulator-metal structure, exploiting THz plasmon polaritons (PPs) with strongly reduced wavelength (λp ≈ λ0/66) compared to the photon wavelength λ0. However, graphene PPs propagate isotropically, complicating the directional control of the THz field, which, on the contrary, can be achieved exploiting anisotropic layered crystals, such as orthorhombic black-phosphorus. Here, we detect PPs, at THz frequencies, in hBN-encapsulated black phosphorus field effect transistors through THz near-field photocurrent nanoscopy. The real-space mapping of the thermoelectrical near-field photocurrents reveals deeply sub-wavelength THz PPs (λp ≈ λ0/76), with dispersion tunable by electrostatic control of the carrier density. The in-plane anisotropy of the dielectric response results into anisotropic polariton propagation along the armchair and zigzag crystallographic axes of black-phosphorus. The achieved directional subwavelength light confinement makes this material system a versatile platform for sensing and quantum technology based on nonlinear optics.
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Optical resonators are fundamental building blocks of photonic systems, enabling meta-surfaces, sensors, and transmission filters to be developed for a range of applications. Sub-wavelength size (< λ/10) resonators, including planar split-ring resonators, are at the forefront of research owing to their potential for light manipulation, sensing applications and for exploring fundamental light-matter coupling phenomena. Near-field microscopy has emerged as a valuable tool for mode imaging in sub-wavelength size terahertz (THz) frequency resonators, essential for emerging THz devices (e.g. negative index materials, magnetic mirrors, filters) and enhanced light-matter interaction phenomena. Here, we probe coherently the localized field supported by circular split ring resonators with single layer graphene (SLG) embedded in the resonator gap, by means of scattering-type scanning near-field optical microscopy (s-SNOM), using either a single-mode or a frequency comb THz quantum cascade laser (QCL), in a detectorless configuration, via self-mixing interferometry. We demonstrate deep sub-wavelength mapping of the field distribution associated with in-plane resonator modes resolving both amplitude and phase of the supported modes, and unveiling resonant electric field enhancement in SLG, key for high harmonic generation.
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The growth of semiconductor nanowires (NWs) has recently opened new paths to silicon integration of device families such as light-emitting diodes, high-efficiency photovoltaics, or high-responsivity photodetectors. It is also offering a wealth of new approaches for the development of a future generation of nanoelectronic devices. Here we demonstrate that semiconductor nanowires can also be used as building blocks for the realization of high-sensitivity terahertz detectors based on a 1D field-effect transistor configuration. In order to take advantage of the low effective mass and high mobilities achievable in III-V compounds, we have used InAs nanowires, grown by vapor-phase epitaxy, and properly doped with selenium to control the charge density and to optimize source-drain and contact resistance. The detection mechanism exploits the nonlinearity of the transfer characteristics: the terahertz radiation field is fed at the gate-source electrodes with wide band antennas, and the rectified signal is then read at the output in the form of a DC drain voltage. Significant responsivity values (>1 V/W) at 0.3 THz have been obtained with noise equivalent powers (NEP) < 2 × 10(-9) W/(Hz)(1/2) at room temperature. The large existing margins for technology improvements, the scalability to higher frequencies, and the possibility of realizing multipixel arrays, make these devices highly competitive as a future solution for terahertz detection.
Assuntos
Nanoestruturas/química , Nanotecnologia/instrumentação , Radiometria/instrumentação , Radiação Terahertz , Transistores Eletrônicos , Desenho de Equipamento , Análise de Falha de Equipamento , Nanoestruturas/ultraestrutura , Doses de Radiação , TemperaturaRESUMO
The on-chip integration of two-dimensional nanomaterials, having exceptional optical, electrical, and thermal properties, with terahertz (THz) quantum cascade lasers (QCLs) has recently led to wide spectral tuning, nonlinear high-harmonic generation, and pulse generation. Here, we transfer a large area (1 × 1 cm2) multilayer graphene (MLG), to lithographically define a microthermometer, on the bottom contact of a single-plasmon THz QCL to monitor, in real-time, its local lattice temperature during operation. We exploit the temperature dependence of the MLG electrical resistance to measure the local heating of the QCL chip. The results are further validated through microprobe photoluminescence experiments, performed on the front-facet of the electrically driven QCL. We extract a heterostructure cross-plane conductivity of kâ¥= 10.2 W/m·K, in agreement with previous theoretical and experimental reports. Our integrated system endows THz QCLs with a fast (â¼30 ms) temperature sensor, providing a tool to reach full electrical and thermal control on laser operation. This can be exploited, inter alia, to stabilize the emission of THz frequency combs, with potential impact on quantum technologies and high-precision spectroscopy.
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Chip-scale, electrically-pumped terahertz (THz) frequency-combs (FCs) rely on nonlinear four-wave-mixing processes, and have a nontrivial phase relationship between the evenly spaced set of emitted modes. Simultaneous monitoring and manipulation of the intermode phase coherence, without any external seeding or active modulation, is a very demanding task for which there has hitherto been no technological solution. Here, a self-mixing intermode-beatnote spectroscopy system is demonstrated, based on THz quantum cascade laser FCs, in which light is back-scattered from the tip of a scanning near-field optical-microscope (SNOM) and the intracavity reinjection monitored. This enables to exploit the sensitivity of FC phase-coherence to optical feedback and, for the first time, manipulate the amplitude, linewidth and frequency of the intermode THz FC beatnote using the feedback itself. Stable phase-locked regimes are used to construct a FC-based hyperspectral, THz s-SNOM nanoscope. This nanoscope provides 160 nm spatial resolution, coherent detection of multiple phase-locked modes, and mapping of the THz optical response of nanoscale materials up to 3.5 THz, with noise-equivalent-power (NEP) ≈400 pW âHz-1 . This technique can be applied to the entire infrared range, opening up a new approach to hyper-spectral near-field imaging with wide-scale applications in the study of plasmonics and quantum science, inter alia.
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Engineering detection dynamics in nanoscale receivers that operate in the far infrared (frequencies in the range 0.1-10 THz) is a challenging task that, however, can open intriguing perspectives for targeted applications in quantum science, biomedicine, space science, tomography, security, process and quality control. Here, we exploited InAs nanowires (NWs) to engineer antenna-coupled THz photodetectors that operated as efficient bolometers or photo thermoelectric receivers at room temperature. We controlled the core detection mechanism by design, through the different architectures of an on-chip resonant antenna, or dynamically, by varying the NW carrier density through electrostatic gating. Noise equivalent powers as low as 670 pWHz-1/2 with 1 µs response time at 2.8 THz were reached.
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Near-field microscopy discloses a peculiar potential to explore novel quantum state of matter at the nanoscale, providing an intriguing playground to investigate, locally, carrier dynamics or propagation of photoexcited modes as plasmons, phonons, plasmon-polaritons or phonon-polaritons. Here, we exploit a combination of hyperspectral time domain spectroscopy nano-imaging and detectorless scattering near-field optical microscopy, at multiple terahertz frequencies, to explore the rich physics of layered topological insulators as Bi2Se3 and Bi2Te2.2Se0.8, hyperbolic materials with topologically protected surface states. By mapping the near-field scattering signal from a set of thin flakes of Bi2Se3 and Bi2Te2.2Se0.8 of various thicknesses, we shed light on the nature of the collective modes dominating their optical response in the 2-3 THz range. We capture snapshots of the activation of transverse and longitudinal optical phonons and reveal the propagation of sub-diffractional hyperbolic phonon-polariton modes influenced by the Dirac plasmons arising from the topological surface states and of bulk plasmons, prospecting new research directions in plasmonics, tailored nanophotonics, spintronics and quantum technologies.
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The scalable synthesis and transfer of large-area graphene underpins the development of nanoscale photonic devices ideal for new applications in a variety of fields, ranging from biotechnology, to wearable sensors for healthcare and motion detection, to quantum transport, communications, and metrology. We report room-temperature zero-bias thermoelectric photodetectors, based on single- and polycrystal graphene grown by chemical vapor deposition (CVD), tunable over the whole terahertz range (0.1-10 THz) by selecting the resonance of an on-chip patterned nanoantenna. Efficient light detection with noise equivalent powers <1 nWHz-1/2 and response time â¼5 ns at room temperature are demonstrated. This combination of specifications is orders of magnitude better than any previous CVD graphene photoreceiver operating in the sub-THz and THz range. These state-of-the-art performances and the possibility of upscaling to multipixel architectures on complementary metal-oxide-semiconductor platforms are the starting points for the realization of cost-effective THz cameras in a frequency range still not covered by commercially available microbolometer arrays.
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Semiconductor nanowire field-effect transistors represent a promising platform for the development of room-temperature (RT) terahertz (THz) frequency light detectors due to the strong nonlinearity of their transfer characteristics and their remarkable combination of low noise-equivalent powers (<1 nW Hz-1/2) and high responsivities (>100 V/W). Nano-engineering an NW photodetector combining high sensitivity with high speed (sub-ns) in the THz regime at RT is highly desirable for many frontier applications in quantum optics and nanophotonics, but this requires a clear understanding of the origin of the photo-response. Conventional electrical and optical measurements, however, cannot unambiguously determine the dominant detection mechanism due to inherent device asymmetry that allows different processes to be simultaneously activated. Here, we innovatively capture snapshots of the photo-response of individual InAs nanowires via high spatial resolution (35 nm) THz photocurrent nanoscopy. By coupling a THz quantum cascade laser to scattering-type scanning near-field optical microscopy (s-SNOM) and monitoring both electrical and optical readouts, we simultaneously measure transport and scattering properties. The spatially resolved electric response provides unambiguous signatures of photo-thermoelectric and bolometric currents whose interplay is discussed as a function of photon density and material doping, therefore providing a route to engineer photo-responses by design.
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Semiconductor heterostructures have enabled a great variety of applications ranging from GHz electronics to photonic quantum devices. While nonlinearities play a central role for cutting-edge functionality, they require strong field amplitudes owing to the weak light-matter coupling of electronic resonances of naturally occurring materials. Here, we ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation. We complement our data by a quantitative theory. Our results highlight a path towards passively mode-locked QCLs based on polaritonic saturable absorbers in a monolithic single-chip design.
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Chemical doping of bulk black phosphorus is a well-recognized way to reduce surface oxidation and degradation. Here, we report on the fabrication of terahertz frequency detectors consisting of an antenna-coupled field-effect transistor (FET) with an active channel of Se-doped black phosphorus. Our devices show a maximum room-temperature hole mobility of 1780 cm2 V-1 s-1 in a SiO2-encapsulated FET. A room-temperature responsivity of 3 V W-1 was observed, with a noise-equivalent power of 7 nW Hz-1/2 at 3.4 THz, comparable with the state-of-the-art room-temperature photodetectors operating in the same frequency range. The inclusion of Se dopants in the growth process of black phosphorus crystals enables the optimization of the transport and optical performances of FETs in the far-infrared with a high potential for the development of BP-based electro-optical devices. We also demonstrate that the flake thickness can be tuned according to the target application. Specifically, thicker flakes (>80 nm) are suitable for applications in which high mobility and high speed are essential, thinner flakes (<10 nm) are more appropriate for applications requiring high on/off current ratios, while THz photodetection is optimal with flakes 30-40 nm thick, due to the larger carrier density tunability.
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Herein, we explore the main features and the prospect of plasmonics with two-dimensional semiconductors. Plasmonic modes in each class of van der Waals semiconductors have their own peculiarities, along with potential technological capabilities. Plasmons of transition-metal dichalcogenides share features typical of graphene, due to their honeycomb structure, but with damping processes dominated by intraband rather than interband transitions, unlike graphene. Spin-orbit coupling strongly affects the plasmonic spectrum of buckled honeycomb lattices (silicene and germanene), while the anisotropic lattice of phosphorene determines different propagation of plasmons along the armchair and zigzag directions. Black phosphorus is also a suitable material for ultrafast plasmonics, for which the active plasmonic response can be initiated by photoexcitation with femtosecond pulses. We also review existing applications of plasmonics with two-dimensional materials in the fields of thermoplasmonics, biosensing, and plasma-wave Terahertz detection. Finally, we consider the capabilities of van der Waals heterostructures for innovative low-loss plasmonic devices.
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Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.