Near-field terahertz probes with room-temperature nanodetectors for subwavelength resolution imaging

Terahertz near-field microscopy of metallic circular split ring resonators with graphene in the gap

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.

Real-Time Measure of the Lattice Temperature of a Semiconductor Heterostructure Laser via an On-Chip Integrated Graphene Thermometer

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 k_⊥= 10.2 W/m·K, in agreement with previous theoretical and experimental reports. Our integrated system endows THz QCLs with a fast (∼30 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.

Terahertz-field-induced optical luminescence from graphene for imaging and near-field visualization of a terahertz field

Graphene-based terahertz (THz)-field-induced optical luminescence (GB-TFIOL) is proposed in this Letter as a novel, to the best of our knowledge, THz imaging technique. We experimentally show that two-dimensional distribution of the optical luminescence from a monolayer graphene traces the beam profile of the pump THz radiation. The atomic thickness of a graphene detector, as well as a strong nonlinear dependence of optical luminescence on THz field, make the GB-TFIOL technique a useful tool for near-field mapping. A proof-of-principle experiment of the visualization of local THz-field enhancement near a metal tip with a 2 µm radius curvature was performed.

Broadband micro-transient absorption spectroscopy enabled by improved lock-in amplification

Recent breakthroughs in material development have increased the demand for characterization methods capable of probing nanoscale features on ultrafast time scales. As the sample reduces to atomically thin levels, an extremely low-level signal limits the feasibility of many experiments. Here, we present an affordable and easy-to-implement solution to expand the maximum sensitivity of lock-in detection systems used in transient absorption spectroscopy by multiple orders of magnitude. By implementation of a tuned RC circuit to the output of an avalanche photodiode, electric pulse shaping allows for vastly improved lock-in detection. Furthermore, a carefully designed "peak detector" circuit provides additional pulse shaping benefits, resulting in even more lock-in detection signal enhancement. We demonstrate the improvement of lock-in detection with each of these schemes by performing benchmark measurements of a white-light continuum signal and micro-transient absorption spectroscopy on a few-layer transition metal dichalcogenide sample. Our results show the practicality of ultrafast pump-probe spectroscopy for many high-sensitivity experimental schemes.

HBN-Encapsulated, Graphene-based, Room-temperature Terahertz Receivers, with High Speed and Low Noise

Uncooled terahertz photodetectors (PDs) showing fast (ps) response and high sensitivity (noise equivalent power (NEP) < nW/Hz^1/2) over a broad (0.5-10 THz) frequency range are needed for applications in high-resolution spectroscopy (relative accuracy ∼10^-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 (∼2000 k_B μm^-2) electronic specific heat of high mobility (>50 000 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.

Thermodynamics of a Phase-Driven Proximity Josephson Junction

We study the thermodynamic properties of a superconductor/normal metal/superconductor Josephson junction in the short limit. Owing to the proximity effect, such a junction constitutes a thermodynamic system where phase difference, supercurrent, temperature and entropy are thermodynamical variables connected by equations of state. These allow conceiving quasi-static processes that we characterize in terms of heat and work exchanged. Finally, we combine such processes to construct a Josephson-based Otto and Stirling cycles. We study the related performance in both engine and refrigerator operating mode.

Reconfigurable MEMS Fano metasurfaces with multiple-input-output states for logic operations at terahertz frequencies

A broad range of dynamic metasurfaces has been developed for manipulating the intensity, phase and wavefront of electromagnetic radiation from microwaves to optical frequencies. However, most of these metasurfaces operate in single-input-output state. Here, we experimentally demonstrate a reconfigurable MEMS Fano resonant metasurface possessing multiple-input-output (MIO) states that performs logic operations with two independently controlled electrical inputs and an optical readout at terahertz frequencies. The far-field behaviour of Fano resonance exhibits XOR and XNOR operations, while the near-field resonant confinement enables the NAND operation. The MIO configuration resembling hysteresis-type closed-loop behaviour is realized through inducing electromechanically tuneable out-of-plane anisotropy in the near-field coupling of constituent resonator structures. The XOR metamaterial gate possesses potential applications in cryptographically secured terahertz wireless communication networks. Furthermore, the MIO features could lay the foundation for the realization of programmable and randomly accessible metamaterials with enhanced electro-optical performance across terahertz, infrared and optical frequencies.

Sub-wavelength terahertz imaging through optical rectification

We record a sub-wavelength terahertz image of a caster sugar grain thanks to optical rectification in the sample excited with a femtosecond laser beam. The lateral spatial resolution of this technique is given by the laser spot size at the sample and here its measured value is 50 μm, i.e. ~λ/12. We give an estimation of the ultimate resolution that could be achieved with this method.

Study of Energy Scattering Relation and RCS Reduction Characteristic of Matrix-Type Coding Metasurface

In this paper, we present a design of the linear polarization conversion metasurface (MS) for the broadband radar cross section (RCS) reduction based on split-ring resonator (SRR) structure in microwave region. The corresponding phase gradient can be obtained through the stable phase difference of basic units of polarization conversion MS. The designed polarization conversion MS is applied in coded electromagnetic (EM) matrix by defining two basic units “0” and “1”, respectively. Based on the principle of planar array theory, a new random coding method named by matrix-type coding is proposed. Correlative RCS reduction mechanism is discussed and verified, which can be used to explore the RCS reduction characteristic. The simulated linear polarization conversion rate of the designed structure is up to 90% in the frequency range of 6–15 GHz, and the RCS reduction results verify the theoretical assumptions. Two kinds of matrix-type coding MS samples are prepared and measured. The experimental results indicate that the reflectance of MS is less than –10 dB on average under normal incidence in frequency range of 5.8–15.5 GHz. The average RCS reduction is essentially more than 10 dB in frequency range of 5.5–15 GHz and the corresponding relative bandwidth is 92.7%, which reasonably agrees with simulation. In addition, excellent RCS reduction characteristic of the designed MS can also be achieved over a wide incident angle.

Phase-resolved terahertz self-detection near-field microscopy

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.

Broadband Polarization Conversion Metasurface Based on Metal Cut-Wire Structure for Radar Cross Section Reduction

A class of linear polarization conversion coding metasurfaces (MSs) based on a metal cut-wire structure is proposed, which can be applied to the reduction properties of radar cross section (RCS). We firstly present a hypothesis based on the principle of planar array theory, and then verify the RCS reduction characteristics using linear polarization conversion coding MSs by simulations and experiments. The simulated results show that in the frequency range of 6⁻14 GHz, the linear polarization conversion ratio reaches a maximum value of 90%, which is in good agreement with the theoretical predictions. For normal incident x- and y-polarized waves, RCS reduction of designed coding MSs 01/01 and 01/10 is essentially more than 10 dB in the above-mentioned frequency range. We prepare and measure the 01/10 coding MS sample, and find that the experimental results in terms of reflectance and RCS reduction are in good agreement with the simulated ones under normal incidence. In addition, under oblique incidence, RCS reduction is suppressed as the angle of incidence increases, but still exhibits RCS reduction effects in a certain frequency range. The designed MS is expected to have valuable potential in applications for stealth field technology.