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Terahertz radiation is extremely suitable for various imaging applications. In real life, these range from food or pharmaceutical quality control to illegal materials or human security control inspections. Despite its great potential, the wide usage and commercialization of terahertz imaging systems are still limited by the lack of compact technologies. The emerging graphene-based devices can efficiently contribute to fill this gap, offering higher versatility, scalability, and superior electronic properties compared to conventional semiconductors. In this work, we study a new scheme for realizing a multielement terahertz sensor, which is capable of multipixel parallel detection. The array consists of linearly distributed antenna-coupled graphene field-effect transistors, which are realized by exploiting a deterministic growth by chemical vapor deposition of single-crystal graphene. This novel growth technique ensures high material quality and offers large adaptability to different electronic device architectures. Relatively uniform terahertz detection performances (with a maximum homogeneity degree of 80 $\%$ ) were obtained with a maximum responsivity of the order of 1 V/W and an estimated response time in the picosecond scale. These detectors have demonstrated to fulfill several main requirements for image sensors (pixel uniformity, operability, and scalability), becoming very promising candidates for the realization of commercial high-resolution room-temperature terahertz cameras.

Refractive freeform components are becoming increasingly relevant for generating controlled patterns of light, because of their capability to spatially-modulate optical signals with high efficiency and low background. However, the use of these devices is still limited by difficulties in manufacturing macroscopic elements with complex, 3-dimensional (3D) surface reliefs. Here, 3D-printed and stretchable magic windows generating light patterns by refraction are introduced. The shape and, consequently, the light texture achieved can be changed through controlled device strain. Cryptographic magic windows are demonstrated through exemplary light patterns, including micro-QR-codes, that are correctly projected and recognized upon strain gating while remaining cryptic for as-produced devices. The light pattern of micro-QR-codes can also be projected by two coupled magic windows, with one of them acting as the decryption key. Such novel, freeform elements with 3D shape and tailored functionalities is relevant for applications in illumination design, smart labels, anti-counterfeiting systems, and cryptographic communication., Comment: 31 pages, 22 figures, Advanced Materials Technologies 2022

Artificial photonic materials, nanofabricated through wavelength-scale engineering, have shown astounding and promising results in harnessing, tuning, and shaping photonic beams. Metamaterials have proven to be often outperforming the natural materials they take inspiration from. In particular, metallic chiral metasurfaces have demonstrated large circular and linear dichroism of light which can be used, for example, for probing different enantiomers of biological molecules. Moreover, the precise control, through designs on demand, of the output polarization state of light impinging on a metasurface, makes this kind of structures particularly relevant for polarization-based telecommunication protocols. The reduced scale of the metasurfaces makes them also appealing for integration with nanomechanical elements, adding new dynamical features to their otherwise static or quasi-static polarization properties. To this end we designed, fabricated and characterized an all-dielectric metasurface on a suspended nanomembrane. Actuating the membrane mechanical motion, we show how the metasurface reflectance response can be modified, according to the spectral region of operation, with a corresponding intensity modulation or polarization conversion. The broad mechanical resonance at atmospheric pressure, centered at about 400 kHz, makes the metasurfaces structure suitable for high-frequency operation, mainly limited by the piezo-actuator controlling the mechanical displacement, which in our experiment reached modulation frequencies exceeding 1.3 MHz.

Even though already in the seventies, right after the invention of the quantum cascade laser (QCL) concept, it was argued that this device could be operated in the THz (far-infrared) range of the electromagnetic spectrum, it was only in 2002 that the first working THz QCL was demonstrated. Soon afterwards, the progress was very rapid; in the space of 2-3 years, operating temperatures were raised, single-mode DFB devices were produced, applications as local oscillators in heterodyne transceivers were implemented, frequency coverage was extended to the whole 1-5 THz region. In the last few years, technological advancement has continued to improve performances: the maximum operating temperature has now reached about 250 K and about 1 W peak output power has been demonstrated. Several beam engineering techniques have been implemented, with the scope of enhancing spectral purity, improving beam quality and achieving vertical emission. In parallel, various approaches have been devised that allow frequency tunability of the emitted light, with the most efficient schemes achieving a tuning range of about 10% of the central emission frequency. Even the generation of frequency combs directly from THz QCLs has been obtained, by employing dispersion compensated waveguides and an intrinsic material non-linearity. This manuscript reviews the physics underlying the operation of THz QCLs, the technology developed to advance laser performances, and highlights the latest most promising progresses in this fascinating area of opto-electronics.

We present the first steps to develop radiation sensors based on the graphene field effect transistor technology. Such a sensor exploits the ambipolar behavior of graphene near its Dirac point and it is not dependent on collecting charges, but it senses ionizing radiation trough the change in conductivity of the graphene layer induced by changes of the electric field. We designed the layout of the sensors with the help of Sentaurs TCAD. We simulated static operations and the dynamic response to radiation and calculated the source–drain current through the graphene layer with a quasi-analytical model. The transistors were produced at the National Enterprise for nanoScience and nanoTechnology by depositing high quality graphene on silicon chips manufactured by the Fondazione Bruno Kessler foundry. To reduce the high contact resistance between graphene and aluminum contacts caused by oxidation of the aluminum surface, we used gold/chromium interfaces. We investigated the sensors behavior by mean of electrical measurements, extracting the graphene properties, such as mobility and doping. We observed modulation of the source–drain current, determined the Dirac point and found the optimal voltage levels to be sensitive to pulsed IR laser light and β -particles.

One of the major issues in graphene-based optoelectronics is to scale-up high-performing devices. In this work, we report an original approach for the fabrication of efficient optoelectronic devices from scalable tungsten disulfide (WS2)/graphene heterostructures. Our approach allows for the patterned growth of WS2 on graphene and facilitates the realization of ohmic contacts. Photodetectors fabricated with WS2 on epitaxial graphene on silicon carbide (SiC) present, when illuminated with red light, a maximum responsivity R ~220 A W-1, a detectivity D* ~2.0 × 109 Jones and a -3 dB bandwidth of 250 Hz. The retrieved detectivity is 3 orders of magnitude higher than that obtained with graphene-only devices at the same wavelength. For shorter illumination wavelengths we observe a persistent photocurrent with a nearly complete charge retention, which originates from deep trap levels in the SiC substrate. This work ultimately demonstrates that WS2/graphene optoelectronic devices with promising performances can be obtained in a scalable manner. Furthermore, by combining wavelength-selective memory, enhanced responsivity and fast detection, this system is of interest for the implementation of 2d-based data storage devices.

Quantum cascade lasers (QCLs) represent a most promising compact source at terahertz (THz) frequencies, but efficiency of their continuous wave (CW) operation still needs to be improved to achieve large-scale exploitation. Here, we demonstrate highly efficient operation of a subwavelength microcavity laser consisting of two evanescently coupled whispering gallery microdisk resonators. Exploiting a dual injection scheme for the laser cavity, single mode CW vertical emission at 3.3 THz is obtained at 10 K with 6.4 mA threshold current and 145 mW/A slope efficiency up to 320 μ W emitted power measured in quasi-CW mode. The tuning of the laser emission directionality is also obtained by independently varying the pumping strength between the microdisks. By connecting the resonators through a suspended gold bridge, the laser out-coupling efficiency in the vertical direction is strongly enhanced. Owing to the high brightness, low-power consumption and CW operation, the proposed microcavity laser design could allow the realization of high-performance CW THz QCLs ready for massive parallelization.

The development of terahertz based technology has given the opportunity for the realization of non destructive techniques capable of gaining meaningful information on delicate systems such as biological samples. Here, the health status of leaves in vivo has been monitored through a portable terahertz imaging system. The data have been extracted and analysed from the images acquired and compared with analogous results reported in the literature on similar systems. The possibilty of extracting additional information from the images regarding leaf details has also been explored.

The interaction between graphene surface plasmons and a semiconductor quantum well has been investigated by means of scattering matrix simulations. Due to the strong confinement factor of the graphene layer, a large Rabi splitting arises from the interaction with intersubband transitions. By varying the Fermi energy in the graphene and the doping in the quantum well, the resulting polariton states show features of strong and ultra-strong coupling. The system has been modeled with the coupled-mode theory to find the highest quality factor for the polariton resonance, reaching a “highly resolved” ultra-strong coupling regime.

The effectiveness of self-mixing interferometry has been demonstrated across the electromagnetic spectrum, from visible to microwave frequencies, in a plethora of sensing applications, ranging from distance measurement to material analysis, microscopy and coherent imaging. Owing to their intrinsic stability to optical feedback, quantum cascade lasers (QCLs) represent a source that offers unique and versatile characteristics to further improve the self-mixing functionality at mid infrared and terahertz (THz) frequencies. Here, we show the feasibility of detecting with nanometer precision deeply subwalength (< {\lambda}/6000) mechanical vibrations of a suspended Si3N4-membrane used as the external element of a THz QCL feedback interferometric apparatus. Besides representing a platform for the characterization of small displacements, our self-mixing configuration can be exploited for the realization of optomechanical systems, where several laser sources can be linked together through a common mechanical microresonator actuated by radiation pressure., Comment: 5 pages, 4 figures

The control of the local strain profile in 2D materials offers an invaluable tool for tailoring the electronic and photonic properties of solid-state devices. In this paper, we demonstrate a local engineering of the exciton photoluminescence (PL) energy of monolayer tungsten disulfide (WS2) by means of strain. We apply a local uniaxial stress to WS2 by exploiting electron-beam patterned and actuated polymeric micrometric artificial muscles (MAMs), which we implement onto monolithic synthetic WS2/graphene heterostructures. We show that MAMs are able to induce an in-plane stress to the top WS2 layer of the van der Waals heterostructure and that the latter can slide on the graphene underneath with negligible friction. As a proof of concept for the local strain-induced PL shift experiments, we exploit a two-MAM configuration in order to apply uniaxial tensile stress on well-defined micrometric regions of WS2. Remarkably, our architecture does not require the adoption of fragile suspended microstructures. We observe a spatial modulation of the excitonic PL energy of the WS2 monolayers under stress, which agrees with the expected strain profile and attains a maximum redshift of about 40 meV at the maximum strain intensity point. After the actuation, a time-dependent PL blueshift is observed in agreement with the viscoelastic properties of the polymeric MAMs. Our approach enables inducing local and arbitrary deformation profiles and circumvents some key limitations and technical challenges of alternative strain engineering methods requiring the 2D material transfer and production of suspended membranes., Comment: This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in APL and may be found at https://aip.scitation.org/doi/10.1063/1.5122262

The terahertz (THz) quantum cascade laser (QCL) provides a versatile tool in a plethora of applications ranging from spectroscopy to astronomy and communications. In many of these fields, compactness, single mode frequency emission, and low threshold are highly desirable. The proposed approach, based on line defects in a photonic crystal (PhC) matrix, addresses all these features while offering unprecedented capabilities in terms of flexibility, light waveguiding, and emission directionality. Nine line-defect QCLs were realized in a triangular lattice of pillars fabricated in the laser active region (AR), centered around ∼2 THz by tuning the photonic design. A maximal 36% threshold reduction was recorded for these ultraflat dispersion line-defect QCLs in comparison to standard metal-metal QCL. The mode selectivity is an intrinsic property of the chosen fabrication design and has been achieved by lithographically scaling the dimension of the defect pillars and by acting on the PhC parameters in order to match the AR emission bandwidth. The measured line-defect QCLs emitted preferentially in the single frequency mode in the propagation direction throughout the entire dynamic range. An integrated active platform with multiple directional outputs was also fabricated as proof-of-principle to demonstrate the potential of this approach. The presented results pave the way for integrated circuitry operating in the THz regime and for fundamental studies on microcavity lasers.

The ability to drive a system with an external input is a fundamental aspect of light-matter interaction. The coherent perfect absorption (CPA) phenomenon extends to the general multibeam interference phenomenology the well known critical coupling concepts. This interferometric control of absorption can be employed to reach full delivery of optical energy to nanoscale systems such as plasmonic nanoparticles, and multi-port interference can be used to enhance the absorption of a nanoscale device when it is embedded in a strongly scattering system, with potential applications to nanoscale sensing. Here we review the two-port CPA in reference to photonic structures which can resonantly couple to the external fields. A revised two-port theory of CPA is illustrated, which relies on the Scattering Matrix formalism and is valid for all linear two-port systems with reciprocity. Through a semiclassical approach, treating two-port critical coupling conditions in a non-perturbative regime, it is demonstrated that the strong coupling regime and the critical coupling condition can indeed coexist; in this situation, termed strong critical coupling, all the incoming energy is converted into polaritons. Experimental results are presented, which clearly display the elliptical trace of absorption as function of input unbalance in a thin metallo-dielectric metamaterial, and verify polaritonic CPA in an intersubband-polariton photonic-crystal membrane resonator. Concluding remarks discuss the future perspectives of CPA with photonic structures., arXiv admin note: substantial text overlap with arXiv:1605.08906

The interplay between interference and absorption leads to interesting phenomena like coherent perfect absorption and coherent perfect transparency (CPA and CPT), which can be exploited for fully optical modulation. While it is known that it is possible to harness CPA and CPT for switching a strong signal beam with a weak control beam, it is not immediate that this process su ers from a fundamental compromise between the device e ciency (quantified by device loss and modulation depth) and the asymmetry between signal and control intensity desired for operation. This article quantifies this compromise and outlines a possible way to overcome it by means of a combination of optical gain and loss in the same photonic component. A general formulation and a specific device realization are both discussed. © 2018 Optical Society of America.

Photodetectors exploiting photoejected hot electrons have the potential to achieve ultrahigh sensitivity and broadband detection capabilities, which are controlled by the structure of the device rather than the bandgap of the employed materials. However, the achievement of photodetectors of long-wavelength photons with both high responsivity and bandwidth is still challenging. Here, a novel class of high-gain photodetectors based on the manipulation of intrinsic hot carriers by exploiting the electromagnetic engineering of a graphene-based active channel is presented. Light field is focused in a split-finger gated structure to create a potential gradient in the channel, which is able to trap and detrap the charges laterally transferred from low resistive Au-graphene interface, finally leading to a high photoconductive gain. Correspondingly, the device activity can be easily switched from photovoltaic to photoconductive, depending on the photoinduced hot-carrier distribution, just by controlling the electric field. The device shows tunable sensitivity, higher energy efficiency, and photoconductive gain. In particular, the responsivity (0.6-6.0 kV W-1) and the noise-equivalent power (less than 0.1 nW Hz-0.5 at room temperature) are significantly improved even at low-energy terahertz band with respect to state-of-the-art devices based on extrinsically coupled hot carriers operating in the near infrared. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

© 2018 OSA. We report on the development of terahertz (THz) saturable-absorbers exploiting printable graphene inks. The achieved 80% transparency modulation at 3.5 THz makes these devices potential candidates as passive components for THz solid-state lasers., EPSRC

In the present article we numerically investigated the magneto-optical behaviour of a sub-wavelength structure composed by a monolayer graphene and a metallic metasurface of optical resonators. Using this hybrid graphene-metal structure, a large increase of the non-reciprocal polarization rotation of graphene can be achieved over a broad range of terahertz frequencies. We demonstrate that the symmetry of the resonator geometry plays a key role for the performance of the system: in particular, increasing the symmetry of the resonator the non-reciprocal properties can be progressively enhanced. Moreover, the possibility to exploit the metallic metasurface as a voltage gate to vary the graphene Fermi energy allows the system working point to be tuned to the desired frequency range. Another peculiar result is the achievement of a structure able to operate both in transmission and reflection with almost the same performance, but in a different frequency range of operation. The described system is hence a sub-wavelength, tunable, multifunctional, effective non-reciprocal element in the terahertz region.

Electromagnetic fields coupled with mechanical degrees of freedom have recently shown exceptional and innovative applications, ultimately leading to mesoscopic optomechanical devices operating in the quantum regime of motion. Simultaneously, micromechanical elements have provided new ways to enhance and manipulate the optical properties of passive photonic elements. Following this concept, in this article we show how combining a chiral metasurface with a GaAs suspended micromembrane can offer new scenarios for controlling the polarization state of near-infrared light beams. Starting from the uncommon properties of chiral metasurface to statically realize target polarization states and circular and linear dichroism, we report mechanically induced, ~300 kHz polarization modulation, which favorably compares, in terms of speed, with liquid-crystals commercial devices. Moreover, we demonstrate how the mechanical resonance can be non-trivially affected by the input light polarization (and chiral state) via a thermoelastic effect triggered by intracavity photons. This work inaugurates the field of Polarization Optomechanics, which could pave the way to fast polarimetric devices, polarization modulators and dynamically tunable chiral state generators and detectors, as well as giving access to new form of polarization nonlinearities and control.

Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. Here we report the fabrication of THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. These results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications, providing unprecedented compactness and resolution., Graphene shows promise for saturable absorption, a key property for ultrafast lasing, yet graphene saturable absorbers operating in the terahertz region suffer from low absorption modulation. Here, the authors report terahertz saturable absorbers based on inkjet printed graphene with 80% transparency modulation.

Terahertz (THz) spectroscopy and imaging have been heralded for some time as potentially revolutionary techniques for biomedical applications. Label-free detection of molecules and recognition of molecular events are often mentioned as the most exciting possibilities. A crucial practical goal, however, is the ability to perform such measurements on tiny amounts of biological fluids or even on individual organic structures. Living cells, for instance, have diameters at most of some tens of micrometers, i.e. at least λ/10 even for few-THz radiation. Furthermore, all analyses relevant for a biological perspective must be performed in a water environment, which presents a strong absorption across the whole THz spectral range, severely limiting the penetration of the electromagnetic field. Here, it is shown how both issues can be overcome with a lab-on-a-chip approach based on a microfluidic platform coupled to a plasmonic antenna. Using a quantum cascade laser as THz illumination source, liquid volumes down to the picoliter range are probed, and direct operation on individual 10-μm diameter microparticles flowing in water is shown. The present demonstration opens the way to the development of THz biosensing of individual living cells and small probe volumes. A THz lab-on-a-chip device based on a microfluidic platform coupled to an integrated plasmonic antenna is shown. Using a quantum cascade laser as illumination source, liquid volumes down to the picoliter range are probed. A proof-of-concept experiment is performed in which single 10-μm diameter (∼ λ/10) microparticles flowing in water are identified and investigated. The present demonstration opens the way to the development of THz biosensing of individual living cells and small probe volumes.

The continuously growing interest in the so called bidimensional (2D) materials has been accompanied by a parallel development of techniques specifically devoted to manipulating and measuring micro-sized objects. Here a flexible microphotoluminescence (μPL) custom-made setup has been built on purpose: every setup component has been optimized for 2D materials; an automatized system able to perform measurements in different positions of the sample and execute mapping have been made; the dependence of the μPL intensity, peak center and other properties such as the Full Width Half Maximum (FWHM) on the position has been then extracted. The attention has been focused on WS 2 and phosphorene. Optical response has been compared between Chemical Vapor Deposition (CVD) grown WS 2 transferred on graphene and sapphire, especially focusing on the changes in the trion peak. Signatures have been found that can be attributed to photoluminescence from thick (more than five layers) phosphorene flakes.

Graphene is establishing itself as a new photonic material with huge potential in a variety of applications ranging from transparent electrodes in displays and photovoltaic modules to saturable absorber in mode-locked lasers. Its peculiar bandstructure and electron transport characteristics naturally suggest graphene could also form the basis for a new generation of high-performance devices operating in the terahertz (THz) range of the electromagnetic spectrum. The region between 300 GHz and 10 THz is in fact still characterized by a lack of efficient, compact, solid state photonic components capable of operating well at 300 K. Recent works have already shown very promising results in the development of high-speed modulators as well as of bolometer and plasma-wave detectors. Furthermore, several concepts have been proposed aiming at the realization of lasers and oscillators. This paper will review the latest achievements in graphene-based THz photonics and discuss future perspectives of this rapidly developing research field. Graphene is establishing itself as a new photonic material with huge potential in a variety of applications ranging from transparent electrodes in displays and photovoltaic modules to saturable absorber in mode-locked lasers. Its peculiar bandstructure and electron transport characteristics naturally suggest graphene could also form the basis for a new generation of high-performance devices operating in the terahertz (THz) range of the electromagnetic spectrum. The region between 300 GHz and 10 THz is in fact still characterized by a lack of efficient, compact, solid state photonic components capable of operating well at 300 K. Recent works have already shown very promising results in the development of high-speed modulators as well as of bolometer and plasma-wave detectors. Furthermore, several concepts have been proposed aiming at the realization of lasers and oscillators. This paper will review the latest achievements in graphene-based THz photonics and discuss future perspectives of this rapidly developing research field. © 2013 IEEE.

In this paper we report about the possibility to control a strong signal beam with a much weaker control beam, resorting to the linear optical phenomena known as coherent perfect absorption and transparency (CPA and CPT). First, analytical formulas for CPA and CPT in a realistic yet prototypical configuration of a substrate-backed optically conducting surface (graphene, metallic metasurface...) are reported. Maximal optical control can be achieved by only imposing the conditions for CPA, since CPT is always present in this system. Secondly, we observe that the performance of passive two-port devices as optical switches is always limited by fundamental energy conservation constraints. Meanwhile, we outline how a hybrid gain/loss optical device reminiscent of parity-time symmetric objects can overcome such limitation, enabling the realization of ideal full optical control.

We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 {\mu}m were used as high quality factor (Q>10^6) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The studied microspheres show monomode and multi-mode lasing action, which can be modulated in the best case up to 10 MHz. We have optically transduced thermally-activated mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which decreases with increasing the size of the microspheres. In a pump-and-probe configuration we observed modulation of the probe signal up to the maximum pump modulation frequency of our experimental setup, i.e., 20 MHz. This modulation decreases with frequency and is unrelated to lasing emission, pump scattering or thermal effects. We associate this effect to free-carrier-dispersion induced by multiphoton pump light absorption. On the other hand, we conclude that, in our current experimental conditions, it was not possible to resonantly excite the mechanical modes. Finally, we discuss on how to overcome these limitations by increasing the modulation frequency of the lasing emission and decreasing the frequency of the mechanical eigenmodes displaying a strong degree of optomechanical coupling., Comment: 17 pages, 5 figures

Resonators and the way they couple to external radiation rely on very different concepts if one considers devices belonging to the photonic and electronic worlds. The terahertz frequency range, however, provides intriguing possibilities for the development of hybrid technologies that merge ideas from both fields in novel functional designs. In this paper, we show that high-quality, subwavelength, whispering-gallery lasers can be combined to form a linear dipole antenna, which creates a very efficient, low-threshold laser emission in a collimated beam pattern. For this purpose, we employ a terahertz quantum-cascade active region patterned into two 19-μm-radius microdisks coupled by a suspended metallic bridge, which simultaneously acts as an inductive antenna and produces the dipole symmetry of the lasing mode. Continuous-wave vertical emission is demonstrated at approximately 3.5 THz in a very regular, low-divergence (±10°) beam, with a high slope efficiency of at least 160 mW A â '1 and a mere 6 mA of threshold current, which is ensured by the ultra-small resonator size (V RES/Î" 3 â ‰10 â '2). The extremely low power consumption and the superior beam brightness make this concept very promising for the development of miniaturized and portable THz sources to be used in the field for imaging and sensing applications as well as for exploring novel optomechanical intracavity effects.

The selfmixer properties of a laser compound cavity are investigated and experimentally exploited to couple the mechanical fluctuations of a silicon nitride membrane to the laser photons and electronical states, producing an active optomechanical system.

In this work we show how a continuous wave, low threshold and well collimated terahertz laser source can be obtained by coupling two sub-wavelength whispering galleries quantum cascade optical resonators with a suspended metallic bridge.

Analytical formulas are derived describing the coherent absorption of light from a realistic multilayer structure composed by an optically conducting surface on a sup- porting substrate. The model predicts two fundamental results. First, the absorption regime named coherent perfect transparency theoretically can always be reached. Second, the optical conductance of the surface can be extrapolated from absorption experimental data even when the substrate thickness is unknown. The theoretical predictions are experimentally verified by analyzing a multilayer graphene structure grown on a silicon carbide substrate. The graphene thickness estimated through the coherent absorption technique resulted in good agreement with the values obtained by two other spectroscopic techniques. Thanks to the high spatial resolution that can be reached and high sensitivity to the probed structure thickness, coherent absorp- tion spectroscopy represents an accurate and non-destructive diagnostic method for the spatial mapping of the optical properties of two-dimensional materials and of metasurfaces on a wafer scale.

In this paper we investigate the effect of a static magnetic field and of optical pumping on the transmittance of a hybrid graphene-split ring resonator metasurface. A significant modulation of the transmitted spectra is obtained, both by optical pumping, and by a combination of optical pumping and magnetostatic biasing. The transmittance modulation features spectral fingerprints that are characteristic of a non-Trivial interplay between the bare graphene response and the split ring resonance.

Nanoplasmonics exploits the coupling between light and collective electron density oscillations (plasmons) to bypass the stringent limits imposed by diffraction. This coupling enables confinement of light to sub-wavelength volumes and is usually exploited in nanostructured metals. Substantial efforts are being made at the current frontier of the field to employ electron systems in semiconducting and semimetallic materials since these add the exciting possibility of realizing electrically tunable and/or active nanoplasmonic devices. Here we demonstrate that a suitable design of the doping profile in a semiconductor nanowire (NW) can be used to tailor the plasmonic response and induce localization effects akin to those observed in metal nanoparticles. Moreover, by field-effect carrier modulation, we demonstrate that these localized plasmon resonances can be spatially displaced along the nanostructure body, thereby paving the way for the implementation of spatially tunable plasmonic circuits., Comment: 12 pages, 3 figures

We demonstrate room-temperature strong coupling between a mid-infrared (? = 9.9 ?m) intersubband transition and the fundamental cavity mode of a metal-insulator-metal resonator. Patterning of the resonator surface enables surface-coupling of the radiation and introduces an energy dispersion which can be probed with angle-resolved reflectivity. In particular, the polaritonic dispersion presents an accessible energy minimum at k = 0 where - potentially - polaritons can accumulate. We also show that it is possible to maximize the coupling of photons into the polaritonic states and - simultaneously - to engineer the position of the minimum Rabi splitting at a desired value of the in-plane wavevector. This can be precisely accomplished via a simple post-processing technique. The results are confirmed using the temporal coupled mode theory formalism and their significance in the context of the strong critical coupling concept is highlighted. © 2014 AIP Publishing LLC.

In this paper we report about linear and nonlinear optical properties of intersubband cavity polariton samples, where the resonant photonic mode is a defect state in a metallodielectric photonic crystal slab. By tuning a single geometric parameter of the resonator, the cavity Q factor can reach values as large as 85, with a consequent large cooperativity for the light-matter interaction. We show that a device featuring large cooperativity leads to sharp saturation, or even bistability, of the polariton states. This nonlinear dynamics occurs at the crossover between the weak-and the strong-coupling regimes, where the weak critical coupling concept plays a fundamental role. In this paper we report about linear and nonlinear optical properties of intersubband cavity polariton samples, where the resonant photonic mode is a defect state in a metallodielectric photonic crystal slab. By tuning a single geometric parameter of the resonator, the cavity Q factor can reach values as large as 85, with a consequent large cooperativity for the light-matter interaction. We show that a device featuring large cooperativity leads to sharp saturation, or even bistability, of the polariton states. This nonlinear dynamics occurs at the crossover between the weak- and the strong-coupling regimes, where the weak critical coupling concept plays a fundamental role.

The technological solutions and optical configurations employed to tune the emission frequency of quantum cascade lasers (QCLs) operating in the Terahertz range are reviewed. The gain spectrum of THz QCLs can be engineered by quantum design to be broadband over a range even larger than 1 THz, or, alternatively, to be electrically tunable, for instance exploiting the Stark shift of the lasing transition. Fabry-Perot lasers can then display very ample multimode spectra, but wide, though discontinuous and unpredictable, tunability through mode hopping can also be achieved. For reliable and repeatable single-mode tuning, QCL emitters based on external cavities can be realized using either a movable mirror to control the cavity length or a rotating grating for wavelength dependent feedback. First-order and second-order distributed feedback (DFB) THz QCLs also provide stable single-mode emission, whose frequency can be tuned by temperature and current, though usually on a more limited range of only a few GHz. It has been recently shown, however, that second-order THz DFB lasers can be coupled to an external microcavity, exploiting the anti-crossing of the respective eigenfrequencies to achieve wider tuning ranges by mechanically changing the size of the external microcavity. As an alternative, if fabricated in a narrow (~lambda/3) ridge configuration, double-metal THz DFB lasers represent an ideal device where to manipulate the lateral size of the lasing mode via electro-mechanical components. In this way, a new and robust method for wide and continuous tuning of the laser emission has been recently demonstrated, displaying a record large bandwidth of over 330 GHz.

We investigate the nonlinear transmission of a ~280-layer turbostratic graphene sheet for near-infrared amplifier laser pulses (775 nm, Ti:sapphire laser) with a duration of 150-fs and 20-fs. Saturable absorption is observed in both cases, however it is not very strong, amounting to ~13% transmittance change for the 20-fs (150-fs) pulses at a peak intensity of 30 GW/cm2 (4 GW/cm2). The dependence on incident peak intensity is reproduced well using a theoretical model for the time-dependent saturable absorption, where the excited carriers vacate the photo-excited energy range within 3-5 fs, which we attribute to energy redistribution due to carrier-carrier scattering. This is also supported by spectrally resolved measurements for the 20-fs pulses, which show a marked dependence of the degree of saturation on the photon energy. A key result is that the shorter pulses do not yield a lower saturation fluence, due to the combined effects of the broader excitation bandwidth, and the rapid and broad energy redistribution. We also predict the potential performance of multilayer graphene samples for removing pedestal and pre-pulse structure from ultrafast high-energy pulses.

We report the external control of the intersubband polariton coupling by manipulating the carrier density in quantum wells resonantly coupled to a GaAs/AlGaAs microcavity. The electrons in the wells were tuned by means of a depletion gate Was or by utilizing charge transfer between the energetically aligned ground subbands of asymmetric tunnel-coupled quantum wells. We propose the use of tunnel-assisted control of the polariton ground state in an asymmetrically coupled quantum well for implementing ultrafast modulation of intersubband polaritons. (c) 2007 Elsevier Ltd. All rights reserved.

Tunable oscillators are a key component of almost all electronic and photonic systems. Yet, a technology capable of operating in the terahertz (THz)-frequency range and fully suitable for widescale implementation is still lacking. This issue is significantly limiting potential THz applications in gas sensing, high-resolution spectroscopy, hyper-spectral imaging, and optical communications. The THz quantum cascade laser is arguably the most promising solution in terms of output power and spectral purity. In order to achieve reliable, repeatable, and broad tunability, here we exploit the strong coupling between two different cavity mode concepts: a distributed feedback one-dimensional photonic resonator (providing gain) and a mechanically actuated wavelength-size microcavity (providing tuning). The result is a continuously tunable, single-mode emitter covering a 162 GHz spectral range, centered on 3.2 THz. Our source has a few tens of MHz resolution, extremely high differential efficiency, and unprecedented compact and simple design architecture. By unveiling the large potential that lies in this technique, our results provide a robust platform for radically different THz systems exploiting broadly tunable semiconductor lasers.

In order to extend the Coherent Perfect Absorption (CPA) phenomenology to broadband operation, the interferometric control of absorption is investigated in two-port systems without port permutation symmetry. Starting from the two-port theory of CPA treated within the Scattering Matrix formalism, we demonstrate that for all linear two-port systems with reciprocity the absorption is represented by an ellipse as function of the relative phase and intensity of the two input beams, and it is uniquely determined by the device single-beam reflectance and transmittance, and by the dephasing of the output beams. The basic properties of the phenomenon in systems without port permutation symmetry show that CPA conditions can still be found in such asymmetric devices, while the asymmetry can be beneficial for broadband operation. As experimental proof, we performed transmission measurements on a metal-semiconductor metamaterial, employing a Mach-Zehnder interferometer. The experimental results clearly evidence the elliptical feature of absorption and trace a route towards broadband operation. In order to extend the Coherent Perfect Absorption (CPA) phenomenology to broadband operation, the interferometric control of absorption is investigated in two-port systems without port permutation symmetry. Starting from the two-port theory of CPA treated within the Scattering Matrix formalism, we demonstrate that for all linear two-port systems with reciprocity the absorption is represented by an ellipse as function of the relative phase and intensity of the two input beams, and it is uniquely determined by the device single-beam reflectance and transmittance, and by the dephasing of the output beams. The basic properties of the phenomenon in systems without port permutation symmetry show that CPA conditions can still be found in such asymmetric devices, while the asymmetry can be beneficial for broadband operation. As experimental proof, we performed transmission measurements on a metal-semiconductor metamaterial, employing a Mach-Zehnder interferometer. The experimental results clearly evidence the elliptical feature of absorption and trace a route towards broadband operation. (C) 2015 Optical Society of America

The discovery of graphene and the related fascinating capabilities have triggered an unprecedented interest in inorganic two-dimensional (2D) materials. Despite the impressive impact in a variety of photonic applications, the absence of energy gap has hampered its broader applicability in many optoelectronic devices. The recent advance of novel 2D materials, such as transition-metal dichalcogenides or atomically thin elemental materials, (e.g. silicene, germanene and phosphorene) promises a revolutionary step-change. Here we devise the first room-temperature Terahertz (THz) frequency detector exploiting few-layer phosphorene, e.g., a 10 nm thick flake of exfoliated crystalline black phosphorus (BP), as active channel of a field-effect transistor (FET). By exploiting the direct band gap of BP to fully switch between insulating and conducting states and by engineering proper antennas for efficient light harvesting, we reach detection performance comparable with commercial detection technologies, providing the first technological demonstration of a phosphorus-based active THz device.

We fabricate and characterize a microscale silicon opto-electro-mechanical system whose mechanical motion is coupled capacitively to an electrical circuit and optically via radiation pressure to a photonic crystal cavity. To achieve large electromechanical interaction strength, we implement an inverse shadow mask fabrication scheme which obtains capacitor gaps as small as 30 nm while maintaining a silicon surface quality necessary for minimizing optical loss. Using the sensitive optical read-out of the photonic crystal cavity, we characterize the linear and nonlinear capacitive coupling to the fundamental omega(m)/2 pi - 63 MHz in-plane flexural motion of the structure, showing that the large electromechanical coupling in such devices may be suitable for realizing efficient microwave-to-optical signal conversion. We fabricate and characterize a microscale silicon opto-electromechanical system whose mechanical motion is coupled capacitively to an electrical circuit and optically via radiation pressure to a photonic crystal cavity. To achieve large electromechanical interaction strength, we implement an inverse shadow mask fabrication scheme which obtains capacitor gaps as small as 30 nm while maintaining a silicon surface quality necessary for minimizing optical loss. Using the sensitive optical read-out of the photonic crystal cavity, we characterize the linear and nonlinear capacitive coupling to the fundamental ωm=2π = 63 MHz in-plane flexural motion of the structure, showing that the large electromechanical coupling in such devices may be suitable for realizing efficient microwave-to-optical signal conversion.

A terahertz quantum cascade laser has been realized from an isotropic disordered hyperuniform design. Such a system presents a photonic band-gap although it is characterized by an efficient depletion of the long range order. Hyperuniform patterns allow greater versatility in engineering band gaps in comparison to standard photonic-crystal materials. Bidimensional hyperuniform patterns were simulated for hexagonal tiles composed of high refractive index disks merged in a low dielectric constant polymeric matrix. Based on this design, quantum cascade lasers were fabricated by standard photolithography, metal evaporation, lift-off and dry-etching techniques in a half-stack bound to continuum active region emitting around 2.9 THz.

Quantum cascade (QC) lasers operating at terahertz frequencies were demonstrated two years ago, and, since then, their development has proceeded at a very rapid pace. The gain medium of the first devices was based on chirped superlattices, and a resonator relying on the surface plasmon concept was employed to achieve a large optical confinement with concomitant low propagation losses. Laser action was obtained at 4.4 THz, in pulsed mode and at temperatures up to 50 K. Improved fabrication allowed continuous-wave (cw) operation and increased the operating temperature to 75 K. Using a similar active region, lasing at 3.5 THz was achieved. More recently, various groups have introduced several new design concepts such as bound-to-continuum transitions and extraction of carriers via resonant phonon scattering, leading to pulsed operation up to 140 K, output powers of up to 50 mW, and cw operation up to 93 K. The lowest emission frequency is now 2.1 THz, tackling the technologically important region of 1.5-2.5 THz. Stable single-mode emission under all operating conditions has also recently become a reality thanks to the adoption of distributed feedback resonators. This rapid and substantial progress underlines the growing potential of QC lasers in THz photonics.

We report on the implementation of a confocal microscopy system based on a 2.9 THz quantum cascade laser source. Lateral and axial resolutions better than 70 mu m and 400 mu m, respectively, are achieved, with a large contrast enhancement compared to the non-confocal arrangement. The capability of resolving overlapping objects lying on different longitudinal planes is also clearly demonstrated.

We demonstrate the integration of Nd3+ doped Barium-Titanium-Silicate microsphere lasers with a Silicon Nitride photonic platform. Devices with two different geometrical configurations for extracting the laser light to buried waveguides have been fabricated and characterized. The first configuration relies on a standard coupling scheme, where the microspheres are placed over strip waveguides. The second is based on a buried elliptical geometry whose working principle is that of an elliptical mirror. In the latter case, the input of a strip waveguide is placed on one focus of the ellipse, while a lasing microsphere is placed on top of the other focus. The fabricated elliptical geometry (ellipticity=0.9) presents a light collecting capacity that is 50% greater than that of the standard waveguide coupling configuration and could be further improved by increasing the ellipticity. Moreover, since the dimensions of the spheres are much smaller than those of the ellipses, surface planarization is not required. On the contrary, we show that the absence of a planarization step strongly damages the microsphere lasing performance in the standard configuration., Comment: 10 pages, 4 figures

We report on room temperature detection of terahertz radiation by means of antenna-coupled field effect transistors (FETs) fabricated using epitaxial graphene grown on silicon carbide. The achieved photoresponsivity (similar to 0.25 V/W) and noise equivalent power (similar to 80 nW/root Hz) result from the combined effect of two independent detection mechanisms: over-damped plasma wave rectification and thermoelectric effects, the latter ascribed to the presence of carrier density junctions along the FET channel. The calculated plasmonic and thermoelectric response reproduces qualitatively well the measured photovoltages; the experimentally observed sign-switch demonstrates the stronger contribution of plasmonic detection compared to the thermoelectric one. These results unveil the potential of plasmonic detectors exploiting epitaxial graphene on silicon carbide for fast large area imaging of macroscopic samples. We report on room temperature detection of terahertz radiation by means of antenna-coupled field effect transistors (FETs) fabricated using epitaxial graphene grown on silicon carbide. The achieved photoresponsivity (∼0.25 V/W) and noise equivalent power (∼80 nW/ Hz) result from the combined effect of two independent detection mechanisms: over-damped plasma wave rectification and thermoelectric effects, the latter ascribed to the presence of carrier density junctions along the FET channel. The calculated plasmonic and thermoelectric response reproduces qualitatively well the measured photovoltages; the experimentally observed sign-switch demonstrates the stronger contribution of plasmonic detection compared to the thermoelectric one. These results unveil the potential of plasmonic detectors exploiting epitaxial graphene on silicon carbide for fast large area imaging of macroscopic samples.

By placing a material in close vicinity of a resonant optical element, its intrinsic optical response can be tuned, possibly to a wide extent. Here, we show that a graphene monolayer, spaced a few tenths of nanometers from a split ring resonator metasurface, exhibits a magneto-optical response which is strongly influenced by the presence of the metasurface itself. This hybrid system holds promises in view of thin optical modulators, polarization rotators, and nonreciprocal devices, in the technologically relevant terahertz spectral range. Moreover, it could be chosen as the playground for investigating the cavity electrodynamics of Dirac fermions in the quantum regime. By placing a material in close vicinity of a resonant optical element, its intrinsic optical response can be tuned, possibly to a wide extent. Here, we show that a graphene monolayer, spaced a few tenths of nanometers from a split ring resonator metasurface, exhibits a magneto-optical response which is strongly influenced by the presence of the metasurface itself. This hybrid system holds promises in view of thin optical modulators, polarization rotators, and nonreciprocal devices, in the technologically relevant terahertz spectral range. Moreover, it could be chosen as the playground for investigating the cavity electrodynamics of Dirac fermions in the quantum regime. (C) 2015 AIP Publishing LLC.

Coherent Perfect Absorption is discussed in the context of strongly-coupled polariton systems. It occurs when the cavity loss rate matches the material one, as demonstrated for intersubband transitions in a photonic crystal resonator.

Quasi-crystal structures do not present a full spatial periodicity but are nevertheless constructed starting from deterministic generation rules. When made of different dielectric materials, they often possess fascinating optical properties, which lie between those of periodic photonic crystals and those of a random arrangement of scatterers. Indeed, they can support extended band-like states with pseudogaps in the energy spectrum, but lacking translational invariance, they also intrinsically feature a pattern of ‘defects’, which can give rise to critically localized modes confined in space, similar to Anderson modes in random structures. If used as laser resonators, photonic quasi-crystals open up design possibilities that are simply not possible in a conventional periodic photonic crystal. In this letter, we exploit the concept of a 2D photonic quasi crystal in an electrically injected laser; specifically, we pattern the top surface of a terahertz quantum-cascade laser with a Penrose tiling of pentagonal rotational symmetry, reaching 0.1–0.2% wall-plug efficiencies and 65 mW peak output powers with characteristic surface-emitting conical beam profiles, result of the rich quasi-crystal Fourier spectrum., Various vertical surface emitting, terahertz quantum-cascade lasers have been proposed recently but these suffer from power cancellations in the far-field and limited extraction efficiencies. Here, Vitiello et al. circumvent these issues using two-dimensional photonic quasi-crystalline resonators.

We report on the demonstration of Terahertz (THz) broadband detectors based on field effect transistors exploiting tapered semiconductor nanowires. The intrinsic asymmetry provided by the nanowires geometry allows to achieve responsivity values as high as 55 V/W (2.5 mA/W) and a noise-equivalent-power of 3 x 10(-10) W/Hz(1/2) independent of the specific gate voltage applied. The possibility to reduce the number of terminals required to the source and drain contacts only and the technological feasibility of multi-pixel arrays are promising for the realization of compact and integrated THz matrix array detection systems. (C) 2014 AIP Publishing LLC.