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Terahertz resonant nanoantennas have recently become a key tool to investigate otherwise inaccessible interactions of such long-wavelength radiation with nano-matter. Because of their high-aspect-ratio rod-shaped geometry, resonant nanoantennas suffer from severe loss, which ultimately limits their field localization performance. Here we show, via a quasi-analytical model, numerical simulations, and experimental evidence, that a proper tapering of such nanostructures relaxes their overall loss, leading to an augmented local field enhancement and a significantly reduced resonator mode volume. Our findings, which can also be extended to more complex geometries and higher frequencies, have profound implications for enhanced sensing and spectroscopy of nano-objects, as well as for designing more effective platforms for nanoscale long-wavelength cavity quantum electrodynamics.

This paper is devoted to reviewing the results achieved so far in the application of the single-pixel imaging technique to terahertz (THz) systems. The use of THz radiation for imaging purposes has been largely explored in the last twenty years, due to the unique capabilities of this kind of radiation in interrogating material properties. However, THz imaging systems are still limited by the long acquisition time required to reconstruct the object image and significant efforts have been recently directed to overcome this drawback. One of the most promising approaches in this sense is the so-called "single-pixel" imaging, which in general enables image reconstruction by patterning the beam probing the object and measuring the total transmission (or reflection) with a single-pixel detector (i.e., with no spatial resolution). The main advantages of such technique are that i) no bulky moving parts are required to raster-scan the object and ii) compressed sensing (CS) algorithms, which allow an appropriate reconstruction of the image with an incomplete set of measurements, can be successfully implemented. Overall, this can result in a reduction of the acquisition time. In this review, we cover the experimental solutions proposed to implement such imaging technique at THz frequencies, as well as some practical uses for typical THz applications.

We demonstrate Yb laser (170fs) propagation in large a core hollow-core fiber (HCF) with 97.4% transmission. It serves as a pre-compression stage to achieve 1.3 cycle pulses in a second step with 70% overall efficiency

The generation of structured light by means of metasurfaces is presented and the applications in the characterizations of polarization rotation and Pancharatnam-Berry phase are discussed.

We report the observation of enhanced third-harmonic generation in a mid-infrared metasurface based on the phase change material Ge 2 Sb 2 Se 4 Te 1 .

We investigated terahertz resonant gold nanocones for out-of-plane field-driven photoemission and compare their performance with a standard non-resonant nanotip.

We exploit the strong field enhancement offered by vertical gold nanocones resonating at 1 THz to induce THz field-driven electron emission. The nanocones are fabricated via an advanced 3D printing technique on a photopolymer and are successively gold coated. We demonstrate the clear advantage offered by nanocones featuring a monopolar resonance at THz frequencies with respect to traditional non-resonant tips via numerical modelling, THz far-field characterization, and the analysis of electron-induced argon gas fluorescence. Finally, we show that a further degree of optimization is enabled by tailoring the collective response of the nanocones when arranged in an array geometry.

We apply the single-pixel imaging technique to retrieve multi-dimensional (space, time/frequency) images at terahertz frequencies by indirectly reconstructing the temporal waveform in each pixel. Moreover, we exploit compressed sensing algorithms to reduce the acquisition time.

We report on the nonlinear optical properties of the phase change material Ge2Sb2Se4Te1 (GSST). A 30-fold enhancement of third-harmonic generation is also demonstrated using a GSST metasurface operating in the mid-wave infrared region.

We numerically and experimentally investigate out-of-plane gold nanostructures resonating at terahertz frequencies for field-driven photoemission and compare their performance with a traditional non-resonant nanotip geometry.

We report on the third-order susceptibilities of the phase change material Ge2Sb2Se4Te1 in the amorphous and crystalline phase states and demonstrate enhancement of third-harmonic generation in the mid-infrared using a metasurface design.

By exploiting stimulated Raman scattering in long nitrogen-filled capillary fibers, we demonstrate continuous tunability of Yb laser systems over the 1.0-1.7 μm range, with conversion efficiency up to 82%, and an up to 10-fold pulse compression.

The results are based on a commercial Yb:KGW regenerative amplifier (Pharos, Light Conversion) delivering 1 mJ, 170 fs pulses at 1 to 6 kHz. This beam is precisely matched to the fundamental mode of a large core HCF ( few-cycle Inc.) of 1 mm inner diameter, by realizing a focal spot diameter of 660 μm at 1/e 2 of intensity (corresponding approximately to a Rayleigh range of 0.3 m).

We present an innovative implementation of the solid-state-biased coherent detection (SSBCD) technique, which we have recently introduced for the reconstruction of both amplitude and phase of ultra-broadband terahertz pulses. In our previous works, the SSBCD method has been operated via a heterodyne scheme, which involves demanding square-wave voltage amplifiers, phase-locked to the THz pulse train, as well as an electronic circuit for the demodulation of the readout signal. Here, we demonstrate that the SSBCD technique can be operated via a very simple homodyne scheme, exploiting plain static bias voltages. We show that the homodyne SSBCD signal turns into a bipolar transient when the static field overcomes the THz field strength, without the requirement of an additional demodulating circuit. Moreover, we introduce a differential configuration, which extends the applicability of the homodyne scheme to higher THz field strengths, also leading a two-fold improvement of the dynamic range compared to the heterodyne counterpart. Finally, we demonstrate that, by reversing the sign of the static voltage, it is possible to directly retrieve the absolute THz pulse polarity. The homodyne configuration makes the SSBCD technique of much easier access, leading to a vast range of field-resolved applications.

In this work, we capitalize on the unique properties of terahertz radiation to investigate nanoparticle-assisted laser-tissue interaction. We demonstrate the promising capability of the proposed technique to perform the simultaneous monitoring of laser-tissue interaction and the three-dimensional visualization of the induced photothermal damage in a non-invasive and noncontact manner.

Ultrashort high-energy pulses at wavelengths longer than 1 µm are now desirable for a vast variety of applications in ultrafast and strong-field physics. To date, the main answer to the wavelength tunability for energetic, broadband pulses still relies on optical parametric amplification (OPA), which often requires multiple and complex stages, may feature imperfect beam quality, and has limited conversion efficiency into one of the amplified waves. In this work, we present a completely different strategy to realize an energy-efficient and scalable laser frequency shifter. This relies on the continuous red shift provided by stimulated Raman scattering (SRS) over a long propagation distance in nitrogen-filled hollow-core fibers (HCF). We show a continuous tunability of the laser wavelength from 1030 nm up to 1730 nm with a conversion efficiency higher than 70% and high beam quality. The highly asymmetric spectral broadening, arising from the spatiotemporal nonlinear interplay between higher-order modes of the HCF, can be readily employed to generate pulses ( ∼ 20 f s ) significantly shorter than the pump ones ( ∼ 200 f s ) with high beam quality, and the pulse energy can further be scaled up to tens of millijoules. We envision that this technique, coupled with the emerging high-power Yb laser technology, has the potential to answer the increasing demand for energetic multi-TW few-cycle sources tunable in the near-IR.

We show how gas-filled hollow-core fibers represent a unique platform to explore exotic nonlinear phenomena. We review our recent results including extreme pulse compression, terahertz and supercontinuum generation.

In view of minimally-invasive clinical interventions, laser tissue soldering assisted by plasmonic nanoparticles is emerging as an appealing concept in surgical medicine, holding the promise of surgeries without sutures. Rigorous monitoring of the plasmonically-heated solder and the underlying tissue is crucial for optimizing the soldering bonding strength and minimizing the photothermal damage. To this end, we propose a non-invasive, non-contact, and non-ionizing modality for monitoring nanoparticle-assisted laser-tissue interaction and visualizing the localized photothermal damage, by taking advantage of the unique sensitivity of terahertz radiation to the hydration level of biological tissue. We demonstrate that terahertz radiation can be employed as a versatile tool to reveal the thermally-affected evolution in tissue, and to quantitatively characterize the photothermal damage induced by nanoparticle-assisted laser tissue soldering in three dimensions. Our approach can be easily extended and applied across a broad range of clinical applications involving laser-tissue interaction, such as laser ablation and photothermal therapies.

We exploit a single-pixel imaging approach to obtain multi-dimensional (space, time/frequency) images at terahertz frequencies by reconstructing the temporal waveform in each pixel. Moreover, we apply compressive sensing to reduce the acquisition time.

We present an energy-efficient and scalable technique to realize energetic sub-100-fs pulses, continuously tunable in the 1030÷1730 nm range, by exploiting stimulated Raman scattering in long N2-filled hollow-core fibers.

We present the solid-state-biased coherent detection technique for ultra-broadband THz pulses operated via a homodyne configuration. This makes our detection method of easy implementation, suitable for cost-effective and portable THz systems.

Pulse compression based on non-linear propagation in a gas-filled hollow core fiber (HCF) is amongst the common techniques to generate few-cycle laser pulses [1]. Although very efficient, the major disadvantage of this method is the need to use expensive noble gases like krypton or xenon to compress low energy pulses [2–4]. Our latest results confirm that certain molecular gases, hydrofluorocarbons, represent an affordable and efficient alternative to the traditional atomic gases [5,6]. Such gases bear the potential to generalize HCF compression to high repetition rate, low intensity laser systems [7].

Phonons are quantized lattice vibrations that represent a major energy dissipation channel in solid-state systems [1], both at the macro- and at the nano-scale. Although the phonon response of a specific nanomaterial is usually considered as its intrinsic fingerprint, here we show how it can be altered by exploiting the unique properties of terahertz (THz) plasmonic nanocavities [2]. Specifically, we obtained such nanocavities from the end-to-end coupling (30-nm gap size) of few-μm-long plasmonic gold nanoantennas. We fabricated a series of plasmonic arrays featuring different nanoantenna lengths, spanning from 4.75 μm to 6.75 μm, thus tuning their resonances between approximately 7 and 9 THz. We tested our approach on cadmium sulphide (CdS) nanocrystals (NCs), spin-coated over the array surfaces (Fig. 1a), since these NCs feature a dipole-active (Frohlich) phonon mode at 7.85 THz. We performed THz transmission measurements using a Fourier-transform THz microscope coupled to synchrotron light (ELETTRA, Trieste), showing the splitting of the nanoantenna resonance into two new vibro-polariton bands, as shown in Fig. 1b. This anti-crossing behaviour represents a distinctive signature of the strong coupling between the plasmon and phonon modes, the splitting (Rabi) at the crossing point being directly related to the coupling strength. More intriguingly, we also observed the phonon resonance modification without any THz illumination, just exploiting the vacuum electric field of the nanocavities [3] (estimated to be as high as 4.6× 105 V/m). To this end, we performed a series of micro-Raman measurements on individual nanocavity areas, finding evidence of the two new hybrid states (P− and P+ in Fig. 1c) even in THz "dark" conditions. The evidence of phonon mode splitting both in THz and Raman characterizations confirms the possibility of altering the intrinsic phonon response of a nanomaterial using properly tailored plasmonic resonators, which could open new avenues for the manipulation of energy dissipation in nanodevices. Novel cavity geometries that promise to further boost the strong vibrational coupling in these systems will be presented on site.

Plasmonic nanoantennas (NAs) have received a growing attention in recent years due to their ability to confine light on sub-wavelength dimensions [1]. More recently, this property has been exploited in the terahertz (THz) frequency range (0.1–10 THz) for enhanced sensing and spectroscopy [2], as well as for more fundamental investigations [3]. These applications typically require high local electric fields that can be achieved by concentrating THz radiation into deeply sub-wavelength volumes located at the NAs extremities. However, the achievable near-field enhancement values are severely limited by the poor resonance quality factor of traditional rod-shaped THz NAs. Unlike what is commonly assumed in the infrared domain [4], here we show that an optimal NA tapering angle can be effectively introduced to obtain higher quality factors and, at least, twofold higher local near-field enhancement in comparison with standard (wire-like) dipolar THz NAs. To evaluate how the tapering angle affects the NA performance, a simplified quasi-analytical model was first developed. Each NA is considered as a truncated cone constituted by a sequence of gold cylinders of increasing radii, so that the effective refractive index of the surface mode propagating along the NA changes gradually along the main axis. Once the reflection coefficients for the surface mode at both extremities are retrieved [5], a NA can be interpreted as a Fabry-Perot resonator and its resonances can be analytically calculated. This model reveals a trade-off between large tapering angles (resulting in a low reflection coefficient at the large extremity) compared to small tapers (which are affected by high propagation losses for the surface mode), leading to an optimal taper angle. FEM-based simulations (COMSOL Multiphysics) were then used to confirm this prediction. 60-nm-thick gold tapered NA dimers were designed with 45-μm-long arms (in order to resonate at around 1 THz) and with their facing tips (100-nm-wide) separated by a 30 nm gap, thus realizing a bowtie geometry (Fig. 1a). We numerically investigated the near-field enhancement in the gap between the NAs, varying the tapering angle α from 0° to 10°.

We demonstrate 33-fold pulse compression employing a 6-m-long hollow-core fiber and chirped mirrors. 1 mJ, 170 fs pulses at 1030 nm are compressed to 5.1 fs (1.5 optical cycles) with 70% efficiency.

The Doppler frequency shift of sound or electromagnetic waves has been widely investigated in many different contexts and, nowadays, represents a formidable tool in medicine, engineering, astrophysics, and optics. Such effect is commonly described in the framework of the universal energy-momentum conservation law. In particular, the rotational Doppler effect has been recently demonstrated using light carrying orbital angular momentum. When a wave undergoes a cyclic adiabatic transformation of its Hamiltonian, it is known to acquire the so-called Pancharatnam–Berry (PB) phase. In this work, an experimental evidence of the direct connection between the high-order PB phase time evolution on the Poincaré sphere and the rotational Doppler frequency shift of light is provided. A metasurface operating at telecom wavelengths is employed to impose a total (spin and orbital) angular momentum (TAM) on the light wave, while two TAM converters ensure a closed cycle on the Poincaré sphere. By rotating one of the converters, a significant Doppler frequency shift is observed without variation of the output TAM. The proposed metasurface-based approach offers new advanced ways to engineer the frequency content of light.

The mid-wave infrared (MWIR) spectral region (3-5 {\mu}m) is important to a vast variety of applications in imaging, sensing, spectroscopy, surgery, and optical communications. Efficient third-harmonic generation (THG), converting light from the MWIR range into the near-infrared, a region with mature optical detection and manipulation technologies, offers the opportunity to mitigate a commonly recognized limitation of current MWIR systems. In this work, we present the possibility of boosting THG in the MWIR through a metasurface design. Specifically, we demonstrate a 30-fold enhancement in a highly nonlinear phase change material Ge2Sb2Se4Te1 (GSST), by patterning arrays of subwavelength cylinders supporting a magnetic dipolar resonance. The unprecedented broadband transparency, large refractive index, and remarkably high nonlinear response, together with unique phase-change properties, make GSST-based metasurfaces an appealing solution for reconfigurable and ultra-compact nonlinear devices operating in the MWIR.

We present an implementation of the single-pixel imaging approach into a terahertz (THz) time-domain spectroscopy (TDS) system. We demonstrate the indirect coherent reconstruction of THz temporal waveforms at each spatial position of an object, without the need of mechanical raster-scanning. First, we exploit such temporal information to realize (far-field) time-of-flight images. In addition, as a proof of concept, we apply a typical compressive sensing algorithm to demonstrate image reconstruction with less than 50% of the total required measurements. Finally, the access to frequency domain is also demonstrated by reconstructing spectral images of an object featuring an absorption line in the THz range. The combination of single-pixel imaging with compressive sensing algorithms allows to reduce both complexity and acquisition time of current THz-TDS imaging systems.

The precise determination of the polarization state of light is fundamental for a vast variety of applications in remote sensing, astronomy, optics and terahertz technology, to name just a few. Typically, polarization characterization is performed by using a combination of multiple optical devices such as beam splitters, polarizers, and waveplates. Moreover, to achieve high-precision, balanced photodetectors and lock-in amplifiers are employed, thus contributing to increasing system complexity. Here, a technique for polarization rotation measurements with a dynamic range of 180° and a sensitivity of about 10−2 degrees is realized using a properly designed metasurface. Such device generates a vector beam with an azimuthally-dependent polarization distribution, as a result of the superposition of two vortex beams carrying opposite orbital angular momenta (ℓ = ±30). After propagation through a linear polarizer, the spatial intensity profile of such a beam turns into 60 lobes. By tracking the displacement of only two of these lobes on a camera, the rotation of the input polarization state can be retrieved with high resolution. The proposed approach offers a new route toward the development of compact high-precision polarimeters and can also be exploited in quantum information processing, optical communications, as well as nonlinear and chiral optics.

We introduce hydrofluorocarbon molecules as an alternative medium to noble gases with low ionization potential like krypton or xenon to compress ultrashort pulses of relatively low energy in a conventional hollow core fiber with subsequent dispersion compensation. Spectral broadening of pulses from two different laser systems exceeded those achieved with argon and krypton. Initially 40 fs, 800 nm, 120 μJ pulses were compressed to few optical cycles duration. With the same approach a compression factor of more than 10 was demonstrated for an ytterbium-based laser (1030 nm, 170 fs, 200 μJ) leading to 15.6 fs.

The accurate determination of the photothermal response of nanomaterials represents an essential aspect in many fields, such as nanomedicine. Specifically, photothermal cancer therapies rely on the precise knowledge of the light-to-heat transfer properties of plasmonic nanoparticles to achieve the desired temperature-induced effects in biological tissues. In this work, we present a novel method for the quantification of the photothermal effect exhibited by nanoparticles in aqueous dispersions. By combining the spatial and temporal thermal dynamics acquired at terahertz frequencies, the photothermal conversion efficiency associated with the geometry of the plasmonic nanoparticles can be retrieved in a noncontact and noninvasive manner. The proposed technique can be extended to the characterization of all those nanomaterials which feature a temperature-dependent variation of the refractive index in the terahertz regime.

We present details regarding the modeling of light-nanomatter interaction in terahertz plasmonic nanocavities.

High-peak-electric-field THz pulses (>200 kV/cm) can be efficiently generated in a recently developed organic crystal pumped by a femtosecond ytterbium laser in a collinear configuration.

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.

We introduce hydrofluorocarbon molecules as a valuable alternative to conventional noble gases to compress 100 pJ level ultrashort pulses. We demonstrate efficient compression down to 15.6 fs for 170 fs, 1030 nm, 200 gJ initial pulses.

Copyright information see: https://www.osapublishing.org/submit/review/copyright_permissions.cfm#oal We present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies.

We show a new contactless, non-invasive temperature and hyperspectral mapping method at terahertz (THz) frequencies capable of imaging the thermal and spatial distribution of a drug composite injected in a biological sample, as well as simultaneously verifying its chemical integrity.

The spectral fingerprint of ibuprofen within the THz frequency window has been retrieved through an ultra-broadband THz Time Domain Spectrometry set-up. The latter implements the Solid State Biased Coherent Detection scheme, based on a compact CMOS-compatible integrated device. Such a technique shows unprecedented advantages in term of bandwidth (greater than 10 THz) over other solid state methods like electro-optic sampling.

Phonons (quanta of collective vibrations) are a major source of energy dissipation and drive some of the most relevant properties of materials. In nanotechnology, phonons severely affect light emission and charge transport of nanodevices. While the phonon response is conventionally considered an inherent property of a nanomaterial, here we show that the dipole-active phonon resonance of semiconducting (CdS) nanocrystals can be drastically reshaped inside a terahertz plasmonic nanocavity, via the phonon strong coupling with the cavity vacuum electric field. Such quantum zero-point field can indeed reach extreme values in a plasmonic nanocavity, thanks to a mode volume well below λ3/107. Through Raman measurements, we find that the nanocrystals within a nanocavity exhibit two new “hybridized” phonon peaks, whose spectral separation increases with the number of nanocrystals. Our findings open exciting perspectives for engineering the optical phonon response of functional nanomaterials and for implementing a novel platform for nanoscale quantum optomechanics., Here the authors show that the dipole-active phonon resonance of semiconducting nanocrystals can be hybridized by a strongly concentrated terahertz vacuum field of a plasmonic nanocavity, thus achieving strong plasmon–phonon coupling even in the absence of direct terahertz illumination.

We demonstrate a novel terahertz radiation-based joint thermal-hyperspectral imaging method for theranostic applications. Hyperspectral imaging of a drug formulation was realized in the stratum granulosum of skin, in the presence of plasmonically heated gold nanoparticles.

We demonstrate the first fully solid-state technique for the coherent detection of ultra-broadband THz pulses (0.1-10 THz), relying on the electric-field-induced second-harmonic generation attained in integrated CMOS-compatible devices.

We present a system for extremely broadband terahertz (THz) generation based on an Ytterbium (Yb) amplified laser emitting 170-fs-long pulses centered at 1030 nm. The pulses are first spectrally broadened in an Ar-filled hollow-core capillary fiber (HCF) and then recompressed down to ∼18 fs with a chirped-mirror pair. Extreme broadband THz pulses of bandwidths up to 60 THz and peak electric field as high as 55 kV/cm are obtained via two-color plasma generation. The combination of high-power Yb laser systems with gas-filled HCF opens the path towards the realization of the next generation high-repetition-rate, extremely broadband, and intense-field THz time-domain spectroscopy systems.

We report on the numerical investigation and analytical modeling of tapered terahertz nanoantennas. We show that the maximum field enhancement at the antenna tip is achieved for small taper angles rather than in a more traditional dipolar (wire-like) geometry.

We report here on the design, fabrication and characterization of out-of-plane arrays of conical nanoantennas resonating in the terahertz frequency range.

A continuous-wave fiber laser characterization based on a Findlay–Clay analysis is proposed. With this procedure, commonly used for solid-state lasers, information about saturation power and reabsorption loss is obtained. The knowledge of these parameters is commonly sufficient for a number of fiber oscillator design issues, but it also allows estimating values for fabrication dependent parameters, such as absorption and emission cross-sections, doping concentrations and dopant effective diffusion areas. As an application example, a single-mode low-power oscillator employing standard and well-known fiber components has been studied, providing values in good agreement with previously reported spectroscopical measurements.

We present a novel class of CMOS-compatible devices aimed to perform the solid-state-biased coherent detection of ultrashort terahertz pulses, i.e., featuring a gap-free bandwidth at least two decades-wide. Such a structure relies on a 1-µm-wide slit aperture located between two parallel aluminum pads, embedded in a 1-µm-thick layer of silicon nitride, and deposited on a quartz substrate. We show that this device can detect ultra-broadband terahertz pulses by employing unprecedented low optical probe energies of only a few tens of nanojoules. This is due to the more than one order of magnitude higher nonlinear coefficient of silicon nitride with respect to silica, the nonlinear material employed in the previous generations. In addition, due to the reduced distance between the aluminum pads, very high static electric fields can be generated within the slit by applying extremely low external bias voltages (in the order of few tens of volts), which strongly enhance the dynamic range of the detected THz waveforms. These results pave the way to the integration of solid-state ultra-broadband detection in compact and miniaturized terahertz systems fed by high repetition-rate laser oscillators and low-noise, low-voltage generators.