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In this article, we investigated the effect of continuous-wave (CW) radiation at 0.6 THz on pathological protein aggregates in the form of amyloid fibrils, i.e., ordered protein complexes linked to neurodegenerative diseases such as Parkinson's and Frontotemporal Dementia. To monitor the effect of terahertz (THz) irradiation, we exploited mid-infrared (mid-IR) vibrational spectroscopy in the amide-I band range, whose lineshape is known to depend on the protein conformation and on how proteins arrange into ordered supramolecular complexes such as fibrils. We coupled the focused THz beam to two different IR-based spectrometers: a conventional Fourier-transform IR (FTIR) Michelson interferometer where the estimated THz electric field is of the order of $\sim$1 $\frac{\text{V}}{{{\text {cm}}}}$; and an atomic force microscopy-assisted (AFM-IR) near-field spectrometer based on a tunable mid-IR quantum cascade laser, where a much higher electric field ($\sim$0.1 $\frac{{{\text {kV}}}}{\text{cm}}$) is mainly achieved thanks to the field enhancement provided by the use of a metallic AFM tip and sample support. In the first case, we interpreted the modification of the amide-I band upon THz irradiation in terms of an increase of the intermolecular forces within fibrils in response to environmental changes induced by THz irradiation (change of hydration). On the other hand, nonthermal effects are observed in the high-THz-field experiments performed on isolated fibril agglomerates in dry condition with the AFM-assisted spectrometer. The IR spectral response upon prolonged THz irradiation contains only the protein contribution and we obtain a different trend compared to the FTIR experiments, i.e., a weakening of the intermolecular forces, here directly induced by THz absorption and not mediated by changes of the environmental conditions. One can envision that further increase of the THz field value, such as with pulsed laser, can lead to the disassembly of protein fibrils.

Static electric fields play a considerable role in a variety of molecular nanosystems as diverse as single-molecule junctions, molecules supporting electrostatic catalysis, and biological cell membranes incorporating proteins. External electric fields can be applied to nanoscale samples with a conductive atomic force microscopy (AFM) probe in contact mode, but typically, no structural information is retrieved. Here we combine photothermal expansion infrared (IR) nanospectroscopy with electrostatic AFM probes to measure nanometric volumes where the IR field enhancement and the static electric field overlap spatially. We leverage the vibrational Stark effect in the polymer poly(methyl methacrylate) for calibrating the local electric field strength. In the relevant case of membrane protein bacteriorhodopsin, we observe electric-field-induced changes of the protein backbone conformation and residue protonation state. The proposed technique also has the potential to measure DC currents and IR spectra simultaneously, insofar enabling the monitoring of the possible interplay between charge transport and other effects.

Few-layer graphene possesses low-energy carriers that behave as massive Fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV, we target these massive quasiparticles in the split bands close to the Kpoint. The low excitation energy weakens some of the Raman processes that are resonant in the visible, and induces a clearer frequency-separation of the substructures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each substructure, and comparing experimental measurements on bilayer graphene with ab initiotheoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron–phonon matrix elements.

We present a theoretical investigation of guided second harmonic generation at THz frequencies in SiGe waveguides embedding n-type Ge/SiGe asymmetric coupled quantum wells to engineer a giant second order nonlinear susceptibility. A characteristic of the chosen material system is the existence of large off-diagonal elements in the χ2tensor, coupling optical modes with different polarization. To account for this effect, we generalize the coupled-mode theory, proposing a theoretical model suitable for concurrently resolving every second harmonic generation interaction among guide-sustained modes, regardless of which χ2tensor elements it originates from. Furthermore, we exploit the presence of off-diagonal χ2elements and the peculiarity of the SiGe material system to develop a simple and novel approach to achieve perfect phase matching without requiring any fabrication process. For a realistic design of the quantum heterostructure we estimate second order nonlinear susceptibility peak values of ∼7 and ∼1.4 × 105pm/V for diagonal and off diagonal χ2elements, respectively. Embedding such heterostructure in Ge-rich SiGe waveguides of thicknesses of the order of 10–15 μm leads to second harmonic generation efficiencies comprised between 0.2 and 2 %, depending on the choice of device parameters. As a case study, we focus on the technologically relevant frequency of 7.1 THz, yet the results we report may be extended to the whole 5–20 THz range.

A parabolic potential that confines charge carriers along the growth direction of quantum wells semiconductor systems is characterized by a single resonance frequency, associated to intersubband transitions. Motivated by fascinating quantum optics applications leveraging on this property, we use the technologically relevant SiGe material system to design, grow, and characterize n-type doped parabolic quantum wells realized by continuously grading Ge-rich Si1−xGexalloys, deposited on silicon wafers. An extensive structural analysis highlights the capability of the ultra-high-vacuum chemical vapor deposition technique here used to precisely control the quadratic confining potential and the target doping profile. The absorption spectrum, measured by means of Fourier transform infrared spectroscopy, revealed a single peak with a full width at half maximum at low and room temperature of about 2 and 5 meV, respectively, associated to degenerate intersubband transitions. The energy of the absorption resonance scales with the inverse of the well width, covering the 2.5–5 THz spectral range, and is almost independent of temperature and doping, as predicted for a parabolic confining potential. On the basis of these results, we discuss the perspective observation of THz strong light–matter coupling in this silicon compatible material system, leveraging on intersubband transitions embedded in all-semiconductor microcavities.

The concept of strong light–matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits the study of the light–matter interaction in single subwavelength-sized nanocavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin (∼150 nm) polymer layer with negligible absorption in the mid-infrared range (5 μm < λ < 12 μm) inside a metal–insulator–metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at λ = 8.3 μm, and the nanocavity is overall 270 nm thick. Acting as a nonperturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using atomic force microscopy (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nanoresonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of a strong light–matter interaction.

The growing need for new and reliable surface sensing methods is arousing interest in the electromagnetic excitations of ultrathin films, i.e., to generate electromagnetic field distributions that resonantly interact with the most significant quasi-particles of condensed matter. In such a context, Bloch surface waves turned out to be a valid alternative to surface plasmon polaritons to implement high-sensitivity sensors in the visible spectral range. Only in the last few years, however, has their use been extended to infrared wavelengths, which represent a powerful tool for detecting and recognizing molecular species and crystalline structures. In this work, we demonstrate, by means of high-resolution reflectivity measurements, that a one-dimensional photonic crystal can sustain Bloch surface waves in the infrared spectral range from room temperature down to 10 K. To the best of our knowledge, this is the first demonstration of infrared Bloch surface waves at cryogenic temperatures. Furthermore, by exploiting the enhancement of the surface state and the high brilliance of infrared synchrotron radiation, we demonstrate that the proposed BSW-based sensor has a sensitivity on the order of 2.9 cm–1for each nanometer-thick ice layer grown on its surface below 150 K. In conclusion, we believe that Bloch surface wave-based sensors are a valid new class of surface mode-based sensors for applications in materials science.

Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition. We demonstrate the high material quality of strain-symmetrized structures and heterointerfaces as well as control over the inter-well coupling and electron tunneling. Motivated by the promising results obtained on ACQWs, which are the basic building block of a cascade structure, we investigate, both experimentally and theoretically, a Ge/SiGe THz QCL design, optimized through a non-equilibrium Green's function formalism.

Photosensitive proteins embedded in the cell membrane (about 5 nm thickness) act as photoactivated proton pumps, ion gates, enzymes, or more generally, as initiators of stimuli for the cell activity. They are composed of a protein backbone and a covalently bound cofactor (e.g. the retinal chromophore in bacteriorhodopsin (BR), channelrhodopsin, and other opsins). The light-induced conformational changes of both the cofactor and the protein are at the basis of the physiological functions of photosensitive proteins. Despite the dramatic development of microscopy techniques, investigating conformational changes of proteins at the membrane monolayer level is still a big challenge. Techniques based on atomic force microscopy (AFM) can detect electric currents through protein monolayers and even molecular binding forces in single-protein molecules but not the conformational changes. For the latter, Fourier-transform infrared spectroscopy (FTIR) using difference-spectroscopy mode is typically employed, but it is performed on macroscopic liquid suspensions or thick films containing large amounts of purified photosensitive proteins. In this work, we develop AFM-assisted, tip-enhanced infrared difference-nanospectroscopy to investigate light-induced conformational changes of the bacteriorhodopsin mutant D96N in single submicrometric native purple membrane patches. We obtain a significant improvement compared with the signal-to-noise ratio of standard IR nanospectroscopy techniques by exploiting the field enhancement in the plasmonic nanogap that forms between a gold-coated AFM probe tip and an ultraflat gold surface, as further supported by electromagnetic and thermal simulations. IR difference-spectra in the 1450–1800 cm–1range are recorded from individual patches as thin as 10 nm, with a diameter of less than 500 nm, well beyond the diffraction limit for FTIR microspectroscopy. We find clear spectroscopic evidence of a branching of the photocycle for BR molecules in direct contact with the gold surfaces, with equal amounts of proteins either following the standard proton-pump photocycle or being trapped in an intermediate state not directly contributing to light-induced proton transport. Our results are particularly relevant for BR-based optoelectronic and energy-harvesting devices, where BR molecular monolayers are put in contact with metal surfaces, and, more generally, for AFM-based IR spectroscopy studies of conformational changes of proteins embedded in intrinsically heterogeneous native cell membranes.

Thermoplasmonics is a method for increasing temperature remotely using focused visible or infrared laser beams interacting with plasmonic nanoparticles. Here, local heating induced by mid-infrared quantum cascade laser illumination of vertical gold-coated nanoantenna arrays embedded into polymer layers is investigated by infrared nanospectroscopy and electromagnetic/thermal simulations. Nanoscale thermal hotspot images are obtained by a photothermal scanning probe microscopy technique with laser illumination wavelength tuned at the different plasmonic resonances of the arrays. Spectral analysis indicates that both Joule heating by the metal antennas and surface-enhanced infrared absorption (SEIRA) by the polymer molecules located in the apical hotspots of the antennas are responsible for thermoplasmonic resonances, that is, for strong local temperature increase. At odds with more conventional planar nanoantennas, the vertical antenna structure enables thermal decoupling of the hotspot at the antenna apex from the heat sink constituted by the solid substrate. The temperature increase was evaluated by quantitative comparison of data obtained with the photothermal expansion technique to the results of electromagnetic/thermal simulations. In the case of strong SEIRA by the C═O bond of poly-methylmethacrylate at 1730 cm–1, for focused mid-infrared laser power of about 20 mW, the evaluated order of magnitude of the nanoscale temperature increase is of 10 K. This result indicates that temperature increases of the order of hundreds of K may be attainable with full mid-infrared laser power tuned at specific molecule vibrational fingerprints.

A compact confocal terahertz microscope working at 0.30 THz based on all-solid-state components is used to locate buried thin metal foils in a painted wood artwork. Metal foils are used for decoration, and their precise localization under the pictorial layer is relevant information for conservation scientists and restorers, which can neither be obtained by X-ray radiography nor by spectroscopic imaging in the infrared, as we directly show here. The confocal microscopy principle based on the spatial pinhole concept is here implemented by positioning the first focus of an ellipsoidal reflector at the phase center of horn antennas coupled to Schottky diode detector and emitter mounted in rectangular waveguide blocks, together with an optical beamsplitter. The second focus of the reflector is mechanically scanned inside the sample in three dimensions. The predictions of diffraction theory for a confocal microscope at an imaging wavelength of 1.00 mm with numerical aperture of 0.53 are verified experimentally (1.2 and 2.8 mm for the lateral and the axial resolution, respectively). These values of resolution allow a precise determination of the position of buried metal foils in an ancient piece of art hence making restoration interventions possible.

Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5 barriers grown on a relaxed Si0.2Ge0.8 buffer on a Si substrate is demonstrated from 6 to 9 um wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red transmission demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8-13 um where many chemical and biological molecules have unique molecular absorption lines for spectroscopic identification.

Heavily-doped semiconductor films are very promising for applications in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work, we investigate, n-type doped Ge epilayers grown on Si substrates. We design and realize Ge nano-antennas on Si substrates demonstrating the presence of localized plasmon resonances, and exploit them for molecular sensing in the mid-infrared.

We present an experimental study of subwavelength hole arrays in thin metal films employed as surface-enhanced optical sensors operating in the mid-infrared. The extremely narrow surface plasmon polariton spectral resonances are fitted to an analytical Fano interference model in the wavelength range 2–10 μm. In general, the resonance frequency shifts after deposition of few-molecule layers (3.2–24 nm thickness) according to electrodynamic polarization models, hence allowing for label-free sensing. The absolute value of the shift is shown to depend on the overlap between the electric field distribution of the specific surface plasmon mode and the molecular layer, as verified by electromagnetic simulations. Biochemical sensor application is finally demonstrated by determining, from a single mid-infrared measurement, both the thickness and the absorption spectrum of phospholipid monolayers and trilayers, obtained by liposome adsorption.

Imaging arrays of direct detectors in the 0.5-5 THz range are being experimentally developed. Terahertz active imaging with amplitude-modulated quantum cascade lasers emitting at 2.5 and 4.4 THz performed by using an antenna-coupled superconducting microbolometer. We then present two room-temperature terahertz detector technologies compatible with monolithic arrays: i) GaAs Schottky diodes with air-bridge sub-micron anodes; ii) high electron mobility transistors with sub-micron Schottky gate. Performances, requirements and fabrication costs of the different detector technologies are compared.

We show how synchrotron radiation (SR) in the terahertz (THz) region provides the possibility to measure the properties of conventional and exotic superconductors in their superconducting state. Indeed, through the coupling of SR and a conventional Michelson interferometer, one can obtain in the THz range a signal-to-noise ratio up to 103. We review the application of this technique to superconductors with a different degree of complexity: the single-gap boron-doped diamond BCS isotropic material; CaAlSi, a superconductor isostructural to MgB2 with a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis; and isotropic V3Si, where superconductivity opens two gaps at the Fermi energy.

The results of pilot experiments carried out at the new infrared beamline SISSI (Source for Imaging and Spectroscopic Studies in the Infrared) operated at the synchrotron laboratory ELETTRA in Trieste, Italy, are presented and compared with the results obtained with conventional IR sources. The main figures of merit of the infrared synchrotron radiation (IRSR) such as brightness, spectral quality, and stability are discussed. Using a pinhole scanned across the IRSR beam, the effective beam size, the intensity, and the lateral distribution for different wavelengths are determined. The results obtained on geological and biological samples are used to illustrate how the broadband nature and high brightness of the IRSR beam allow IR spectroscopy experiments on diffraction-limited sample areas in both the mid-IR and far-IR regions.

We show how synchrotron radiation (SR) in the terahertz (THz) region provides the possibility to measure the properties of conventional and exotic superconductors in their superconducting state. Indeed, through the coupling of SR and a conventional Michelson interferometer, one can obtain in the THz range a signal-to-noise ratio up to 103. We review the application of this technique to superconductors with a different degree of complexity: the single-gap boron-doped diamond BCS isotropic material; CaAlSi, a superconductor isostructural to MgB2 with a slight anisotropy between the gap in the hexagonal planes and that along the orthogonal c axis; and isotropic V3Si, where superconductivity opens two gaps at the Fermi energy.

Most of the recently discovered superconductors, like all cuprates, MgB2, or NaxCoO2∙nH2O, are uniaxial crystals with metallic ab planes orthogonal to an insulating, or sometimes poorly conducting, c axis. The reflectivity Rab(omega) provides, through the optical conductivity sigmaab(omega), valuable information on the low-energy electrodynamics of these intriguing materials. However, in the far infrared, Rab(omega) ~ or = 1 and a slight contamination from the c-axis response or an unsuitable choice of the extrapolation to omega=0 may lead to large errors on sigmaab(omega). These issues are quantitatively discussed here with particular regard to La2−xSrxCuO4+y, one of the most-studied high-Tc materials.

Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001) substrates by means of ultrahigh vacuum chemical vapor deposition. We demonstrate the high material quality of strain-symmetrized structures and heterointerfaces as well as control over the inter-well coupling and electron tunneling. Motivated by the promising results obtained on ACQWs, which are the basic building block of a cascade structure, we investigate, both experimentally and theoretically, a Ge/SiGe THz QCL design, optimized through a non-equilibrium Green’s function formalism.

Mid-infrared intersubband absorption from p-Ge quantum wells with Si0.5Ge0.5barriers grown on a relaxed Si0.2Ge0.8buffer on a Si substrate is demonstrated from 6 to 9 μm wavelength at room temperature and can be tuned by adjusting the quantum well thickness. Fourier transform infra-red transmission demonstrate clear absorption peaks corresponding to intersubband transitions among confined hole states. The work indicates an approach that will allow quantum well intersubband photodetectors to be realized on Si substrates in the important atmospheric transmission window of 8-13 μm where many chemical and biological molecules have unique molecular absorption lines for spectroscopic identification.

Heavily-doped semiconductor films are very promising for applications in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work, we investigate, n-type doped Ge epilayers grown on Si substrates. We design and realize Ge nano-antennas on Si substrates demonstrating the presence of localized plasmon resonances, and exploit them for molecular sensing in the mid-infrared.