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We report on the design, fabrication, and spectroscopic characterization of a 1D Photonic Cristal (1DPC) sustaining Bloch Surface Waves (BSWs) in the mid-infrared. The reported all-dielectric 1DPC structure shows potential for label-free biosensing applications to medical diagnostics.

We present infrared (IR) nanospectroscopy results obtained on single hundred-nanometer-sized lipid microvesicles. The use of an IR nanospectroscopy platform allowed us to identify heterogeneities of the molecular cargo of the microvesicles that would not be possible by conventional IR spectroscopy.

Surface current patterns in single patch antennas are imaged by means of photoexpansion nanospectroscopy performed with an atomic force microscopy probe and a broadly tunable quantum cascade laser system. The local ohmic losses lead to temperature increase of a thin polymer layer positioned in the cavity, which then expands.

A thin layer of a mid-IR transparent polymer is inserted inside a metallic optical nano-resonator, where a photonic mode and an intersubband transition are strongly coupled together. Acting as a non-perturbative transducer, and without modifying the optical response, the polymer allows the detection of absorbed optical power in the form of thermal expansion due to heat dissipation. This innovative approach permits to identify - even at the single nano-resonator level - the typical anti crossing characteristic of the polaritonic system, and to perform direct imaging of the electromagnetic fields buried within the inside of an otherwise inaccessible optical nanocavity.

The strong light-matter coupling is an exotic phenomenon of interest not only for basic physics, but also because it has potential for improving optoelectronic devices. When dealing with conduction band electrons in semiconductor quantum wells (QWs), intersubband (ISB) transitions can couple to the optical resonances of plasmonic antennas. In the case of strong light-matter coupling, a characteristic anticrossing of the "light" and "matter" excitations is observed. It stems from the appearance of new eigenmodes, called cavity polaritons.

Exploiting the coupling of laser beams to atomic force microscopy probes, both scattering and absorption of mid-infrared radiation (wavelengths of 5 to $11\ \mu \mathrm{m}$ ) have been measured in the near field region of single patch-antennas, strongly coupled to the intersubband transition of an InGaAs/AlInAs quantum well.

Polymerization of 3',5' cyclic guanosine monophosphate (3',5' cGMP) was previously reported to afford short RNA sequences from a plausible mildly activated prebiotic precursor in a non-enzymatic and template-free manner. In the current paper we analyze the reaction with PAGE as well as by infrared micro-and nanospectroscopy on time scales not considered before. We provide evidence that a preliminary oligomerization step in drystate allows for a continued synthesis of RNA oligomers even in the presence of subsequently added water. On a longer time scale, the oligomerization yield oscillates in an apparently chaotic fashion hinting at the importance of some non-reversible phase-separation processes on the experimentally observed outcome of the polymerization/degradation chemistries. The long-lasting (hundreds of hours) intrinsic sustainability and resilience of the process gives a robust prebiotic potential to this unique oligomerization reaction, which could lead to the very first oligonucleotide sequences on the primordial Earth.

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.

Light conversion into dc current is of paramount interest for a wide range of upcoming energy applications. Here we integrated dipole antennas with field-effect transistors based on a two-dimensional electron gas, with the specific aim of rectifying free-space radiation exploiting both artificial and natural nonlinearities. In the present work, resonant conditions of antenna-coupled field-effect rectifiers have been identified in a terahertz experiment based on the well-established GaAs transistor technology. Rectification of free-space radiation has been observed in a broad 0.15–0.40 THz range by implementing quasi-optical coupling with a substrate lens to an AlGaAs/GaAs heterostructure transistor into the gap of a cross-dipole antenna. The short- and the open-circuit resonances have been clearly identified through a comparison between experimental photocurrent spectra, electromagnetic simulations, and antenna models. The former depends only on the dipole antenna geometry, while the latter is determined ...

Conformational changes of the light-sensitive membrane receptor and proton pump bacteriorhodopsin is investigated by tip-enhanced mid-infrared nanospectroscopy on individual cell membrane patches, with a dramatic increase of sensitivity (in terms of number of probed molecules) compared with standard mid-infrared spectroscopy.

We explore the possibility of detecting anomalous structures buried under the skin surface by studying the deviations from the ideal Airy pattern of the point-spread function (PSF) of a terahertz microscope that includes the skin as one of the reflecting surfaces of the optical system. Using a custom terahertz microscope with a monochromatic point source emitting at 0.611 THz, we record the PSF images with a microbolometer camera. Skin simulants based on collagen gel, with and without artificial buried structures, have been analyzed. The geometrical features characterizing the PSF deformations have been extracted automatically from the PSF images. A machine learning algorithm applied to these geometrical features produces a reliable classification of targets with or without buried structures with error below 5%. It can even classify targets with anisotropic buried structures according to their different orientation.

The quest for novel plasmonic materials has been a lively area of research over the last few years. In the mid-infrared (mid-IR) spectral region, in particular, localized plasmon resonances in nanoparticles and nano-antennas hold promise for enhanced IR spectroscopies, with key applications in biology, medicine, and security. In this frame, the development of a CMOS-compatible plasmonic platform in the mid-IR could have disruptive effects for future technologies, allowing for cost-effective sensing devices integrated with electronics [1-2]. We report on the growth, fabrication and optical characterization of heavily-doped Ge antennas integrated on a Si substrate and we exploit them for the sensing of solid-phase and liquid-phase analytes [3-5]. Epitaxial Ge is grown on Si by plasma-enhanced chemical vapor deposition, exploiting phosphorous as the dopant and achieving plasma frequencies up to 1950 cm-1 (5.1 µm wavelength) corresponding to activated doping of 8.1 x 1019 cm–3. Through modelling and simulation of a series of key figures of merit we demonstrate that n-Ge has performance as a mid-infrared plasmonic material that is within an order of magnitude of noble metals such as gold and with properly designed antennas has the potential to provide enhancements of up to 3 orders of magnitude. We demonstrate two-wire gap antennas fabricated by electron-beam lithography and reactive ion etching techniques, displaying localized plasmon resonances in the important 8 to 13 µm molecular fingerprint region. We target the sensing of a thin polydimethylsiloxane (PDMS) layer (thickness of about 40 nm) and demonstrate experimentally an enhancement of two orders of magnitude in the collected signal for resonances around 800 cm–1 (12.5 µm wavelength), as derived from a comparison with the results of detailed numerical simulations. Further measurements of the ~700 cm–1 (14.3 µm wavelength) C–Cl bond duplet of 2-chloroethyl methyl sulfide, a mustard chemical warfare (CW) agent simulant, demonstrates potential security applications. This part of the mid-infrared spectrum has a host of molecular absorption lines important for the sensing of both CW agents and explosives, both of which are of interest for airport and counter terrorism security screening. Our results represent a first experimental benchmark for group-IV mid-IR plasmonics and confirm that future CMOS sensing platforms could benefit significantly from plasmonic enhancements provided by integrated heavily-doped Ge-based devices. The research leading to these results has received funding from the European Union’s Seventh Framework Programme under grant agreement n°613055. [1] R. Soref, Nature Phot. 4, 495-497 (2010). [2] R. Soref, J. Hendrickson, and J.W. Cleary, Opt. Exp. 20, 3814-3824 (2012). [3] L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D.J. Paul, M. Ortolani, and P. Biagioni, Nano Lett. 15, 7225-7231 (2015). [4] P. Biagioni, J. Frigerio, A. Samarelli, K. Gallacher, L. Baldassarre, E. Sakat, E. Calandrini, R.W. Millar, V. Giliberti, G. Isella, D.J. Paul, and M. Ortolani, J. Nanophot. 9, 093789 (2015). [5] A. Samarelli, J. Frigerio, E. Sakat, L. Baldassarre, K. Gallacher, M. Finazzi, G. Isella, M. Ortolani, P. Biagioni, and D.J. Paul, Thin Solid Films 602, 52 – 55 (2016). Figure 1

The channelrhodopsin-ChR2 is a light-sensitive transmembrane protein that acts as a selective ion channel between the intra- and the extra-cellular environments. In the last decade, ChR2 has proven to be essential for optogenetics, because, if expressed in mammalian neural cells, it enables the control of neuronal activity in response to visible light. Mid-infrared difference spectroscopy can probe the functional conformational changes of light-sensitive proteins, however intrinsic limitations of standard IR spectroscopy in terms of diffraction, and therefore number of probed proteins, require that the mid-IR experiments be performed on huge numbers of lipid membrane patches with overexpressed proteins. In this work, we apply for the first time IR difference nanospectroscopy, based on the use of mid-IR lasers and an atomic force microscope (AFM), to single membrane patches containing ChR2, obtaining relevant spectroscopy results for optogenetic applications and, more generally, for future experimental studies of light-sensitive proteins at the nanoscale.

We use mid-infrared laser absorption nanospectroscopy and nanoimaging, assisted by atomic force microscopy, to study individual hundred-nanometer-sized cell-membrane patches and microvesicles loaded with proteins. The patches and the microvesicles were sparsely deposited on flat gold surfaces and exposed to air for up to 48 hours. The viability of the transmembrane protein Channelrhodopsin, contained in individual membrane patches with lateral size of 500 nm and thickness of 7 nm, was verified by absorption-difference nanospectroscopy. The physical integrity of spherical microvesicles, produced by mouse cell cultures, of diameters distributed around 160 nm was observed by nanoimaging performed 24 hours after drop-casting. The protein cargo of an individual microvesicle was identified by the acquisition of the local infrared absorption spectrum both immediately after deposition and again 24 hours after deposition. The results suggest that infrared nanospectroscopy and nanoimaging may be regarded as potentially useful techniques to assess the stability of e.g. individual hundred-nanometer-sized virus particles deposited on solid surfaces exposed to air for many hours.

We apply mid-infrared absorption microspectroscopy to investigate the light-induced conformational changes of bacteriorhodopsin embedded in native purple membranes deposited on gold substrates. The difference spectra (visible light ON-visible light OFF) obtained on thin films of bacteriorhodopsin display main spectral features connected to the light-induced proton pump process, representing a promising preliminary step towards the investigation of light-sensitive proteins by means of mid-IR tip-enhanced nanospectroscopy.

Photo-Thermal Induced Resonance (PTIR) nanospectroscopy, tuned towards amide-I absorption, was used to study the distribution of proteic material in 34 different HeLa cells, of which 18 were chemically stressed by oxidative stress with Na3AsO3. The cell nucleus was found to provide a weaker amide-I signal than the surrounding cytoplasm, while the strongest PTIR signal comes from the perinuclear region. AFM topography shows that the cells exposed to oxidative stress undergo a volume reduction with respect to the control cells, through an accumulation of the proteic material around and above the nucleus. This is confirmed by the PTIR maps of the cytoplasm, where the pixels providing a high amide-I signal were identified with a space resolution of ∼300 × 300 nm. By analyzing their distribution with two different statistical procedures we found that the probability to find protein clusters smaller than 0.6 μm in the cytoplasm of stressed HeLa cells is higher by 35% than in the control cells. These results indicate that it is possible to study proteic clustering within single cells by label-free optical nanospectroscopy.

We present a compact, all solid-state THz confocal microscope operating at 0.30 THz that achieves super-resolution by using the knife-edge scan approach. In the final reconstructed image, a lateral resolution of 60 μm ≈ λ/17 is demonstrated when the knife-edge is deep in the near-field of the sample surface. When the knife-edge is lifted up to λ/4 from the sample surface, a certain degree of super-resolution is maintained with a resolution of 0.4 mm, i.e. more than a factor 2 if compared to the diffraction-limited scheme. The present results open an interesting path towards super-resolved imaging with in-depth information that would be peculiar to THz microscopy systems.

Cell membranes are intrinsically heterogeneous, as the local protein and lipid distribution is critical to physiological processes. Even in template systems embedding a single protein type, like purple membranes, there can be a different local response to external stimuli or environmental factors, resulting in heterogeneous conformational changes. Despite the dramatic advances of microspectroscopy techniques, the identification of the conformation heterogeneity is still a challenging task. Tip-enhanced infrared nanospectroscopy is here used to identify conformational changes connected to the hydration state of the transmembrane proteins contained in a 50 nm diameter cell membrane area, without the need for fluorescent labels. In dried purple membrane monolayers, areas with fully hydrated proteins are found among large numbers of molecules with randomly distributed hydration states. Infrared nanospectroscopy results are compared to the spectra obtained with diffraction-limited infrared techniques based on the use of synchrotron radiation, in which the diffraction limit still prevents the observation of nanoscale heterogeneity.

A plasmonic analogue of electromagnetically induced transparancy is activated and tuned in the terahertz (THz) range in asymmetric metamaterials fabricated from high critical temperature (T-c) superconductor thin films. The asymmetric design provides a near-field coupling between a superradiant and a subradiant plasmonic mode, which has been widely tuned through super-conductivity and monitored by Fourier transform infrared spectroscopy. The sharp transparency window that appears in the extinction spectrum exhibits a relative modulation up to 50% activated by temperature change. The interplay between ohmic and radiative damping, which can be independently tuned and controlled, allows for engineering the electromagnetically induced transparency of the metamaterial far beyond the current state-of-the-art, which relies on standard metals or low-T-c superconductors.

Plasmonic nanoantenna designs are quickly evolving in the direction of practical molecular sensing applications hence their wavelength range is being extended from the visible towards the mid-infrared. The problem of obtaining, in the mid-infrared, the same degree of plasmonic confinement obtained with gold in the visible range is related to the perfect conductor behavior of metals at long wavelengths. Here we fabricated bow-tie nanoantennas made of bottom-up assembled “metallic germanium” with free electron density of the order of 1020 cm-3 and therefore short plasma wavelength of 4.5 μm. We demonstrate the existence in the antenna gaps of confined hotspots with radius of the order of 100 nm, which we imaged by near-field photoexpansion microscopy at a wavelength of 5.8 μm in order to provide a clear proof of strong field confinement in the mid-infrared.

By use of resonantly-enhanced mechanical photoexpansion technique we have performed mid-infrared vibrational spectroscopy and imaging on the nanoscale of individual purple membranes deposited on ultraflat gold substrate. The observed Amide I and Amide II bands of proteins are in good agreement with the far-field infrared transmission spectra previously collected on large numbers of membranes. A comparison between the near-field signal of single purple membranes and that of two overlapping membranes is presented. The results are discussed by adopting simple thermal and electromagnetic considerations.

Test structures comprehending several combinations of FET detector sizes and bow-tie antennas were designed and fabricated in a 0.13 μm standard CMOS technology. Measurement results from these structures provide a quantitative comparison basis for the design of a future real-time high-frame rate THz camera, providing an insight on the optimization of the FET size.

The intrinsic linewidth of Surface Plasmon Polariton resonance in a micrometric metal mesh has been measured with a collimated mid-infrared beam, provided by an External-Cavity, tunable Quantum Cascade Laser. The use of a collimated beam yields a resonance linewidth of 12 cm-1, much narrower than by conventional black-body sources. This opens more favorable perspectives to the use of metal meshes as sensors and detectors.

We first report evidence by using terahertz (THz) spectroscopy of plasmonic excitations in a topological insulator (TI) that was engineered in thin micro-ribbon arrays. Plasmons are due to Dirac quasi-particles of the two-dimensional electron gas which forms at the surface of TIs.

We have fabricated AlGaAs/InGaAs/AlGaAs heterostructure field effect transistors (HFET) with integrated on-chip antennas and we have measured their optical responsivity when mounted in a in-house developed quasi-optical package with a silicon substrate lens.

The interaction between whey carrier protein β-lactoglobulin B and (-)-epicatechin, a major dietary flavonoid with a wide range of health-promoting biological activities, was investigated by Fourier transform infrared spectroscopy in physiological conditions. Amide I spectra of epicatechin - β-lactoglobulin complexes, in D2O buffer solutions, pD= 6.8, at molar ratios from 0.5:1 to 15:1, were measured by using a cell device specifically created. Changes in secondary structure elements at increasing epicatechin concentrations were quantified. Two different trends were observed for the intensities of β-sheet, random coil, and side chain contributions. At molar ratios ≤2 the β-exposed strand contributions (1625 cm−1) increased at the expence of the β-antiparallel sheet band (1637 cm−1). At molar ratios >2 the intensities of both β structures slightly decreased. The same behaviour was observed for the side chain contributions (band around 1610 ÷ 1620 cm−1). In addition, a conformational transition to a slightly opened structure, followed by aggregate formation at the highest molar ratios, were revealed. The results suggest that binding of epicatechin to β-lactoglobulin in physiological conditions occurs at the surface of the protein molecule, resulting in protein dissociation at molar ratios ≤2 with minor changes in secondary structure. This finding provides further evidence for the possibility of successful use of the protein as a carrier of flavonoids, epicatechin included.

The intrinsic linewidth and angular dispersion of Surface Plasmon Polariton resonance of a micrometric metal mesh have been measured with a collimated mid-infrared beam, provided by an External Cavity tunable Quantum Cascade Laser. We show that the use of a collimated beam yields an observed resonance linewidth γ = 12 cm(-1) at the resonance frequency ν0 = 1658 cm(-1), better by an order of magnitude than with a non-collimated beam. The extremely narrow plasmon resonance attained by our mesh is then exploited to reconstruct, by varying the QCL angle of incidence θ, the angular intensity distribution f(θ) of a globar at the focal plane of a conventional imaging setup. We thus show that f(θ) is better reproduced by a Gaussian distribution than by a uniform one, in agreement with ray-tracing simulation.

International audience; Functionalized scanning probe tips hold great promise for the controlled delivery of signals from and to selected nanoscale volumes. Here, a new nanotechnological approach for the functionalization of scanning probe tips is presented, targeting the transfer of the optical, electrical, thermal, or chemical properties of precisely characterized epitaxial semiconductor layers to the nanoscale probe tip. Homogeneously doped, strain-relaxed heteroepitaxial germanium layers, several micrometers thick, are first grown on silicon wafers by low-energy plasma-enhanced chemical vapor deposition and then employed to functionalize the probe apex. This choice of materials and growth technique enables the future scalability of the tip functionalization process toward high production volumes. The fabricated probe tips are investigated in a transmission electron microscope, revealing that the crystal structure and the homogeneous doping level of the epitaxial layers are unchanged after the probe-tip functionalization process. These doped-germanium tips are also tested as nanoemitters of light at telecom wavelengths (1.55 µm) and applied as scattering probes for near-field mid-infrared microscopy. The reported combination of heteroepitaxial growth and a nanofabrication approach can be extended to a variety of epitaxial materials, thus enabling a new generation of scanning probes for the investigation of nanostructured materials and devices.

Thin metal films with subwavelength hole arrays (meshes) have been fabricated for operation as optical sensors. Mid-infrared spectroscopy and electromagnetic simulations demonstrate that free-standing meshes are particularly suitable for sensing solid nanopollutants.

We studied terahertz current oscillations induced by a frequency-tunable radiation source in a AlGaAs/InGaAs/AlGaAs heterostructure field effect transistor channel. A planar antenna was integrated on-chip, and a substrate lens was used for broadband coupling of free-space radiation at 0.18-0.72 THz to the channel ends. Through spectral analysis of the detection signal, we identified two different mixing mechanisms: one related to channel current oscillations and the other to modulation of the gate-to-channel potential. Depending on gate bias and radiation frequency, the two mechanisms either compete or cooperate, leading to responsivity up to 300 V/W and noise equivalent power of 1 nW/Hz(0.5) (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4717464]