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The proton pump Bacteriorhodopsin (BR) undergoes repeated photocycles including reversible conformational changes upon visible light illumination. Exploiting the sensitivity of infrared (IR) spectra to the conformation, we have determined the reaction kinetic parameters of the conductive intermediate M for the wild-type protein and for its slow mutant D96N during its photocycle. Time-resolved IR micro-spectroscopy using an in-house developed confocal laser microscope operating in the mid-IR is employed to record absorption changes of 10−4 at wavelengths λ1 = 6.08 μm and λ2 = 6.35 μm, assigned to backbone and retinal structural modifications, respectively. Protein samples were embedded in dried lipid bilayers deposited on ultraflat gold supports to enhance the surface field. The signals were analyzed according to a simplified photocycle model with only two dominant states: the dark-adapted state BR* and the intermediate M. We obtained the excitation and relaxation times of the intermediate M from exponential fits to the absorption change time traces. Our results constitute a first step towards future plasmonic-assisted nanoscale time-resolved mid-IR spectrometers for the characterization of bioelectronic and light-harvesting nanodevices based on BR.

Recently, 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 to study the light-matter interaction in single subwavelength-sized nano-cavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin ($\approx$ 150 nm) polymer layer with negligible absorption in the mid-IR (5 $\mu$m < $\lambda$ < 12 $\mu$m) 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 $\lambda$ = 8.3 $\mu$m, and the nano-cavity is overall 270 nm thick. Acting as a non-perturbative 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 an atomic force microscope (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nano-resonator 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 strong light-matter interaction.

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.

Extracellular vesicles are membrane-delimited structures, involved in several inter-cellular communication processes, both physiological and pathological, since they deliver complex biological cargo. Extracellular vesicles have been identified as possible biomarkers of several pathological diseases, thus, their characterization is fundamental in order to gain a deep understanding of their function and of the related processes. Traditional approaches for the characterization of the molecular content of the vesicles require a large quantity of sample, thereby providing an average molecular profile, while their heterogeneity is typically probed by non-optical microscopies that, however, lack the chemical sensitivity to provide information of the molecular cargo. Here, we perform a study of individual microvesicles, a subclass of extracellular vesicles generated by the outward budding of the plasma membrane, released by two cultures of glial cells under different stimuli, by applying a state-of-the-art infrared nanospectroscopy technique based on the coupling of an atomic force microscope and a pulsed laser, which combines the label-free chemical sensitivity of infrared spectroscopy with the nanometric resolution of atomic force microscopy. By correlating topographic, mechanical and spectroscopic information of individual microvesicles, we identified two main populations in both families of vesicles released by the two cell cultures. Subtle differences in terms of nucleic acid content among the two families of vesicles have been found by performing a fitting procedure of the main nucleic acid vibrational peaks in the 1000–1250 cm−1 frequency range.

Optogenetics is a revolutionary method for studying neural activity by exploiting the optical stimulation of neurons made possible by artificial genetic expression of light-sensitive transmembrane ``gate'' proteins. Evidence of functional conformational changes of the gate proteins can be obtained by difference infrared (IR) absorption spectroscopy triggered by light stimuli. Here we investigate the effect on the photocycle of the prototype optogenetic protein channelrhodopsin (ChR2) of gold surfaces placed in close proximity to the protein molecules. In order to do this, we bring difference IR spectroscopy to the nanoscale, using a platform based on the coupling of a tunable mid-IR quantum cascade laser and an atomic force microscope (AFM). Sensitivity to individual subwavelength-sized membrane patches, embedding fewer than a hundred ChR2 molecules, is achieved by taking advantage of a plasmonic field enhancement in the nanogap between the gold-coated AFM tip and an ultraflat gold surface used as a sample support. We obtain relative difference absorption variations smaller than ${10}^{\ensuremath{-}2}$ and benchmark nanospectroscopy difference spectra against those taken with the Fourier transform IR (FTIR) spectroscopic technique on the same sample. We identify distinct simultaneous processes in the ChR2 photocycle by performing singular value decomposition of the FTIR difference spectra, and use this procedure to compare IR nanospectroscopy data on individual membrane patches in contact with gold surfaces with those obtained on thick stacks of membrane patches unexposed to metal surfaces. ChR2 proteins maintain their gate function when placed in a 14-nm-wide gap between two gold surfaces, apart from minor modifications to their kinetics. Our results are relevant to optogenetic applications that require physical contact between nanoscale metallic probes and electrodes and the membrane of single neuronal cells. More generally, our work paves the way towards the spectroscopic study of transmembrane proteins at the nanoscale.

We apply polarized Attenuated Total Reflection Fourier Transform Infrared spectroscopy (ATR-FTIR) to study conformational changes of the prototype photosensitive protein bacteriorhodopsin (BR). The experiments are performed on hundreds-nanometer-thick films of stacked membrane patches embedding BR molecules. Moreover, we report our results on functionalization of silicon surfaces that are preliminary to further experiments involving faced-down silicon substrates on ATR crystal in order to increase the sensitivity down to very thin protein films.

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 ...

A novel spectroscopy technique to enable the rapid characterization of discrete mid-infrared integrated photonic waveguides is demonstrated. The technique utilizes lithography patterned polymer blocks that absorb light strongly within the molecular fingerprint region. These act as integrated waveguide detectors when combined with an atomic force microscope that measures the photothermal expansion when infrared light is guided to the block. As a proof of concept, the technique is used to experimentally characterize propagation loss and grating coupler response of Ge-on-Si waveguides at wavelengths from 6 to 10 µm. In addition, when the microscope is operated in scanning mode at fixed wavelength, the guided mode exiting the output facet is imaged with a lateral resolution better than 500 nm i.e. below the diffraction limit. The characterization technique can be applied to any mid-infrared waveguide platform and can provide non-destructive in-situ testing of discrete waveguide components.

Scanning-probe-assisted mid-infrared nano-spectroscopy is employed to reveal the polaritonic dispersion of individual MIM (metal-insulator-metal) square patch antennas whose modes can be strongly coupled to a mid-infrared intersubband transition. The patch antenna side length L sets the resonances between λ = 5.5 μm and 12.5 μm. The active region consists of a highly doped AlInAs/InGaAs/AlInAs single quantum well that presents an intersubband transition at 1190 cm−1 (λ = 8.4 μm). When the patch antenna optical resonance approaches and matches the intersubband transition frequency (L ∼ 1.8 μm), a clear anticrossing behavior—evidence of strong coupling—is observed in the near-field scattering phase spectra of individual antennas. The measured Rabi splitting is 4.5 THz. The near-field scattering spectra agree with the far-field extinction spectra acquired on arrays of identical antennas.

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.

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), channel rhodopsin, 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^{-1} range 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..., Comment: published under ACS AuthorChoice license

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.

The temperature increase and temperature gradients induced by mid-infrared laser illumination of vertical gold nanoantenna arrays embedded into polymer layers was measured directly with a photothermal expansion nanoscope. Nanoscale thermal hotspot images and local temperature increase spectra were both obtained, the latter by broadly tuning the emission wavelength of a quantum cascade laser. The spectral analysis indicates that plasmon-enhanced mid-infrared vibrations of molecules located in the antenna hotspots are responsible for some of the thermoplasmonic resonances, while Joule heating in gold is responsible for the remaining resonances. In particular, plasmonic dark modes with low scattering cross-section mostly produce surface-enhanced infrared absorption (SEIRA), while bright modes with strong radiation coupling produce Joule heating. The dark modes do not modify the molecular absorption lineshape and the related temperature increase is chemically triggered by the presence of molecules with vibrational fingerprints resonant with the plasmonic dark modes. The bright modes, instead, are prone to Fano interference, display an asymmetric molecular absorption lineshape and generate heat also at frequencies far from molecular vibrations, insofar lacking chemical specificity. For focused mid-infrared laser power of 50 mW, the measured nanoscale temperature increases are in the range of 10 K and temperature gradients reach 5 K/$\mu$m in the case of dark modes resonating with strong infrared vibrations such as the C=O bond of poly-methylmethacrylate at 1730 cm$^{-1}$., Comment: 17 pages, 4 figures

Hexagonal Boron Nitride (hBN) is a layered van der Waals material able to sustain hyperbolic phonon-polaritons within its mid-infrared reststrahlen bands. We study the effect of a metallic substrate adjacent to hBN flakes on the polariton dispersion and on the standing wave patterns in nanostructures by means of mid-infrared nanospectroscopy and nanoimaging. We exploit the gold-coated tip apex for atomic force microscopy to launch polaritons in thin hBN flakes. The photo-thermal induced mechanical resonance is used to detect the amplitude profile of polariton standing waves with a lateral resolution of 30 nm. We observe the polariton excitation spectra on hBN flakes as thin as 4 nm, thanks to the infrared field enhancement in the nanogap between the gold-coated tip apex and an ultraflat gold substrate. The data indicate no major effect of remote screening of the free electrons in gold on the phonon-polariton excitation that appears robust also against geometrical imperfections.

Despite the recent introduction of heavily doped semiconductors for mid-infrared plasmonics, it still remains an open issue whether such materials can compete with noble metals. A whole set of figures of merit are employed to thoroughly assess the use of heavily doped Ge on Si as a mid-infrared plasmonic material and benchmark it against standard noble metals such as Au. A full-wave electrodynamics framework is used to model and design high-performance, silicon-foundry compatible mid-infrared plasmonic sensors based on experimental material data reaching plasma wavelengths down to λp ∼ 3.1 μm. It is finally shown that Ge sensors can provide signal enhancements for vibrational spectroscopy above the 3 orders of magnitude, thus, representing a promising alternative to noble metals, leveraging the full compatibility with the silicon foundry microfabrication processes.

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.

Summary form only given. According to rigorous diffraction theory, monochromatic electromagnetic waves reflected from a material surface form a mixture of propagating and evanescent waves. During propagation, light fields suffer a progressive spatial filtering that depends exponentially on the size of the subwavelength image detail, a peculiar multiscale distortion in the image that spans the near-field of the sources, occurs in vacuum and with no dissipation, and has never been reported before.Here we explore this transition from super-resolved to diffraction-limited imaging for THz frequencies (? = 1 mm). The experiment is carried out using a knife-edge scan, that allows subwavelength beam profiling in the near-field of emitters [1], here enacted remotely to produce super-resolved images on planes at different distances from the emitters [2]. Compared to visible light imaging, the millimetric wavelength expands the near-field region to the point that single near-field imaging planes can be scanned using a mechanical free-standing blade.

International audience; A novel scanning probe tip made of heavily doped semiconductor is fabricated and used instead of standard gold-coated tips in infrared scattering-type near-field microscopy. Midinfrared near-field microscopy experiments are conducted on ZnO nanowires with a lateral resolution better than 100 nm, using tips made of heavily electron-doped germanium with a plasma frequency in the midinfrared (plasma wavelength of 9.5 μm). Nanowires embedded in a dielectric matrix are imaged at two wavelengths, 11.3 and 8.0 μm, above and below the plasma wavelength of the tips. An opposite sign of the imaging contrasts between the nanowire and the dielectric matrix is observed at the two infrared wavelengths, indicating a clear role of the free-electron plasma in the heavily doped germanium tip in building the imaging contrast. Electromagnetic simulations with a multispherical dipole model accounting for the finite size of the tip are well consistent with the experiments. By comparison of the simulated and measured imaging contrasts, an estimate for the local free-carrier density in the investigated ZnO nanowires in the low 10 19 cm −3 range is retrieved. The results are benchmarked against the scattering intensity and phase maps obtained on the same sample with a gold-coated probe tip in pseudoheterodyne detection mode.

Ge-on-Si has been demonstrated as a platform for Si foundry compatible plasmonics. We use laser thermal annealing to demonstrate activated doping levels >1020 cm−3 which allows most of the 3 to 20 μm mid-infrared sensing window to be covered with enhancements comparable to gold plasmonics.

Germanium-on-silicon rib waveguides are modelled, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the waveguide losses at 5.8 μm wavelength.

Germanium-on-silicon waveguides are designed, fabricated and characterized with a novel near-field infrared spectroscopy technique that allows on-chip investigation of the in-coupling efficiency. On-chip propagation along bends and straight sections up to 0.8 mm is demonstrated around λ = 6 μm.

Terahertz imaging has the potential to identify and decipher portions of ancient manuscripts, which may be unreadable at infrared and visible wavelengths. We use a scanning confocal terahertz microscope to scan a medieval parchment with music notes and pentagrams written with different inks. The microscope is based on a continuous-wave solid-state source at 0.3 THz, emitting in the free space with a horn antenna, and a high numerical-aperture ellipsoidal reflector. We present terahertz images with diffraction-limited lateral resolution of approximately 0.5 mm, where the different inks all give similar high contrast. Symbols written on the “verso” side of the parchment, barely glimpsed in the near-infrared photograph, leave a clear imprint in the terahertz images. Artifacts due to imperfect flatness of the parchment are also briefly discussed.

We present mid-infrared vibrational spectroscopy and imaging at the nanoscale of individual cell membranes deposited on ultraflat gold substrate by use of resonantly-enhanced mechanical photoexpansion technique. This platform allows one to measure the energy absorbed by the sample by monitoring its local thermal expansion with a nanometer atomic force microscope tip. The observed Amide-I and Amide-II bands of proteins in the spectrum acquired on individual purple membrane flakes, filled with bacteriorhodopsin (bR) molecules, are in good agreement with the far-field infrared spectrum collected on large numbers of membranes. Differences among the relative intensity of the two Amide bands in the near- and far-field spectra are attributed to different orientation of bR protein molecules in the two samples. Strong vibrational contrast imaging at the Amide-I of proteins with a lateral resolution of around 50 nm is reported for individual flakes of both purple membranes and artificial lipid vesicles loaded with channelrhodospin molecules.

Imaging with structured illumination allows for the retrieval of subwavelength features of an object by conversion of evanescent waves into propagating waves. In conditions in which the object plane and the structured-illumination plane do not coincide, this conversion process is subject to progressive filtering of the components with high spatial frequency when the distance between the two planes increases, until the diffraction-limited lateral resolution is restored when the distance exceeds the extension of evanescent waves. We study the progressive filtering of evanescent waves by developing a remote super-resolution terahertz imaging system operating at a wavelength ?=1.00 mm, based on a freestanding knife edge and a reflective confocal terahertz microscope. In the images recorded with increasing knife-edge-to-object-plane distance, we observe the transition from a super-resolution of ?/17?60 ?m to the diffraction-limited lateral resolution of ?x?? expected for our confocal microscope. The extreme nonparaxial conditions are analyzed in detail, exploiting the fact that, in the terahertz frequency range, the knife edge can be positioned at a variable subwavelength distance from the object plane. Electromagnetic simulations of radiation scattering by the knife edge reproduce the experimental super-resolution achieved.

High n-type doping in germanium is essential for many electronic and optoelectronic applications especially for high performance Ohmic contacts, lasing and mid-infrared plasmonics. We report on the combination of in situ doping and excimer laser annealing to improve the activation of phosphorous in germanium. An activated n-doping concentration of 8.8 x 10(19) cm(-3) has been achieved starting from an incorporated phosphorous concentration of 1.1 x 10(20) cm(-3). Infrared reflectivity data fitted with a multi-layer Drude model indicate good uniformity over a 350 nm thick layer. Photoluminescence demonstrates clear bandgap narrowing and an increased ratio of direct to indirect bandgap emission confirming the high doping densities achieved.

Despite the recent introduction of heavily-doped semiconductors for mid-infrared plasmonics, it still remains an open point whether such materials can compete with noble metals. We employ a whole set of figures of merit to thoroughly assess the use of heavily-doped Ge on Si as a mid-infrared plasmonic material and benchmark it against standard noble metals such as Au. In doing this, we design and model high-performance, CMOS compatible mid-infrared plasmonic sensors based on experimental material data reaching plasma frequencies up to about 1950 cm−1. We demonstrate that plasmonic Ge sensors can provide signal enhancements for vibrational spectroscopy above 3 orders of magnitude, thus representing a viable alternative to noble metals.

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.

We discuss a new type of nanoscale photon source constituted by a micro-pyramid made of epitaxial semiconductor material (Ge) attached to a silicon cantilever optimized for Atomic Force Microscopy and shaped by means of low-current Focused Ion Beam (FIB) milling. The optical characterization of these tips demonstrates sizeable photoluminescence emission at the direct energy gap of Ge (1550nm). We have checked by independent measurements on the nano-patterned epitaxial material prior to the tip shaping process that the photoluminescence yield is not significantly affected by FIB milling, indicating that the optical properties of the material in this wavelength range are preserved. We envisage an application of this tool as a scanning probe in apertureless scanning near-field optical microscopy. Display Omitted Photoluminescence emission from a scanning probe tip made of epitaxial semiconductor materialPatterning by EBL of epitaxial SiGe semiconductor material filmFabrication method to realize free-standing semiconductor nanostructuresSemiconductor scanning nanoscale photon source for apertureless-SNOM (a-SNOM)

There is significant interest to develop cheap CMOS compatible sensors that operate in the mid-infrared (MIR). To meet these requirements, Ge-on-Si is proving to be an exciting platform. There is the potential to realize waveguide integrated quantum well infrared photodetectors (QWIPs) based on Ge quantum wells (QWs). Intersubband absorption from p-Ge QWs has been demonstrated in the important atmospheric transmission window of 8-13 μm. An alternative strategy for sensing in the MIR is demonstrated through highly n-type doped Ge plasmonic antennas. These antennas demonstrate vibrational sensing of polydimethylsiloxane (PDMS) spin coated layers at 12.5 μm wavelength. These demonstrate enhanced sensing capabilities due to the localized hot spots of the antenna resonant modes.

We present the responsivity spectra in the 0.2–0.4 THz range at low temperature of a AlGaAs/GaAs field-effect transistor connected to an on-chip integrated antenna. We fabricated asymmetric plasmonic microcavities to obtain down-converted signals from the intrinsic nonlinearity of 2D electron plasma oscillations. Finite-element modeling simulations of the 2D electron oscillations and electromagnetic calculation of the antenna spectral response suggest that the device may work as ultra-broadband nonlinear terahertz detector.