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We present n-doped germanium-on-silicon (Ge-on- Si) antennas fabricated with CMOS mainstream methods, showing resonances between 0.1 and 0.7 THz for sensing applications. After surface functionalization with α-lipoic acid, a red-shift of the antenna resonance is observed, as demonstrated by THz time domain spectroscopy. Similar results are obtained with silicon nitride (Si 3 N 4 )-coated antennas, enabling microfluidic integration. Finally, we compare antennas of bow-tie slots to antennas fabricated via maskless photolithography.

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.

In the last decade, silicon photonics has undergone an impressive development driven by an increasing number of technological applications. Plasmonics has not yet made its way to the microelectronic industry, mostly because of the lack of compatibility of typical plasmonic materials with foundry processes. In this framework, we have developed a plasmonic platform based on heavily n-doped Ge grown on silicon substrates. We developed growth protocols to reach n-doping levels exceeding 1020 cm-3, allowing us to tune the plasma wavelength of Ge in the 3-15 μm range. The plasmonic resonances of Ge-on-Si nanoantennas have been predicted by simulations, confirmed by experimental spectra and exploited for molecular sensing. Our work represents a benchmark for group-IV mid-IR plasmonics.

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.

The realization of a Ge/SiGe THz emitter is of great interest since it can help to reach room-temperature operation due to the peculiar electron-phonon interaction in nonpolar crystals. Here we present Ge/SiGe quantum-well building blocks epitaxially grown on silicon wafers in order to understand limitations of this material platform in the perspective of realizing a Si-based laser.

n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.

The fabrication of highly doped and high quality Ge layers is a challenging and hot topic for advancements in nanoelectronics, photonics and radiation detectors. In this article, we report on a simple method for junction formation consisting in sputter depositing or evaporating a thin pure Sb layer on Ge followed by cycles of Pulsed Laser Melting (PLM). We show that PLM promotes an efficient diffusion of high Sb concentrations into the melted Ge subsurface layer, followed by a fast epitaxial regrowth. The resulting layer is perfectly pseudomorphic to the Ge substrate, preserving the high strain level induced by the Sb, having covalent radius higher than Ge. In addition, it shows extremely high active concentrations up to 3 × 1020 cm−3 and a record low resistivity of 1.4 × 10-4 Ohm cm. The carrier mobility is also in line with the extrapolation of literature data with no signs of mobility degradation. Furthermore, infrared reflectivity confirms the good optical quality of the doped layers and demonstrates, for the first time, plasma wavelengths in Ge below 3 µm. These results are highly relevant for nanolectronic and plasmonic applications.

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 high material quality of strain-symmetrized structures and heterointerfaces as well as control over 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

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.

Germanium-on-silicon, mid-infrared waveguides are used to demonstrate molecular fingerprint sensing of poly(methyl methacrylate) between 7.5 and 10 μm wavelength. The results are compared to Fourier transform infrared spectroscopy measurements, highlighting the potential of the platform for the identification of analytes.

Ge-on-Si waveguides with losses below 5 dB/cm across 7.5 to 11 µm wavelength are demonstrated. Sidewall etch roughness was measured using atomic force microscopy to investi- gate the waveguide loss mechanisms. Mid-infrared spectroscopy of poly(methyl methacrylate) was demonstrated using the Ge waveguides for mid-infrared sensing.

Low loss Ge-on-Si waveguides are demonstrated in the 8 – 14 μm atmospheric transmission window, a technology that will enable detection and sensing of unique molecular vibrations. Such a low cost platform would have applications in key markets such as pollution monitoring, explosives detection and point of care diagnostics. Rib-waveguides are fabricated using electron beam lithography and dry etching. The waveguides propagation losses are characterized using the Fabry-Perot technique, and are found to be below 5 dB/cm across the measurement range of 7.5 to 11 μm wavelength, reaching as low as ~ 1 dB/cm. The contribution to the losses are analyzed using the experimentally measured Si substrate losses, and the calculated scattering losses from an analytical model. The results verify the feasibility of the Ge-on-Si platform for integrated mid-infrared photonics and sensing.

Kinetic Inductance Detectors made of High-TC superconductor YBCO are designed and fabricated, in view as possible application as high sensitivity, low cost and high-manageability terahertz camera for imaging.

For the last decade, germanium has been proposed as an excellent material for passive mid-infrared (MIR) integrated photonics. This technology allows for label-free sensing in the molecular fingerprint regime (6.7–20 μm), where molecules can be uniquely identified by their absorption spectra. Such a platform has the potential to enable low cost, miniaturized mid-infrared sensors for use in crucial applications such as explosives detection, pollution monitoring and detection of breath biomarkers for point of care diagnostics. There have now been a number of demonstrations of waveguides up to 8.5 μm wavelength using Ge [1] and SiGe [2] waveguides. Previously, we have demonstrated the first low loss Ge-on-Si waveguides from 7.5 to 11 μm, with losses as low as ∼1 dB/cm [3]. Here, we demonstrate their potential for sensing applications by evanescently sensing unique vibrations in poly(methyl methacrylate) (PMMA) polymers, in the spectral region of 7.5–10 μm wavelength.

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

In this work we present model calculations and experimental verification of the giant nonlinear coefficients provided by intersubband transitions in hole-doped Ge/SiGe and Si/SiGe asymmetric quantum wells in the mid-infrared. We develop a model for the valence band-structure of the SiGe alloy including heavy-hole, light-hole and split-off bands for any value of Ge concentration, so as to obtain heterojunction parameters for the calculation of the wavefunction of confined hole states in quantum wells.

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

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.

Germanium-on-silicon, mid-infrared waveguides operating up to 11 micron wavelength are demonstrated for the first time, with propagation losses below 5.5 dB/cm. The results indicate that the platform could be suitable for sensing applications in the 8–13 micron atmospheric window of the molecular fingerprint region.

SrRuO3 (SRO) is a perovskite increasingly used in oxide-based electronics both for its intrinsic metallicity, which remains unaltered in thin films and for the ease of deposition on dielectric perovskites like SrTiO3, (STO) to implement SRO/STO microcapacitors and other devices. In order to test the reliability of SRO/STO also as high-current on-chip conductor, when the SRO dimensions are pushed to the nanoscale, here we have measured the electrodynamic properties of arrays of nanoribbons, fabricated by lithography starting from an ultrathin film of SRO deposited on a STO substrate. The nanoribbons are 6 or 4 nm thick, 400, 200 and 100 nm wide and 5 mm long. The measurements have been performed by infrared spectroscopy, a non-contact weakly perturbing technique which also allows one to separately determine the carrier density and their scattering rate or mobility. Far-infrared reflectivity spectra have been analyzed by Rigorous Coupled-Wave Analysis (RCWA) and by an Effective Medium Theory, obtaining consistent results. With the radiation polarized along the nanoribbons, we obtain a carrier density similar to that of a flat film used as reference, which in turn is similar to that of bulk SRO. Moreover, in the nanoribbons the carrier scattering rate is even smaller than in the unpatterned film by about a factor of 2. This shows that the transport properties of SRO deposited on STO remain at least unaltered down to nanometric dimensions, with interesting perspectives for implementing on-chip nano-interconnects in an oxide-based electronics. When excited in the perpendicular direction, the nanoribbons appear instead virtually transparent to the radiation field, as predicted by RCWA.

International audience; Midinfrared plasmonic sensing allows the direct targeting of unique vibrational fingerprints of molecules. While gold has been used almost exclusively so far, recent research has focused on semiconductors with the potential to revolutionize plasmonic devices. We fabricate antennas out of heavily doped Ge films epitaxially grown on Si wafers and demonstrate up to 2 orders of magnitude signal enhancement for the molecules located in the antenna hot spots compared to those located on a bare silicon substrate. Our results set a new path toward integration of plasmonic sensors with the ubiquitous CMOS platform.

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.

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.

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.

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)

International audience; In this work, the growth and the fabrication of heavily doped germanium plasmonic antennas for mid-infrared applications are reported. By tuning the phosphorus doping concentration and the antenna geometrical parameters, plasma frequencies for targeting the 8–15 μm spectral region are achieved. 1 μm thick, heavily doped (2.3 × 1019 cm− 3) germanium was used to fabricate dipole antennas of 800 nm width with a gap spacing of 300 nm which demonstrate resonance frequencies around 13 μm and 13.5 μm for 2 μm and 3 μm long structures, respectively. This technology has the potential to be used for mid-infrared sensing applications of hazardous gases and liquids.

We study plasmonic nanoantennas for molecular sensing in the mid-infrared made of heavily doped germanium, epitaxially grown with a bottom-up doping process and featuring free carrier density in excess of 1020 cm-3. The dielectric function of the 250 nm thick germanium film is determined, and bow-tie antennas are designed, fabricated, and embedded in a polymer. By using a near-field photoexpansion mapping technique at λ = 5.8 μm, we demonstrate the existence in the antenna gap of an electromagnetic energy density hotspot of diameter below 100 nm and confinement volume 105 times smaller than λ3.

In this study, a chemical stabilization method, that preserves the tensile strength of artificially aged paper pretreated with an aqueous solution of ionic liquid, is presented. Pure cotton cellulose paper samples were soaked with cholinium glycinate ionic-liquid solution either before or after the artificial aging process, which was based on thermal degradation in dry air. The tensile strength of artificially aged paper was measured by using the double-folding technique. The role of ionic liquids was investigated by mid-infrared and terahertz time-domain absorption spectroscopy. It was found that the tensile strength of pretreated samples is higher than that of other aged samples. A model for changes in the cellulose structure caused by oxidation and by the binding of ionic liquid molecules is proposed, based on the analysis of the mid-infrared absorption bands in the carbonyl/carboxyl region at 1590-1750 cm-1. Terahertz spectroscopy data indicate that the ionic liquid molecules penetrate in the larger size porosity, acting as binders among cellulose fibers to maintain the tensile strength of paper. © 2016 American Chemical Society.

The pressure dependencies at room temperature of Raman and infrared (IR) data on VO 2 are presented up to 19 and 15 GPa, respectively, together with preliminary X-ray diffraction (XRD) data collected up to 42 GPa. Both Raman and XRD indicate the persistence of a monoclinic lattice up the highest pressures. Observed modifications in the pressure dependence of Raman phonon frequencies, IR spectral weight, and unit cell volume at ∼10 GPa allow us to identify two regimes at low and high pressure and to conjecture a transition to a new unknown monoclinic phase where applying pressure activates a metallization mechanism.

Transition metal oxides often display insulator to metal transitions (MIT) with conductivity jumps up to several orders of magnitude, which makes them very attractive both for technological applications and for the several fundamental issues they raise. Information about the mechanism inducing the MIT may be gained by means of infrared (IR) spectroscopy. We present here the capabilities of the IR synchrotron source SISSI (Synchrotron Infrared Source for Spectroscopy and Imaging), at the Italian storage ring ELETTRA. We demonstrate SISSI to be a powerful source for optical studies of pressure-induced insulator-to-metal transitions, reporting on V3O5 and (V0.989Cr0.011)2O3 as examples.

The pressure-induced changes in the optical response of magnetite were studied by infrared reflectivity measurements at room temperature. The pressure-induced redshift of the pronounced midinfrared absorption band confirms the polaronic transport mechanism. The low-frequency conductivity increases and the phonon modes harden with increasing pressure. We discuss our results in terms of the proposed pressure-induced charge transfer among the different Fe sites.

International audience; Heavily doped Ge thin films grown on different substrates have been investigated by infrared (IR) reflectometry. Screened plasma frequency and losses have been determined to assess the possibilities and limitations of Ge for mid infrared plasmonic.

Vanadium sesquioxide V2O3 is considered a textbook example of Mott-Hubbard physics. In this paper, we present an extended optical study of its whole temperature/doping phase diagram as obtained by doping the pure material with M = Cr or Ti atoms (V1-x M-x)(2)O-3. We reveal that its thermodynamically stable metallic and insulating phases, although macroscopically equivalent, show very different low-energy electrodynamics. The Cr and Ti doping drastically change both the antiferromagnetic gap and the paramagnetic metallic properties. A slight chromium content induces a mesoscopic electronic phase separation, while the pure compound is characterized by short-lived quasiparticles at high temperature. This study thus provides a new comprehensive scenario of the Mott-Hubbard physics in the prototype compound V2O3.

International audience; We address the behavior of mid-infrared localized plasmon resonances in elongated germanium antennas integrated on silicon substrates. Calculations based on Mie theory and on the experimentally retrieved dielectric constant allow us to study the tunability and the figures of merit of plasmon resonances in heavily-doped germanium and to preliminarily compare them with those of the most established plasmonic material, gold.

International audience; We investigate the mid-infrared response of heavily doped germanium films and periodic arrays of nanoantennas by both continuous wave transmission/reflection spectroscopy and ultrafast pump-probe spectroscopy. We compare the data to finite-element modeling electromagnetic simulations of the subwavelength nanostructures. The plasma frequency of the doped or optically activated semiconductor extends in the 1000-2000 cm-1 range thus enabling access to molecular fingerprints.

In this work we present an integrated biosensor that enables FTIR (Fourier Transform-Infrared) detection of analytes contained in diluted solutions. The fabricated nanosensor allows for the detection of proteins through the identification of the fine structure of their amide I and II bands, up to the nanomolar concentration range. We exploited two distinct effects to enhance the sensitivity: (i) the concentration effect due to the presence of the superhydrophobic surface that conveys molecules dispersed in solution directly inside the focus of a FTIR spectromicroscope; (ii) the plasmonic resonance of the nanoantenna array that provides electromagnetic field enhancement in the amide I and II spectral region (1500-1700 cm-1). We demonstrate the detection of ferritin in the nanomolar concentration range, a blood protein that is usually available in small amounts in typical blood samples.

International audience; We introduce an all-semiconductor platform for mid-infrared plasmonics based on epitaxial heavily-doped Ge-on-Si. We design, fabricate, and characterize plasmonic Ge antenna arrays to assess the potential of this platform for mid-infrared sensing applications.

We report pressure-dependent reflection and transmission measurements on ZnCr$_2$Se$_4$, HgCr$_2$S$_4$, and CdCr$_2$O$_4$ single crystals at room temperature over a broad spectral range 200-24000 cm$^{-1}$. The pressure dependence of the phonon modes and the high-frequency electronic excitations indicates that all three compounds undergo a pressure-induced structural phase transition with the critical pressure 15 GPa, 12 GPa, and 10 GPa for CdCr$_2$O$_4$, HgCr$_2$S$_4$, and ZnCr$_2$Se$_4$, respectively. The eigenfrequencies of the electronic transitions are very close to the expected values for chromium crystal-field transitions. In the case of the chalcogenides pressure induces a red shift of the electronic excitation which indicates a strong hybridization of the Cr d-bands with the chalcogenide bands.

The potentialities of the Raman spectroscopy in providing a deep insight into the microscopic anion-cation interactions in ionic liquids are discussed. We report on and discuss Raman measurements mainly collected on a particular class of ionic liquids called protic ionic liquids (PILs), based on the nitrate anion, NO3 −. Together with the spectroscopic investigation, first principle studies have been carried out for most of the measured samples. The combined experimental-theoretical approach allows us to deeply investigate the effects induced by the length of the alkyl chain on the optical properties of PILs. We also focus and report on systematic temperature (300–450 K) and pressure (0–10 GPa) dependent studies. The extended P–T region explored allows to observe liquid–solid and several solid–solid transitions and to draw general remarks on the phase diagram of PILs. Finally, exploiting the high sensitivity of the Raman spectroscopy to protein secondary and tertiary structures, the efficacy of PILs as protein refolding enhancers is also discussed.

We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C${}_{22}$H${}_{14}$) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the observed phonon peaks. We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a $\ensuremath{\sim}20%$ compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure, without any evidence of structural phase transitions. The Gr\"uneisen parameter of the 1380 cm${}^{\ensuremath{-}1}$ ${a}_{1}$ Raman peak (${\ensuremath{\gamma}}_{p}=0.1$) is much lower than the effective value (${\ensuremath{\gamma}}_{d}=0.8$) due to K doping. Therefore, doping and pressure have very different effects and it can be argued that softening of the 1380 cm${}^{\ensuremath{-}1}$ mode is probably due to coupling with electronic states in K-doped solid picene.

We are developing an all-semiconductor plasmonic platform for mid-infrared sensing which includes growth of epitaxial n-doped germanium films, spectroscopic test and electromagnetic design of plasmonic antennas.

The optical conductivity and the spectral weight of four topological insulators with increasing chemical compensation (Bi2Se3, Bi2-xCaxSe3, Bi2Se2Te, Bi2Te2Se) have been measured from 5 to 300 K and from sub-THz to visible frequencies. The effect of compensation is clearly observed in the infrared spectra, through the suppression of an extrinsic Drude term and the appearance of strong absorption peaks, that we assign to electronic transitions among localized states. From the far-infrared spectral weight of the most compensated sample (Bi2Te2Se) one can estimate a density of charge-carriers in the order of 10^17/cm^3 in good agreement with transport data. Those results demonstrate that the low-energy electrodynamics in single crystals of topological insulators, even at the highest degree of compensation presently achieved, is still affected by extrinsic charge excitations., 5 pages, 4 figures

Nature Communications 1, Article number: 105 (2010); published: 02 November 2010; updated: 17 January 2012. In Figure 2 of this Article, panel labels c and d were inadvertently switched. A typographical error was also introduced in the last sentence of the legend, which should have read 'The scale bar in panel c represents 10 μm'.

V2O3 is the prototype system for the Mott transition, one of the most fundamental phenomena of electronic correlation. Temperature, doping or pressure induce a metal to insulator transition (MIT) between a paramagnetic metal (PM) and a paramagnetic insulator (PI). This or related MITs have a high technological potential, among others for intelligent windows and field effect transistors. However the spatial scale on which such transitions develop is not known in spite of their importance for research and applications. Here we unveil for the first time the MIT in Cr-doped V2O3 with submicron lateral resolution: with decreasing temperature, microscopic domains become metallic and coexist with an insulating background. This explains why the associated PM phase is actually a poor metal. The phase separation can be associated with a thermodynamic instability near the transition. This instability is reduced by pressure which drives a genuine Mott transition to an eventually homogeneous metallic state., Paper plus supplementary material