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We investigate local heat generation by molecules at the apex of polymer-embedded vertical antennas excited at resonant mid-infrared wavelengths, exploiting the surface enhanced infrared absorption effect. The embedding of vertical nanoantennas in a non-absorbing polymer creates thermal isolation between the apical hotspot and the heat sink represented by the substrate. Vibrational mid-infrared absorption by strongly absorbing molecules located at the antenna apex then generates nanoscale temperature gradients at the surface. We imaged the thermal gradients by using a nano-photothermal expansion microscope, and we found values up to 10 K/μm in conditions where the radiation wavelength resonates with both the molecule vibrations and the plasmonic mode of the antennas. Values up to 1000 K/μm can be foreseen at maximum quantum cascade laser power. The presented system provides a promising thermoplasmonic platform for antenna-assisted thermophoresis and resonant mid-infrared photocatalysis. [ABSTRACT FROM AUTHOR]

We have measured the electric signal downconverted from a terahertz frequency by an unbiased high mobility two-dimensional electron-gas (2DEG) device. The 2DEG was confined in an asymmetric plasmonic microcavity, and the radiation frequency was continuously tuned in the 0.2-0.4 THz range. The presence of resonant peaks at three frequencies corresponding to three plasma oscillation modes of the ungated 2DEG clearly points to the intrinsic nature of the hydrodynamic nonlinearity responsible for the downconversion as opposed to previously proposed plasmonic cavity configurations where the 2DEG oscillates under the metal gate that also acts as the source of the nonlinearity. [ABSTRACT FROM AUTHOR]

The channel of a high electron mobility transistor can work as a resonant microcavity for plasma waves, provided that the plasmon decay length is much larger than the cavity length. We have performed a spectroscopic study in the 0.15–0.4 THz range of the power absorbed by the micrometric channel of a two-dimensional electron gas (2DEG) transistor, where the active layer is formed by a remotely doped AlGaAs/InGaAs/AlGaAs quantum well where the electron mobility increases with decreasing temperature. The radiation emitted by a tunable frequency-multiplied THz oscillator was coupled to the cavity by an integrated lens-antenna optical system. The rectified signal is measured as a function of frequency and a strong increase upon cooling to 10 K is found at specific radiation frequencies, indicating the formation of standing plasma waves in the microcavity formed by the channel. [ABSTRACT FROM AUTHOR]

We have developed a confocal laser microscope operating in the mid-infrared range for the study of light-sensitive proteins, such as rhodopsins. The microscope features a co-aligned infrared and visible illumination path for the selective excitation and probing of proteins located in the IR focus only. An external-cavity tunable quantum cascade laser provides a wavelength tuning range (5.80–6.35 µm or 1570–1724 cm−1) suitable for studying protein conformational changes as a function of time delay after visible light excitation with a pulsed LED. Using cryogen-free detectors, the relative changes in the infrared absorption of rhodopsin thin films around 10−4 have been observed with a time resolution down to 30 ms. The measured full-width at half maximum of the Airy disk at λ = 6.08 µm in transmission mode with a confocal arrangement of apertures is 6.6 µm or 1.1λ. Dark-adapted sample replacement at the beginning of each photocycle is then enabled by exchanging the illuminated thin-film location with the microscope mapping stage synchronized to data acquisition and LED excitation and by averaging hundreds of time traces acquired in different nearby locations within a homogeneous film area. We demonstrate that this instrument provides crucial advantages for time-resolved IR studies of rhodopsin thin films with a slow photocycle. Time-resolved studies of inhomogeneous samples may also be possible with the presented instrument. [ABSTRACT FROM AUTHOR]

We fabricated a two-dimensional-electron-gas field effect transistor with an asymmetric terahertz antenna connected to the channel terminals and a gate length of 1 μm. We investigated frequency mixing in the transistor's channel by measuring, with a quasi-optical setup, the heterodyne, second- and third-order subharmonic mixing signal at 0.592 THz. The dependence on the gate voltage and on the radiation power of both the local-oscillator and the radio-frequency signals was studied for all mixing orders. The conditions for full-plasmonic-mixing are fulfilled in our transistor at room temperature. [ABSTRACT FROM AUTHOR]

Background: Extracellular Vesicles (EVs) are sub-micrometer lipid-bound particles released by most cell types. They are considered a promising source of cancer biomarkers for liquid biopsy and personalized medicine due to their specific molecular cargo, which provides biochemical information on the state of parent cells. Despite this potential, EVs translation process in the diagnostic practice is still at its birth, and the development of novel medical devices for their detection and characterization is highly required. Results: In this study, we demonstrate mid-infrared plasmonic nanoantenna arrays designed to detect, in the liquid and dry phase, the specific vibrational absorption signal of EVs simultaneously with the unspecific refractive index sensing signal. For this purpose, EVs are immobilized on the gold nanoantenna surface by immunocapture, allowing us to select specific EV sub-populations and get rid of contaminants. A wet sample-handling technique relying on hydrophobicity contrast enables effortless reflectance measurements with a Fourier-transform infrared (FTIR) spectro-microscope in the wavelength range between 10 and 3 µm. In a proof-of-principle experiment carried out on EVs released from human colorectal adenocarcinoma (CRC) cells, the protein absorption bands (amide-I and amide-II between 5.9 and 6.4 µm) increase sharply within minutes when the EV solution is introduced in the fluidic chamber, indicating sensitivity to the EV proteins. A refractive index sensing curve is simultaneously provided by our sensor in the form of the redshift of a sharp spectral edge at wavelengths around 5 µm, where no vibrational absorption of organic molecules takes place: this permits to extract of the dynamics of EV capture by antibodies from the overall molecular layer deposition dynamics, which is typically measured by commercial surface plasmon resonance sensors. Additionally, the described metasurface is exploited to compare the spectral response of EVs derived from cancer cells with increasing invasiveness and metastatic potential, suggesting that the average secondary structure content in EVs can be correlated with cell malignancy. Conclusions: Thanks to the high protein sensitivity and the possibility to work with small sample volumes—two key features for ultrasensitive detection of extracellular vesicles- our lab-on-chip can positively impact the development of novel laboratory medicine methods for the molecular characterization of EVs. [ABSTRACT FROM AUTHOR]

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 dry state 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 100nm and confinement volume 105 times smaller than λ³. [ABSTRACT FROM AUTHOR]

The great potential of Dirac electrons for plasmonics and photonics has been readily recognized after their discovery in graphene, followed by applications to smart optical devices. Dirac carriers are also found in topological insulators (TIs)—quantum systems having an insulating gap in the bulk and intrinsic Dirac metallic states at the surface. Here, the plasmonic response of ring structures patterned in Bi2Se3 TI films is investigated through terahertz (THz) spectroscopy. The rings are observed to exhibit a bonding and an antibonding plasmon modes, which we tune in frequency by varying their diameter. An analytical theory based on the THz conductance of unpatterned films is developed, which accurately describes the strong plasmon–phonon hybridization and Fano interference experimentally observed as the bonding plasmon is swiped across the prominent 2 THz phonon exhibited by this material. This work opens the road for the investigation of plasmons in topological insulators and for their application in tunable THz devices. [ABSTRACT FROM AUTHOR]

The front cover artwork is provided by Judit Šponer (Institute of Biophysics, Czech Academy of Sciences). The image shows that polymerization of H‐form 3',5' cyclic guanosine monophosphate (GMP) exhibits an oscillatory behavior when studying the reaction on longer time scales (of several days). Domains of the material, differing in the level of hydration, interlock like pieces of a puzzle, resulting in a complex chemical system working far from equilibrium. Read the full text of the Article at 10.1002/syst.202000011. [ABSTRACT FROM AUTHOR]

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/Hz0.5 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Heavily doped semiconductor thin films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in the 5 to 50μm wavelength range at least. In this work, we investigate the electrodynamics of heavily n-type-doped germanium epilayers at infrared frequencies beyond the assumptions of the Drude model. The films are grown on silicon and germanium substrates, are in situ doped with phosphorous in the 1017 to 1019 cm-3 range, then screened plasma frequencies in the 100 to 1200cm-1 range were observed. We employ infrared spectroscopy, pump-probe spectroscopy, and dc transport measurements to determine the tunability of the plasma frequency. Although no plasmonic structures have been realized in this work, we derive estimates of the decay time of mid-infrared plasmons and of their figures of merit for field confinement and for surface plasmon propagation. The average electron scattering rate increases almost linearly with excitation frequency, in agreement with quantum calculations based on a model of the ellipsoidal Fermi surface at the conduction band minimum of germanium accounting for electron scattering with optical phonons and charged impurities. Instead, we found weak dependence of plasmon losses on neutral impurity density. In films where a transient plasma was generated by optical pumping, we found significant dependence of the energy relaxation times in the few-picosecond range on the static doping level of the film, confirming the key but indirect role played by charged impurities in energy relaxation. Our results indicate that underdamped mid-infrared plasma oscillations are attained in n-type-doped germanium at room temperature. [ABSTRACT FROM AUTHOR]