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.
The terahertz (THz) portion of the electromagnetic spectrum provides specific spectroscopic information for substance
identification. It has been shown that the spectral features of explosive materials might be used for detection and
identification at stand-off distances. We report on the development of a THz spectrometer for explosive detection and
identification. The system is based on THz quantum cascade lasers working at different frequencies. These are used for
illumination of the object under test. The reflected and backscattered radiation from the object under test is detected with
a sensitive heterodyne receiver. As a first step a single frequency, liquid-cryogen free heterodyne receiver operating at
2.5 THz has been developed. In order to realize maximum sensitivity a phonon-cooled NbN hot electron bolometric
mixer with a quantum cascade laser as local oscillator were chosen. The concept of the system and first results will be
Photoconductivity spectra of unstressed and stressed Ge:Ga detectors were measured. The experiments were performed
with a polarizing step scan Fourier transform spectrometer using the synchrotron facility BESSY, which was operated in
a dedicated mode with a low momentum compaction factor. By this way powerful and coherent synchrotron radiation
below 50 cm-1 was generated. We observed a significant response of unstressed and stressed Ge:Ga detectors below
50 cm-1 and 25 cm-1, respectively. This response can be attributed to transitions between bound excited states or from
bound excited states to the valence band. The results indicate that in germanium detectors a fraction of the recombining
holes is captured into bound excited states.
We report on the first successful generalized Mueller-matrix ellipsometry measurements in the THz-frequency domain using the high-brilliance THz synchrotron radiation source IRIS at the electron storage ring BESSY, Germany. Generalized Ellipsometry, which is known as a powerful tool for measurement of optical constants including anisotropy and which was previously used in the FIR to VUV spectral range, is now employed for the first time to investigate condensed matter samples in the frequency range from 0.9 to 8 THz (30 to 650 cm-1). Exemplarily, results obtained from bound and unbound charge-carrier investigations in low-dimensional semi- and superconducting systems are presented. Future applications of this technique for investigation of charge-carrier dynamics in magnetic fields are envisioned.
Terahertz scanning near-field infrared microscopy (SNIM) below 1 THz is demonstrated. The near-field technique benefits from the broadband and highly brilliant coherent synchrotron radiation (CSR) from an electron storage ring and from a detection method based on locking on to the intrinsic time structure of the synchrotron radiation. The scanning microscope utilizes conical waveguides as near-field probes with apertures smaller than the wavelength. Different cone approaches have been investigated to obtain maximum transmittance. Together with a Martin-Puplett spectrometer the set-up enables spectroscopic mapping of the transmittance of samples well below the diffraction limit. Spatial resolution down to about λ/40 at 2 wavenumbers (0.06 THz) is derived from the transmittance spectra of the near-field probes. The potential of the technique is exemplified by imaging biological samples. Strongly absorbing living leaves have been imaged in transmittance with a spatial resolution of 130 μm at about 12 wavenumbers (0.36 THz). The THz near-field images reveal distinct structural differences of leaves from different plants investigated. The technique presented also allows spectral imaging of bulky organic tissues. Human teeth samples of various thicknesses have been imaged between 2 and 20 wavenumbers (between 0.06 and 0.6 THz). Regions of enamel and dentin within tooth samples are spatially and spectrally resolved, and buried caries lesions are imaged through both the outer enamel and into the underlying dentin.
The visible infrared thermal imaging spectrometer (VIRTIS) is one of the principal payloads to be launched in 2003 on ESA's Rosetta spacecraft. Its primary scientific objective s are to map the surface of the comet Wirtanen, monitor its temperature, and identify the solids and gaseous species on the nucleus and in the coma. VIRTIS will also collet data on two asteroids, one of which has been identified as Mimistrobell. The data is collected remotely using a mapping spectrometer co-boresighted with a high spectral resolution spectrometer. The mapper consists of a Shafer telescope matched to an Offner grating spectrometer capable of gathering high spatial, medium spectral resolution image cubes in the 0.25 to 5 micrometers waveband. The high spectral resolution spectrometer uses an echelle grating and a cross dispersing prism to achieve resolving powers of 1200 to 300 in the 1.9 to 5 micrometers band. Both sub-systems are passively cooled to 130 K and use two Sterling cycle coolers to enable two HgCdTe detector arrays to operate at 70 K. The mapper also uses a silicon back-side illuminated detector array to cover the ultra-violet to near-infrared optical band.