High sensitivity static Fourier transform spectrometer.

Static Fourier transform spectrometers (S-FTSs) are well-consolidated instruments providing high throughput and high spectral resolution in a narrow spectral band. They use two reflective gratings as dispersive elements in a Michelson interferometer. Gratings allow high spectral dispersion and consequently high resolution, but, due to the light diffused from their grooves, they are one of the main noise sources in the reconstructed spectrum. In this work, we compare the signal-to-noise ratio performance of a prism-based S-FTS with that of a grating-based S-FTS. As a primary advantage, prisms give intrinsically lower diffused light than gratings. Furthermore, they do not have multiple diffracted orders, reducing thereafter the optical constraints on the instrumental baffling.

Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions.

Charge transport processes at interfaces play a crucial role in many processes. Here, the first soft x-ray second harmonic generation (SXR SHG) interfacial spectrum of a buried interface (boron-Parylene N) is reported. SXR SHG shows distinct spectral features that are not observed in x-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 Å, with changes of less than 1 Å resulting in easily detectable SXR SHG spectral shifts (ca. hundreds of milli-electron volts).

Wavefront-propagation simulations supporting the design of a time-delay compensating monochromator beamline at FLASH2.

Wavefront-propagation simulations have been performed to complete the design of a monochromator beamline for FLASH2, the variable-gap undulator line at the soft X-ray free-electron laser in Hamburg (FLASH). Prior to propagation through the beamline optical elements, the parameters of the photon source were generated using the GENESIS code which includes the free-electron laser experimental data. Threshold tolerances for the misalignment of mirror angles are calculated and, since diffraction effects were included in the simulations, the minimum quality with respect to the slope errors required for the optics is determined.

Generation of deep ultraviolet sub-2-fs pulses.

We demonstrate the generation of few-cycle deep ultraviolet pulses via frequency upconversion of 5-fs near-infrared pulses in argon using a laser-fabricated gas cell. The measured spectrum extends from 210 to 340 nm, corresponding to a transform-limited pulse duration of 1.45 fs. We extract from a dispersion-free second-order cross-correlation measurement a pulse duration of 1.9 fs, defining a new record in the deep ultraviolet spectral range.

Ultra-Fast-VUV Photoemission Study of UV Excited 2-Nitrophenol.

The initial deactivation pathways of gaseous 2-nitrophenol excited at 268 nm were investigated by time-resolved photoelectron spectroscopy (TRPES) with femtosecond-VUV light, produced by a monochromatized high harmonic generation source. TRPES allowed us to obtain new, valuable experimental information about the ultrafast excited-state dynamics of 2-nitrophenol in the gas phase. In accord with recent ab initio on-the-fly nonadiabatic molecular dynamic simulations, our results validate the occurrence of an ultrafast intersystem crossing leading to an intermediate state that decays on a subpicosecond time scale with a branched mechanisms. Two decay pathways are experimentally observed. One probably involves proton transfer, leading to the most stable triplet aci-form of 2-nitrophenol; the second pathway may involve OH rotation. We propose that following intersystem crossing, an ultrafast fragmentation channel leading to OH or HONO loss could also be operative.

Grating configurations to compress free-electron laser pulses.

The optical layout of soft X-ray grating compressors designed to provide both positive and negative group-delay dispersion (GDD) is discussed. They are tailored for chirped-pulse-amplification experiments with seeded free-electron laser sources. Designs with plane or concave gratings are discussed, depending on the sign of the GDD to be introduced.

Double-grating monochromatic beamline with ultrafast response for FLASH2 at DESY.

The preliminary design of a monochromatic beamline for FLASH2 at DESY is presented. The monochromator is tunable in the 50-1000 eV energy range with resolving power higher than 1000 and temporal response below 50 fs over the whole energy range. A time-delay-compensated configuration using the variable-line-spacing monochromator design with two gratings is adopted: the first grating disperses the radiation on its output plane, where the intermediate slit performs the spectral selection; the second grating compensates for the pulse-front tilt and for the spectral dispersion due to diffraction from the first grating.

Few-femtosecond extreme-ultraviolet pulses fully reconstructed by a ptychographic technique.

Sub-10-fs pulses tunable in the extreme-ultraviolet (XUV) spectral region are particularly important in many research fields: from atomic and molecular spectroscopy to the study of relaxation processes in solids and transition phase processes, from holography to free-electron laser injection. A crucial prerequisite for all applications is the accurate measurement of the temporal characteristics of these pulses. To fulfill this purpose, many phase retrieval algorithms have been successfully applied to reconstruct XUV attosecond pulses. Nevertheless, their extension to XUV femtosecond pulses is not trivial and has never been investigated/reported so far. We demonstrate that ultrashort XUV pulses, produced by high-order harmonic generation, spectrally filtered by a time-delay compensated monochromator, can be fully characterized, in terms of temporal intensity and phase, by employing the ptychographic reconstruction technique while other common reconstruction algorithms fail. This allows us to report on the generation and complete temporal characterization of XUV pulses with duration down to 5 fs, which constitute the shortest XUV pulse ever achieved via a time-delay compensated monochromator.

Cost-effective plane-grating monochromator design for extreme-ultraviolet application.

The optical design of a plane-grating monochromator mainly intended for high resolution in the extreme ultraviolet and soft x-ray is presented. The configuration has three optical elements. It uses a uniform line-spaced plane grating illuminated in the converging light coming from a focusing concave mirror and an additional plane mirror that is needed to change the grating subtended angle to keep the system in focus on a fixed slit. The parameters of the focusing mirror are determined to introduce a coma that compensates for the coma given by the grating. A monochromator for the 12-50 eV region is designed for application to high-order laser harmonics.

Direct Imaging of Transient Fano Resonances in N_{2} Using Time-, Energy-, and Angular-Resolved Photoelectron Spectroscopy.

Autoionizing Rydberg states of molecular N_{2} are studied using time-, energy-, and angular-resolved photoelectron spectroscopy. A femtosecond extreme ultraviolet pulse with a photon energy of 17.5 eV excites the resonance and a subsequent IR pulse ionizes the molecule before the autoionization takes place. The angular-resolved photoelectron spectra depend on pump-probe time delay and allow for the distinguishing of two electronic states contributing to the resonance. The lifetime of one of the contributions is determined to be 14±1 fs, while the lifetime of the other appears to be significantly shorter than the time resolution of the experiment. These observations suggest that the Rydberg states in this energy region are influenced by the effect of interference stabilization and merge into a complex resonance.

Comb-locked cavity ring-down spectrometer.

Extreme frequency accuracy and high sensitivity are obtained with a novel comb-locked cavity-ring-down spectrometer operating in the near-infrared from 1.5 to 1.63 µm. A key feature of our approach is the tight frequency locking of the probe laser to the comb, ensuring very high reproducibility and accuracy to the frequency axis upon scanning the comb repetition rate, as well as an efficient light injection into a length-swept high-finesse passive cavity containing the gas sample. Spectroscopic tests on the (30012) ← (00001) P14e line of CO2 at ∼1.57 µm demonstrate an accuracy of ∼17 kHz on the line center frequency in a Doppler broadening regime over the time scale of about 5 min, corresponding to four consecutive spectral scans of the absorption line. Over a single scan, which consists of 1500 spectral points over 75 s, the limit of detection is as low as 5.7 × 10(-11) cm(-1).

Grating configurations to compress extreme-ultraviolet ultrashort pulses.

It is here discussed the design of ultrafast extreme-ultraviolet (XUV) grating compressors that can be used to condition the spectral phase of ultrashort chirped pulses to compensate for the phase chirp and get closer to the Fourier limit. We discuss the two configurations that can be used to realize the compressor, the classical diffraction mount, and the off-plane one. The concept is applied to the realization of a XUV compressor with applications to free-electron lasers.

Transmittance and optical constants of Ca films in the 4-1000 eV spectral range.

The low expected absorption of Ca in the extreme ultraviolet (EUV) makes it an attractive material for multilayers and filters because most materials in nature strongly absorb the EUV. Few optical constant data had been reported for Ca. In this research, Ca films of various thicknesses were deposited on grid-supported C films and their transmittance measured in situ from the visible to the soft x-rays. The measurement range contains M2,3 and L2,3 absorption edges. Transmittance measurements were used to obtain the Ca extinction coefficient k. A minimum k of 0.017 was obtained at ∼23 eV, which makes Ca a promising low-absorption material for EUV coatings. A second spectral range of interest for its low absorption is below the Ca L3 edge at ∼343 eV. Measured k data and extrapolations were used to calculate the refractive index n using Kramers-Krönig relations. This is the first self-consistent data set on Ca covering a wide spectral range including the EUV.

Static, refractive and monolithic Fourier transform spectrometer: development and prototyping.

Static Fourier transform spectrometers are devices that can be realized as monolithic and compact assemblies. In the "grating-based" monolithic version, they are usually realized gluing together a beam-splitter with two reflective diffraction gratings using spacers as connecting elements. In this work we present the development and test of an alternative form of this kind of instrument in which the dispersive elements are Littrow's prisms and are glued to the splitting element, forming in this way a robust and filled structure with no air gaps. The device can work in the visible/near infrared spectral region with a resolution power that varies across the spectral range due to the dispersion of the used glasses. The absence of hollow regions inside the monolithic block makes the device extremely robust and protects the optical surfaces inside the interferometer from possible contaminations. The device can be easily miniaturized, as it does not require spacers or structural elements other than just the optical parts. The tested instrument works in the 470-850 nm wavelength range with a variable resolution between 3000 and 300.

Raman Spectroscopy Applied to Early Detection of Clostridium Infection in Milk.

Detecting Clostridium in milk presents a significant challenge for the dairy industry given that traditional methods are time-consuming and not specific for these bacteria. Microbiological techniques are expensive and require qualified personnel. Clostridium, in the form of spores, can withstand pasteurization and revert to its vegetative form during cheese aging. These gas-producing bacteria are known for their production of carbon dioxide and hydrogen, causing the formation of slits, cracks, and irregular eyes in hard and semi-hard cheeses. However, gas analysis in the vial headspace of appropriate culture can be exploited to specifically detect Clostridium presence, since the closest competing bacterial Bacilli produces only carbon dioxide. The aim of this paper is to present a Raman-spectroscopy-based instrument for a rapid, inexpensive identification of Clostridium in milk with a limit of detection of 29 spores/L. The proposed measurement procedure is analog to that routinely used, based on the most probable number method. The Raman-based instrument speeds up the detection of a vial's positivity. A test conducted with Clostridium spores demonstrated its effectiveness in almost halving the time needed for the measurement campaign compared to the traditional method.

Synechococcus sp. PCC7335 responses to far-red enriched spectra and anoxic/microoxic atmospheres: Potential for astrobiotechnological applications.

Recently, cyanobacteria have gained attention in space exploration to support long-term crewed missions via Bioregenerative Life Support Systems. In this frame, cyanobacteria would provide biomass and profitable biomolecules through oxygenic photosynthesis, uptaking CO2, and releasing breathable O2. Their growth potential and organic matter production will depend on their ability to photoacclimate to different light intensities and spectra, maximizing incident light harvesting. Studying cyanobacteria responses to different light regimes will also benefit the broader field of astrobiology, providing data on the possibility of oxygenic photosynthetic life on planets orbiting stars with emission spectra different than the Sun. Here, we tested the acclimation and productivity of Synechococcus sp. PCC7335 (hereafter PCC7335), capable of Far-Red Light Photoacclimation (FaRLiP) and type III chromatic acclimation (CA3), in an anoxic, CO2-enriched atmosphere and under a spectrum simulating the low energetic light regime of an M-dwarf star, also comparable to a subsuperficial environment. When exposed to the light spectrum, with few photons in the visible (VIS) and rich in far-red (FR), PCC7335 did not activate FaRLiP but acclimated only via CA3, achieving a biomass productivity higher than expected, considering the low VIS light availability, and a higher production of phycocyanin, a valuable pigment, with respect to solar light. Its growth or physiological responses of PCC7335 were not affected by the anoxic atmosphere. In these conditions, PCC7335 efficiently produced O2 and scavenged CO2. Results highlight the photosynthetic plasticity of PCC7335, its suitability for astrobiotechnological applications, and the importance to investigate biodiversity of oxygenic photosynthesis for searching life beyond Earth.

Grazing-incidence spectrometer for soft X-ray solar imaging spectroscopy.

The design and realization of a stigmatic grazing-incidence instrument for space applications to solar imaging spectroscopy is presented. We propose an optical layout in which imaging and spectral capabilities are decoupled by the use of crossed cylindrical mirrors. The design consists of a double telescope and a spectrograph: telescope I consists of a single cylindrical mirror with parabolic section, focusing the radiation on the entrance slit of the spectrograph in the spectral dispersion plane; telescope II consists of two cylindrical mirrors with aspherical section in Wolter configuration focusing the radiation on the spectrograph focal plane in the direction perpendicular to the spectral dispersion plane; the spectrograph consists of a grazing-incidence spherical variable-line-spaced grating with flat-field properties. Telescope II is crossed with respect to the grating and telescope I, i.e., is mounted with its tangential planes coincident with the grating equatorial plane, to decouple spectral and spatial focusing properties. The spectral resolution is preserved also for off-axis angles. The instrument that has been realized operates in the 4-26 nm spectral range and has a field of view of 0.5 deg to image the full Sun disk.

Active-grating monochromator for the spectral selection of ultrashort pulses.

Active gratings have been used to realize a grazing-incidence double-grating monochromator for the spectral selection of ultrashort pulses while preserving the temporal duration by compensating for the pulse-front tilt. The active grating consists of a bimorph deformable mirror on the top of which a diffraction grating with laminar profile is realized by UV lithography. The time-delay compensated configuration has been tested with ultrashort pulses at 800 nm. The feasibility of this configuration for the extreme-ultraviolet spectral region has been demonstrated by ray tracing studies.

Growth and Photosynthetic Efficiency of Microalgae and Plants with Different Levels of Complexity Exposed to a Simulated M-Dwarf Starlight.

Oxygenic photosynthetic organisms (OPOs) are primary producers on Earth and generate surface and atmospheric biosignatures, making them ideal targets to search for life from remote on Earth-like exoplanets orbiting stars different from the Sun, such as M-dwarfs. These stars emit very low light in the visible and most light in the far-red, an issue for OPOs, which mostly utilize visible light to photosynthesize and grow. After successfully testing procaryotic OPOs (cyanobacteria) under a simulated M-dwarf star spectrum (M7, 365-850 nm) generated through a custom-made lamp, we tested several eukaryotic OPOs: microalgae (Dixoniella giordanoi, Microchloropsis gaditana, Chromera velia, Chlorella vulgaris), a non-vascular plant (Physcomitrium patens), and a vascular plant (Arabidopsis thaliana). We assessed their growth and photosynthetic efficiency under three light conditions: M7, solar (SOL) simulated spectra, and far-red light (FR). Microalgae grew similarly in SOL and M7, while the moss P. patens showed slower growth in M7 with respect to SOL. A. thaliana grew similarly in SOL and M7, showing traits typical of shade-avoidance syndrome. Overall, the synergistic effect of visible and far-red light, also known as the Emerson enhancing effect, could explain the growth in M7 for all organisms. These results lead to reconsidering the possibility and capability of the growth of OPOs and are promising for finding biosignatures on exoplanets orbiting the habitable zone of distant stars.

Oxygenic photosynthetic responses of cyanobacteria exposed under an M-dwarf starlight simulator: Implications for exoplanet's habitability.

Introduction: The search for life on distant exoplanets is expected to rely on atmospheric biosignatures detection, such as oxygen of biological origin. However, it is not demonstrated how much oxygenic photosynthesis, which on Earth depends on visible light, could work under spectral conditions simulating exoplanets orbiting the Habitable Zone of M-dwarf stars, which have low light emission in the visible and high light emission in the far-red/near-infrared. By utilizing cyanobacteria, the first organisms to evolve oxygenic photosynthesis on our planet, and a starlight simulator capable of accurately reproducing the emission spectrum of an M-dwarf in the range 350-900 nm, we could answer this question. Methods: We performed experiments with the cyanobacterium Chlorogloeopsis fritschii PCC6912, capable of Far-Red Light Photoacclimation (FaRLiP), which allows the strain to harvest far-red in addition to visible light for photosynthesis, and Synechocystis sp. PCC6803, a species unable to perform this photoacclimation, comparing their responses when exposed to three simulated light spectra: M-dwarf, solar and far-red. We analysed growth and photosynthetic acclimation features in terms of pigment composition and photosystems organization. Finally, we determined the oxygen production of the strains directly exposed to the different spectra. Results: Both cyanobacteria were shown to grow and photosynthesize similarly under M-dwarf and solar light conditions: Synechocystis sp. by utilizing the few photons in the visible, C. fritschii by harvesting both visible and far-red light, activating the FaRLiP response. Discussion: Our results experimentally show that an M-dwarf light spectrum could support a biological oxygen production similar to that in solar light at the tested light intensities, suggesting the possibility to discover such atmospheric biosignatures on those exoplanets if other boundary conditions are met.