Exploring the Charge Density Wave Phase of 1T-TaSe_{2}: Mott or Charge-Transfer Gap?

1T-TaSe_{2} is widely believed to host a Mott metal-insulator transition in the charge density wave (CDW) phase according to the spectroscopic observation of a band gap that extends across all momentum space. Previous investigations inferred that the occurrence of the Mott phase is limited to the surface only of bulk specimens, but recent analysis on thin samples revealed that the Mott-like behavior, observed in the monolayer, is rapidly suppressed with increasing thickness. Here, we report combined time- and angle-resolved photoemission spectroscopy and theoretical investigations of the electronic structure of 1T-TaSe_{2}. Our experimental results confirm the existence of a state above E_{F}, previously ascribed to the upper Hubbard band, and an overall band gap of ∼0.7 eV at Γ[over ¯]. However, supported by density functional theory calculations, we demonstrate that the origin of this state and the gap rests on band structure modifications induced by the CDW phase alone, without the need for Mott correlation effects.

Hyperspectral imaging with a TWINS birefringent interferometer.

We introduce a high-performance hyperspectral camera based on the Fourier-transform approach, where the two delayed images are generated by the Translating-Wedge-Based Identical Pulses eNcoding System (TWINS) [Opt. Lett. 37, 3027 (2012)], a common-path birefringent interferometer that combines compactness, intrinsic interferometric delay precision, long-term stability and insensitivity to vibrations. In our imaging system, TWINS is employed as a time-scanning interferometer and generates high-contrast interferograms at the single-pixel level. The camera exhibits high throughput and provides hyperspectral images with spectral background level of -30dB and resolution of 3 THz in the visible spectral range. We show high-quality spectral measurements of absolute reflectance, fluorescence and transmission of artistic objects with various lateral sizes.

Versatile Control of

We demonstrate quantum control of ^{9}Be^{+} ions directly implemented by an optical frequency comb. Based on numerical simulations of the relevant processes in ^{9}Be^{+} for different magnetic field regimes, we demonstrate a wide applicability when controlling the comb's spectral properties. We introduce a novel technique for the selective and efficient generation of a spectrally tailored narrow-bandwidth optical frequency comb near 313 nm. We experimentally demonstrate internal state control and internal-motional state coupling of ^{9}Be^{+} ions implemented by stimulated-Raman manipulation using a spectrally optimized optical frequency comb. Our pulsed laser approach is a key enabling step for the implementation of quantum logic and quantum information experiments in Penning traps.

Fourier transform spectroscopy in the vibrational fingerprint region with a birefringent interferometer.

We introduce a birefringent interferometer for Fourier transform (FT) spectroscopy in the mid-infrared, covering the vibrational fingerprint region (5-10 µm, 1000-2000 cm-1), which is crucial for molecular identification. Our interferometer employs the crystal calomel (Hg2Cl2), which combines high birefringence (ne-no≈0.55) with a broad transparency range (0.38-20 µm). We adopt a design based on birefringent wedges, which is simple and compact and guarantees excellent delay accuracy and long-term stability. We demonstrate FTIR spectroscopy, with a frequency resolution of 3 cm-1, as well as two-dimensional IR (2DIR) spectroscopy. Our setup can be extended to other spectroscopic modalities such as vibrational circular dichroism and step-scan FT spectroscopy.

Narrowband carrier-envelope phase stable mid-infrared pulses at wavelengths beyond 10 µm by chirped-pulse difference frequency generation.

We report on the generation of narrowband carrier-envelope phase stable mid-infrared (MIR) pulses between 10 and 15 µm. High pulse energies and narrow bandwidths are required for the selective nonlinear excitation of collective modes of matter that is not possible with current sources. We demonstrate bandwidths of <2% at 12.5 µm wavelength through difference frequency generation between two near-infrared (NIR) pulses, which are linearly chirped. We obtain a reduction in bandwidth by one order of magnitude, compared to schemes that make use of transform-limited NIR pulses. The wavelength of the narrowband MIR pulse can be tuned by changing the optical delay between the two chirped NIR pulses.

Spin-Hall Voltage over a Large Length Scale in Bulk Germanium.

We exploit the spin-Hall effect to generate a uniform pure spin current in an epitaxial n-doped Ge channel, and we detect the electrically induced spin accumulation, transverse to the injected charge current density, with polar magneto-optical Kerr microscopy at a low temperature. We show that a large spin density up to 400 µm^{-3} can be achieved at the edges of the 100-µm-wide Ge channel for an applied electric field lower than 5 mV/µm. We find that the spin density linearly decreases toward the center of the Ge bar, due to the large spin diffusion length, and such a decay is much slower than the exponential one observed in III-V semiconductors, allowing very large spin accumulations over a length scale of tens of micrometers. This lays the foundation for multiterminal spintronic devices, where different spin voltages can be exploited as inputs for magnetologic gates on the same Ge platform.

Stimulated Raman gas sensing by backward UV lasing from a femtosecond filament.

We perform a proof-of-principle demonstration of chemically specific standoff gas sensing, in which a coherent stimulated Raman signal is detected in the direction anticollinear to a two-color laser excitation beam traversing the target volume. The proposed geometry is intrinsically free space as it does not involve back-scattering (reflection) of the signal or excitation beams at or behind the target. A beam carrying an intense mid-IR femtosecond (fs) pulse and a parametrically generated picosecond (ps) UV Stokes pulse is fired in the forward direction. A fs filament, produced by the intense mid-IR pulse, emits a backward-propagating narrowband ps laser pulse at the 337 and 357 nm transitions of excited molecular nitrogen, thus supplying a counter-propagating Raman pump pulse. The scheme is linearly sensitive to species concentration and provides both transverse and longitudinal spatial resolution.

Optical stark effects in j-aggregate-metal hybrid nanostructures exhibiting a strong exciton-surface-plasmon-polariton interaction.

We report on the observation of optical Stark effects in J-aggregate-metal hybrid nanostructures exhibiting strong exciton-surface-plasmon-polariton coupling. For redshifted nonresonant excitation, pump-probe spectra show short-lived dispersive line shapes of the exciton-surface-plasmon-polariton coupled modes caused by a pump-induced Stark shift of the polariton resonances. For larger coupling strengths, the sign of the Stark shift is reversed by a transient reduction in normal mode splitting. Our studies demonstrate an approach to coherently control and largely enhance optical Stark effects in strongly coupled hybrid systems. This may be useful for applications in ultrafast all-optical switching.

Tracking the primary photoconversion events in rhodopsins by ultrafast optical spectroscopy.

Opsins are a broad class of photoactive proteins, found in all classes of living beings from bacteria to higher animals, which work either as light-driven ion pumps or as visual pigments. The photoactive function in opsins is triggered by the ultrafast isomerization of the retinal chromophore around a specific carbon double bond, leading to a highly distorted, spectrally red-shifted photoproduct. Understanding, by either experimental or computational methods, the time course of this photoisomerization process is of utmost importance, both for its biological significance and because opsin proteins are the blueprint for molecular photoswitches. This paper focuses on the ultrafast 11-cis to all-trans isomerization in visual rhodopsins, and has a twofold goal: (i) to review the most recent experimental and computational efforts aimed at exposing the very early phases of photoconversion; and (ii) discuss future advanced experiments and calculations that will allow an even deeper understanding of the process. We present high time resolution pump-probe data, enabling us to follow the wavepacket motion through the conical intersection connecting excited and ground states, as well as femtosecond stimulated Raman scattering data allowing us to track the subsequent structural evolution until the first stable all-trans photoproduct is reached. We conclude by introducing computational results for two-dimensional electronic spectroscopy, which has the potential to provide even greater detail on the evolution of the electronic structure of retinal during the photoisomerization process.

Hot exciton dissociation in polymer solar cells.

The standard picture of photovoltaic conversion in all-organic bulk heterojunction solar cells predicts that the initial excitation dissociates at the donor/acceptor interface after thermalization. Accordingly, on above-gap excitation, the excess photon energy is quickly lost by internal dissipation. Here we directly target the interfacial physics of an efficient low-bandgap polymer/PC(60)BM system. Exciton splitting occurs within the first 50 fs, creating both interfacial charge transfer states (CTSs) and polaron species. On high-energy excitation, higher-lying singlet states convert into hot interfacial CTSs that effectively contribute to free-polaron generation. We rationalize these findings in terms of a higher degree of delocalization of the hot CTSs with respect to the relaxed ones, which enhances the probability of charge dissociation in the first 200 fs. Thus, the hot CTS dissociation produces an overall increase in the charge generation yield.

Explaining the temperature dependence of spirilloxanthin's S* signal by an inhomogeneous ground state model.

We investigate the nature of the S* excited state in carotenoids by performing a series of pump-probe experiments with sub-20 fs time resolution on spirilloxanthin in a polymethyl-methacrylate matrix varying the sample temperature. Following photoexcitation, we observe sub-200 fs internal conversion of the bright S2 state into the lower-lying S1 and S* states, which in turn relax to the ground state on a picosecond time scale. Upon cooling down the sample to 77 K, we observe a systematic decrease of the S*/S1 ratio. This result can be explained by assuming two thermally populated ground state isomers. The higher lying one generates the S* state, which can then be effectively frozen out by cooling. These findings are supported by quantum chemical modeling and provide strong evidence for the existence and importance of ground state isomers in the photophysics of carotenoids.

Ultrafast-laser-induced backward stimulated Raman scattering for tracing atmospheric gases.

By combining tunable broadband pulse generation with the technique of nonlinear spectral compression we demonstrate a prototype scheme for highly selective detection of air molecules by backward stimulated Raman scattering. The experimental results allow to extrapolate the laser parameters required for standoff sensing based on the recently demonstrated backward atmospheric lasing.

Coherent synthesis of ultra-broadband optical parametric amplifiers.

We report on coherent synthesis of two ultra-broadband optical parametric amplifiers, each compressed by chirped mirror pairs, resulting in almost-octave-spanning (520-1000 nm) spectra supporting nearly single-cycle sub-4 fs pulse duration. Synthesized pulse timing is locked to less than 30 as by a balanced optical cross-correlator. The synthesized pulse is characterized by two-dimensional spectral interferometry and has a 3.8 fs duration.

Double stapled haemorrhoidopexy for haemorrhoidal prolapse: indications, feasibility and safety.

AIM: Selected patients with haemorrhoidal prolapse undergoing double stapled anopexy with the procedure for prolapse and haemorrhoids (PPH03) were studied. METHOD: Between March 2007 and March 2010, 235 patients referred with haemorrhoids were included in the study. Patients with obstructed defaecation were excluded. At surgery intraoperative evaluation for double stapled anopexy was carried out based on the criteria of prolapse occupying half or more of the anal circumference and redundant prolapsed tissue determined by the circular anal dilator. Patients fulfilling these criteria were submitted for double stapled anopexy with the PPH03 stapler. All clinical and operative data were recorded in a prospectively maintained database. RESULTS: Among the 142 patients with haemorrhoidal prolapse having surgery 91 had a single and 51 a double stapled technique. The mean operative time was 34.8 min with no major or minor intraoperative complications. Recurrence at 48 months was 1.9% and the mean satisfaction score was 8.9. CONCLUSION: The double stapled PPH03 technique in selected cases was as safe and effective as a single stapling technique with a lower incidence of recurrence over a medium-term follow-up.

Background-free broadband CARS spectroscopy from a 1-MHz ytterbium laser.

We introduce a novel configuration for broadband, time-resolved CARS spectroscopy/microscopy in which pump, Stokes and probe pulses are all derived from a single femtosecond Yb:KYW laser. The 1-MHz repetition rate of the system allows very intense CARS signals to be obtained over short acquisition times, while a delayed probe pulse ensures an efficient non-resonant background suppression.

Femtosecond stimulated Raman spectrometer in the 320-520nm range.

Multi-µJ narrow-bandwidth (≈ 10 cm(-1)) picosecond pulses, broadly tunable in the visible-UV range (320-520 nm), are generated by spectral compression of femtosecond pulses emitted by an amplified Ti:sapphire system. Such pulses provide the ideal Raman pump for broadband femtosecond stimulated Raman spectroscopy, as here demonstrated on a heme protein.

Arthroscopic patellar reinsertion of the MPFL in acute patellar dislocations.

PURPOSE: The objective of this study was to evaluate the mid-term results of a new technique for the arthroscopic repair of MPFL after an acute patellar dislocation (APD). MATERIALS: The series included 17 patients (11 men and 6 women) with a first episode of acute patellar dislocation; treated over a period of 6 years. Re-dislocation, subjective symptoms and functional limitations were evaluated at an average follow-up of 2.2 years (1-5.5). The patients were evaluated with the Lysholm and the Kujala scoring systems. RESULTS: At follow-up, no re-dislocation was reported. Only one patient referred an episode of patellar instability, without a distinct dislocation. The postoperative median Lysholm score was 90 (72-100). The median Kujala score was 92 (75-100). Fourteen out of 17 patients were able to return to sports at the same level as before. CONCLUSION: When the MPFL is avulsed from the patella, the proposed technique has the advantage of restoring tension of the ligament through reattachment at the patellar border with two trans-patellar sutures. The full-arthroscopic approach has the advantage of being less invasive and having a shorter recovery time.

Heterotopic pancreatic tissue in the gallbladder. Two case reports and brief review of the literature.

Heterotopic pancreas tissue in the gallbladder is a rare benign condition with no clinical relevance and found incidentally in most cases. Only 28 cases of heterotopic pancreas are reported until 2007 in the worldwide literature. The preoperative diagnosis is very hard, so that it could be considered in differential diagnosis with other benign and malignant disease of gallbladder and biliary tree. The surgical approach for the excision of a undefined lesion and the histological identification is a correct procedure in suspect case. We report 2 cases of heterotopic pancreatic tissue of the gallbladder.

Phototoxicity induced in living HeLa cells by focused femtosecond laser pulses: a data-driven approach.

Nonlinear optical microscopy is a powerful label-free imaging technology, providing biochemical and structural information in living cells and tissues. A possible drawback is photodamage induced by high-power ultrashort laser pulses. Here we present an experimental study on thousands of HeLa cells, to characterize the damage induced by focused femtosecond near-infrared laser pulses as a function of laser power, scanning speed and exposure time, in both wide-field and point-scanning illumination configurations. Our data-driven approach offers an interpretation of the underlying damage mechanisms and provides a predictive model that estimates its probability and extension and a safety limit for the working conditions in nonlinear optical microscopy. In particular, we demonstrate that cells can withstand high temperatures for a short amount of time, while they die if exposed for longer times to mild temperatures. It is thus better to illuminate the samples with high irradiances: thanks to the nonlinear imaging mechanism, much stronger signals will be generated, enabling fast imaging and thus avoiding sample photodamage.

Femtosecond laser written optical waveguide amplifier in phospho-tellurite glass.

We report on the first demonstration of an optical waveguide amplifier in phospho-tellurite glass providing net gain at 1.5 µm. The device was fabricated using a high repetition rate femtosecond laser and exhibited internal gain across 100-nm bandwidth covering the entire C + L telecom bands.