Four-wave mixing in a silicon microring resonator using a self-pumping geometry

We report on four-wave mixing in a silicon microring resonator using a self-pumping scheme instead of an external laser. The ring resonator is inserted in an external-loop cavity with a fibered semiconductor amplifier as a source of gain. The silicon microring acts as a filter and we observe lasing in one of the microring's resonances. We study correlations between signal and idler generated beams using a Joint Spectral Density experiment

Generation of microwave fields in cavities with laser-excited nonlinear media: competition between the second- and third-order optical nonlinearities

We discuss a scheme for the parametric amplification of the quantum fluctuations of the electromagnetic vacuum in a three-dimensional microwave resonator, and report the preliminary measurements to test its feasibility. In the present experimental scheme, the fundamental mode of a microwave cavity is nonadiabatically perturbed by modulating the index of refraction of the nonlinear optical crystal enclosed therein. Intense, multi-GHz laser pulses, such as those delivered by a mode-locked laser source, impinge on the crystal to accomplish the n-index modulation. We theoretically analyze the process of parametric generation, which is related to the third-order nonlinear coefficient \u3c7(3) of the nonlinear crystal, and assess the suitable experimental conditions for generating real photons from the vacuum. Second-order nonlinear processes are first analyzed as a possible source of spurious photons in quantum vacuum experiments when an ideal, mode-locked laser source is considered. The combination of a crystal non-null \u3c7(2) coefficient and a real mode-locked laser system\u2014i.e. one featuring offset-fromcarrier noise and unwanted secondary oscillations\u2014is also experimentally investigated, paving the way for future experiments in three-dimensional cavities

High Gain Solid-State Amplifiers for Picosecond Pulses

We review solid-state laser amplifiers for generation of intense picosecond pulses, in various regimes from single shot to repetition rates of GHz. Such laser sources are becoming increasingly attractive for many industrial and scientific applications. In particular, we have exploited the technology of side-pumped grazing-incidence bounce amplifiers. Such amplifiers yield very high gain per pass, up to several thousands, and offer excellent beam quality preservation owing to the total reflection leading to left-right inversion. This technology allows the realization of compact, efficient and modular amplifiers, significantly simpler than, for example, cavity-based regenerative schemes. Starting from robust, low-power diode-pumped solid-state oscillators, using programmable pulse-pickers one can select either a single pulse or a properly shaped pulse train for further amplification and compensation of envelope distortions due to gain saturation. For single pulse amplification it is preferred to start with a relatively low-repetition-rate oscillator (< 100 MHz). Picosecond fiber oscillators are most promising in this respect. Using quasi-cw diode arrays as the pump source of Nd:YVO4 slab amplifier, starting from ≈ 1 nJ, 10-ps pulse seed, amplified pulse energy as high as 200 μJ at 1 kHz can be obtained. Efficient harmonic and traveling-wave parametric generation are readily achieved with such high pulse peak powers. Some other applications require instead the amplification of pulse trains, that can be conveniently extracted and amplified from a low-power oscillator at the desired repetition rate. For example, starting from a 20-mW, 5-GHz picosecond oscillator we amplified trains of few thousands of pulses up to 2 mJ with three slab amplifiers (as much as 300 mJ were achieved with two additional Nd:YAG flash-lamp-pumped post-amplifiers). Such pulse trains are very effective for synchronous pumping of optical parametric oscillators, lowering significantly their threshold with respect to the traveling-wave geometry. When multi-MHz picosecond pulses are required, cw diode arrays are chosen as pump sources for the slab amplifiers. An 8-W, 8-ps laser system has been demonstrated starting from a 50-mW cw oscillator, at 150 MHz. Owing to the effective gain shaping of the tightly pumped amplifier, no significant thermal distortion were detected, allowing nearly diffraction limited operation. Although high power picosecond oscillators have been demonstrated lately, this result is interesting since it suggests an alternative way for power-scaling of picosecond sources without pushing delicate intracavity components (such as semiconductor saturable absorbers) to the damage limit. Numerical models of the amplifiers and their dynamics are also reviewed. The effects of amplified spontaneous emission are discussed, as well as the most effective methods for its suppression

CaloCube: a novel calorimeter for high-energy cosmic rays in space

In order to extend the direct observation of high-energy cosmic rays up to the PeV region, highly performing calorimeters with large geometrical acceptance and high energy resolution are required. Within the constraint of the total mass of the apparatus, crucial for a space mission, the calorimeters must be optimized with respect to their geometrical acceptance, granularity and absorption depth. CaloCube is a homogeneous calorimeter with cubic geometry, to maximise the acceptance being sensitive to particles from every direction in space; granularity is obtained by relying on small cubic scintillating crystals as active elements. Different scintillating materials have been studied. The crystal sizes and spacing among them have been optimized with respect to the energy resolution. A prototype, based on CsI(Tl) cubic crystals, has been constructed and tested with particle beams. Some results of tests with different beams at CERN are presented.Comment: Seven pages, seven pictures. Proceedings of INSTR17 Novosibirs

CaloCube: an innovative homogeneous calorimeter for the next-generation space experiments

The direct measurement of the cosmic-ray spectrum, up to the knee region, is one of the instrumental challenges for next generation space experiments. The main issue for these measurements is a steeply falling spectrum with increasing energy, so the physics performance of the space calorimeters are primarily determined by their geometrical acceptance and energy resolution. CaloCube is a three-year R&D project, approved and financed by INFN in 2014, aiming to optimize the design of a space-born calorimeter. The peculiarity of the design of CaloCube is its capability of detecting particles coming from any direction, and not only those on its upper surface. To ensure that the quality of the measurement does not depend on the arrival direction of the particles, the calorimeter will be designed as homogeneous and isotropic as possible. In addition, to achieve a high discrimination power for hadrons and nuclei with respect to electrons, the sensitive elements of the calorimeter need to have a fine 3-D sampling capability. In order to optimize the detector performances with respect to the total mass of the apparatus, which is the most important constraint for a space launch, a comparative study of different scintillating materials has been performed using detailed Monte Carlo simulation based on the FLUKA package. In parallel to simulation studies, a prototype consisting in 14 layers of 3 x 3 CsI(Tl) crystals per layer has been assembled and tested with particle beams. An overview of the obtained results during the first two years of the project will be presented and the future of the detector will be discussed too

Replication fork stalling in WRN-deficient cells is overcome by prompt activation of a MUS81-dependent pathway

Failure to stabilize and properly process stalled replication forks results in chromosome instability, which is a hallmark of cancer cells and several human genetic conditions that are characterized by cancer predisposition. Loss of WRN, a RecQ-like enzyme mutated in the cancer-prone disease Werner syndrome (WS), leads to rapid accumulation of double-strand breaks (DSBs) and proliferating cell nuclear antigen removal from chromatin upon DNA replication arrest. Knockdown of the MUS81 endonuclease in WRN-deficient cells completely prevents the accumulation of DSBs after fork stalling. Also, MUS81 knockdown in WS cells results in reduced chromatin recruitment of recombination enzymes, decreased yield of sister chromatid exchanges, and reduced survival after replication arrest. Thus, we provide novel evidence that WRN is required to avoid accumulation of DSBs and fork collapse after replication perturbation, and that prompt MUS81-dependent generation of DSBs is instrumental for recovery from hydroxyurea-mediated replication arrest under such pathological conditions

Femtosecond Mamyshev fiber oscillator started by ultra-low power microchip laser seeder at two different wavelengths: a comparison

A 1-W average output power, sub-60-fs femtosecond Mamyshev fiber oscillator was reliably started by a passively Q-switched sub-ns microchip laser generating four-wave-mixing signals in a polarization-maintaining passive fiber, at either 1064 nm or at 1033 nm. We show experimentally that seeding at 1033 nm provides much higher quality pulses with a duration as short as 41 fs and minimal satellites. The evolution toward the gain-managed nonlinear amplification regime clearly takes place when seeding the oscillator closer to the peak gain of the Yb-doped fiber

DNA2 drives processing and restart of reversed replication forks in human cells

Accurate processing of stalled or damaged DNA replication forks is paramount to genomic integrity and recent work points to replication fork reversal and restart as a central mechanism to ensuring high-fidelity DNA replication. Here, we identify a novel DNA2- and WRN-dependent mechanism of reversed replication fork processing and restart after prolonged genotoxic stress. The human DNA2 nuclease and WRN ATPase activities functionally interact to degrade reversed replication forks with a 5'-to-3' polarity and promote replication restart, thus preventing aberrant processing of unresolved replication intermediates. Unexpectedly, EXO1, MRE11, and CtIP are not involved in the same mechanism of reversed fork processing, whereas human RECQ1 limits DNA2 activity by preventing extensive nascent strand degradation. RAD51 depletion antagonizes this mechanism, presumably by preventing reversed fork formation. These studies define a new mechanism for maintaining genome integrity tightly controlled by specific nucleolytic activities and central homologous recombination factors

Versatile OSCAT time-domain THz spectrometer

: We report on a compact and versatile time-domain spectrometer operating in the THz spectral region from 0.2 to 2.5 THz based on ultrafast Yb:CALGO laser and photo-conductive antennas. The spectrometer operates with the optical sampling by cavity tuning (OSCAT) method based on laser repetition rate tuning, which allows at the same time the implementation of a delay-time modulation scheme. The whole characterization of the instrument is presented and compared to the classical THz time-domain spectroscopy implementation. THz spectroscopic measurements on a 520-μm thick GaAs wafer substrate together with water vapor absorption measurements are also reported to further validate the instrument capabilities

Narrowband, mid-infrared, seeded optical parametric generator based on non-critical CdSiP2 pumped by 120-ps, single longitudinal mode 1,064 nm pulses

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