Giant All-Optical Modulation of Second-Harmonic Generation Mediated by Dark Excitons.

All-optical control of nonlinear photonic processes in nanomaterials is of significant interest from a fundamental viewpoint and with regard to applications ranging from ultrafast data processing to spectroscopy and quantum technology. However, these applications rely on a high degree of control over the nonlinear response, which still remains elusive. Here, we demonstrate giant and broadband all-optical ultrafast modulation of second-harmonic generation (SHG) in monolayer transition-metal dichalcogenides mediated by the modified excitonic oscillation strength produced upon optical pumping. We reveal a dominant role of dark excitons to enhance SHG by up to a factor of ∼386 at room temperature, 2 orders of magnitude larger than the current state-of-the-art all-optical modulation results. The amplitude and sign of the observed SHG modulation can be adjusted over a broad spectral range spanning a few electronvolts with ultrafast response down to the sub-picosecond scale via different carrier dynamics. Our results not only introduce an efficient method to study intriguing exciton dynamics, but also reveal a new mechanism involving dark excitons to regulate all-optical nonlinear photonics

The effects of cryotherapy versus cryostretching on clinical and functional outcomes in athletes with acute hamstring strain

Purpose: Hamstring strain is a common sport injury that results in pain and functional limitation. Despite its high frequency in active populations, there is no agreement regarding the best method used for early intervention of hamstring strain. The aim of the present study was to compare the effects of cryotherapy and cryostretching on clinical and functional outcomes in athletes with acute hamstring strain. Materials and methods: Thirty seven elite athletes with an acute grade I or II hamstring strain were randomly assigned to either cryotherapy (n = 19) or cryostretching (n = 18) group, receiving 5 sessions of supervised treatment plus home-based intervention monitored by the therapist. Pre-treatment to post-treatment changes in pain, active and passive knee extension range of motion and functional status were compared between the two groups. Results: Compared to cryotherapy, cryostretching resulted in larger improvement of function and passive knee extension range of motion. Changes in active knee extension range of motion and pain severity were not significantly different between the two groups. Conclusion: A rehabilitation protocol involving gentle stretching following cryotherapy is more effective than cryotherapy alone in the improvement of function and passive knee range of motion in patients with grade I and II hamstring strain

Femtosecond optical parametric generators and amplifiers for the near infrared based on BiB 3 O 6

ABSTRACT We describe the most relevant optical properties of BiB 3 O 6 (BIBO), that make this material an attractive candidate for nonlinear frequency conversion of laser light in general, and down-conversion of femtosecond laser sources operating near 800 nm in particular. Experimental results are presented in the high-energy, low-repetition-rate (1 kHz) regime, demonstrating the unique versatility of BIBO for efficient generation of femtosecond pulses in the infrared up to ~3 μm

High-power Femtosecond Optical Parametric Amplification at 1 kHz in BiB 3 O 6 pumped at 800 nm

Abstract: Substantial power scaling of a travelling-wave femtosecond optical parametric amplifier, pumped near 800 nm by a 1 kHz Ti:sapphire laser amplifier, is demonstrated using monoclinic BiB 3 O 6 in a two stage scheme with continuum seeding. Total energy output (signal plus idler) exceeding 1 mJ is achieved, corresponding to an intrinsic conversion efficiency of ≈32% for the second stage. The tunability extends from 1.1 to 2.9 µm. The high parametric gain and broad amplification bandwidth of this crystal allowed the maintenance of the pump pulse duration, leading to pulse lengths less than 140 fs, both for the signal and idler pulses, even at such high output levels

High-power Femtosecond Optical Parametric Amplification at 1 kHz in BiB 3 O 6 pumped at 800 nm

Abstract: Substantial power scaling of a travelling-wave femtosecond optical parametric amplifier, pumped near 800 nm by a 1 kHz Ti:sapphire laser amplifier, is demonstrated using monoclinic BiB 3 O 6 in a two stage scheme with continuum seeding. Total energy output (signal plus idler) exceeding 1 mJ is achieved, corresponding to an intrinsic conversion efficiency of ≈32% for the second stage. The tunability extends from 1.1 to 2.9 µm. The high parametric gain and broad amplification bandwidth of this crystal allowed the maintenance of the pump pulse duration, leading to pulse lengths less than 140 fs, both for the signal and idler pulses, even at such high output levels

Difference frequency generation in monolayer MoS2

| openaire: EC/H2020/820423/EU//S2QUIP | openaire: EC/H2020/834742/EU//ATOPDifference frequency generation has long been employed for numerous applications, such as coherent light generation, sensing and imaging. Here, we demonstrate difference frequency generation down to atomic thickness in monolayer molybdenum disulfide. By mixing femtosecond optical pulses at wavelength of 406 nm with tunable pulses in the spectral range of 1300-1520 nm, we generate tunable pulses across the spectral range of 550-590 nm with frequency conversion efficiency up to ∼2 × 10-4. The second-order nonlinear optical susceptibility of monolayer molybdenum disulfide, χ(2)eff, is calculated as ∼1.8 × 10-8 m V-1, comparable to the previous results demonstrated with second harmonic generation. Such a highly efficient down-conversion nonlinear optical process in two-dimensional layered materials may open new ways to their nonlinear optical applications, such as coherent light generation and amplification.Peer reviewe