Combined optical nonlinearity of bound and free electrons in a fast-ionizing medium driven by ultrashort, mid-infrared (mid-IR) pulses gives rise to a vast variety of ultrafast nonlinear-optical scenarios, producing bright, broadband radiation in spectral ranges as different as ultraviolet (UV) and terahertz (THz). Given its enormous bandwidth, a quantitative experimental analysis of this type of nonlinear response is anything but simple. Here, we confront this challenge by ultrabroadband spectral measurements performed across the spectral range from the UV to the millimeter-wave (MMW) band jointly with beam profile analysis in the THz-to-MMW band and direct time-domain field waveform characterization. As one of the most striking results, the nonlinear response of a fast-ionizing gas driven by a two-color field, consisting of a high-peak-power sub-100-fs mid-IR pulse and its second harmonic, is shown to provide a source of a bright multiband supercontinuum (SC) radiation, whose spectrum spans over about 14 octaves, stretching from below 300 nm all the way beyond 4.3 mm. The MMW-to-THz part of this SC is emitted, as direct measurements show, in the form of half-cycle field waveforms that can be focused to yield a field strength of ≈ 5 M V / c m . At least 1.5% of the MMW–THz supercontinuum energy is emitted in the MMW range, giving rise to MMW field strengths up to 100 kV/cm in the beam waist region.

Raman spectroscopy is a powerful tool for molecular chemical analysis and bioimaging, which shows an astonishing sensitivity when combined with a huge enhancement by the coherence and surface effects. Noble metal nanoparticles have been commonly used for the spontaneous surface-enhanced Raman scattering (SERS) and for the surface-enhanced coherent anti-Stokes Raman scattering (SECARS) spectroscopies, as they provide large enhancement factors via the electromagnetic and chemical mechanisms. Recently, two-dimensional (2D) semiconductors, such as monolayer molybdenum disulfide (MoS2), were used for potential SERS applications as cheaper substrates compared to noble metal nanoparticles. However, the coherent enhancement of SECARS on 2D materials has not been previously explored. Here we present the experimental SECARS measurements of pyridine–ethanol solutions containing 2D MoS2 nanocrystals with the giant chemical enhancement factor of 109 over coherent anti-Stokes Raman scattering (CARS), which is attributed to the charge transfer states and resonant MoS2 excitation. As a comparison, the SERS signals on MoS2 using incoherent nonresonant excitation show at least 2 orders of magnitude smaller enhancement. Time-resolved SECARS measurements directly reveal the increased vibrational dephasing rates, which provide strong evidence for the charge transfer in the pyridine–ethanol–MoS2 system.

Coherent-wake plasma emission induced by ultrashort mid-infrared laser pulses on a solid target is shown to give rise to high-brightness, high-order harmonic radiation, offering a promising source of attosecond pulses and a probe for ultrafast subrelativistic plasma dynamics. With 80-fs, 0.2-TW pulses of 3.9-μm radiation used as a driver, optical harmonics up to the 34th order are detected, with their spectra stretching from the mid-infrared region to the extreme ultraviolet region. The harmonic spectrum is found to be highly sensitive to the chirp of the driver. Particle-in-cell analysis of this effect suggests, in agreement with the generic scenario of coherent-wake emission, that optical harmonics are radiated as trains of extremely short, attosecond ultraviolet pulses with a pulse-to-pulse interval varying over the pulse train. A positive chirp of the driver pulse can partially compensate for this variation in the interpulse separation, allowing harmonics of the highest orders to be generated in the plasma emission spectrum.

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Featured Application: remote spectroscopic mapping of gas emissions and leaks. Abstract:We examine the concentration dependence of the Coherent Anti-Stokes Raman Scattering (CARS) signal obtained for gas mixtures at various conditions using the Femtosecond Adaptive Spectroscopic Technique (FAST). We use the CARS signal of the Q-branch vibrational oscillation of molecular oxygen (1556 cm −1 ) to confirm the quadratic dependence of the coherent signal on the number of molecules in a test volume. In addition, we demonstrate multi-shot FAST CARS imaging of a gas flow in free space by raster-scanning the area of interest.

As the Coronavirus 2019 pandemic creates worldwide shortages of personal protective equipment, hospitals have increasingly turned to sterilization and re-use protocols, often without significant data supporting the specific methodologies. When using UV-C irradiation, previously shown to be effective for decontaminating hard surfaces, modeling shows the importance of accounting for the porosity and non-uniform curvature of the N95 masks in decontamination procedures. Data shows a standard incident dose of 1 J/cm^2 delivered to both front and back surfaces is more than 500x higher than the known kill dose. However, modeling indicates this would undertreat 40\% of the mask material due to the curvature, path-length attenuation and scatter. Multidirectional UV-C irradiation employing dose calibrated exposures can adjust for this loss and best decontaminate masks. Such protocols can be rapidly implemented in thousands of hospitals across the world equipped with UV-C irradiation lamps without the need for additional capital equipment purchases.

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We investigate the possibility of tailoring coherent Raman generated spectra via adaptive wavefront optimization. Our technique combines a spatial light modulator and a spectrometer providing a feedback loop. The algorithm is capable of controlling the Raman generation, producing broader spectra and an improved overall efficiency, and increasing the intensity of high-order sidebands. Moreover, by wavefront optimization we can extend the generated spectra towards the blue spectral region and increase the total power of generated sidebands. Mutual coherence and equal frequency separation of the multiple Raman sidebands are of interest for the synthesis of ultrashort light pulses with the total spectral bandwidth extending over ultraviolet, visible and near-infrared wavelengths.