Frequency and amplitude modulation of ultra-compact terahertz quantum cascade lasers using an integrated avalanche diode oscillator

Mode-locked comb sources operating at optical frequencies underpin applications ranging from spectroscopy and ultrafast physics, through to absolute frequency measurements and atomic clocks. Extending their operation into the terahertz frequency range would greatly benefit from the availability of compact semiconductor-based sources. However, the development of any compact mode-locked THz laser, which itself is inherently a frequency comb, has yet to be achieved without the use of an external stimulus. High-power, electrically pumped quantum cascade lasers (QCLs) have recently emerged as a promising solution, owing to their octave spanning bandwidths, the ability to achieve group-velocity dispersion compensation and the possibility of obtaining active mode-locking. Here, we propose an unprecedented compact architecture to induce both frequency and amplitude self-modulation in a THz QCL. By engineering a microwave avalanche oscillator into the laser cavity, which provides a 10 GHz self-modulation of the bias current and output power, we demonstrate multimode laser emission centered around 3 THz, with distinct multiple sidebands. The resulting microwave amplitude and frequency self-modulation of THz QCLs opens up intriguing perspectives, for engineering integrated self-mode-locked THz lasers, with impact in fields such as nano- and ultrafast photonics and optical metrology

The Salmonella enterica Pan-genome

Salmonella enterica is divided into four subspecies containing a large number of different serovars, several of which are important zoonotic pathogens and some show a high degree of host specificity or host preference. We compare 45 sequenced S. enterica genomes that are publicly available (22 complete and 23 draft genome sequences). Of these, 35 were found to be of sufficiently good quality to allow a detailed analysis, along with two Escherichia coli strains (K-12 substr. DH10B and the avian pathogenic E. coli (APEC O1) strain). All genomes were subjected to standardized gene finding, and the core and pan-genome of Salmonella were estimated to be around 2,800 and 10,000 gene families, respectively. The constructed pan-genomic dendrograms suggest that gene content is often, but not uniformly correlated to serotype. Any given Salmonella strain has a large stable core, whilst there is an abundance of accessory genes, including the Salmonella pathogenicity islands (SPIs), transposable elements, phages, and plasmid DNA. We visualize conservation in the genomes in relation to chromosomal location and DNA structural features and find that variation in gene content is localized in a selection of variable genomic regions or islands. These include the SPIs but also encompass phage insertion sites and transposable elements. The islands were typically well conserved in several, but not all, isolates—a difference which may have implications in, e.g., host specificity

High harmonic interferometry of the Lorentz force in strong mid-infrared laser fields

The interaction of intense mid-infrared laser fields with atoms and molecules leads to a range of new opportunities, from the production of bright, coherent radiation in the soft x-ray range to imaging molecular structures and dynamics with attosecond temporal and sub-angstrom spatial resolution. However, all these effects, which rely on laser-driven recollision of an electron removed by the strong laser field and the parent ion, suffer from the rapidly increasing role of the magnetic field component of the driving pulse: the associated Lorentz force pushes the electrons off course in their excursion and suppresses all recollision-based processes, including high harmonic generation, elastic and inelastic scattering. Here we show how the use of two non-collinear beams with opposite circular polarizations produces a forwards ellipticity which can be used to monitor, control, and cancel the effect of the Lorentz force. This arrangement can thus be used to re-enable recollision-based phenomena in regimes beyond the long-wavelength breakdown of the dipole approximation, and it can be used to observe this breakdown in high-harmonic generation using currently-available light sources

Schemes for generation of isolated attosecond pulses of pure circular polarization

We propose and analyze two schemes capable of generating isolated attosecond pulses of pure circular polarization, based on results of numerical simulations. Both schemes utilize the generation of circularly polarized high-order-harmonics by crossing two circularly polarized counter-rotating pulses in a noncollinear geometry. Our results show that in this setup isolation of a single attosecond pulse can be achieved either by restricting the driver pulse duration to a few cycles or by temporally delaying the two crossed driver pulses. We further propose to compensate the temporal walk-off between the pulses across the focal spot and increasing the conversion efficiency by using angular spatial chirp to provide perfectly matched pulse fronts. The isolation of pure circularly polarized attosecond pulses, along with the opportunity to select their central energy and helicity in the noncollinear technique, opens new perspectives from which to study ultrafast dynamics in chiral systems and magnetic materials

Spatio-temporally focused femtosecond laser pulses for nonreciprocal writing in optically transparent materials

Simultaneous spatial and temporal focusing (SSTF)provides precise control of the pulse front tilt (PFT) necessary to achieve nonreciprocal writing in glass wherein the material modification depends on the sample scanning direction with respect to the PFT. The PFT may be adjusted over several orders of magnitude. Using SSTF nonreciprocal writing is observed for a large range of axial focal positions within the sample, and nonreciprocal ablation patterns on the surface of the sample are revealed. Further, the lower numerical aperture (0.03 NA) utilized with SSTF increases the rate of writing