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Microresonator based dual frequency combs offer an integrated photonic solution to a wide range of technological challenges, including spectroscopy, range finding, material characterization and hyperspectral imaging. These applications require a high level of mutual coherence between combs, but achieving such stability can be demanding. Here we experimentally demonstrate that comb generation exploiting the rich structure of the nonlinear electro-optic tensor in lithium niobate can generate ultra-stable dual combs, with the two combs naturally having orthogonal polarizations. We theoretically explore the application of our technique to dual frequency comb generation from the far infrared (IR) to the ultraviolet (UV). Our combs have relative linewidths as low as ~400 μHz, and require no stabilization or post-processing methods. Dual frequency combs are a powerful tool for a range of optical measurements and technologies. The authors here generate orthogonally polarized dual combs with exceptionally high relative stability. [ABSTRACT FROM AUTHOR]

The information contained in population genomic data can tell us much about the past ecology and evolution of species. We leveraged detailed phenotypic and genomic data of nearly all living kākāpō to understand the evolution of its feather color polymorphism. The kākāpō is an endangered and culturally significant parrot endemic to Aotearoa New Zealand, and the green and olive feather colorations are present at similar frequencies in the population. The presence of such a neatly balanced color polymorphism is remarkable because the entire population currently numbers less than 250 birds, which means it has been exposed to severe genetic drift. We dissected the color phenotype, demonstrating that the two colors differ in their light reflectance patterns due to differential feather structure. We used quantitative genomics methods to identify two genetic variants whose epistatic interaction can fully explain the species' color phenotype. Our genomic forward simulations show that balancing selection might have been pivotal to establish the polymorphism in the ancestrally large population, and to maintain it during population declines that involved a severe bottleneck. We hypothesize that an extinct apex predator was the likely agent of balancing selection, making the color polymorphism in the kākāpō a "ghost of selection past." The kākāpō is an endangered and culturally significant parrot endemic to Aotearoa New Zealand. This genomic, phenotypic and simulation study of most living kākāpō explores the evolution of green and olive feather coloration, concluding that this polymorphism is likely a remnant of past predation. [ABSTRACT FROM AUTHOR]

The authors discuss a study by B. Redding et al. in the journal "Proceedings of the National Academy of Sciences of the USA" on a chaotic cavity semiconductor laser that offers a solution to the problem of imaging artifacts, such as speckle, caused by spatiotemporal coherence from the uncontrolled scattering of laser light and image degradation caused by multipath interference. Topics include multmode laser sources, the steady-state ab initio laser theory (SALT), and chaotic laser cavities.

The author discusses a research paper within the issue on research by B. Peng et al. reporting the onset of lasing in a pair of coupled resonators when the total loss of the system is increased. Topics include Peng et al.'s non-Hermitian system design, which makes such lasing possible, modeling of corresponding gain and loss processes through Hamiltonians that are non-Hermitian, exceptional points (EPs), and a subset of non-Hermitian systems with a so-called parity-time (PT) symmetry.

The nonlinear scattering of a linear optical wave from a conservative soliton has been widely studied in optical fibers as a mechanism for nonlinear frequency conversion. Here we extend this analysis to consider the scattering of an externally injected probe wave from a dissipative cavity soliton circulating in a Kerr microresonator. We demonstrate, both theoretically and experimentally, that this nonlinear interaction can be harnessed for useful expansion of the soliton frequency comb via the formation of a secondary idler comb. We explore the physics of the process, showing that the phase detuning of the injected probe from a cavity resonance plays a key role in setting the central frequency of the idler comb, thus providing a convenient parameter through which to control the spectral envelope of that comb. Our results elucidate the dynamics that govern the interactions between dissipative Kerr cavity solitons and externally injected probe waves, and could prove useful in the design of future Kerr frequency comb systems by enabling the possibility to provide high-power comb lines in a specified spectral region simply through the injection of a suitably chosen probe. Tuneable microresonator frequency combs offer low-power, coherent light with a small device footprint. Here, the concept of controlling the comb frequency by detuning the probe phase is translated from photonic crystal fibres to a Kerr microresonator. [ABSTRACT FROM AUTHOR]

Titanium doped sapphire (Ti:sapphire) is a laser gain material with broad gain bandwidth benefiting from the material stability of sapphire. These favorable characteristics of Ti:sapphire have given rise to femtosecond lasers and optical frequency combs. Shaping a single Ti:sapphire crystal into a millimeter sized high quality (Q) whispering gallery mode resonator (Q ≈ 108) reduces the lasing threshold to 14.2 mW and increases the laser slope efficiency to 34%. The observed lasing can be both multi‐mode and single‐mode. This is the first demonstration of a Ti:sapphire whispering‐gallery laser. Furthermore, a novel method of evaluating the gain in Ti:sapphire in the near infrared region is demonstrated by introducing a probe laser with a central wavelength of 795 nm. This method results in decreasing linewidth of the modes excited with the probe laser, consequently increasing their Q. These findings open avenues for the usage of whispering gallery mode resonators as cavities for the implementation of compact Ti:sapphire lasers. Moreover, Ti:sapphire whispering‐gallery laser can also be utilized as an amplifier inside its gain bandwidth by implementing a pump–probe configuration. [ABSTRACT FROM AUTHOR]

Photon pairs and heralded single photons, obtained from cavity-assisted parametric down conversion (PDC), play an important role in quantum communications and technology. This motivated a thorough study of the spectral and temporal properties of parametric light, both above the optical parametric oscillator (OPO) threshold, where the semiclassical approach is justified, and deeply below it, where the linear cavity approximation is applicable. The pursuit of a higher two-photon emission rate leads into an interesting intermediate regime, between above OPO threshold and very low pump power, where the OPO still operates considerably below the threshold but the nonlinear cavity phenomena cannot be neglected anymore. Here, we investigate this intermediate regime and show that the spectral and temporal properties of the photon pairs, as well as their emission rate, may significantly differ from the widely accepted linear model. The observed phenomena include frequency pulling and broadening in the temporal correlation for the down-converted optical fields. These factors need to be taken into account when devising practical applications of the high-rate cavity-assisted spontaneous PDC sources. [ABSTRACT FROM AUTHOR]

We propose a dielectric rod waveguide antenna (DRW) integrated with a photomixer as a THz emitter. This represents a different approach as opposed to the classical solution of a substrate lens. Main goals are an inexpensive alternative to substrate lenses, reduction of both reflections on the semiconductor-air interface and scattering of terahertz-generated power into the substrate. A radiation pattern measured at 137 GHz is shown as a proof-of-concept. In order to increase radiated power, the improvement of the rod antenna is discussed. Finally, as an application example, evanescent coupling of the DRW into a high index whispering gallery mode resonator is shown. [ABSTRACT FROM PUBLISHER]

Ti:sapphire, whispering-gallery laser, whispering-gallery modes Keywords: Ti:sapphire; whispering-gallery laser; whispering-gallery modes EN Ti:sapphire whispering-gallery laser whispering-gallery modes 1 1 1 04/25/22 20220415 NES 220415 A fresh approach is taken to titanium sapphire by manufacturing it into a whispering-gallery laser. The photo shows the resonator fluorescing when pumped evanescently through a prism with green laser light. [Extracted from the article]

We predict and investigate the emission of high-order harmonics by atoms that cross intense laser hot spots that last for a nanosecond or longer. An atom that moves through a nanometer-scale hot spot at characteristic thermal velocity can emit high-order harmonics in a similar fashion to an atom that is irradiated by a short-duration (picosecond-scale) laser pulse. We analyze the collective emission from a thermal gas and from a jet of atoms. In both cases, the line shape of a high-order harmonic exhibits a narrow spike with spectral width that is determined by the bandwidth of the driving laser. Finally, we discuss a scheme for producing long-duration laser hot spots with intensity in the range of the intensity threshold for high-harmonic generation. In the proposed scheme, the hot spot is produced by a long laser pulse that is consecutively coupled to a high-quality micro-resonator and a metallic nano-antenna. This system may be used for generating ultra-narrow bandwidth extreme-ultraviolet radiation through frequency up-conversion of a low-cost compact pump laser. [ABSTRACT FROM AUTHOR]