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1. Room temperature terahertz semiconductor frequency comb

   https://doi.org/10.1038/s41467-019-10395-7  

   Lu, Quanyong; Wang, Feihu; Wu, Donghai; Slivken, Steven; Razeghi, Manijeh (June 2019, Nature Communications)

   Abstract A terahertz (THz) frequency comb capable of high-resolution measurement will significantly advance THz technology application in spectroscopy, metrology and sensing. The recently developed cryogenic-cooled THz quantum cascade laser (QCL) comb has exhibited great potentials with high power and broadband spectrum. Here, we report a room temperature THz harmonic frequency comb in 2.2 to 3.3 THz based on difference-frequency generation from a mid-IR QCL. The THz comb is intracavity generated via down-converting a mid-IR comb with an integrated mid-IR single mode based on distributed-feedback grating without using external optical elements. The grating Bragg wavelength is largely detuned from the gain peak to suppress the grating dispersion and support the comb operation in the high gain spectral range. Multiheterodyne spectroscopy with multiple equally spaced lines by beating it with a reference Fabry-Pérot comb confirms the THz comb operation. This type of THz comb will find applications to room temperature chip-based THz spectroscopy. 

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2. Mid-infrared cross-comb spectroscopy

   https://doi.org/10.1038/s41467-023-36811-7  

   Liu, Mingchen; Gray, Robert M.; Costa, Luis; Markus, Charles R.; Roy, Arkadev; Marandi, Alireza (February 2023, Nature Communications)

   Abstract Dual-comb spectroscopy has been proven beneficial in molecular characterization but remains challenging in the mid-infrared region due to difficulties in sources and efficient photodetection. Here we introduce cross-comb spectroscopy, in which a mid-infrared comb is upconverted via sum-frequency generation with a near-infrared comb of a shifted repetition rate and then interfered with a spectral extension of the near-infrared comb. We measure CO₂absorption around 4.25 µm with a 1-µm photodetector, exhibiting a 233-cm⁻¹instantaneous bandwidth, 28000 comb lines, a single-shot signal-to-noise ratio of 167 and a figure of merit of 2.4 × 10⁶Hz^1/2. We show that cross-comb spectroscopy can have superior signal-to-noise ratio, sensitivity, dynamic range, and detection efficiency compared to other dual-comb-based methods and mitigate the limits of the excitation background and detector saturation. This approach offers an adaptable and powerful spectroscopic method outside the well-developed near-IR region and opens new avenues to high-performance frequency-comb-based sensing with wavelength flexibility. 

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3. The Time Programmable Frequency Comb: Generation and Application to Quantum-Limited Dual-Comb Ranging

   https://doi.org/10.48550/arXiv.2205.01147  

   Caldwell, E.D.; Sinclair, L.C.; Newbury, N.R.; Deschenes, J-D (July 2022, ArXivorg)

   The classic self-referenced frequency comb acts as an unrivaled ruler for precision optical metrology in both time and frequency. Two decades after its invention, the frequency comb is now used in numerous active sensing applications. Many of these applications, however, are limited by the tradeoffs inherent in the rigidity of the comb output and operate far from quantum-limited sensitivity. Here we demonstrate an agile programmable frequency comb where the pulse time and phase are digitally controlled with +/- 2 attosecond accuracy. This agility enables quantum-limited sensitivity in sensing applications since the programmable comb can be configured to coherently track weak returning pulse trains at the shot-noise limit. To highlight its capabilities, we use this programmable comb in a ranging system, reducing the detection threshold by ~5,000-fold to enable nearly quantum-limited ranging at mean pulse photon number of 1/77 while retaining the full accuracy and precision of a rigid frequency comb. Beyond ranging and imaging, applications in time/frequency metrology, comb-based spectroscopy, pump-probe experiments, and compressive sensing should benefit from coherent control of the comb-pulse time and phase. 

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4. The effect of comb architecture on complex coacervation

   https://doi.org/10.1039/c7ob01314k  

   Johnston, Brandon M.; Johnston, Cameron W.; Letteri, Rachel A.; Lytle, Tyler K.; Sing, Charles E.; Emrick, Todd; Perry, Sarah L. (January 2017, Organic & Biomolecular Chemistry)

   Complex coacervation is a widely utilized technique for effecting phase separation, though predictive understanding of molecular-level details remains underdeveloped. Here, we couple coarse-grained Monte Carlo simulations with experimental efforts using a polypeptide-based model system to investigate how a comb-like architecture affects complex coacervation and coacervate stability. Specifically, the phase separation behavior of linear polycation-linear polyanion pairs was compared to that of comb polycation-linear polyanion and comb polycation-comb polyanion pairs. The comb architecture was found to mitigate cooperative interactions between oppositely charged polymers, as no discernible phase separation was observed for comb-comb pairs and complex coacervation of linear-linear pairs yielded stable coacervates at higher salt concentration than linear-comb pairs. This behavior was attributed to differences in counterion release by linear vs. comb polymers during polyeletrolyte complexation. Additionally, the comb polycation formed coacervates with both stereoregular poly( l -glutamate) and racemic poly( d , l -glutamate), whereas the linear polycation formed coacervates only with the racemic polyanion. In contrast, solid precipitates were obtained from mixtures of stereoregular poly( l -lysine) and poly( l -glutamate). Moreover, the formation of coacervates from cationic comb polymers incorporating up to ∼90% pendant zwitterionic groups demonstrated the potential for inclusion of comonomers to modulate the hydrophilicity and/or other properties of a coacervate-forming polymer. These results provide the first detailed investigation into the role of polymer architecture on complex coacervation using a chemically and architecturally well-defined model system, and highlight the need for additional research on this topic. 

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5. Grappling with homoplasy: taxonomic refinements and reassignments in the ant genera Camponotus and Colobopsis (Hymenoptera: Formicidae)

   https://doi.org/10.3897/asp.79.e66978  

   Ward, Philip S.; Boudinot, Brendon E. (April 2021, Arthropod Systematics & Phylogeny)

   null (Ed.)

   Camponotus and Colobopsis are widely distributed and species-rich genera in the ant tribe Camponotini. Molecular phylogenetic studies demonstrate that they are not sister taxa, but several lineages within each genus have converged to a remarkable degree, confounding the taxonomy of these ants. Based on multiple lines of evidence, including worker and male morphology, we demonstrate that: (1) three species of “Camponotus” belonging to the subgenus Myrmotemnus, including its type species, are in fact members of the genus Colobopsis ; (2) four species previously assigned to Colobopsis belong to the subgenus Myrmamblys of Camponotus ; and (3) three Nearctic taxa recently placed in Colobopsis are members of the genus Camponotus and closely related to Camponotus clarithorax . These taxonomic findings yield the following new or revived combinations: Colobopsis moeschi ( comb. nov. ), Colobopsis moeschi lygaea ( comb. nov. ), Colobopsis nutans ( comb. nov. ), Colobopsis nutans cleliae ( comb. nov. ), and Colobopsis reichenspergeri ( comb. nov. ); Camponotus apostemata ( comb. nov. ), Camponotus aurelianus ( comb. rev. ), Camponotus cavibregma ( comb. nov. ), Camponotus horrens ( comb. rev. ), Camponotus politae ( comb. rev. ), Camponotus trajanus ( comb. rev. ), and Camponotus yogi ( comb. rev. ). A further consequence is the following generic synonymy (senior synonym listed first): Colobopsis = Myrmotemnus syn. nov. , and Camponotus = Dolophra syn. rev. At the species level, we argue that Camponotus apostemata and Camponotus cavibregma are junior synonyms ( syn. nov. ) of Camponotus yogi , and Camponotus quercicola is a junior synonym ( syn. nov. ) of Ca. laevigatus . Taxonomic comments are also provided on some members of the Camponotus reticulatus group, with Camponotus adustus ( stat. nov. ) and Ca. leucodiscus ( stat. rev. ) being recognized as distinct species rather than subspecies of Ca. bellus . A male-based diagnosis of the Camponotini is provided, and differences between the males of Colobopsis and Camponotus are documented and illustrated for the first time. This study reveals new character systems of potential value to the systematics of these ants, including features of the male genitalia, and emphasizes the value of reciprocal illumination between phylogenomics and critical morphological analysis. 

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