Subharmonic instabilities in Kerr microcombs.

We report experimental observation of subharmonic mode excitation in primary Kerr optical frequency combs generated using crystalline whispering-gallery mode resonators. We show that the subcombs can be controlled and span a single or multiple free spectral ranges around the primary comb modes. In the spatial domain, the resulting multiscale combs correspond to an amplitude modulation of intracavity roll patterns. We perform a theoretical analysis based on eigenvalue decomposition that evidences the mechanism leading to the excitation of these combs.

The profile and persistence of clinically critical antibiotic resistance genes and human pathogenic bacteria in manure-amended farmland soils.

Introduction: Microbial contamination in farmlands is usually underestimated and understudied. Different fertilization times and manure origins might introduce and change the microorganism diversity in farmland soils and thus might influence the abundance and persistence of microbial contamination including antibiotic resistance genes (ARGs), human bacterial pathogens (HBPs), and virulence factor genes (VFGs). Methods: A 0.5-/1.5-year fertilization experiment was performed, and metagenomic sequencing was conducted to quantify microbial contamination. The resistomes of soil samples revealed that ARGs against antibiotics which were extensively used in veterinary medicine as well as clinically critical ARGs (CCARGs) persisted in manure-amended soils. Here the extended-spectrum beta-lactamase and carbapenemase bla genes, the high-level mobilized colistin resistance gene mcr, the tigecycline resistance gene tet(X), and the vancomycin resistance gene van, all of which can circumvent the defense line of these "last-resort" antibiotics were selected to investigate CCARG pollution in farm environments. Results: A total of 254 potential HBPs and 2106 VFGs were detected in soil samples. Overall, our results revealed that (1) farmland soils could serve as a reservoir of some important bla, mcr, tet(X), and van gene variants, (2) the diversity and relative abundance of HBPs and VFGs increased significantly with incremental fertilization times and were discrepant among different manureamended soils, and (3) most CCARGs and VFGs coexisted in HBPs. Disscusion: The results of this study suggested a biological risk of manure in spreading antimicrobial resistance and pathogenicity.

Fluctuations and correlations in Kerr optical frequency combs with additive Gaussian noise.

We investigate the effects of environmental stochastic fluctuations on Kerr optical frequency combs. This spatially extended dynamical system can be accurately studied using the Lugiato-Lefever equation, and we show that when additive noise is accounted for, the correlations of the modal field fluctuations can be determined theoretically. We propose a general theory for the computation of these field fluctuations and correlations, which is successfully compared to numerical simulations.

On the transition to secondary Kerr combs in whispering-gallery mode resonators.

We demonstrate that extended dissipative structures in Kerr-nonlinear whispering-gallery mode resonators undergo a spatiotemporal instability, as the pumping parameters are varied. We show that the dynamics of the patterns beyond this bifurcation yield specific Kerr comb and sub-comb spectra that can be subjected to a phase of frequency-locking when optimal conditions are met. Our numerical results are found to be in agreement with experimental measurements.

Spontaneous generation of orbital angular momentum crystals using a monolithic Nd:YAG nonplanar ring laser.

We report the emission of localized orbital angular momentum (OAM) crystals in a millimeter-size monolithic Nd:YAG nonplanar ring laser. Narrow-linewidth single-frequency lasing in the kilohertz level featuring crystal-like vortices is obtained via phase locking of Laguerre-Gaussian modes in the cavity. It is found that the spatially degenerate OAM of high-order LG modes can be easily broken by superimposing a low-order mode, leading to crystal-like vortices. Our theoretical analysis is found to be in agreement with the experimental results for both intensity and interference patterns.

Direct generation of a narrow-linewidth Laguerre-Gaussian vortex laser in a monolithic nonplanar oscillator.

Vortex laser beams carrying orbital angular momentum have been attracting a lot of interest in recent years. Here we demonstrate the direct generation of a vortex laser in a monolithic nonplanar ring cavity. The unidirectional and single-frequency operation of Laguerre-Gaussian modes is observed and characterized. Fork interferograms have been obtained using a simple interferometer based on a plano-concave lens, and the topological charge of vortex beam is determined. A spectral linewidth as narrow as 2.3 kHz is measured by beating with a reference laser. We believe that such a high coherent vortex laser can be beneficial for numerous applications, including precision measurements and optical communications.

Dependence of quality factor on surface roughness in crystalline whispering-gallery mode resonators.

We present an experimental study of the variation of quality factor (Q-factor) of WGM resonators as a function of surface roughness. We consider mm-size whispering-gallery mode resonators manufactured with fluoride crystals, featuring Q-factors of the order of 1 billion at 1550 nm. The experimental procedure consists of repeated polishing steps, after which the surface roughness is evaluated using profilometry by white-light phase-shifting interferometry, while the Q-factors are determined using the cavity-ring-down method. This protocol permits us to establish an explicit curve linking the Q-factor of the disk-resonator to the surface roughness of the rim. We have performed measurements with four different crystals, namely, magnesium, calcium, strontium, and lithium fluoride. We have thereby found that the variations of Q-factor as a function of surface roughness is universal, in the sense that it is globally independent of the bulk material under consideration. We also discuss our experimental results in the light of theoretical estimates of surface scattering Q-factors already published in the literature.

Phase-locking transition in Raman combs generated with whispering gallery mode resonators.

We investigate the mechanisms leading to phase locking in Raman optical frequency combs generated with ultrahigh Q crystalline whispering gallery mode disk resonators. We show that several regimes can be triggered depending on the pumping conditions, such as single-frequency Raman lasing, multimode operation involving more than one family of cavity eigenmodes, and Kerr-assisted Raman frequency comb generation. The phase locking and coherence of the combs are experimentally monitored through the measurement of beat signal spectra. These phase-locked combs, which feature high coherence and wide spectral spans, are obtained with pump powers in the range of a few tens of mW. In particular, Raman frequency combs with multiple free-spectral range spacings are reported, and the measured beat signal in the microwave domain features a 3 dB linewidth smaller than 50 Hz, thereby indicating phase locking.

Universal nonlinear scattering in ultra-high Q whispering gallery-mode resonators.

Universal nonlinear scattering processes such as Brillouin, Raman, and Kerr effects are fundamental light-matter interactions of particular theoretical and experimental importance. They originate from the interaction of a laser field with an optical medium at the lattice, molecular, and electronic scale, respectively. These nonlinear effects are generally observed and analyzed separately, because they do not often occur concomitantly. In this article, we report the simultaneous excitation of these three fundamental interactions in mm-size ultra-high Q whispering gallery mode resonators under continuous wave pumping. Universal nonlinear scattering is demonstrated in barium fluoride and strontium fluoride, separately. We further propose a unified theory based on a spatiotemporal formalism for the understanding of this phenomenology.

Giant thermo-optical relaxation oscillations in millimeter-size whispering gallery mode disk resonators.

In this Letter, we show that giant thermo-optical oscillations can be triggered in millimeter (mm)-size whispering gallery mode (WGM) disk resonators when they are pumped by a resonant continuous-wave laser. Our resonator is an ultrahigh-Q barium fluoride cavity that features a positive thermo-optic coefficient and a negative thermo-elastic coefficient. We demonstrate for the first time, to our knowledge, that the complex interplay between these two thermic coefficients and the intrinsic Kerr nonlinearity yields very sharp slow-fast relaxation oscillations with a slow timescale that can be exceptionally large, typically of the order of 1 s. We use a time-domain model to gain understanding into this instability, and we find that both the experimental and theoretical results are in excellent agreement. The understanding of these thermal effects is an essential requirement for every WGM-related application and our study demonstrates that even in the case of mm-size resonators, such effects can still be accurately analyzed using nonlinear time-domain models.

Spectro-temporal dynamics of Kerr combs with parametric seeding.

We report a joint theoretical and experimental investigation of the parametric seeding of a primary Kerr optical frequency comb. Electro-optic modulation sidebands matching multiple free-spectral ranges of an ultrahigh-Q millimeter-size magnesium fluoride disk resonator are used as seed signals. These seed signals interact through four-wave mixing with the spectral components of a stable primary comb and give rise to complex spectro-temporal patterns. We show that the new frequency combs feature multiscale frequency spacing, with major frequency gaps in the order of a few hundred gigahertz, and minor frequency spacing in the order of a few tens of gigahertz. The experimental results are in agreement with numerical simulations using the Lugiato-Lefever equation. We expect such versatile and coherent optical frequency combs to have potential applications in optical communications systems where frequency management assigns predefined spectral windows at the emitter stage.

Kerr optical frequency comb generation in strontium fluoride whispering-gallery mode resonators with billion quality factor.

We report the fabrication for the first time of a strontium fluoride (SrF(2)) whispering-gallery mode resonator with quality factor in excess of 1 billion. The millimeter-size disk-resonator is polished until the surface roughness decreases down to a root-mean square value of 1.2 nm, as measured with a vertical scanning profilometer. We also demonstrate that this ultrahigh Q resonator allows for the generation of a normal-dispersion Kerr optical frequency comb at 1550 nm.

On the dispersion management of fluorite whispering-gallery mode resonators for Kerr optical frequency comb generation in the telecom and mid-infrared range.

Optical whispering gallery mode (WGM) resonators have been very attracting platforms for versatile Kerr frequency comb generations. We report a systematic study on the material dispersion of various optical materials that are capable of supporting quality factors above 109. Using an analytical approximation of WGM resonant frequencies in disk resonators, we investigate the effect of the geometry and transverse mode order on the total group-velocity dispersion (GVD). We demonstrate that the major radii and the radial mode indices play an important role in tailoring the GVD of WGM resonators. In particular, our study shows that in WGM disk-resonators, the polar families of modes have very similar GVD, while the radial families of modes feature dispersion values that can differ by up to several orders of magnitude. The effect of these giant dispersion shifts are experimentally evidenced in Kerr comb generation with magnesium fluoride. From a more general perspective, this critical feature enables to push the zero-dispersion wavelength of fluorite crystals towards the mid-infrared (mid-IR) range, thereby allowing for efficient Kerr comb generation in that spectral range. We show that barium fluoride is the most interesting crystal in this regard, due to its zero dispersion wavelength (ZDW) at 1.93 µm and an optimal dispersion profile in the mid-IR regime. We expect our results to facilitate the design of different platforms for Kerr frequency comb generations in both telecommunication and mid-IR spectral ranges.

Barium fluoride whispering-gallery-mode disk-resonator with one billion quality-factor.

We demonstrate a monolithic optical whispering-gallery-mode resonator fabricated with barium fluoride (BaF2) with an ultra-high quality (Q) factor above 109 at 1550 nm, and measured with both the linewidth and cavity-ring-down methods. Vertical scanning optical profilometry shows that the root mean square surface roughness of 2 nm is achieved for our mm-size disk. To the best of our knowledge, we show for the first time that one billion Q-factor is achievable by precision polishing in relatively soft crystals with mohs hardness of 3. We show that complex thermo-optical dynamics can take place in these resonators. Beside usual applications in nonlinear optics and microwave photonics, high-energy particle scintillation detection utilizing monolithic BaF2 resonators potentially becomes feasible.

Continuous tuning of double resonance-enhanced second harmonic generation in a dispersive dielectric resonator.

We report a method for continuous tuning of ultraviolet (UV) radiation of second harmonic generation in a dispersive beta barium borate (BBO) whispering gallery mode resonator. The doubly resonant enhancement in a high quality factor resonator leads to high conversion efficiency but the resonator dispersion severely limits practical tuning range. By simultaneously varying the temperature of the resonator and the mechanical stress on the disk, we were able to experimentally demonstrate a continuous tuning range of 70 GHz of 317 nm laser light at 0.74%/mW conversion efficiency. The achieved tuning range is at least 35 times wider than that by either mechanical or temperature tuning alone.

Phase noise performance comparison between optoelectronic oscillators based on optical delay lines and whispering gallery mode resonators.

We investigate the phase noise performance of optoelectronic oscillators when the optical energy storage elements are in the following three configurations: a high-Q whispering gallery mode resonator, an optical delay-line and a combination of both elements. The stability properties of these various optical elements are first characterized, and then systematically compared in the optical and in the microwave frequency domains. Subsequently, the spectral purity of the oscillator is theoretically and experimentally examined for each case. When the resonator is used as both delay and filtering element inside the delay-line based oscillator, the generated spurious modes are highly rejected. A spur rejection by more than 53 dB has been demonstrated for the first-neighboring spur.

Polarization conversion loss in birefringent crystalline resonators.

Whispering gallery modes in birefringent crystalline resonators are investigated. We experimentally investigate the XY-cut resonators made with LiNbO3, LiTaO3, and BBO and observe strong influence of the resonator's shape and birefringence on the quality factor of the extraordinary polarized modes. We show that extraordinary modes can have lower Q and even be suppressed owing to polarization conversion loss. The ordinary ray modes retain the high Q due to inhibited reflection phenomenon.

Distribution and sources of organochlorine pesticides in agricultural soils from central China.

There is little information on the organochlorine pesticides (OCPs) residues in agricultural soils of Wuhan, the largest city in central China. Surface soil samples were collected from agricultural soils in Wuhan and analyzed to determine twenty-one OCPs. According to the measured concentrations and detection frequencies, dichlorodiphenyltrichloroethanes (DDTs), hexachlorocyclohexanes (HCHs), heptachlor (HEPT), hexachlorobenzene (HCB) and aldrin were the predominant compounds in soil. DDTs accounted for 77.10% of total OCPs, followed by HCHs (7.83%), aldrin (4.21%), HEPTs (2.82%) and HCB (1.53%). The total DDT concentrations ranged from nd to 1198.0ngg(-1) and the main contaminated areas were distributed in Hannan and Xinzhou districts of Wuhan. The total HCH concentrations ranged from nd to 100.58ngg(-1) in soil and relatively higher levels were observed in soil samples from Huangpi and Hannan districts. Source analysis showed that OCPs residues except heptachlor originated mainly from historical application, besides slight recent introduction at some sites. Based on the China National Soil Quality Standard, DDT pollution in most samples of Wuhan agricultural soils could be considered as no and low contamination, while the level of HCHs was classified as no pollution. Our study indicated that there existed potential exposure risk of OCPs in Wuhan agricultural soils although the use of OCPs has been banned.

Whispering gallery mode resonators augmented with engraved diffraction gratings.

We report the demonstration of whispering gallery mode (WGM) resonators augmented with diffraction gratings. We apply focused ion beam (FIB) methods to precisely engrave a surface grating directly into the perimeter of a crystalline disc. The grating provides a simple and highly directional free-space coupling mechanism with superior stability to evanescent coupling techniques. These integrated gratings can also provide control of the resonance spectrum, significantly reducing the mode density. Our FIB fabrication process does not introduce significant loss; Q≃3×10(7) has been demonstrated. The wavelength dependence of the diffraction angle was found to be in excellent agreement with grating theory. The versatility of spectral control and far-field grating coupling will have significant impact in WGM resonator applications in lasers, sensors, and optoelectronics.

Thermal bistability-based method for real-time optimization of ultralow-threshold whispering gallery mode microlasers.

A method based on thermal bistability for ultralow-threshold microlaser optimization is demonstrated. When sweeping the pump laser frequency across a pump resonance, the dynamic thermal bistability slows down the power variation. The resulting line shape modification enables a real-time monitoring of the laser characteristic. We demonstrate this method for a functionalized microsphere exhibiting a submicrowatt laser threshold. This approach is confirmed by comparing the results with a step-by-step recording in quasi-static thermal conditions.