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The series of precisely spaced, sharp spectral lines that form an optical frequency comb is enabling unprecedented measurement capabilities and new applications in a wide range of topics that include precision spectroscopy, atomic clocks, ultracold gases, and molecular fingerprinting. A new optical frequency comb generation principle has emerged that uses parametric frequency conversion in high resonance quality factor (Q) microresonators. This approach provides access to high repetition rates in the range of 10 to 1000 gigahertz through compact, chip-scale integration, permitting an increased number of comb applications, such as in astronomy, microwave photonics, or telecommunications. We review this emerging area and discuss opportunities that it presents for novel technologies as well as for fundamental science.

Optical frequency combs allow for the precise measurement of optical frequencies and are used in a growing number of applications. The new class of Kerr-frequency comb sources, based on parametric frequency conversion in optical microresonators, can complement conventional systems in applications requiring high repetition rates such as direct comb spectroscopy, spectrometer calibration, arbitrary optical waveform generation and advanced telecommunications. However, a severe limitation in experiments working towards practical systems is phase noise, observed in the form of linewidth broadening, multiple repetition-rate beat notes and loss of temporal coherence. These phenomena are not explained by the current theory of Kerr comb formation, yet understanding this is crucial to the maturation of Kerr comb technology. Here, based on observations in crystalline MgF 2 and planar Si 3 N 4 microresonators, we reveal the universal, platform-independent dynamics of Kerr comb formation, allowing the explanation of a wide range of phenomena not previously understood, as well as identifying the condition for, and transition to, low-phase-noise performance. Based on observations in crystalline MgF 2 and planar Si 3 N 4 microresonators, scientists reveal that the existence of multiple and broad-beat notes in a Kerr-frequency comb is due to the formation dynamics of the comb itself. This work identifies the conditions requires for low-phase-noise performance and also helps to elucidate a number of yet-unexplained phenomena.

Evergreen conifers in boreal forests can survive extremely cold (freezing) temperatures during long dark winter and fully recover during summer. A phenomenon called “sustained quenching” putatively provides photoprotection and enables their survival, but its precise molecular and physiological mechanisms are not understood. To unveil them, here we have analyzed seasonal adjustment of the photosynthetic machinery of Scots pine ( Pinus sylvestris ) trees by monitoring multi-year changes in weather, chlorophyll fluorescence, chloroplast ultrastructure, and changes in pigment-protein composition. Analysis of Photosystem II and Photosystem I performance parameters indicate that highly dynamic structural and functional seasonal rearrangements of the photosynthetic apparatus occur. Although several mechanisms might contribute to ‘sustained quenching’ of winter/early spring pine needles, time-resolved fluorescence analysis shows that extreme down-regulation of photosystem II activity along with direct energy transfer from photosystem II to photosystem I play a major role. This mechanism is enabled by extensive thylakoid destacking allowing for the mixing of PSII with PSI complexes. These two linked phenomena play crucial roles in winter acclimation and protection. Evergreen conifers rely on ‘sustained quenching’ to protect their photosynthetic machinery during long, cold winters. Here, Bag et al. show that direct energy transfer (spillover) from photosystem II to photosystem I triggered by loss of grana stacking in chloroplast is the major component of sustained quenching in Scots pine.

Optical clocks show unprecedented accuracy, surpassing that of previously available clock systems by more than one order of magnitude. Precise intercomparisons will enable a variety of experiments, including tests of fundamental quantum physics and cosmology and applications in geodesy and navigation. Well-established, satellite-based techniques for microwave dissemination are not adequate to compare optical clocks. Here, we present phase-stabilized distribution of an optical frequency over 920 kilometers of telecommunication fiber. We used two antiparallel fiber links to determine their fractional frequency instability (modified Allan deviation) to 5 × 10⁻¹⁵ in a 1-second integration time, reaching 10⁻¹⁸ in less than 1000 seconds. For long integration times τ, the deviation from the expected frequency value has been constrained to within 4 × 10⁻¹⁹ The link may serve as part of a Europe-wide optical frequency dissemination network.

A fine-tooth comb Optical frequency 'combs' are light sources that emit at discrete, equally spaced frequencies, so the spectrum has a characteristic comb-like appearance. Frequency combs have revolutionized the fields of spectroscopy and metrology: clocks using the technology now beat atomic clocks, such as the current caesium standard, for accuracy. But the instrumentation required to generate a frequency comb is cumbersome and complex, usually involving a bulky femtosecond laser. Del'Haye et al . have now developed a radically different approach to comb generation: a tiny disc-like resonator structure on a silicon chip is simply illuminated by a conventional laser diode. The resulting interaction between the laser light and the resonator gives rise to an optical frequency comb emitting in the infrared. The simplicity of the scheme — and the prospects of a reduction in size, cost and power — should enhance the utility of optical frequency combs in a broad number of fields. A tiny disc-like structure on a silicon chip is simply illuminated by a conventional laser diode, and the resulting interaction between the laser light and the resonator gives rise to an optical frequency comb that emits in the infrared. The simplicity of the scheme, and the reduction in size, cost and power, should enhance the utility of optical frequency combs in a broad number of fields. Optical frequency combs 1 , 2 , 3 provide equidistant frequency markers in the infrared, visible and ultraviolet 4 , 5 , and can be used to link an unknown optical frequency to a radio or microwave frequency reference 6 , 7 . Since their inception, frequency combs have triggered substantial advances in optical frequency metrology and precision measurements 6 , 7 and in applications such as broadband laser-based gas sensing 8 and molecular fingerprinting 9 . Early work generated frequency combs by intra-cavity phase modulation 10 , 11 ; subsequently, frequency combs have been generated using the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized 12 . Here we report a substantially different approach to comb generation, in which equally spaced frequency markers are produced by the interaction between a continuous-wave pump laser of a known frequency with the modes of a monolithic ultra-high- Q microresonator 13 via the Kerr nonlinearity 14 , 15 . The intrinsically broadband nature of parametric gain makes it possible to generate discrete comb modes over a 500-nm-wide span (∼70 THz) around 1,550 nm without relying on any external spectral broadening. Optical-heterodyne-based measurements reveal that cascaded parametric interactions give rise to an optical frequency comb, overcoming passive cavity dispersion. The uniformity of the mode spacing has been verified to within a relative experimental precision of 7.3 × 10 -18 . In contrast to femtosecond mode-locked lasers 16 , this work represents a step towards a monolithic optical frequency comb generator, allowing considerable reduction in size, complexity and power consumption. Moreover, the approach can operate at previously unattainable repetition rates 17 , exceeding 100 GHz, which are useful in applications where access to individual comb modes is required, such as optical waveform synthesis 18 , high capacity telecommunications or astrophysical spectrometer calibration 19 .

Although invented for precision measurements of single atomic transitions, frequency combs have also become a versatile tool for broadband spectroscopy in recent years. Here, we present a novel and simple approach for broadband spectroscopy, combining the accuracy of an optical fibre-laser-based frequency comb with the ease of use of a tunable external cavity diode laser. The scheme enables broadband and fast spectroscopy of more than 4 THz bandwidth at scanning speeds up to 1 THz s −1 at sub-MHz resolution. We use this method for spectroscopy of microresonator modes and precise measurements of their dispersion, which is relevant in the context of broadband optical frequency comb generation, having recently been demonstrated in these devices. Moreover, we find excellent agreement between measured microresonator dispersion with predicted values from finite element simulations, and we show that microresonator dispersion can be tailored by adjusting their geometrical properties. Spectroscopy that combines the accuracy of a frequency comb with the ease of use of a tunable, external cavity diode laser is demonstrated, enabling precise dispersion measurements of microresonator modes.

The mid-infrared spectral range ( λ ~2–20 μm) is of particular importance as many molecules exhibit strong vibrational fingerprints in this region. Optical frequency combs—broadband optical sources consisting of equally spaced and mutually coherent sharp lines—are creating new opportunities for advanced spectroscopy. Here we demonstrate a novel approach to create mid-infrared optical frequency combs via four-wave mixing in a continuous-wave pumped ultra-high Q crystalline microresonator made of magnesium fluoride. Careful choice of the resonator material and design made it possible to generate a broadband, low-phase noise Kerr comb at λ =2.5 μm spanning 200 nm (≈10 THz) with a line spacing of 100 GHz. With its distinguishing features of compactness, efficient conversion, large mode spacing and high power per comb line, this novel frequency comb source holds promise for new approaches to molecular spectroscopy and is suitable to be extended further into the mid-infrared. Optical frequency combs are vital tools for precision measurements, and extending them further into the mid-infrared 'molecular fingerprint' range will open new avenues for spectroscopy. Using crystalline microresonators, Wang et al . demonstrate Kerr combs at 2.5 μm as a promising route into the mid-infrared.

The mechanism and kinetics of electron transfer in isolated D1/$D2-cyt_{b559}$photosystem (PS) II reaction centers (RCs) and in intact PSII cores have been studied by femtosecond transient absorption and kinetic compartment modeling. For intact PSII, a component of ≈1.5 ps reflects the dominant energy-trapping kinetics from the antenna by the RC. A 5.5-ps component reflects the apparent lifetime of primary charge separation, which is faster by a factor of 8-12 than assumed so far. The 35-ps component represents the apparent lifetime of formation of a secondary radical pair, and the ≈200-ps component represents the electron transfer to the$Q_{A}$acceptor. In isolated RCs, the apparent lifetimes of primary and secondary charge separation are ≈3 and 11 ps, respectively. It is shown (i) that pheophytin is reduced in the first step, and (ii) that the rate constants of electron transfer in the RC are identical for PSII cores and for isolated RCs. We interpret the first electron transfer step as electron donation from the primary electron donor$Chl_{acc D1}$. Thus, this mechanism, suggested earlier for isolated RCs at cryogenic temperatures, is also operative in intact PSII cores and in isolated RCs at ambient temperature. The effective rate constant of primary electron transfer from the equilibrated$RC^{*}$excited state is 170-180$ns^{-1}$, and the rate constant of secondary electron transfer is 120-130$ns^{-1}$.

Light-harvesting complex II (LHCII) is the major antenna complex in higher plants and green algae. It has been suggested that a major part of the excited state energy dissipation in the so-called “non-photochemical quenching” (NPQ) is located in this antenna complex. We have performed an ultrafast kinetics study of the low-energy fluorescent states related to quenching in LHCII in both aggregated and the crystalline form. In both sample types the chlorophyll (Chl) excited states of LHCII are strongly quenched in a similar fashion. Quenching is accompanied by the appearance of new far-red (FR) fluorescence bands from energetically low-lying Chl excited states. The kinetics of quenching, its temperature dependence down to 4 K, and the properties of the FR-emitting states are very similar both in LHCII aggregates and in the crystal. No such FR-emitting states are found in unquenched trimeric LHCII. We conclude that these states represent weakly emitting Chl–Chl charge-transfer (CT) states, whose formation is part of the quenching process. Quantum chemical calculations of the lowest energy exciton and CT states, explicitly including the coupling to the specific protein environment, provide detailed insight into the chemical nature of the CT states and the mechanism of CT quenching. The experimental data combined with the results of the calculations strongly suggest that the quenching mechanism consists of a sequence of two proton-coupled electron transfer steps involving the three quenching center Chls 610/611/612. The FR-emitting CT states are reaction intermediates in this sequence. The polarity-controlled internal reprotonation of the E175/K179 aa pair is suggested as the switch controlling quenching. A unified model is proposed that is able to explain all known conditions of quenching or non-quenching of LHCII, depending on the environment without invoking any major conformational changes of the protein.

Photosystem I (PSI) is a large pigment-protein complex that unites a reaction center (RC) at the core with ~100 core antenna chlorophylls surrounding it. The RC is composed of two cof actor branches related by a pseudo-C2 symmetry axis. The ultimate electron donor, P₇₀₀ (a pair of chlorophylls), and the tertiary acceptor, Fx (a Fe₄S₄ cluster), are both located on this axis, while each of the two branches is made up of a pair of chlorophylls (ec2 and ec3) and a phylloquinone (PhQ). Based on the observed biphasic reduction of Fx , it has been suggested that both branches in PSI are competent for electron transfer (ET), but the nature and rate of the initial electron transfer steps have not been established. We report an ultrafast transient absorption study of Chlamydomonas reinhardtii mutants in which specific amino acids donating Ç-bonds to the 13¹ -keto oxygen of either ec3A (PsaA-Tyr696) or ec3B (PsaBTyr676) are converted to Phe, thus breaking the Ç-bond to a specific ec3 cofactor. We find that the rate of primary charge separation (CS) is lowered in both mutants, providing direct evidence that the primary ET event can be initiated independently in each branch. Furthermore, the data provide further support for the previously published model in which the initial CS event occurs within an ec2/ec3 pair, generating a primary ec2⁺ec3⁻ radical pair, followed by rapid reduction by P₇₀₀ in the second ET step. A unique kinetic modeling approach allows estimation of the individual ET rates within the two cofactor branches.