Structures and Properties of High-Concentration Doped Th:CaF2 Single Crystals for Solid-State Nuclear Clock Materials.

Thorium-doped vacuum ultraviolet (VUV) transparent crystals is a promising candidate for establishing a solid-state nuclear clock. Here, we report the research results on high-concentration doping of 232Th:CaF2 single crystals. The structures, defects, and VUV transmittance performances of highly doped Th:CaF2 crystals are investigated by theoretical and experimental methods. The defect configurations formed by Th and the charge compensation mechanism (Ca vacancy or interstitial F atoms) located at its first nearest neighbor position are mainly considered and studied. The preferred defect configuration is identified according to the doping concentration dependence of structural changes caused by the defects and the formation energies of the defects at different Ca or F chemical potentials. The cultivated Th:CaF2 crystals maintain considerable high VUV transmittance levels while accommodating high doping concentrations, showcasing an exceptional comprehensive performance. The transmittances of 1-mm-thick samples with doping concentrations of 1.91 × 1020 and 2.76 × 1020 cm-3 can reach ∼62% and 53% at 150 nm, respectively. The VUV transmittance exhibits a weak negative doping concentration dependence. The system factors that may cause distortion and additional deterioration of the VUV transmittance are discussed. Balancing and controlling the impacts of various factors will be of great significance for fully exploiting the advantages of Th:CaF2 and other Th-doped crystals for a solid-state nuclear optical clock.

Absolute Frequency Measurement of

We report the precision measurement of the absolute frequencies, hyperfine splitting, and 2P fine structure splitting in cold atoms of ^{6}Li. Using the stabilized optical frequency comb and developed heterodyne detection technique, the photon shot-noise limited optical spectroscopy is achieved. The measurement of absolute frequencies of D_{1} lines is reached with an uncertainty of about 1 kHz, which is 1 order of magnitude more accurate than previous measurements. The hyperfine splitting of the D_{1} line and 2P fine structure splitting of ^{6}Li are 26.103 1 (14) and 10 052.780 4 (18) MHz, respectively, in agreement with recent theoretical calculations. Our results could provide a benchmark to test the theory at the higher precision and help to resolve large discrepancies among previous experiments.

Laser frequency instability of 2 × 10-16 by stabilizing to 30-cm-long Fabry-Pérot cavities at 578 nm.

Laser light at 578 nm is frequency-stabilized to two independent 30-cm-long Fabry-Pérot cavities. To achieve a thermal-noise-limited cavity length stability, the geometry and support configuration of the Fabry-Pérot cavities are optimized. The fractional frequency instability of each cavity-stabilized laser system is 2 × 10-16 at 1 s averaging time, approaching to the thermal-noise-induced length instability of the reference cavity. The most probable linewidth of each laser system is about 0.2 Hz, and the laser frequency noise at Fourier frequency of 1 Hz is 0.1 Hz/√Hz.

Systematic evaluation of a 171Yb optical clock by synchronous comparison between two lattice systems.

Optical clocks are the most precise measurement devices. Here we experimentally characterize one such clock based on the 1S0-3P0 transition of neutral 171Yb atoms confined in an optical lattice. Given that the systematic evaluation using an interleaved stabilization scheme is unable to avoid noise from the clock laser, synchronous comparisons against a second 171Yb lattice system were implemented to accelerate the evaluation. The fractional instability of one clock falls below 4 × 10-17 after an averaging over a time of 5,000 seconds. The systematic frequency shifts were corrected with a total uncertainty of 1.7 × 10-16. The lattice polarizability shift currently contributes the largest source. This work paves the way to measuring the absolute clock transition frequency relative to the primary Cs standard or against the International System of Units (SI) second.

Coherence transfer of subhertz-linewidth laser light via an optical fiber noise compensated by remote users.

We present a technique for the coherence transfer of laser light through a fiber link, where the optical phase noise induced by environmental perturbation via the fiber link is compensated by remote users. When compensating the fiber noise by remote users, the time base at the remote site independent from that at the local site does not destroy the performance of the fiber output light. Using this technique, we demonstrate the transfer of subhertz-linewidth laser light through a 25-km-long, lab-based spooled fiber. After being compensated, the relative linewidth between the fiber input and output light is 1 mHz, and the relative frequency instability is 4×10-17 at 1 s averaging time and scales down to 2×10-19 at 800 s averaging time. The frequency uncertainty of the light after transferring through the fiber relative to that of the input light is 3.0×10-19. This system is suitable for the simultaneous transfer of an optical signal to a number of end users within a city.

1 Hz linewidth Ti:sapphire laser as local oscillator for (40)Ca(+) optical clocks.

A Ti:sapphire laser at 729 nm is frequency stabilized to an ultra-stable ultra-low thermal expansion coefficient (ULE) cavity by means of Pound-Drever-Hall method. An acousto-optic modulator is used as the fast frequency feedback component. 1 Hz linewidth and 2 × 10(-15) frequency stability at 1-100 s are characterized by optical beating with a separated Fabry-Perot cavity stabilized diode laser. Compared to the universal method that the error signal feedback to inject current of a diode laser, this scheme is demonstrated to be simple and also effective for linewidth narrowing. The temperature of zero coefficient of the thermal expansion of the ULE cavity is measured with the help of a femto-second frequency comb. And the performance of the laser is well defined by locking it to the unperturbed clock transition line-center of 4 S1/2-3 D5/2 clock transition of a single laser cooled (40)Ca(+) ion. A Fourier-transform limited resonance of 6 Hz (Δv/v = 1.5 × 10(-14)) is observed. This laser is also used as the local oscillator for the comparison experiment of two (40)Ca(+) ion optical clocks and improves the stability of comparison for an order of magnitude better than the previous results.

0.26-Hz-linewidth ultrastable lasers at 1557 nm.

Narrow-linewidth ultrastable lasers at 1.5 µm are essential in many applications such as coherent transfer of light through fiber and precision spectroscopy. Those applications all rely on the ultimate performance of the lasers. Here we demonstrate two ultrastable lasers at 1557 nm with a most probable linewidth of 0.26 Hz by independently frequency-stabilizing to the resonance of 10-cm-long ultrastable Fabry-Pérot cavities at room temperature. The fractional frequency instability of each laser system is nearly 8 × 10(-16) at 1-30 s averaging time, approaching the thermal noise limit of the reference cavities. A remarkable frequency instability of 1 × 10(-15) is achieved on the long time scale of 100-4000 s.

Design, synthesis and structure-activity relationship of oxazolidinone derivatives containing novel S4 ligand as FXa inhibitors.

A novel series of potent and efficacious factor Xa inhibitors which possesses pyrrole/indole/thiazole moieties as S4 binding element was identified. Compound 7b showed strong human factor Xa inhibitory activity (IC50 = 2.01 nM) and anticoagulant activities in both human (PTCT2 = 0.15 µM, APPTCT2 = 0.30 µM) and rabbit plasma (PTCT2 = 0.46 µM, APPTCT2 = 0.75 µM). The SARs analyses indicated that the size and water solubility of different alkylamino group at the position of S4 ligand were responsible for the anticoagulant activity.

Thermal analysis of optical reference cavities for low sensitivity to environmental temperature fluctuations.

The temperature stability of optical reference cavities is significant in state-of-the-art ultra-stable narrow-linewidth laser systems. In this paper, the thermal time constant and thermal sensitivity of reference cavities are analyzed when reference cavities respond to environmental perturbations via heat transfer of thermal conduction and thermal radiation separately. The analysis as well as simulation results indicate that a reference cavity enclosed in multiple layers of thermal shields with larger mass, higher thermal capacity and lower emissivity is found to have a larger thermal time constant and thus a smaller sensitivity to environmental temperature perturbations. The design of thermal shields for reference cavities may vary according to experimentally achievable temperature stability and the coefficient of thermal expansion of reference cavities. A temperature fluctuation-induced length instability of reference cavities as low as 6 × 10(-16) on a day timescale can be achieved if a two-layer thermal shield is inserted between a cavity with the coefficient of thermal expansion of 1 × 10(-10) /K and an outer vacuum chamber with temperature fluctuation amplitude of 1 mK and period of 24 hours.

Simple and compact 1-Hz laser system via an improved mounting configuration of a reference cavity.

We report an improved mounting configuration for a passive optical cavity used for laser frequency stabilization. The associated reduction of the vibration sensitivity of the effective cavity length has led to a simple and compact reference cavity system for laser stabilization at the level of 1 Hz linewidth.

First international comparison of femtosecond laser combs at the International Bureau of Weights and Measures.

The first international comparison of femtosecond laser combs has been carried out at the International Bureau of Weights and Measures (BIPM). Three comb systems were involved: BIPM-C1 and BIPM-C2 from the BIPM and ECNU-C1 from the East China Normal University (ECNU). The agreement among the three combs was found to be on the subhertz level in the vicinity of 563 THz. A frequency difference measurement scheme was demonstrated that is suitable for general comb comparisons.

Optical frequency synthesis and comparison with uncertainty at the 10(-19) level.

A femtosecond laser-based optical frequency synthesizer is referenced to an optical standard, and we use it to demonstrate the generation and control of the frequency of electromagnetic fields over 100 terahertz of bandwidth with fractional uncertainties approaching 1 part in 10(19). The reproducibility of this performance is verified by comparison of different types of femtosecond laser-based frequency synthesizers from three laboratories.

Comparison of 127I2-stabilized frequency-doubled Nd:YAG lasers at the Bureau International des Poids et Mesures.

A frequency comparison was carried out between iodine-stabilized Nd:YAG lasers at 532 nm from the Bureau International des Poids et Mesures, the Centre for Metrology and Accreditation, the Czech Metrology Institute, and the Bureau National de Métrologie-Institut National de Métrologie. The frequency differences between lasers, as well as the frequency reproducibility of each system,were investigated. Pressure-, modulation-, and power-induced shifts were studied. A frequency dispersion (1 sigma) of 3.5 kHz (6.2 x 10(-12) in relative terms) with an average reproducibility for each laser of the order of 0.4 kHz (7.1 x 10(-13) in relative terms) was observed over the duration of the comparison. Relative stabilities better than 1 x 10(13) at 1 s were demonstrated for the third-harmonic systems.

Femtosecond-laser-based optical clockwork with instability

Two octave-spanning optical-frequency combs (750-MHz comb spacing) are phase locked to a common continuous-wave laser diode. The measured instability of the heterodyne beat between the two combs demonstrates that the intrinsic fractional frequency noise of a comb is

Subfemtosecond timing jitter between two independent, actively synchronized, mode-locked lasers.

With the implementation of a fast-bandwidth servo, along with improved laser construction and associated better passive stability, we have achieved subfemtosecond relative timing jitter between two independent, actively synchronized, mode-locked Ti:sapphire lasers. Timing jitter of 0.58 fs is obtained with a 160-Hz observation bandwidth over several seconds. Within a 2-MHz observation bandwidth, the timing jitter is 1.75 fs. Excellent repeatability and rapid speed in setting an arbitrary time delay between two pulses are also demonstrated.