Stability improvement of 40Ca+ optical clock by using a transportable ultra-stable cavity.

The instability of the clock laser is one of the primary factors limiting the instability of the optical clocks. We present an ultra-stable clock laser based on a 30-cm-long transportable cavity with an instability of ∼3 × 10-16 at 1 s-100 s. The cavity is fixed by invar poles in three orthogonal directions to restrict the displacement, meeting the requirements of transportability and low vibration sensitivity. By applying the ultra-stable laser to a transportable 40Ca+ optical clock with a systematic uncertainty of 4.8 × 10-18 and using the real-time feedback algorithm to compensate the linear shift of the clock laser, the short-term stability of the transportable 40Ca+ optical clock has been greatly improved from 4.0×10-15/τ/s to 1.16×10-15/τ/s, measured at ∼100 s-1000 s of averaging time, enriching its applications in metrology, optical frequency comparison, and time keeping.

The space cold atom interferometer for testing the equivalence principle in the China Space Station.

The precision of the weak equivalence principle (WEP) test using atom interferometers (AIs) is expected to be extremely high in microgravity environment. The microgravity scientific laboratory cabinet (MSLC) in the China Space Station (CSS) can provide a higher-level microgravity than the CSS itself, which provides a good experimental environment for scientific experiments that require high microgravity. We designed and realized a payload of a dual-species cold rubidium atom interferometer. The payload is highly integrated and has a size of [Formula: see text]. It will be installed in the MSLC to carry out high-precision WEP test experiment. In this article, we introduce the constraints and guidelines of the payload design, the compositions and functions of the scientific payload, the expected test precision in space, and some results of the ground test experiments.

The space cold atom interferometer for testing the equivalence principle in the China Space Station.

The precision of the weak equivalence principle (WEP) test using atom interferometers (AIs) is expected to be extremely high in microgravity environment. The microgravity scientific laboratory cabinet (MSLC) in the China Space Station (CSS) can provide a higher-level microgravity than the CSS itself, which provides a good experimental environment for scientific experiments that require high microgravity. We designed and realized a payload of a dual-species cold rubidium atom interferometer. The payload is highly integrated and has a size of 460 mm × 330 mm × 260 mm. It will be installed in the MSLC to carry out high-precision WEP test experiment. In this article, we introduce the constraints and guidelines of the payload design, the compositions and functions of the scientific payload, the expected test precision in space, and some results of the ground test experiments.

Time delay interferometry with a transfer oscillator.

In this work, we experimentally perform time delay interferometry by using a transfer oscillator, which is capable of reducing the laser frequency noise and the clock noise simultaneously in the post processing. The iodine frequency reference is coherently downconverted to the microwave frequency using a laser frequency comb. The residual noise of the downconversion network is 5 × 10-6Hz/Hz1/2 at 0.7 mHz, and 4 × 10-6Hz/Hz1/2 at 0.1 Hz, indicating high homology between the optical frequency and the microwave frequency. We carry out time delay interferometry with the aid of the electrical delay module, which can introduce large time delays. The results show that the laser frequency noise and the clock noise can be reduced simultaneously by ten and three orders of magnitude, respectively, in the frequency band from 0.1 mHz to 0.1 Hz. The performance of the noise reduction can reach 6 × 10-8Hz/Hz1/2 at 0.1 mHz, and 7 × 10-7Hz/Hz1/2 at 1 mHz, meeting the requirements of the space-borne gravitational wave detection. Our work will be able to offer an alternative method for the frequency comb-based time delay interferometry in the future space-borne gravitational wave detectors.

Molecularly engineered CMC-caged PNIPAM for broadband light management in energy-saving window.

Dynamic transparent-opaque transition behavior endows the stimuli-chromic materials with the solar modulation capability. However, these materials commonly involve the high manufacturing cost and complexity, the additional consumption of electric energy for solar modulation, or the weak effectiveness of light management. Herein, we develop a low-cost yet broadband light management sodium carboxymethyl cellulose-caging-poly(N-isopropylacrylamide) thermochromic composite (i.e., CMC/PNIPAM), where the nanoscale-skeleton CMC molecules well cage the PNIPAM molecules, which enables the homogeneous dispersion and sufficient distribution of the PNIPAM nanogels in the system. The CMC/PNIPAM features the excellent solar-modulation capability (including optical transmittance modulation of 68.17% and infrared transmittance modulation of 48.50%) and a low phase temperature of 30 °C, as well as the long-term stability of dynamic transparent-opaque transition. Such merits of the broadband light management, low cost, simply fabrication as well as scaling up, make the CMC/PNIPAM function as a promising candidate for the energy-saving buildings and construction.

Transparent, smooth, and sustainable cellulose-derived conductive film applied for the flexible electronic device.

A high-performance flexible conductive substrate is one of the key components for developing promising wearable devices. Concerning this, a sustainable, flexible, transparent, and conductive cellulose/ZnO/AZO (CZA) film was developed in this study. The cellulose was used as the transparent substrate. The added AZO was as the conductive layer and ZnO functioned as an interface buffer layer. Results showed that the interface buffer layer of ZnO effectively alleviated the intrinsic incompatibility of organic cellulose and inorganic AZO, resulting in the improvement of the performance of CZA film. In compared with the controlled cellulose/AZO (CA) film with 365 Ω/sq sheet resistance and 87% transmittance, this CZA film featured a low conductive sheet resistance of 115 Ω/sq and high transmittance of 89%, as well as low roughness of 1.85 nm Moreover, the existence of conducive ZnO buffer layer enabled the conductivity of CZA film to be stable under the bending treatment. Herein, a flexible electronic device was successfully prepared with the biomass materials, which would be available by a roll-to-roll production process.

Quantum cascade laser-based mid-IR frequency metrology system with ultra-narrow linewidth and 1 × 10⁻¹³-level frequency instability.

We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and frequency metrology with quantum cascade lasers (QCLs). We have implemented frequency stabilization of a QCL to an ultra-low expansion (ULE) reference cavity, via upconversion to the near-IR spectral range, at a level of 1×10(-13). The absolute frequency of the QCL is measured relative to a hydrogen maser, with instability <1×10(-13) and inaccuracy 5×10(-13), using a frequency comb phase stabilized to an independent ultra-stable laser. The QCL linewidth is determined to be 60 Hz, dominated by fiber noise. Active suppression of fiber noise could result in sub-10 Hz linewidth.

Silicon single-crystal cryogenic optical resonator: erratum.

We correct fit formulas from a previous paper [Opt. Lett.39, 3242 (2014)10.1364/OL39.005896OPLEDP0146-9592] for the coefficient of thermal expansion αreson(T).

A compact, robust, and transportable ultra-stable laser with a fractional frequency instability of 1 × 10(-15.).

We present a compact and robust transportable ultra-stable laser system with minimum fractional frequency instability of 1 × 10(-15) at integration times between 1 and 10 s. The system was conceived as a prototype of a subsystem of a microwave-optical local oscillator to be used on the satellite mission Space-Time Explorer and QUantum Equivalence Principle Space Test (STE-QUEST) (http://sci.esa.int/ste-quest/). It was therefore designed to be compact, to sustain accelerations occurring during rocket launch, to exhibit low vibration sensitivity, and to reach a low frequency instability. Overall dimensions of the optical system are 40 cm × 20 cm × 30 cm. The acceleration sensitivities of the optical frequency in the three directions were measured to be 1.7 × 10(-11)/g, 8.0 × 10(-11)/g, and 3.9 × 10(-10)/g, and the absolute frequency instability was determined via a three-cornered hat measurement. Two additional cavity-stabilized lasers were used for this purpose, one of which had an instability σy < 4 × 10(-16) at 1 s integration time. The design is also appropriate and useful for terrestrial applications.

Silicon single-crystal cryogenic optical resonator.

We report on the demonstration and characterization of a silicon optical resonator for laser frequency stabilization, operating in the deep cryogenic regime at temperatures as low as 1.5 K. Robust operation was achieved, with absolute frequency drift less than 20 Hz over 1 h. This stability allowed sensitive measurements of the resonator thermal expansion coefficient (α). We found that α=4.6×10(-13) K(-1) at 1.6 K. At 16.8 K α vanishes, with a derivative equal to -6×10(-10) K(-2). The temperature of the resonator was stabilized to a level below 10 µK for averaging times longer than 20 s. The sensitivity of the resonator frequency to a variation of the laser power was also studied. The corresponding sensitivities and the expected Brownian noise indicate that this system should enable frequency stabilization of lasers at the low-10(-17) level.

Radiation hardness of high-Q silicon nitride microresonators for space compatible integrated optics.

Integrated optics has distinct advantages for applications in space because it integrates many elements onto a monolithic, robust chip. As the development of different building blocks for integrated optics advances, it is of interest to answer the important question of their resistance with respect to ionizing radiation. Here we investigate effects of proton radiation on high-Q (θ(106)) silicon nitride microresonators formed by a waveguide ring. We show that the irradiation with high-energy protons has no lasting effect on the linear optical losses of the microresonators.

Spectrally narrow, long-term stable optical frequency reference based on a Eu3+:Y2SiO5 crystal at cryogenic temperature.

Using an ultrastable continuous-wave laser at 580 nm we performed spectral hole burning of Eu(3+):Y(2)SiO(5) at a very high spectral resolution. The essential parameters determining the usefulness as a macroscopic frequency reference, linewidth, temperature sensitivity, and long-term stability, were characterized using a H-maser stabilized frequency comb. Spectral holes with a linewidth as low as 6 kHz were observed and the upper limit of the drift of the hole frequency was determined to be 5±3 mHz/s. We discuss the necessary requirements for achieving ultrahigh stability in laser frequency stabilization to these spectral holes.

Non-degenerated nonclassical photon pairs in a hot atomic ensemble.

We experimentally demonstrate the generation of non-degenerate nonclassical paired photons in a hot atomic ensemble using off-axis four-wave mixing. The time-resolved second-order correlated function between the Stokes photon and the anti-Stokes photon is given. The two-photon correlation between the photons obtained in this experiment is 1.87+/- 0.04, which leads to the violation of Cauchy-Schwarz inequality by a factor of 1.69+/- 0.14.

Generating Bragg solitons in a coherent medium.

In this paper we discuss the possibility of producing Bragg solitons in an electromagnetically induced transparency medium. We show that a coherent medium can be engineered to be a Bragg grating with a large Kerr nonlinearity through proper arrangements of light fields. The parameters of the medium can be easily controlled through adjusting the intensities and detunings of lasers. This scheme may provide an opportunity to study the dynamics of Bragg solitons with low power lights.