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<jats:p>Cold atom clocks have made remarkable progresses in the last two decades and played critical roles in precision measurements. Primary Cs fountain frequency standards have achieved a total uncertainty of a few parts in 10<jats:sup>16</jats:sup>, and the best optical clock has reached a type B uncertainty below 10<jats:sup>−18</jats:sup>. Besides applications in the metrology, navigation, etc., ultra-stable and ultra-accurate atomic clocks have also become powerful tools in the basic scientific investigations. In this paper, we focus on the recent developments in the high-performance cold atomic clocks which can be used as frequency standards to calibrate atomic time scales. The basic principles, performances, and limitations of fountain clocks and optical clocks based on signal trapped ion or neutral atoms are summarized. Their applications in metrology and other areas are briefly introduced.</jats:p>

<jats:p>We proposed a sandwich structure to realize broadband asymmetric transmission (AT) for both linearly and circularly polarized waves in the near infrared spectral region. The structure composes of a silica substrate and two sand-clock-like gold layers on the opposite sides of the substrate. Due to the surface plasmons of gold, the structure shows that the AT parameters of linearly and circularly polarized waves can reach 0.436 and 0.403, respectively. Meanwhile, a broadband property is presented for the AT parameter is over 0.3 between 320 THz and 340 THz. The structure realizes a diode-like AT for linearly wave in forward and circularly wave in backward, respectively. The magnetic dipoles excited by current in the two gold layers contribute to the broadband AT. The current density in top and bottom metallic layers illustrates the mechanism of the polarization conversion for broadband AT in detail.</jats:p>

<jats:p>A tunable dual-band terahertz absorber is designed and investigated. The unit cell of the proposed absorber consists of a graphene monolayer on a guided-mode resonant filter. The graphene absorber presents &gt;40% absorption at two resonance frequencies, which is attributed to the guided mode resonances with different mode numbers. The electric field intensity distribution is analyzed to disclose the physical mechanism of such a dual-band absorption effect. Furthermore, the influence of optical properties of graphene, including Fermi level and relaxation time, on the absorption spectra are investigated. Finally, the influence of geometric parameters on the absorption spectrum is studied, which will provide useful guidance for the fabrication of this absorber. We believe that the results may be useful for developing the next-generation graphene-based optoelectronic devices.</jats:p>

<jats:p>Simultaneous bandwidth (BW) enhancement and time-delay signature (TDS) suppression of chaotic lasing over a wide range of parameters by mutually coupled semiconductor lasers (MCSLs) with random optical injection are proposed and numerically investigated. The influences of system parameters on TDS suppression (characterized by autocorrelation function (ACF) and permutation entropy (PE) around characteristic time) and chaos BW are investigated. The results show that, with the increasing bias current, the ranges of parameters (detuning and injection strength) for the larger BW (&gt; 20 GHz) are broadened considerably, while the parameter range for optimized TDS (&lt; 0.1) is not shrunk obviously. Under optimized parameters, the system can simultaneously achieve two chaos outputs with enhanced BW (&gt; 20 GHz) and perfect TDS suppression. In addition, the system can generate two-channel high-speed truly physical random number sequences at 200 Gbits/s for each channel.</jats:p>

<jats:p>The nonparaxial propagation of radially polarized chirped Airy beams (RPCAiBs) in uniaxial crystal orthogonal to the optical axis is analytically studied. The effects of the first and the second order chirp factors, and the ratio of the extraordinary refractive index to the ordinary refractive index on the nonparaxial evolution of RPCAiBs in different observation planes are investigated in detail. The results show that when one parameter changes, different components behave differently, and even the same component has various behavior in different directions. The initial slope of RPCAiBs in the <jats:italic>x</jats:italic>-direction varies more with the first order chirp factor than that in the <jats:italic>y</jats:italic>-direction. Meanwhile, with the second order chirp factor becoming larger, the depth of the focus of the <jats:italic>y</jats:italic>-component in the <jats:italic>x</jats:italic>-direction decreases while that in the <jats:italic>y</jats:italic>-direction has no difference. In addition, the different ratios of the extraordinary refractive index to the ordinary refractive index change the difference of the depth of the focus between the <jats:italic>x</jats:italic>- and the <jats:italic>y</jats:italic>-components.</jats:p>

<jats:p>Broadband nonlinear frequency conversions of optical waves are widely employed in multiple areas of optics and photonics. However, the broadening of conversion bandwidth is often at a cost of reduction in efficiency, which may induce a limitation on practical applications. Here we theoretically propose a novel design of LiNbO<jats:sub>3</jats:sub> ridge waveguides on LiTaO<jats:sub>3</jats:sub> substrates which can be used for efficient and broadband second harmonic generation. Through group velocity engineering of the ridge waveguides, acceptance bandwidth over 20 nm with a high conversion efficiency of &gt; 25 %W<jats:sup>−1</jats:sup> ⋅ cm<jats:sup>−2</jats:sup> is achieved at telecom-band.</jats:p>

<jats:p>We report on a novel architecture to suppress the multi-pulse formation in an all-polarization-maintaining figure-9 erbium-doped fiber laser under high pump power. A 2×2 fiber coupler is introduced into the phase-biased nonlinear amplifying loop mirror to extract part of intracavity laser power as a laser output, and the dependence of output couple ratio of fiber coupler on the mode-locking state is experimentally investigated. The intracavity nonlinear effect is mitigated by lowering the intracavity laser power, which is conducive to avoiding the multi-pulse formation. In the meantime, the loss-imbalance induced by fiber coupler is helpful in improving the self-starting ability. With the proposed laser structure, the multiple pulse formation can be suppressed and high power single pulse train can be obtained. The laser emits three pulse trains which is convenient for some applications. Finally, the output power values of three ports are 5.3 mW, 51.3 mW, and 13.2 mW, respectively. The total single pulse output power is 69.8 mW, which is more than 10 times the result without OC<jats:sub>2</jats:sub>. The total slope efficiency is about 10.1%. The repetition rate of three pulse trains is 21.17 MHz, and the pulse widths are 2.8 ps, 2.63 ps, and 6.66 ps, respectively.</jats:p>

<jats:p>We investigate a graphene-coated nanowire waveguide (GCNW) composed of two suspended wedge porous silicon nanowires and a thin Ag partition. The plasmonic characteristics of the proposed structure in terahertz (THz) frequency band are simulated by the finite element method (FEM). The parameters including the gap between the nanowires and Ag partition, the height of the nanowire, the thickness of the Ag partition, and the Fermi level of graphene, are optimized. The simulation results show that a normalized mode field area of ∼ 10<jats:sup>−4</jats:sup> and a figure of merit of ∼ 100 can be achieved. Compared with the cylindrical GCNW and isolated GCNW, the proposed wedge GCNW has good electric field enhancement. A waveguide sensitivity of 32.28 is obtained, which indicates the prospects of application in refractive index (RI) sensing in THz frequency band. Due to the adjustable plasmonic characteristics by changing the Fermi level (<jats:italic>E</jats:italic> <jats:sub>F</jats:sub>), the proposed structure has promising applications in the electro-optic modulations, optical interconnects, and optical switches.</jats:p>

<jats:p>Phase modulation is a crucial step when the frequency-based wavefront optimization technique is exploited to measure the optical transmission matrix (TM) of a scattering medium. We report a simple but powerful method, direct digital frequency synthesis (DDS) technology to modulate the phase front of the laser and measure the TM. By judiciously modulating the phase front of a He–Ne laser beam, we experimentally generate a high quality focus at any targeted location through a 2 mm thick 120 grit ground glass diffuser, which is commercially used in laser display and laser holographic display for improving brightness uniformity and reducing speckle. The signal to noise ratio (SNR) of the clear round focus is ∼ 50 and the size is about 44 μm. Our study will open up new avenues for enhancing light energy delivery to the optical engine in laser TV to lower the power consumption, phase compensation to reduce the speckle noise, and controlling the lasing threshold in random lasers.</jats:p>

<jats:p>Ultra-cold atoms provide ideal platforms for interferometry. The macroscopic matter-wave property of ultra-cold atoms leads to large coherent length and long coherent time, which enable high accuracy and sensitivity to measurement. Here, we review our efforts to improve the performance of the interferometer. We demonstrate a shortcut method for manipulating ultra-cold atoms in an optical lattice. Compared with traditional ones, this shortcut method can reduce the manipulation time by up to three orders of magnitude. We construct a matter-wave Ramsey interferometer for trapped motional quantum states and significantly increase its coherence time by one order of magnitude with an echo technique based on this method. Efforts have also been made to enhance the resolution by multimode scheme. Application of a noise-resilient multi-component interferometer shows that increasing the number of paths could sharpen the peaks in the time-domain interference fringes, which leads to a resolution nearly twice compared with that of a conventional double-path two-mode interferometer. With the shortcut method mentioned above, improvement of the momentum resolution could also be fulfilled, which leads to atomic momentum patterns less than 0.6 <jats:italic>ℏk<jats:sub>L</jats:sub> </jats:italic>. To identify and remove systematic noises, we introduce the methods based on the principal component analysis (PCA) that reduce the noise in detection close to the <jats:inline-formula> <jats:tex-math> <?CDATA $1/\sqrt{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:msqrt> <mml:mn>2</mml:mn> </mml:msqrt> </mml:mrow> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cpb_30_1_014210_ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> of the photon-shot noise and separate and identify or even eliminate noises. Furthermore, we give a proposal to measure precisely the local gravity acceleration within a few centimeters based on our study of ultracold atoms in precision measurements.</jats:p>