On Estimators of a Spectral Density Function

This thesis presents two main approaches to estimating the spectral density of a stationary time series, that are based on the classical periodogram. Both of these are related to the non-parametric density estimation. One is the kernel spectral density estimator while the other one is the Bernstein polynomial spectral density estimator. We have also introduced the method to determine the optimal smoothing parameters for estimating the spectral density of a stationary zero-mean process. Finally, the thesis concludes with a simulation study in order to examine the finite sample properties of the proposed spectral density estimators

Controlling the polarization of nitrogen ion lasing

Air lasing provides a promising technique to remotely produce coherent radiation in the atmosphere and attracts continuous attention. However, the polarization properties of N2+ lasing with seeding has not been understood since it was discovered ten years ago, in which the behaviors appear disordered and confusing. Here, we performed an experimental and theoretical investigation on the polarization properties of N2+ lasing and successfully revealed its underlying physical mechanism. We found that the optical gain is anisotropic owing to the permanent alignment of N2+ induced by the preferential ionization of the pump light. As a result, the polarization of N2+ lasing tends to align with that of the pump light after amplification, which becomes more pronounced with increasing amplification factor. Based on the permanent alignment of N2+, we built a theoretical model that analytically interpreted and numerically reproduced the experimental observations, which points out the key factors for controlling the polarization of N2+ lasing.Comment: 12 pages, 4 figure

High-Power and Ultralong-Life Aqueous Zinc-Ion Hybrid Capacitors Based on Pseudocapacitive Charge Storage

© 2019, © 2019, The Author(s). Rechargeable aqueous zinc-ion hybrid capacitors and zinc-ion batteries are promising safe energy storage systems. In this study, amorphous RuO2·H2O for the first time was employed to achieve fast and ultralong-life Zn2+ storage based on a pseudocapacitive storage mechanism. In the RuO2·H2O||Zn zinc-ion hybrid capacitors with Zn(CF3SO3)2 aqueous electrolyte, the RuO2·H2O cathode can reversibly store Zn2+ in a voltage window of 0.4–1.6 V (vs. Zn/Zn2+), delivering a high discharge capacity of 122 mAh g−1. In particular, the zinc-ion hybrid capacitors can be rapidly charged/discharged within 36 s with a very high power density of 16.74 kW kg−1 and a high energy density of 82 Wh kg−1. Besides, the zinc-ion hybrid capacitors demonstrate an ultralong cycle life (over 10,000 charge/discharge cycles). The kinetic analysis elucidates that the ultrafast Zn2+ storage in the RuO2·H2O cathode originates from redox pseudocapacitive reactions. This work could greatly facilitate the development of high-power and safe electrochemical energy storage.[Figure not available: see fulltext.]

Population Redistribution among Multiple Electronic States of Molecular Nitrogen Ions in Strong Laser Fields

We carry out a combined theoretical and experimental investigation on the population distributions in the ground and excited states of tunnel ionized N2 molecules at various driver wavelengths in the near- and mid-infrared range. Our results reveal that efficient couplings (i.e., population exchanges) between the ground state and the excited states occur in strong laser fields. The couplings result in the population inversion between the ground and the excited states at the wavelengths near 800 nm, which is verified by our experiment by observing the amplification of a seed at ~391 nm. The result provides insight into the mechanism of free-space nitrogen ion lasers generated in remote air with strong femtosecond laser pulses.Comment: 18 pages, 4 figure

Amplification of light pulses with orbital angular momentum (OAM) in nitrogen ions lasing

Nitrogen ions pumped by intense femtosecond laser pulses give rise to optical amplification in the ultraviolet range. Here, we demonstrated that a seed light pulse carrying orbital angular momentum (OAM) can be significantly amplified in nitrogen plasma excited by a Gaussian femtosecond laser pulse. With the topological charge of +1 and -1, we observed an energy amplification of the seed light pulse by two orders of magnitude, while the amplified pulse carries the same OAM as the incident seed pulse. Moreover, we show that a spatial misalignment of the plasma amplifier with the OAM seed beam leads to an amplified emission of Gaussian mode without OAM, due to the special spatial profile of the OAM seed pulse that presents a donut-shaped intensity distribution. Utilizing this misalignment, we can implement an optical switch that toggles the output signal between Gaussian mode and OAM mode. This work not only certifies the phase transfer from the seed light to the amplified signal, but also highlights the important role of spatial overlap of the donut-shaped seed beam with the gain region of the nitrogen plasma for the achievement of OAM beam amplification.Comment: 10 pages, 7 figure

Structured air lasing of N2+

Structured light has attracted great interest in scientific and technical fields. Here, we demonstrate the first generation of structured air lasing in N2+ driven by 800 nm femtosecond laser pulses. By focusing a vortex pump beam at 800 nm in N2 gas, we generate a vortex superfluorescent radiation of N2+ at 391 nm, which carries the same photon orbital angular momentum as the pump beam. With the injection of a Gaussian seed beam at 391 nm, the coherent radiation is amplified, but the vorticity is unchanged. A new physical mechanism is revealed in the vortex N2+ superfluorescent radiation: the vortex pump beam transfers the spatial spiral phase into the N2+ gain medium, and the Gaussian seed beam picks up the spatial spiral phase and is then amplified into a vortex beam. Moreover, when we employ a pump beam with a cylindrical vector mode, the Gaussian seed beam is correspondingly amplified into a cylindrical vector beam. Surprisingly, the spatial polarization state of the amplified radiation is identical to that of the vector pump beam regardless of whether the Gaussian seed beam is linearly, elliptically, or circularly polarized. Solving three-dimensional coupled wave equations, we show how a Gaussian beam becomes a cylindrical vector beam in a cylindrically symmetric gain medium. This study provides a novel approach to generating structured light via N2+ air lasing.Comment: 18 pages, 5 figures, 3 equation

Bandgap engineering of polymetric carbon nitride copolymerized by 2,5,8-triamino-tri-s-triazine (melem) and barbituric acid for efficient nonsacrificial photocatalytic H2O2 production

Photocatalytic production of H2O2 from water and oxygen utilizing polymetric carbon nitride (PCN) is a promising alternative to the energy-consuming anthraquinone method. However, insufficient oxidation potential and limited light-absorption have restricted its further improvement. Herein, PCN with sufficient oxidation potential and improved visible-light usage (up to 550 nm) was prepared by co-polymerization of 2,5,8-triamino-tri-s-triazine (melem) and barbituric acid (BA). With the loading of Na2CoP2O7 as a water-oxidation co-catalyst, this novel PCN system showed a record-high apparent quantum efficiency (420 nm) of 8.0 % and a solar-to-chemical conversion efficiency of 0.30 % for H2O2 production. This improvement is attributed to the introduced O 2p states by CO groups remained in the PCN matrix, leading to a positive valence band maximum of 1.85 eV (vs. SHE). The co-polymerization of BA and melem combined with Na2CoP2O7 loading also suppressed the charge recombination, resulting in a rapid stepwise one-electron to one-electron reaction for efficient H2O2 production

Photoexcited single metal atom catalysts for heterogeneous photocatalytic H2O2 production: Pragmatic guidelines for predicting charge separation

A systematic investigation of electronic configuration and excitation properties is extremely urgent for establishing a guideline to boost H2O2 production with metal single-atom photocatalysts (M-SAPCs). Herein, a series of metal-ion incorporated M-SAPCs was prepared, isolating of three transition metals (Fe, Co, Ni) and two main-group metals (In, Sn) single site by pyridinic N atoms in polymeric carbon nitride (PCN) skeleton. The models in which metal ions are isolated by non-defected g-C3N4 units (Melem_3M) are consistent with the practically prepared M-SAPC in terms of band structures and electronic configurations. Transition density and molecular orbital analysis revealed that the atomically dispersed In (III) and Sn (IV) significantly improve the charge separation with an ideal electronic configuration for the end-on adsorption of oxygen for a boosted 2e−. The experimental charge separation properties and photocatalytic activities of M-SAPC showed good accordance with the computed charge transfer profiles of Melem_3 M, manifesting the rationalities and validities of as-proposed guidelines