[Spatiotemporal Distribution and Health Impacts of PM and O in Beijing, from 2014 to 2020].

In China, fine particulate matter (PM) and tropospheric ozone (O) have become major air pollutants that threaten human health. Since 2013, the government has strengthened air pollution controls in Beijing and achieved significant effects. A spatial-temporal analysis was conducted of the distribution and health impacts of PM and O in Beijing, using data collected from 34 air quality monitoring sites between 2014 and 2020.

[Relationship Between Relative Crop Yield/Woody Plant Biomass and Ground-level Ozone Pollution in China].

Currently, ground-level ozone (O) pollution is increasingly serious in China, and highly threatens plant productivity. In this study, we summarized the relationship between relative crop yield or woody plant biomass and O metrics, including M7 (the mean of hourly concentrations from 09:00 to 16:00), SUM06 (sum of hourly concentrations above 60 nmol·mol), W126 (Sigmoidal weighted sum of the hourly concentrations during a specified period), POD[phytotoxic O dose above a threshold flux of nmol·(m·s)], and the most commonly used AOT40 (accumulated O concentrations over an hourly threshold of 40 nmol·mol), and proposed their critical level to protect plants against O damage. Based on the AOT40 metric, we found that the O risk threshold that caused a 5% decrease in yield was 5.

Precision saturated absorption spectroscopy of H.

In our previous work on the Lamb-dips of the ν fundamental band transitions of H, the saturated absorption spectrum was obtained by third-derivative spectroscopy using frequency modulation with an optical parametric oscillator (OPO). However, frequency modulation also caused errors in the absolute frequency determination. To solve this problem, we built a tunable offset locking system to lock the pump frequency of the OPO to an iodine-stabilized Nd:YAG laser.

Relative carrier-envelope phase stabilization of hybridly synchronized ultrafast Yb and Er fiber-laser systems with the feed-forward scheme.

Stabilization of the relative carrier-envelope (CE) phase for hybridly synchronized two-color fs Yb and Er fiber-laser systems is demonstrated for the first time by utilizing the feed-forward scheme based on an acousto-optic frequency shifter. The slow drift issues arising from the feed-forward scheme are solved by adding the in-loop relative CE frequency coarse stabilization via modulating the laser pump current. Sub-fs timing locking between the two-color pulses is still maintained due to the fast response and large locking range of hybrid synchronization.

Terahertz plasmonic waveguide based on metal rod arrays for nanofilm sensing.

A high-aspect-ratio metallic rod array is demonstrated to generate and propagate highly confined terahertz (THz) surface plasmonic waves under end-fire excitation. The transverse modal power distribution and spectral properties of the bound THz plasmonic wave are characterized in two metallic rod arrays with different periods and in two configurations with and without attaching a subwavelength superstrate. The integrated metallic rod array-based waveguide can be used to sense the various thin films deposited on the polypropylene superstrate based on the phase-sensitive mechanism.

Adiabatic pulse propagation in a dispersion-increasing fiber for spectral compression exceeding the fiber dispersion ratio limitation.

Adiabatic soliton spectral compression in a dispersion-increasing fiber (DIF) with a linear dispersion ramp is studied both numerically and experimentally. The anticipated maximum spectral compression ratio (SCR) would be limited by the ratio of the DIF output to the input dispersion values. However, our numerical analyses indicate that SCR greater than the DIF dispersion ratio is feasible, provided the input pulse duration is shorter than a threshold value along with adequate pulse energy control.

Hybrid terahertz plasmonic waveguide for sensing applications.

The suitability of a terahertz plasmonic sensor for sensing applications is successfully demonstrated using a hybrid planar waveguide composed of a subwavelength plastic ribbon waveguide and a diffraction metal grating. The subwavelength-confined terahertz plasmons on the hybrid waveguide resonantly reflect from the periodic metal structure under phase-matched conditions and perform resonant transmission dips. The resonant plasmonic frequencies are found to be strongly dependent on the refractive indices and thicknesses of analytes laid on the hybrid planar waveguide.

Precision frequency metrology of helium 2(1)S(0)→2(1)P(1) transition.

We report a precision frequency measurement of the (4)He 2(1)S(0)→2(1)P(1) transition at 2058 nm. The saturated absorption spectroscopy is performed in a rf discharge sealed-off cell with a volume Bragg grating-based Tm:Ho:YLF laser. The absolute transition frequency measured using a fiber optical frequency comb is 145 622 892 822 (183) kHz with a relative uncertainty of 1.

Subwavelength confined terahertz waves on planar waveguides using metallic gratings.

A terahertz plasmonic waveguide is experimentally demonstrated using a plastic ribbon waveguide integrated with a diffraction metal grating to approach subwavelength-scaled confinement and long-distance delivery. Appropriately adjusting the metal-thickness and the periodical slit width of a grating greatly improves both guiding ability and field confinement in the hybrid waveguide structure. The measured lateral decay length of the bound terahertz surface waves on the hybrid waveguide can be reduced to less than λ/4 after propagating a waveguide of around 50mm-long in length.

High-resolution sub-Doppler Lamb dips of the ν2 fundamental band of H3(+).

The high-resolution sub-Doppler Lamb dips of the ν2 fundamental band transitions of H3(+) have been observed using an extended negative glow discharge tube as an ion source and a periodically poled lithium niobate optical parametric oscillator as a radiation source. The absolute frequency of the R(1,0) transition was measured to be 81,720,371.550 MHz with an accuracy of 250 kHz using an optical frequency comb.

Terahertz refractive index sensors using dielectric pipe waveguides.

A dielectric pipe waveguide is successfully demonstrated as a terahertz refractive index sensor for powder and liquid-vapor sensing. Without additional engineered structures, a simple pipe waveguide can act as a terahertz resonator based on anti-resonant reflecting guidance, forming multiple resonant transmission-dips. Loading various powders in the ring-cladding or inserting different vapors into the hollow core of the pipe waveguide leads to a significant shift of resonant frequency, and the spectral shift is related to the refractive-index change.

Application of metal-clad antiresonant reflecting hollow waveguides to tunable terahertz notch filter.

A novel tunable terahertz notch filter is demonstrated using antiresonant reflecting hollow waveguides with movable metal layers outside dielectric claddings. Based on the Fabry-Pérot resonance of the dielectric cladding, multiple deep notches are observed in a broad THz transmission spectrum. Continuous shift of notch frequencies is for the first time experimentally observed by lateral translation of metal layers from dielectric claddings.

Subwavelength film sensing based on terahertz anti-resonant reflecting hollow waveguides.

A simple dielectric hollow-tube has been experimentally demonstrated at terahertz range for bio-molecular layer sensing based on the anti-resonant reflecting wave-guidance mechanism. We experimentally study the dependence of thin-film detection sensitivity on the optical geometrical parameters of tubes, different thicknesses and tube wall refractive indices, and on different resonant frequencies. A polypropylene hollow-tube with optimized sensitivity of 0.

Modal characteristics of antiresonant reflecting pipe waveguides for terahertz waveguiding.

Modal characteristics of the THz pipe waveguide, which is a thin pipe consisting of a large air core and a thin dielectric layer with uniform but low index, are investigated. Modal indices and attenuation constants are calculated for various core diameters, cladding thicknesses, and cladding refractive indices. Numerical results reveal that the guiding mechanism of the leaky core modes, which transmit most of the power in the air-core region, is that of the antiresonant reflecting guiding.

A terahertz plastic wire based evanescent field sensor for high sensitivity liquid detection.

A highly sensitive detection method based on the evanescent wave of a terahertz subwavelength plastic wire was demonstrated for liquid sensing. Terahertz power spreading outside the wire core makes the waveguide dispersion sensitive to the cladding index variation, resulting in a considerable deviation of waveguide dispersion. Two liquids with transparent appearances, water and alcohol, are easily distinguished based on the waveguide dispersion, which is consistent with theoretical predictions.

Semi-automatic, octave-spanning optical frequency counter.

This work presents and demonstrates a semi-automatic optical frequency counter with octave-spanning counting capability using two fiber laser combs operated at different repetition rates. Monochromators are utilized to provide an approximate frequency of the laser under measurement to determine the mode number difference between the two laser combs. The exact mode number of the beating comb line is obtained from the mode number difference and the measured beat frequencies.

Determination of absolute mode number using two mode-locked laser combs in optical frequency metrology.

The mode number of the comb line involved in the optical frequency measurement was determined using two mode-locked, erbium-doped fiber laser combs operated at different repetition rates independently of the frequency fluctuation of the laser under measurement (LUM). A simple measurement process is presented to determine the difference in mode number between the two laser combs and the comb mode number, yielding the absolute frequency of the LUM.

Absolute frequency measurement of rubidium 5S-7S two-photon transitions with a femtosecond laser comb.

The absolute frequencies of rubidium 5S-7S two-photon transitions at 760 nm are measured to an accuracy of 20 kHz with an optical frequency comb based on a mode-locked femtosecond Ti:sapphire laser. The rubidium 5S-7S two-photon transitions are potential candidates for frequency standards and serve as important optical frequency standards for telecommunication applications. The accuracy of the hyperfine constant of the 7S1/2 state is improved by a factor of 5 in comparison with previous results.

Diode-laser frequency stabilization by two-frequency Doppler-broadened absorption spectroscopy.

We demonstrate a robust method for frequency stabilization of a diode laser by two Doppler-broadened absorption spectra of the cesium D2 line. This technique employs an acousto-optical modulator to generate another frequency component from a diode laser to perform the spectroscopy. The 852-nm diode laser with frequency stabilization at the zero crossing of the error signal showed a peak-to-peak fluctuation of 800 kHz compared with a frequency-stabilized femtosecond laser over a 2-h period.