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Graphene exhibits great potential as an additive to enhance the anti-wear and friction reduction capacity of moving mechanical components in a synergistic mechanism with the base oil. This paper considers the effect of different factors such as the number of base oil molecules, graphene content, normal load, sliding velocity and the presence of graphene. The synergistic mechanism of graphene and base oil is investigated by experiments and molecular dynamics (MD) simulations. The results show that the friction and wear reduction is due to the formation of the load-supporting graphene layers and sufficient base oil molecules between Fe slabs. Graphene can stably adsorb on the rubbing surfaces lubricated by the base oil, confirming that graphene can form a physical deposition film on rubbing surfaces. Low friction and wear can be achieved with higher sliding velocity and lower load. Furthermore, compared to the sliding velocity, the load significantly affects the mean square displacement of base oil and oleic acid molecules. These outcomes provide a better understanding of the tribological properties of graphene as a lubricant additive.

The high cabled rate of urban power grid aggravates the risk of resonance and harmonic amplification. In addition, the system side of DC receiving end urban power grid contains converter station high-power harmonic sources, while the user side contains other nonlinear user harmonic sources. There are two-way conduction characteristics of harmonics in transmission lines. The problem of resonance and harmonic amplification in distribution network is aggravated by high cable ratio. Therefore, the response of the system side injected harmonic voltage at the end of the line and the potential harmonic amplification problem after the user side injected harmonic current are studied respectively. The influence of key factors (line type, length, load rate and power factor) on harmonic propagation is analyzed. The characteristic harmonic of converter station is not amplified and the most severe condition of amplification is summarized. The harmonic limit of the harmonic injection terminal is also proposed.

Among the methods used for mitigation of harmonic resonance, modal analysis theory and modal sensitivity can be used to illuminate the nature and properties of this phenomenon, and are thus widely applied. In this paper, an improved modal sensitivity-based method is proposed to reliably suppress the harmonic resonance. First, we show that the current resonance mitigation schemes that are guided by traditional modal sensitivity are not always robust in some cases. Then, through a theoretical analysis, the roots of the shortcomings of the traditional modal sensitivity are determined. The key factor is that the modal sensitivity of a network component is not always constant but tends to vary with adjustment of the component parameters. Finally, to quantify this sensitivity variation and improve the robustness of the analysis results, a second-order modal sensitivity and a critical index are proposed. The results of the test system indicate that the proposed indexes can accurately show the effects of each network component on the resonance, thus are useful in the design of the corresponding mitigation scheme.

Practicability of CsSnI3 has been limited by the lack of stability in humid environment. Here we calculated phase stability, the impact of H2O embedded in CsSnI3 and surface adsorption by the first-principles calculations. Total energy results show that γ-CsSnI3 and Y-CsSnI3 are more stable. However, the γ-CsSnI3 is hydrophilic-like and Y-CsSnI3 is hydrophobic-like. Further studies show that H2O tend to adsorb at the hollow sites on the (001) surface. The strong coupling between O and Cs atoms and hydrogen bonding render the deformation of (001) surface. These results are partially explained the phase instability of perovskites in air.

With the increasing connecting of photovoltaic systems, the harmonic pollution in power grid is deteriorated. The evaluation of the harmonic emission levels for photovoltaic system is significant for harmonic mitigation. However, when the background harmonics are unstable, traditional evaluation methods have large calculation errors. To solve this problem, a method based on the relevant vector machine is proposed in this paper. Compared with the traditional vector machine based method, the novel one can reach a high calculation accuracy, even when the background harmonics is relatively unstable. The validity for our proposed method is verified by simulation analysis.

We propose a new scheme of attosecond pulse generation by starting from a narrow-band transform-limited high-power solid-state laser and phase lock the fundamental and its first four cascaded harmonics generated by the second-order nonlinear optical processes. The relative phase among the optical fields of the harmonics can be maintained a constant at least for thousands of nanosecond pulses. The worst-case relative phase fluctuation is 0.04π rad. It is shown that sub-single-cycle (~ 0.37 cycle) sub-femtosecond (360 attosecond) pulses with carrier-envelope phase (CEP) control can be generated in this manner. The peak intensity of each pulse exceeds 1014 W/cm2 when it is focused to a spot size of 20 μm. We also demonstrate synthesis of square and saw-tooth waveforms.

In this investigation, we propose and demonstrate a stable and wavelength-tunable erbium-doped fiber (EDF) ring laser scheme with external-injected Fabry-Perot laser diode (FP-LD) technology, in single-longitudinal-mode (SLM) output behavior. Here, the output power and side-mode suppression ratio (SMSR) of the proposed laser scheme can be obtained between –3.9 and 1.3 dBm and 30.1 and 50.5 dB in the operating wavelengths of 1523.65 and 1561.50 nm with 1.12 nm tuning step, respectively, according to the output mode-spacing of FP-LD used. Besides, the lasing stabilities of output power and wavelength are also investigated and discussed.

We show experimentally and theoretically that the spectral profile of the seed pulse of an Yb-doped fibre-based chirped pulse amplifier (CPA) plays a dominant role for achieving an ultrashort compressed pulse. The picosecond seed pulse from an all-normal dispersion Yb-doped fibre laser can be compressed to 350 fs with a peak power of 25 MW. Better compressed pulse quality and a shorter pulse was achieved by the spectrally shaped CPA system as opposed to the system without spectral shaping.

We investigated the characteristics of sub-picosecond radiation pulses from mid-size-gap multi-energy arsenic-ion-implanted GaAs photoconductive antennas in terahertz time-domain spectroscopy experiments. At fixed fluence excitation, both the bandwidth and the negative peak of terahertz waveform minimum reveal increasing trends with increases in the antenna's gap. We reproduce both the measured terahertz waveforms and increasing trends numerically, by calculating a model that incorporates the trap-enhanced bias field with wave equation. We deduce from our simulation that the gap-dependent behavior stems from the fact that an antenna with a larger gap has a stronger trap-enhanced bias field near the anode edge and thereby induces a larger space-charge screening effect and bandwidth.

Deep x-ray lithography (DXL) was used to fabricate two-dimensional photonic crystals (PCs) for a terahertz high-pass filter. Instead of the conventional poly(methyl methacrylate) resist, high-sensitivity SU-8 resist was used to pattern the ultra-deep (1 mm), and high aspect ratio (>30) PC structures. In this study, an x-ray mask with thick Si membrane was used in the DXL process owing to the high-sensitivity nature of SU-8. The robust mask structure can significantly improve the fabrication yield of the conventional x-ray mask with a very thin (< 2 µm) membrane. Preliminary results demonstrate that SU-8 has poor pattern definition after DXL process, probably due to its high sensitivity. This phenomenon can be eliminated by dissolving oxygen into the resist to 'quench' the excessive photochemical reaction. Thanks to the high adhesion property of SU-8, the sticking problem of the high aspect ratio crystals is easily solved with natural drying by reducing the unbalanced capillary force. After optimizing all the processes, the proposed DXL SU-8 technique successively fabricated the terahertz crystals with high efficiency, high precision (< 1 µm) and high surface quality (Ra ~12 nm). The resist pillars were then deposited with a 300 nm thick gold layer for subsequent terahertz measurement. The measurement results show that the PC structure acts as a high-pass filter in the terahertz range, agreeing with the simulation results.

We performed experimental and theoretical investigations on the transverse-mode emission behavior of a broad-area oxide-confined vertical-cavity surface-emitting laser (VCSEL). The far-field emission pattern with y-junction structures is observed in the VCSEL with a 20-µm-diameter aperture when the injection current is significantly above the thermal roll-over point. In order to provide a quantitative understanding of these emission characteristics, we present a numerical simulation to model and fit the experimental results. The numerical simulation adopts the high-order Laguerre-Gaussian mode due to the assumptions of a parabolic refractive-index profile and ring-shaped carrier distribution in the laser. From the numerical simulation results, the formation of y-junction structures is inferred to be caused by the superposition of two high-order Laguerre-Gaussian modes. The results of this numerical simulation are in good agreement with the experimental findings.

We present experimental data characterizing the first-observed transverse mode with y-junction structures in a broad-area (20 µm in diameter) oxide-confined vertical-cavity surface-emitting laser at a 22.3 mA injection current which is larger than the thermal roll-over point. The formation of the y-junction structured pattern, which can be observed in far-field images, is due to an interaction between adjacent high-order transverse modes.

We report on the ultrahigh-resolution optical coherence tomography (OCT) with a novel high-power supercontinuum (SC) light source generated by noise-like pulses from an Yb-doped fiber laser. The SC spectrum is flat with a bandwidth of 420 nm centered around ~1.3 μm. The light source is successfully employed in a time-domain OCT (TD-OCT), achieving an axial resolution of 2.3 μm. High resolution fiber-based spectral-domain OCT (SD-OCT) imaging of bio-tissue was also demonstrated.

An average power as high as 60 W with 73 W of pumping was achieved for an ytterbium-doped fibre-based dual-stage amplifier (MOFA) system seeded by a diode-pumped solid-state (DPSS) laser. The corresponding optical conversion efficiency is 80%. The laser system generates a steady pulse train with a pulse width of 11 ps at a repetition rate of 250 MHz or a peak power of 21.8 kW. Moreover, the output beam quality M2 ≈ 1.6. The length and pumping power for the Yb-doped fibres were optimized to suppress stimulated Raman scattering (SRS) and amplified spontaneous emission (ASE) while maintaining desirable output characteristics.

We investigate generation of the third harmonic (TH; λ = 355 nm) signal by two-colour excitation (λ = 1064 nm and its second harmonic, λ = 532 nm) in argon gas, with emphasis on the influence of relative phases and intensities of the two-colour pump on the third-order nonlinear frequency conversion process. Perturbative nonlinear optics predicts that the TH signal will oscillate periodically with the relative phases of the two-colour driving laser fields due to the interference of TH signals from a direct third-harmonic-generation (THG) channel and a four-wave mixing (FWM) channel. For the first time, we show unequivocal experimental evidence of this effect. A modulation level as high as 0.35 is achieved by waveform control of the two-colour laser field. The modulation also offers a promising way to retrieve the relative phase value of the two-colour laser field.

We report the generation of tunable high-energy noise-like pulses with a super-broadband spectrum from a Yb-doped dispersion-mapped fiber ring laser. Self-starting noise-like operation can be maintained over a relatively large range of pumping powers (4–13 W). The corresponding output power varies from 0.1 to 1.45 W. The maximum 3 dB spectral bandwidth of the noise-like pulses is about 48.2 nm while the output energy is as high as 47 nJ, limited by optical damage of the components. The central wavelength of the noise-like pulses can be tuned easily over ~12 nm. The bandwidth and duration of the generated wave packets can also be controlled. The use of a negative dispersion-delay line and spectral filter are found to be important for generating such high-power noise-like operation. Experimental results are in good agreement with theoretical simulations.

Super-fine ink-jet (SIJ) printing technology is applied to the fabrication of terahertz metamaterials. A silver film is fabricated using an SIJ printer with silver paste ink, and it is confirmed that the film behaves as a good conductor in the terahertz frequency region. Then, basic terahertz metamaterials such as metal wire-grid structures and split-ring resonators are printed on high-resistivity silicon substrates. The terahertz responses of the printed samples agree with those expected from their structures. SIJ printing is one of the ideal methods for fabricating terahertz metamaterials owing to its rapidity, simplicity, flexibility, and sufficient accuracy.

A strained saturable Bragg reflector (SSBR) for passive mode-locking of Ti:sapphire lasers was investigated by the frequency resolved optical gating (FROG) technique. Incident pulses of several wavelengths and with zero, positive, or negative chirp were employed. A considerable pulse shortening near the excitonic resonance of the strained quantum well was observed and attributed to anomalous dispersion due to resonance absorption. On the long wavelength side of exciton resonance, however, the chirp of the reflected pulse showed a weak wavelength dependence attributed to the material dispersion of SSBR in pulse broadening was obtained.

Nonlinear optical properties of the undoped hybrid vapor phase epitaxy-grown (GaN film sample) were investigated and analyzed using a tunable femtosecond laser as an excitation source. As the laser wavelength was tuned from 762–796 nm, the sample emitted signals corresponding to the second-harmonic wavelength from 381–398 nm with a quadratic power dependence. A strong three-photon-process-related signal at 367 nm with cubic power dependence was simultaneously observed for the first time.

A pump-probe study of GaAs grown by molecular beam epitaxy at low temperatures (LT-GaAs) is performed. Ultrashort carrier lifetimes of the as-grown LT-GaAs increase from < 0.13 ps (measurement limitation) to 1.8±0.2 ps as the growth temperature is increased from 200°C to 320°C. The carrier lifetime was found to be approximately inversely proportional to the antisite defect concentration. This trend is found to be in reasonably good agreement with the Schokley–Read–Hall model. The decreasing trend in the amplitudes of continuous-wave and transient reflectivities (ΔR/R) as a function of the growth temperature for the LT-GaAs is explained as an induced absorption caused by dense arsenic antisite defects. The sign of the transient ΔR/R reversed for LT-GaAs grown at 200°C. This is tentatively attributed to the band gap renormalization effect.

We report the effects of annealing temperature on the near-band-gap transmittance, the absorption coefficient, and the evolution of shallow-level defects of arsenic-ion-implanted GaAs (hereafter referred to as GaAs:As+) using Fourier-transform infrared spectroscopy. The maximum above-band-gap transmittance and absorption coefficient are determined to be 0.4 and 6×103 cm-1, respectively. An anomalous absorption peak at the photon energy of 1.37–1.4 eV for the GaAs:As+ samples annealed at lower than 500°C is observed. By either fitting the absorption curve, the band-gap energy of rapid-thermal-annealed GaAs:As+ is evaluated, which blue shifts from 1.36 eV to 1.41 eV as the annealing temperature increases from 300°C to 800°C. The slightly rising absorption spectra at the near-band-gap region suggest the existence of defects and scattering centers. The activation energies and formatting mechanisms of the dense defects in the Rapid-thermal-annealed GaAs:As+ samples were identified. By characterizing the differential absorption spectra, the eVolution of some defect-related absorption bands during the high-temperature annealing process were observed and the correlated structural phenomena were explained.

Optically excited THz-radiation from ten molecular-beam epitaxy (MBE)-grown strained multiple quantum wells (MQWs) on a (100)-oriented semi-insulating GaAs substrate is studied in a 1-T magnetic field. Wavelength-dependent THz-radiation clearly exhibits a resonance behavior at the excitation wavelength near 830 nm, the peak wavelength of photoluminescence (PL). We also observe and explain an emission spectrum from the MQW with slightly enhanced higher-frequency components compared to that of bulk GaAs. In the thin MQW sample, the THz emission is still significant. This indicates the possibility of designing MQWs as efficient THz emitters in the future.

Terahertz (THz) radiation is generated efficiently from a bulk InAs mirror with a shallow incidence angle inside the cavity of a femtosecond, mode-locked Ti:sapphire laser self-started by a strained saturable Bragg reflector. A magnetic field is also applied to the InAs mirror to enhance THz radiation.

We demonstrate a new configuration for intracavity generation of THz radiation. A magnetic-field-biased saturable Bragg reflector (SBR) located inside the femtosecond laser cavity is the emitter, while a strained saturable Bragg reflector (SSBR) achieves self-started mode-locking without focusing. The calibrated power of the emitted THz radiation is estimated to be approximately 45 nW with a peak frequency at 0.72 THz and width of approximately 0.7 THz under a 0.88 T magnetic field. The quadratic dependence of THz-radiation power by the SBR on the magnetic field is also observed for the first time.

We demonstrate a new scheme for the generation of high average power femtosecond pulses by incorporating an intracavity amplifier and a strained saturable Bragg reflector with low saturation fluence for self-starting mode locking. When the Ti:sapphire oscillator and intracavity amplifier are pumped at 10 W and 15 W, respectively, the average output power is as high as 1.62 W. The pulse duration is about 145 fs, and the peak power reaches 160 kW at a 68 MHz repetition rate. The pulse-formation time of 400 µs is sufficiently short to sustain stable mode locking.

We present a new, more versatile scheme for maintaining phase coherence between optical probe pulses and microwave signals under test in a laser-diode-based optoelectronic sampling system. This approach takes advantage of the relative ease of changing the repetition rate of the gain-switched laser diode (GSLD). The microwave signal to be sampled is used as the clock signal for phase-locking of a voltage controlled-oscillator (VCO) which drives the GSLD. Phase-tracking and display of waveforms at frequencies up to 8 GHz have been realized. It is also shown that lower phase noise sidebands for the phase-locked VCO can be realized by locking the higher harmonics of the laser pulse to the microwave clock.

We report a laser-diode-based system for optoelectronic synchronization of two distributed microwave oscillators at 10-GHz. The relative phase between the 10-GHz signals can be continuously tuned over a complete cycle by an optical delay line. Phase-locking of the two oscillators with a frequency offset of 10 kHz is also demonstrated. This method can be used to synchronize millimeter-wave oscillators for distributed architectures via fiber-optic links in the future.

A relative frequency statilization technique, using double fiber resonators as the frequency discriminator, has been demonstrated. By employing the frequency-division-multiplexing technique (FDM) to the laser diodes, the frequency offset between the tested laser diodes has been stabilized to Δν/ν≤3.0×10-9.

We demonstrated a near-infrared transmittance setup for EL2 concentration mapping in undoped semi-insulating (SI) GaAs wafers. The light source was a power-stabilized laser diode (λ=1.3 µm) operating in the light emitting diode (LED) mode. The sensitivity of the apparatus was as low as 1014 cm-3 for typical commercial wafers 350 µm in thickness.