Applied Physics B最新文献

This paper demonstrates the characteristics of the THz parametric frequency upconversion detection based on the stimulated polariton scattering in KTiOAsO₄ (KTA) crystal pumped by a 1064 nm pulsed laser (420 ps, 1 Hz). The detectable spectral ranges can be from 3.54 to 4.03 THz, from 4.08 to 4.50 THz, from 4.71 to 5.16 THz, and from 5.87 to 6.32 THz. The minimum detectable energy is 0.24 fJ at 4.92 THz with a dynamic range of 57 dB.

A compact high-energy nanosecond (ns) slab Yb:YAG Master Oscillator Power-Amplifier (MOPA) laser system with high beam quality is proposed and demonstrated. The MOPA system consists of a Q-switched rod Yb:YAG oscillator and two cascaded large aspect ratio slab (CLARS) Yb:YAG amplifiers. The oscillator delivers a pulse energy of 2.4 mJ at a pulse repetition rate (PRR) of 100 Hz, with a pulse duration of 52 ns. The seed pulse is then amplified to 138 mJ with a gain factor of 57.5. The experimental data is in good agreement with the numerical simulation. The average beam quality factor β is 1.62. This laser system is applicable in scientific and industrial fields. The layout area of the entire system is only 900 mm × 450 mm.

We present a comparative study of continuous-wave (CW) and pulsed optical tweezers for trapping polystyrene beads with radii of 250 nm, 500 nm, and 1 μm, using five different laser power settings. In this study we also emphasize the importance of using femtosecond laser, which uses intense peak power pulses to generate larger gradient forces than a CW laser at very low and same average power. We could also achieve stable trapping of 250 nm particle at very low average powers of femtosecond laser. A Ti: Sapphire (MIRA 900 F) laser, capable of seamlessly switching between CW and pulsed modes, was used to ensure identical experimental conditions for both cases. The trap strength in each mode was determined by fitting the power spectrum to a Lorentzian curve. Our results show that pulsed tweezers are more effective for smaller particles, while for larger particles, both CW and pulsed tweezers perform similarly at lower laser powers. However, as the power increases, pulsed tweezers provide more stable trapping.

In this paper, a sensing detection method based on the fiber wedge angle microstructure for coupling excitation of microtubular cavity resonance was proposed. The theoretical model of coupling resonance between the fiber wedge end-face and microtubular cavity was established. The sensing detection mechanism of WGM resonance in the capillary microreactor was investigated. Through the simulation analysis of the factors influencing coupling efficiency and the sensing sensitivity by the capillary microcavity WGM coupling resonant structure, the structural parameters of the coupling resonant system were optimized. The experimental system was established for testing resonant excitation characteristics under different coupling structure parameters, and detecting the RI sensing of solution. The proposed capillary micro-cavity resonant detection system has good robustness, simple resonant excitation structure, and easy integration and practical application. The established theoretical model and analysis results provide new ideas for the application of optical fiber microstructure in capillary microreactor biochemical assay applications.

Magnetic nanoparticles and an in-line no-core fiber (NCF) Mach Zehnder interferometer are combined to create a unique magnetic field filter that is experimentally demonstrated and theoretically investigated by COMSOL6.1. Multiphysics, multimode interference (MMI), and self-image form the basis of the operation. The cascaded single-mode, no-core, single-mode (SNCS) fiber design of a tunable filter powered by magneto-optical fluids (MOF) was described using the technique of finite element modeling (FEM). Three materials were used as MOFs. The maximum wavelength tunability of the filter is around 113 nm. From 1538 to 1651 nm, the transmission light is increased and red shifted when the magnetic field increases from 0 to 75 mT, as a result of the RI of the surrounding MOF being increased. Experimentally found the maximum tunability of the magnetic field filter was about 19.2 nm (1537.7–1556.9 nm) at concentration 0.4%. When the magnetic field is changed from 0 to 30 mT, change 5 mT each step. The relationships between the transmission dips and magnetic field exhibit a very strong linear response, which is a necessary condition for the practical filter. This device has a wide tuning range, is very reliable, and is inexpensive when compared to other tuning methods. The device can be used in optical communication, fiber laser technology, spectroscopy, and fiber sensors. To the best of our knowledge, this is the first No-Core fiber-based magnetic field MMI tunable filter to be produced experimentally.

Laser processing parameters are an important factor determining the surface quality during the process of micro pit surface texture forming. Box-Behnken (BBD) and orthogonal method were used to design the parameters of laser machining, as well as the relationship between energy, frequency, pulse width and repetition times on micro pit forming were studied. The response surface method (RSM) model and BP neural network model optimized by gray wolf algorithm (GWO) were established respectively, and the processing parameters were predicted and optimized. The optimal laser processing parameters obtained by GWO-BP model were 333.5 μm and 28.8 μm, which are higher than 254.2 μm and 21.4 μm obtained by RSM model. The results showed that the prediction and optimization ability of the GWO-BP model was better than that of the RSM model, and the BP model optimized by GWO could provide a more effective prediction method for realizing the optimal laser micro pit machining.

The quantitative measurements of the soot volume fraction can deliver important validation data for detailed models of soot inception, growth, and oxidation in hydrocarbon-fueled flames – a process still not fully understood. Therefore, the sensitive and non-intrusive optical measurement of soot levels in fuel-rich steady flames is highly desirable. This work presents the first in situ Cavity Ring-Down Extinction (CRDE) setup for monitoring soot concentration levels in the low-ppb range in premixed ethylene/air stagnation flames stabilized on a flat flame burner at elevated pressure. The high-reflectivity CRD mirrors were mounted in nitrogen-flushed chambers attached to and separated from the flame area by small diameter apertures. CFD simulations supported the dimensioning of the purge flows and visualized possible perturbation of the cold- and hot-gas (from the flame) flows inside the burner housing. Within the uncertainties of the present experimental conditions and based on results from other labs for similar flame conditions, the pressure dependence of the evaluated soot volume fraction f_V was fitted to a power-law ansatz with an exponent n between 2.1 and 2.6 depending on the equivalence ratio. A global thermal rate constant for surface growth, ({k}_{text{SG}}approx 49pm 20 {text{s}}^{-{1}}), was found for a measured flame temperature of 1750 ± 100 K in an atmospheric pressure flame, in gross agreement with results in the literature, which demonstrates the potential of the current setup for soot diagnostics in laminar premixed elevated pressure flames. A detection limit for f_V of above 40 ppt has been determined from a comparative measurement of CO₂ having a small absorption coefficient at the laser wavelength of 1064 nm.

We report efficient generation of ultrashort laser pulses in the deep ultraviolet (DUV) from a quasi-phase-matched (QPM) optical parametric oscillator (OPO) based on a periodically poled KTP (PPKTP) crystal. Up-conversion through frequency-doubling of the signal pulses (2ω_S) and sum-frequency generation between the pump and signal pulses (ω_P + ω_S) produces femtosecond pulses in the green and in the blue, respectively. A third-order QPM is responsible for the green-pulse generation and a fifth-order QPM for the blue. The cavity mirrors have two high-reflection bands, which support the oscillation of the OPO signal (ω_S) in the near infrared and the sum-frequency generation of ω_P + ω_S in the blue, respectively. The interaction between the blue and green pulses enables further cascading by a sum-frequency generation process, producing the DUV pulses. Cavity-length detuning facilitates the large tuning range of the DUV generation.