Applied Physics B最新文献

We theoretically investigate photo-excitation of electron–hole pairs and high harmonic generation in the bulk of zinc oxide (ZnO) subjected to intense femto-second laser pulses with mid-infrared wavelength. The main microscopic mechanism of solid-state HHG is identified by separating resonant from non-resonant non-linear optical responses in the photo-excited solid. We obtain an effective light-matter interaction in which electrons are subject to weak atto-second pulse train with the second harmonic of the drive laser frequency being the repetition frequency in the train. Under a condition of constructive interference of electronic transitions between consecutive half-cycles of the drive laser pulse, resonant-like excitation of electron–hole pairs occurs which contribute to the multi-cycle accumulation of photo-current. The inter-band motion of charge carriers creates rapidly oscillating electric dipole moment, which emits continuum bremsstrahlung-like radiation during each half-cycle of the drive laser pulse. If the laser-cycles of the drive pulse are identical, the emissions from consecutive half-cycles are phase-coherent, and their mutual interference produces a frequency comb in the spectral domain, which is composed of the odd-integer harmonics of the drive laser frequency. The spectral intensity of each harmonic scales with the fourth power of the total number of cycles in the drive pulse. In contrast, if consecutive cycles in the pulse are non-identical, the interference pattern in the frequency domain is less well defined and blurred. The importance of the pulse envelope for producing clean frequency combs as observed in the experiments is discussed. Good semi-quantitative agreement with the experimental data is found: clean and well defined odd-order harmonic peaks extending well beyond the band edge of ZnO are exhibited for laser linearly polarized at right angles to the optical axis of the crystal.

Characterizing both the radial (p) and azimuthal (ℓ) indices of Laguerre-Gaussian (LG) beams remains a challenge mainly due to their transverse amplitude profiles. We report a simple yet robust method for simultaneously identifying both mode indices using an off-axis parabolic mirror (OPM). The far-field diffraction pattern produced by an incident LG beam on OPM exhibits characteristic spot distributions, which can be analyzed to determine the magnitude and sign of the topological charge, as well as the radial index. Furthermore, we show that this technique exhibits excellent tolerance to beam misalignment. Experimental results show good agreement with numerical simulations.

Because of their versatility and scalability, Microwave Kinetic Inductance Detectors (MKIDs) represent a rapidly developing class of cryogenic superconducting detectors. In the context of optical photon detectors, the efficiency of MKIDs faces limitations due to the reflection of visible and near-infrared (VIS-NIR) photons by low resistivity metallic films, which prevents a significant portion of incident photons from being absorbed and detected. To address this issue, we propose the use of granular Aluminum (grAl), a disordered superconductor which can achieve resistivity as high as  10 m(Omega cdot)cm. We measure optical transmission and reflection of grAl thin films in the wavelength range 400 nm to 1100 nm, from which we infer their absorption. Compared to other commonly used superconductors in the MKIDs community, for grAl we observe comparable or superior absorption, depending on the wavelength and the film thickness, which makes it a promising alternative material.

We have observed the Rabi oscillations of the population of (^{87}Rb) atoms in the hyperfine ground state, when a cold atom cloud prepared in the hyperfine ground state ((F = 1)) was exposed to a microwave (MW) radiation field resonant to the (|F = 1, m_{F} = 0>) (rightarrow) (|F = 2, m_{F} = 0>) transition. After exposure to MW field, the atom cloud was imaged using the absorption probe method. The absorption images have revealed that population excited to the hyperfine state ((F = 2)) varied with the position in the atom cloud, showing the position dependent atom-field coupling strength. At a given position in the atom cloud, the excited population shows Rabi oscillations between two hyperfine states. Thus, we have demonstrated that by measuring the position dependent Rabi frequency, the spatial variation of magnetic field can be measured using a cold atom cloud.

Fiber-Optic Laser-Induced Breakdown Spectroscopy (FO-LIBS) technology offers excellent remote diagnostic capabilities and flexibility in complex environments, making it highly promising for monitoring the elemental distribution in wall materials of future fusion devices. This study focused on the low-pressure conditions, where a FO-LIBS experimental system was developed to systematically analyze the temporal evolution of Cu and Mo plasma spectra under pressures ranging from 0.2 to 20 Pa. The results demonstrated that the intensities of key spectral lines, such as Cu I and Mo I, show linear growth with increasing laser energy within the specified pressure range. Additionally, the intensities of these characteristic spectral lines decrease significantly as pressure rises, becoming much weaker than at atmospheric pressure. The presence of an argon atmosphere further reduces these spectral line intensities. Time-resolved measurements indicate that plasma lifetimes are approximately 400 ns under 0.2 Pa, which happens earlier than under atmospheric pressure. Calculated electron densities, estimated using the Stark broadening method, correspond with the trends in spectral line intensity variations. This research provides optimized FO-LIBS parameter selection for in situ elemental diagnostics under low-pressure environments in fusion devices.

A dual-wavelength monolithic Y-branch distributed Bragg reflector diode laser at 633 nm is presented, which is suitable for shifted excitation Raman difference spectroscopy to effectively address fluorescence and background interference. The device provides 30 mW optical output power at an electrical power consumption of less than 1 W. At a spectral distance of 0.4 nm (10 cm⁻¹), both laser emission wavelengths show narrowband operation with spectral widths of 12 pm (0.3 cm⁻¹) and side mode suppression ratios of more than 40 dB. Performing shifted excitation Raman difference spectroscopy measurements on a highly fluorescent soil sample exemplarily showed the efficient separation of characteristic Raman signals of the soil constituents quartz and calcite from intense fluorescence interference with a 17-fold improvement in the signal-to-background noise ratio in comparison to the individual Raman measurements.

We theoretically investigated the effect of non-uniform distribution of acceptor-doped atoms on high-order harmonic generation (HHG) in doped semiconductors, employing two models related to actual doped semiconductors: linear and Gaussian distributions. Compared with uniformly doped semiconductors, we found that non-uniformly doped semiconductors with impurities localized on one side of the atomic chain exhibit enhanced below-band-gap harmonics and suppressed the second plateau of harmonics. Additionally, we observed distinct even harmonics in the low-energy region of HHG from non-uniformly doped semiconductors, as non-uniform doping breaks the translational symmetry of the crystal lattice. Moreover, by analyzing the energy band structure, the time-dependent population imaging (TDPI) and time-frequency analysis, we have illustrated the reasons for the harmonic differences between non-uniformly and uniformly doped semiconductors.

In this article, an integrated optical fiber sensor is designed and experimentally demonstrated for simultaneous measurement of magnetic field, displacement, and temperature. Only a long-period fiber grating (LPFG) is inscribed onto a single-mode fiber (SMF), and the LPFG is affixed through a hole in a magneto-strictive material, which is employed to detect magnetic field. Then, a balloon-shaped structure, achieved by utilizing the hole, has ability to monitor displacement and temperature. The highest sensitivities of the sensor are obtained as 1.039 nm/mT, − 467 pm/μm and 42.92 pm/℃. By solving the matrix equation, simultaneous measurement of above-mentioned parameters is achieved. Compared with previously reported sensors, the unique structures have advantages of compact volume, high sensitivity and robust, which satisfied demands of multi-parameter detecting under condition of high-intensity magnetic field and strong electromagnetic interference.

Orientation of the principal optical axes, dispersion of the refractive indices along the three principal optical directions in the spectral range of 480–1014 nm, thermal expansion coefficients for directions along optical indicatrix axes N_p and N_m, and the principal thermooptic coefficients at a wavelengths of 632.8 nm and 1064 nm are determined for Yb³⁺:Gd_2−xY_xSiO₅ (x = 0.385) monoclinic crystal (space group of P2₁/c). The principal thermooptic coefficients are found to be negative. Thermal lensing was characterized in a continuous-wave N_p-cut, N_m-cut, and N_g-cut Yb³⁺:Gd_1.615Y_0.385SiO₅ diode-pumped at wavelength of 0.976 μm and operating at wavelength of 1.090 μm. The measured thermal lens is found to be positive. The sensitivity factors of the thermal lens were obtained.

The structure of a terahertz (THz) laser with a plasmonic waveguide based on an electron-rich GaAs/AlGaAs heterojunction is proposed. Modeling of electromagnetic modes was carried out taking into account the spatial dispersion of permittivity. Based on self-consistent calculations of electron states, taking into account the influence of the spatial charge distribution on the conduction band profile, it is shown that the current–voltage characteristic of the laser has a hysteresis type. The generation of the plasmonic mode occurs in the hysteresis region at frequencies of 1.8–4.1 THz. The gain of the plasmonic mode can reach 10⁵ cm^–1 at a temperature of 77 K.