Applied Physics B

Một kỹ thuật sử dụng công tắc silicon nhạy sáng được mô tả nhằm khắc phục các hiệu ứng electron lạc trong camera streak. Kỹ thuật này được áp dụng trong các nghiên cứu huỳnh quang ở picosecond về quá trình truyền tải điện tích nội phân tử trong một hợp chất thuộc loại A-(CH2)3-D (A: anthracene, D: dimethylaniline), nơi trạng thái kích thích cục bộ và trạng thái exciplex được hình thành một cách tuần tự.

This paper reports on the experimental investigation and the numerical simulation of the spatio-temporal dynamics of the pulse formation in nanosecond optical parametric oscillators. The temporal evolution of the spatial intensity distribution is measured with a fast two-dimensional CCD-camera. The measurements are performed for a signal-resonant nanosecond pulsed optical-parametric-oscillator (OPO) which consists of a 12 mm long, critically phase-matched beta-barium-borate (BBO)-crystal in a 40 mm long cavity of two plane mirrors. The OPO was pumped by the third harmonic of a flashlamp-pumped Nd:YAG-Laser at a repetition rate of 10 Hz. At pump energies close to the OPO threshold the emitted OPO-radiation has an almost gaussian intensity distribution throughout the entire pulse. The beam quality factor M2 remains below 2.2. At high pump energies the OPO oscillation also starts with an almost gaussian beam-profile. During the build-up and parametric amplification of the pulse the fields experience, however, a spatially inhomogeneous gain, caused by walk-off in the birefringent crystal, pump-depletion, and back-conversion. The spatial intensity distribution thus becomes asymmetric and the M2 value increases. The measurements are compared with the results of detailed numerical calculations. The model takes the amplification of the OPO-radiation in the nonlinear crystal, and the properties of the OPO cavity as well as the diffraction of the beams during propagation into account. The spatio-temporal dynamics of the pulse formation predicted by the numerical model are in good agreement with the experimental results.

Chụp cắt lớp quang học (OCT) đã cho thấy tiềm năng cho các ứng dụng quan trọng trong lĩnh vực bảo tồn nghệ thuật và khảo cổ học nhờ khả năng hình ảnh hóa vi cấu trúc bên dưới bề mặt một cách không xâm lấn. Tuy nhiên, độ sâu thâm nhập của nó trong các vật thể được vẽ bị hạn chế do tính chất tán xạ mạnh mẽ của các loại sơn của các nghệ sĩ. Quang phổ phản xạ VIS-NIR (400–2.400 nm) của nhiều loại sơn được chế tạo từ các chất màu của các nghệ sĩ lịch sử đã được đo. Cửa sổ quang phổ tốt nhất để sử dụng OCT cho việc hình ảnh hóa cấu trúc bên dưới của các bức tranh được phát hiện là khoảng 2,2 μm. Cửa sổ quang phổ này cũng sẽ phù hợp nhất cho việc hình ảnh hồng ngoại trực tiếp của các phác thảo chuẩn bị dưới các lớp sơn. Các quang phổ phản xạ từ một mẫu lớn các chất màu đã được xác minh về hóa học cung cấp thông tin về tính trong suốt quang phổ của các chất màu/sơn của các nghệ sĩ lịch sử, cũng như một bộ tham khảo về quang phổ để nhận diện chất màu. Các kết quả của bài báo cho thấy rằng các nguồn băng tần rộng ở ~2 μm là rất được ưa chuộng cho các ứng dụng OCT trong nghệ thuật và tiềm năng trong khoa học vật liệu nói chung.

A single-particle fluorescence spectrometer (SPFS) and an aerodynamic particle sizer were used to measure the fluorescence spectra and particle size distribution from the particulate emissions of 12 different burning materials in a tube furnace to simulate open-air burning of garbage. Although the particulate emissions are likely dominated by particles <1 μm diameter, only the spectra of supermicron particles were measured here. The overall fluorescence spectral profiles exhibit either one or two broad bands peaked around 300–450 nm within the 280–650 nm spectral range, when the particles are illuminated with a 263-nm laser. Different burning materials have different profiles, some of them (cigarette, hair, uniform, paper, and plastics) show small changes during the burning process, and while others (beef, bread, carrot, Styrofoam, and wood) show big variations, which initially exhibit a single UV peak (around 310–340 nm) and a long shoulder in visible, and then gradually evolve into a bimodal spectrum with another visible peak (around 430–450 nm) having increasing intensity during the burning process. These spectral profiles could mainly derive from polycyclic aromatic hydrocarbons with the combinations of tyrosine-like, tryptophan-like, and other humic-like substances. About 68 % of these single-particle fluorescence spectra can be grouped into 10 clustered spectral templates that are derived from the spectra of millions of atmospheric aerosol particles observed in three locations; while the others, particularly these bimodal spectra, do not fall into any of the 10 templates. Therefore, the spectra from particulate emissions of burning materials can be easily discriminated from that of common atmospheric aerosol particles. The SFFS technology could be a good tool for monitoring burning pit emissions and possibly for distinguishing them from atmospheric aerosol particles.

Optical Kerr nonlinearity (n2) in n-type indium tin oxide (ITO) films coated on glass substrates has been measured using Z-scans with 200-fs laser pulses at wavelengths ranging from 720 to 780 nm. The magnitudes of the measured nonlinearity in the ITO films were found to be dependent on the carrier concentration with a maximum n2-value of 4.1×10-5 cm2/GW at 720-nm wavelength and an electron density of Nd=5.8×1020 cm-3. The Kerr nonlinearity was also observed to be varied with the laser wavelength. By employing a femtosecond time-resolved optical Kerr effect (OKE) technique, the relaxation time of OKE in the ITO films is determined to be ∼1 ps. These findings suggest that the Kerr nonlinearity in ITO can be tailored by controlling the carrier concentration, which should be highly desirable in optoelectronic devices for ultrafast all-optical switching.

The unimolecular decay of the hydrogen-bonded phenol(H2O)4 + cluster ion was studied both experimentally and theoretically. The ion is formed by resonant two-color two-photon ionization of the neutral cluster. If the second color exceeds the dissociation threshold of the ion, fragmentation occurs on a microsecond time scale. By analyzing the time-of-flight (TOF) signal of the metastable ions at different excess energies the dependence of the fragmentation rate on the excess energy can be deduced. These experimental rates are compared to microcanonical decay constants k(E) calculated from Rice–Ramsperger–Kassel–Marcus (RRKM) theory based on ab initio data solely.

Heating and UV illumination reduce the coercive field of lithium niobate crystals. It is found that both effects add up, leading to the conclusion that the underlying mechanisms are different. We present arguments that heating leads to an increased ionic conductivity that changes defect structures which pin domain walls, while UV illumination enables electrical screening of the pinning defects by photo-excited electrons.

laser by nonlinear upconversion, is reported for the first time in AgGaSe2 crystal under near noncritical phase-matching, pumped by an electro-optically Q-switched Nd:YAG laser operated at 1.318 μm. The capability for low-level infrared signal detection at room temperature with a fast response was also studied.