Clustered and integrated fluorescence spectra from single atmospheric aerosol particles excited by a 263- and 351-nm laser at New Haven,CT, and Adelphi,MD

Fluorescence spectra from individual micron-sized atmospheric aerosol particles were measured by a Dual-wavelength-excitation Particle Fluorescence Spectrometer (DPFS). Particles were drawn into our laboratory at Adelphi, MD, an urban site in the Washington, DC, metroplex and within the Yale University campus at New Haven, CT. Two fluorescence spectra were obtained for every individual particle as it was moving through the DPFS system and excited sequentially by single laser pulses at 263 and 351 nm. There were around ten to a few hundred particles detected per second and up to a few million per day within the 1–10 μm particle size range. The majority of the particles have weak fluorescence, but 10–50% of the particles have fluorescence signals above the noise level at both sites at different time period. For the first time, these Ultra Violet laser-induced-fluorescence (UV-LIF) spectra from individual particles were integrated every 10 min, which forms a group of about a few thousand to a few tens of thousand particles, to provide the averaged background atmospheric fluorescence spectral profiles which may be helpful in the development of bioaerosol detection systems, particularly those systems based on integrated fluorescence from a group of aerosol particles, such as Light Detection And Rangeing (LIDAR) remotor biosensor and the point sensor based on collected particles on substrate. These integrated spectral profiles had small variations from time to time and were distinguishable from that of the bacterial simulant B. subtilis. Also for the first time, the individual spectra excited by a 351 nm laser were grouped using unstructured hierarchical cluster analysis, with parameters chosen so that spectra clustered into 8 main categories. They showed less spectral variations than that excited by a 263-nm laser. Over 98% of the spectra were able to be grouped into 8 clusters, and over 90% of the fluorescent particles were in clusters 3–5 with a fluorescence emission peak around 420–470 nm; these were mostly from biological and organic carbon-containing compounds. Integrated fluorescence spectral profiles and averaged spectra for each cluster show high similarity between New Haven, CT and Adelphi, MD.     

Performance comparison of self-focusing with 1053- and 351-nm laser pulses

Hole boring characteristics of laser beams are studied using two different laser wavelengths in preformed plasmas with overdense regions. We have shown that a whole beam self-focusing is created in plasma with a considerable density scale length using a 1 microm wavelength laser. The whole beam self-focusing of this type could be used for guiding the ultrahigh intense laser pulse to a highly compressed core for studying the feasibility of a fast ignitor. There is a clear difference in the hole-boring characteristics between two laser wavelengths at 1053 and 351 nm, both in the experiment and the simulation. Using the third-harmonic laser, a whole beam self-focusing is never created. The 351-nm laser beam broke up into filaments resulting in plasma jets observed in our interferogram.     

Frequency stabilization of a 214.5-nm ultraviolet laser

An improved method for stabilizing a frequency-quadrupled 214.5-nm tunable diode laser system is reported. Improvements to the method include a homemade logic circuit and the use of a Fabry-Perot optical spectrum analyzer as a transfer cavity. Lasers locked with this method exhibit megahertz-level frequency stability measured with an optical frequency comb referenced to a cesium atomic standard. The laser can be locked for hours to days, depending on experiment requirements. Being relatively inexpensive, stable, and robust, the control method can be applied to stabilizing essentially all lasers of deep ultraviolet wavelengths.     

Single-shot acquisition of time-resolved fluorescence spectra using a multiple delay optical fiber bundle

We present a technique for simultaneous acquisition of time-resolved fluorescence spectra using a customized optical fiber bundle with multiple collection fibers of different length. These fibers were aligned at the output plane of a spectrograph corresponding to different wavelengths. Fluorescence signal dispersed by the spectrograph was converted into a pulse train owing to time delays introduced by the length differences between each fiber and subsequently detected by a photomultiplier and digitizer. The performance of the technique was tested with standard fluorescent dyes, and the results are in good agreement with literature values.     

Characteristics of blood fluorescence spectra using low-level, 457.9-nm excitation from Ar+ laser

We measured the fluorescence spectra of the whole blood, the red blood cell (RBC) and the hemoglobin using 457.9-nm Ar+ laser excitation. It was found that the fluorescence spectra of the whole blood and the RBC have much similarities in the intensity, the emission peaks and the emitting region, and abundant peaks can be found. But for the hemoglobin, fluorescence could only be found in the wavelength range 580-650 nm. It was concluded that in the wavelength range of 650-850 nm, the fluorescence spectra were emitted by the new fluorophores generated by the breakdown of some weak bonds on the RBC membrane,such as the C-C bond and the C-N bond. In the wavelength range of 590 - 650 nm, the fluorescence spectra are mainly emitted by the hemoglobin, but the hemoglobin solution of cracked RBC has a strong quencher effect on the fluorescence spectrum. The experimental result and the theoretical analysis are meaningful for the medical diagnostics and the therapy.     

Seven-photon ultraviolet upconversion emission of Er3+ induced by 1,540-nm laser excitation

The multi-photon ultraviolet upconversion emission properties and synergistic effect are investigated in BaSr₂Y₆O₁₂:Er³⁺ phosphor. The deep-ultraviolet emissions centered at 274, 297 and 324-nm are observed under the 1,540-nm excitation, which results from a seven-, six- and six-photon upconversion process, respectively. A synergistic effect is found, which shows that the red emission intensity under 351- and 1,540-nm dual excitation is 4.7 % time stronger than the sum of red emission intensities under the 351 and 1,540-nm single excitation. This phenomenon is attributed to the ⁴I_13/2 and ⁴I_11/2 levels of Er³⁺ from non-radiative transition process under the 351-nm excitation are excited again to ⁴F_9/2 level by absorbing 1,540-nm photon in the 351- and 1,540-nm dual-excitation process.     

Single-shot detection of electrons generated by individual photons in a tunable lateral quantum dot

We demonstrate single-shot detection of single electrons generated by single photons using an electrically tunable quantum dot and a quantum point contact charge detector. By tuning the quantum dot in a Coulomb blockade before the photoexcitation, we observe the trapping and subsequent resetting of single photogenerated electrons. The photogenerated electrons can be stored in the dot for a tunable time range from shorter to longer than the spin-flip time T1. We combine this trap-reset technique with spin-dependent tunneling under magnetic fields to observe the spin-dependent photon detection within the T1.     

Frequency locking of a 399-nm laser referenced to fluorescence spectrum of an ytterbium atomic beam

The laser cooling of ytterbium(Yb) atoms needs a 399-nm laser which operates on the strong1S0-1P1 transition and can be locked at the desired frequencies for different Yb isotopes.We demonstrate a frequency locking method using the fluorescence spectrum of an Yb atomic beam as a frequency reference.For unresolved fluorescence peaks,we make the spectrum of the even isotopes vanish by using the strong angular-dependence of the fluorescence radiations;the remained closely-spaced peaks are thus clearly resolved and able to serve as accurate frequency references.A computer-controlled servo system is used to lock the laser frequency to a single fluorescence peak of interest,and a frequency stability of 304 kHz is achieved.This frequency-locked laser enables us to realize stable blue magneto-optic-traps(MOT) for all abundant Yb isotopes.     

Study of the interaction of antigens and antibodies by photoluminescence spectra excited by ultraviolet laser radiation

Photoluminescence spectra of isotonic solutions of antigens, i.e., “Taeniarhynchus saginatus” (polysaccharide) larvae and aqueous solutions of antibodies to them, extracted from calf blood serum are measured. Photoluminescence spectra were excited by ultraviolet radiation (266 nm) of the fourth harmonic of the yttrium aluminum garnet laser. The spectra of these objects are strong bands with intensity maxima at wavelengths of 341 and 332 nm for serum and polysaccharide, respectively. It was found that mixing of polysaccharide and serum solutions in a ratio 1:6 results in a drastic decrease in the photoluminescence intensity spectrum, which indicates bonding of corresponding molecular structures.     

激光对含偏心核球形粒子的辐射俘获力

利用偏心球形粒子对任意角度入射有形波束散射的理论,从广义米理论出发,根据电磁场的动量守恒及麦克斯韦张量,推导了任意入射波束对偏心球形粒子辐射俘获力的级数表达式,并以高斯波束为例,就离轴入射有吸收偏心球形粒子时的辐射俘获力进行了数值模拟,讨论了束腰半径、吸收系数、内核的相对大小及位置对俘获情况的影响. 关键词： 广义米理论 偏心球 辐射俘获力 光镊     

Demonstration of long-term reliability of a 266-nm, continuous-wave, frequency-quadrupled solid-state laser using beta-BaB(2)O(4)

We report what we believe to be the first operation of more than 1000 h of a 266-nm (cw) frequency-quadrupled solid-state laser with a 100-mW output. We used beta-BaB(2)O(4)(BBO) crystal grown by the Czochralski method to double the green-light (532-nm) wavelength, using an external resonant cavity. The green light was generated with an intracavity frequency-doubled Nd:YVO(4)laser pumped by a 4-W laser diode. When the incident 532-nm power on the external resonant doubler was 500 mW, we generated 100 mW of cw 266-nm radiation with the BBO crystal. The degradation rate seems to be proportional to the strength of the UV optical electric field. We also obtained a relative intensity noise of -130dB/Hz at frequencies of 2 to 10 MHz for 266-nm laser light.     

Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays

We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under ∼100 W/cm² laser radiation, the PL intensity (I_PL) increases with irradiation time upto about 500 s and thereafter declines linearly. The wavelength of the PL emission (λ_peak) exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm² 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I_PL and λ_peak are significantly different; I_PL increases to a saturation level, and the magnitude of the blue-shift in λ_peak is reduced. We discuss possible mechanisms underlying these results.     

SO2气体激光诱导色散荧光时间断层扫描研究

以纳秒Nd:YAG激光器的四倍频（266 nm）为激发源,利用门选通增强光学多通道光谱分析仪 (OMA),研究了SO2分子第一激发带粒子的荧光辐射与碰撞弛豫相结合的复杂退激发过程。通过对SO2分子第一激发带的激发及碰撞弛豫过程的时间断层扫描分析,可以将激光诱导色散荧光谱中以305.6 nm、337.2 nm为中心的荧光包络和以424.7 nm为中心的规则序列分别归属于B1B1、A1A2低振动能级和a3B1基振动能级到基电子态X1A1不同振动能级的荧光跃迁,由此可以确定大气污染气体SO2的诱导荧光的灵敏检测波长为425 nm;由规则序列的实验数据可以计算出SO2分子基电子态X1A1的对称振动和弯曲振动模式的基振动角频率分别为ω1= 1151.8±0.6 cm-1和ω2= 517.8±0.6 cm-1,两振动模式的非谐性常数分别为 = 8±0.6 cm-1和 = 9.2±0.6 cm-1。     

脉冲激光激发的原子共振荧光

本文从含有弛豫项的光与二能级原子相互作用的布洛赫(Block)方程出发,利用计算机进行数值求解,在强共振激发条件下,给出了几种不同的激光脉冲(包括常见的高斯脉冲)激发的原子共振荧光谱,获得了原子在瞬态辐射过程中的一些新现象。     

Generation of ultraviolet (335-nm) light by intracavity frequency doubling from active mode-locking action of an external-cavity AlGaInP diode laser

An intracavity frequency-doubling scheme was merged with an actively mode-locked AlGaInP laser diode with an external cavity to provide efficient generation of 335-nm ultraviolet light. Intense pulses of <16ps were successfully generated inside the cavity at subgigahertz repetition frequencies. We applied those pulses to intracavity second-harmonic generation from a 5-mm-long LiIO(3) crystal, which resulted in average ultraviolet power of 70muW for an average fundamental power of 73 mW.     

Spectral attenuation of a 400-nm laser pulse propagating through a plasma filament induced by an intense femtosecond laser pulse

The spectral attenuation of a 400-nm probe laser propagating through a femtosecond plasma in air is studied. Defocusing effect of the low-density plasma is an obvious effect by examining the far-field patterns of the 400-nm pulse.Besides, the energy of 400-nm pulse drops after interaction with the plasma, which is found to be another effect leading to the attenuation. To reveal the physical origin behind the energy loss, we measure fluorescence emissions of the interaction area. The fluorescence is hardly detected with the weak 400-nm laser pulse, and the line spectra from the plasma filament induced by the 800-nm pump pulse are clearly shown. However, when the 400-nm pulse propagates through the plasma filament, the fluorescence at 391 nm from the first negative band system of N_2~+ is enhanced, while that from the second positive band of neutral N_2 at 337 nm remains constant. Efficient near-resonant absorption of the 400-nm pulse by the first negative band system occurs inside the plasma, which results in the enhanced fluorescence. Furthermore, the spectral attenuation of the 400-nm probe laser is measured as a function of the pump–probe time delay as well as the pump-pulse energy.