Dinh C. Nguyen

Detection of single molecules of phycoerythrin in hydrodynamically focused flows by laser-induced fluorescence

Single molecules of B-phycoerythrin were detected by laser-induced fluorescence in hydrodynamically focused flows as each molecule transited a focused laser beam. Phycoerythrin is a large protein molecule containing the equivalent of 25 rhodamine-6G chromophores. Single molecule detection is documented by the following: (1 )the number of counts per molecules is in agreement with the expected number, (2) the number of molecules per second is in agreement with the concentration and the flow rate, and (3) the time interval distribution between detected molecules is In agreement with the concentration and the flow rate. The molecular transit time through the 1.1-pL probe volume was 180 micros.

Blue-green (450-nm) upconversion Tm(3+):YLF laser.

We report an upconversion laser that is pumped by two colors-one red and one near infrared-and emits in the blue-green region.

Photocathodes for free electron lasers

Abstract Many different photocathodes have been used as electron sources for FELs and other electron accelerator systems. In choosing one, a compromise between lifetime and quantum efficiency has been unavoidable. High quantum efficiency photocathodes such as K 2 CsSb, Cs 3 Sb, and cesiated GaAs have short operational lifetimes and require an ultrahigh-vacuum environment. Long lifetime photocathodes such as LaB 6 , Cu, and Y have relatively low quantum efficiencies. However, recently, cesium telluride was found to be an exception. Initial results from CERN and now at Los Alamos have shown that Cs 2 Te is reasonably rugged with a high quantum efficiency below 270 nm. Further studies carried out at Los Alamos have determined that its performance as an electron source for the Los Alamos Advanced FEL is excellent.

Suppression of intensity fluctuations in free space high-speed optical communication based on spectral encoding of a partially coherent beam

A new concept of a free space, high-speed (Gbps) optical communication system based on spectral encoding of radiation from a broadband pulsed laser is developed. It is shown that, in combination with the use of partially coherent laser beams and a relatively slow photosensor, scintillations can be suppressed by orders of magnitude for distances of more than 10 km.

Cesium telluride photocathodes

Cesium telluride (Cs2Te) photocathodes, with quantum efficiencies (QEs) of 15%–18% at 251 nm, were fabricated by vapor deposition of Te and Cs onto a Mo substrate and used as an electron source for the Los Alamos Advanced Free‐Electron Laser. In the fabrication chamber, the spectral response from 251 to 578 nm was measured before and after a controlled exposure of several photocathodes to air. The 251‐nm QE dropped by about a factor of 20 when exposed to 2×10−4 Torr of air for 1 h. Heating degraded photocathodes to 150–200 °C partially rejuvenated their QEs to about 60% of the value before air exposure. The performance of Cs2Te as a source of electrons for accelerators was evaluated in the photoinjector stage of the Advanced Free‐Electron Laser. The response time, saturation level, and dark current of cesium telluride photocathodes and the emittance and energy spread of the resulting electron beam were determined to be sufficient for free electron laser applications.

Spectroscopy and dynamics of upconversion in Tm3+: YLiF4

Abstract Spectroscopic and dynamic properties of Tm3+: YLiF4 as an upconversion material with blue emission are presented. Absorption, fluorescence and excitation spectra at low temperatures were used to determine the crystal field levels of all manifolds except 1S0 of Tm3+. The observed energy levels were fitted in a least-square analysis to determine crystal-field parameters of the S4 site of Tm3+ in YLiF4 with a standard deviation of 16 cm−1 between the calculated and measured energy levels. Judd-Ofelt calculations usuing measured integrated absorption yield the intensity parameters μ2,4,6 for Tm3+ in YLiF4. From these parameters, the calculated stimulated emission cross section of the 1D2→3F4 transition is 2.4 × 10−19 cm2, in good agreement with the measured value of 3 × 10−19 cm2. An estimate of the dipole-dipole interaction which gives rise to quenching of the excited state was derived from the fluorescence decay curves at different concentrations.

First lasing of the regenerative amplifier FEL

The Regenerative Amplifier Free-Electron Laser (RAFEL) is a high-gain RF-linac FEL capable of producing high optical power from a compact design. The combination of a high-gain and small optical feedback enables the FEL to reach saturation and produce a high optical power and high extraction efficiency without risk of optical damage to the mirrors. This paper summarizes the first lasing of the Regenerative Amplifier FEL and describes recent experimental results. The highest optical energy achieved thus far at 16.3 {micro}m is 1.7 J over an 9-{micro}s macropulse, corresponding to an average power during the macropulse of 190 kW. They deduce an energy of 1.7 mJ in each 16 ps micropulse, corresponding to a peak power of 110 MW.

Initial performance of Los Alamos Advanced Free Electron Laser

Abstract The Los Alamos compact Advanced Free Electron Laser (AFEL) has lased at 4.7 and 5.2 μm with a 1-cm period wiggler and a high-brightness electron beam at 16.8 and 15.8 MeV, respectively. The measured electron beam normalized emittance is 1.3 π mm mrad at a peak current of 100 A, corresponding to a beam brightness greater than 2 × 10 12 A/m 2 rad 2 . Initial results indicate that the AFEL small signal gain is ∼ 8% at 0.3 nC (30 A peak). The maximum output energy is 7 mJ over a 2-μs macropulse. The AFEL performance can be significantly enhanced by improvements in the rf and drive laser stability.

Resonantly enhanced sum-frequency generation in adsorbed monolayers of rhodamine 6G

Abstract Resonantly enhanced sum frequency generation in sub-monolayers of rhodamine 6G dye is reported. With the use of two lasers, one of the input frequencies, ω 1 , is in resonance with the S 0 →S 1 transition and the output sum frequency, ω 1 + ω 2 , is in resonance with the S 0 →S 2 transition of the dye. The sum-frequency signal is enhanced by more than two orders of magnitude relative to resonantly enhanced second-harmonic generation where only the output frequency is in resonance with S 0 →S 2 .

Performance of the high brightness linac for the advanced free-electron laser initiative at Los Alamos☆

Abstract The AFEL accelerator has produced beams of 1 nC with peak currents greater than 100 A and a normalized, rms emittance less than 2π mm mrad. The 1300 MHz standing-wave accelerator uses on-axis coupling cells. The electron source is a photoinjector with a CsK 2 Sb photocathode. The photoinjector is an integral part of a single 11-cell accelerator structure. The accelerator operates between 12 and 18 MeV. The beam emittance growth in the accelerator is minimized by using a photoinjector, a focusing solenoid to correct the emittance growth due to space charge, and a special design of the coupling slots between accelerator cavities to minimize quadrupole effects. This paper describes the experimental results and compares those results with PARMELA simulation. The simulation code PARMELA was modified for this effort. This modified version uses SUPERFISH files for the accelerator cavity fields, MAFIA files for the fields due to the coupling slots in the accelerator cells, and POISSON files for the solenoid field in the gun region.