Medical Library

Critical separation for efficient Tm(3+)-Tm(3+) energy transfer evidenced in nanostructured Tm(3+):Al(2)O(3) thin films.

Nanostructured amorphous Al oxide (a-Al(2)O(3)) thin films doped with Tm(3+) were synthesized by alternate pulsed laser deposition. The Tm(3+) ions have been deposited in layers with in-depth separation ranging from 0.75 to 6 nm. The films show two broad emission bands originated from the Tm(3+) (3)H(4)–>(3)F(4) and (3)F(4)–>(3)H(6) transitions. Their intensity increases at a similar rate and the lifetimes are not modified as the layer separation decreases down to 1.5 nm, suggesting that there is no concentration quenching. At the critical value of 1.5 nm the onset of Tm(3+)-Tm(3+) energy transfer is evidenced by a sharp decrease of the emission intensity and lifetime. Below this critical value, the rate at which the intensity increases for the (3)F(4)–>(3)H(6) transition is much higher than that for the (3)H(4)–>(3)F(4) transition, evidencing quenching of the (3)H(4)–>(3)F(4) transition through a cross-relaxation mechanism. The control of the Tm(3+) ions in the nanometer scale allows evidencing the onset of energy transfer processes among ions and offers a route to optimize compact photonic gain integrated devices.

Xiao Z, Serna R, Xu F, Afonso CN.

Wavelength and intensity switching in directly coupled semiconductor microdisk lasers.

We demonstrate output wavelength and intensity switching in a three-element directly coupled microdisk device consisting of one spiral microdisk coupled to two semicircle microdisks. The gapless coupling mechanism used allows individual elements to achieve lasing while achieving optimal transfer of optical power between adjacent microdisks. By controlling the transparency of the center element via injection current, the edge elements can be allowed to exchange their amplified spontaneous emission. In this manner, on-off-on switching of the output intensity, as well as discontinuous shifts in the output wavelength, can be achieved as a function of increasing injection current.

Fernandes GE, Guyot L, Chern GD, Kneissl M, Johnson NM, Song Q, Xu L, Chang RK.

Interferometric polarization coherent anti-Stokes Raman scattering (IP-CARS) microscopy.

We report a novel interferometry-based polarization coherent anti-Stokes Raman scattering (IP-CARS) implementation for effectively suppressing the nonresonant background while significantly amplifying the resonant signal for vibrational imaging. By modulating the phase difference between the two interference CARS signals generated from the same sample and measuring the peak-to-peak intensity of the periodically modulated interference CARS signal, the IP-CARS technique yields a sixfold improvement in the signal-to-background ratio compared with conventional CARS while providing an approximately 20-fold amplification of the resonant CARS signal compared with conventional polarization CARS. We demonstrate this method by imaging 4.69 mum polystyrene beads and unstained human epithelial cells immersed in water.

Lu F, Zheng W, Sheppard C, Huang Z.

Precision steering of an optical trap by electro-optic deflection.

We designed, constructed, and tested a single-beam optical trapping instrument employing twin electro-optic deflectors (EODs) to steer the trap in the specimen plane. Compared with traditional instruments based on acousto-optic deflectors (AODs), EOD-based traps offer a significant improvement in light throughput and a reduction in deflection-angle (pointing) errors. These attributes impart improved force and position resolution, making EOD-based traps a promising alternative for high-precision nanomechanical measurements of biomaterials.

Valentine MT, Guydosh NR, Gutiérrez-Medina B, Fehr AN, Andreasson JO, Block SM.

Spiral-path high-sensitivity silicon photonic wire molecular sensor with temperature-independent response.

We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 mum diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of ~10 fg of streptavidin protein is demonstrated with near temperature-independent response.

Densmore A, Xu DX, Janz S, Waldron P, Mischki T, Lopinski G, Delâge A, Lapointe J, Cheben P, Lamontagne B, Schmid JH.

Fiber-based source of photon pairs at telecom band with high temporal coherence and brightness for quantum information processing.

We experimentally demonstrate a bright pulsed source of correlated photon pairs at the 1550 nm telecom band by pumping 300 m dispersion-shifted fiber with a 4 ps pulse train. We investigate the coherence property of the source by measuring the second-order intensity correlation function g((2)) of individual signal (idler) photons. A preliminary Hong-Ou-Mandel-type two-photon interference experiment with two such sources confirms the high temporal and spatial coherence of the source. The source is suitable for multiphoton quantum interference of independent sources, which is required in quantum information processing.

Li X, Yang L, Cui L, Ou ZY, Yu D.

Highly efficient, low-cost femtosecond Cr(3+):LiCAF laser pumped by single-mode diodes.

We describe efficient continuous-wave (cw) and cw mode-locked operations of a Cr(3+):LiCAF laser, pumped by inexpensive single spatial mode laser diodes. Up to 280 mW of cw output power was obtained with 570 mW of absorbed pump power, with a corresponding slope efficiency of 54%. Continuous tuning between 765 and 865 nm was demonstrated using a fused silica prism. A semiconductor saturable absorber mirror was used to initiate and sustain stable, self-starting, mode-locked operation. In cw mode locking, the laser produced 72 fs (FWHM) duration pulses, and 1.4 nJ pulse energy, at an average output power of 178 mW. Electrical to optical conversion efficiencies of 7.8% in mode-locked operation and 12.2% in cw operation were demonstrated.

Demirbas U, Sennaroglu A, Kärtner FX, Fujimoto JG.

Enhanced chaos synchronization in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers with polarization-preserved injection.

We experimentally study chaos synchronization in unidirectionally coupled vertical-cavity surface-emitting semiconductor lasers (VCSELs) with polarization-preserved and polarization-selected optical injection. The measurements show, in agreement with theoretical predictions, that the maximum cross coefficient of 0.884 obtained with polarization-preserved optical injection is significantly higher than the maximum cross coefficient of 0.724 obtained with polarization-selected optical injection.

Hong Y, Lee MW, Paul J, Spencer PS, Shore KA.

Continuous sorting of Brownian particles using coupled photophoresis and asymmetric potential cycling.

Feasible sorters need to function rapidly and permit the input and delivery of particles continuously. Here, we describe a scheme that incorporates (i) restricted spatial input location and (ii) orthogonal sort and movement direction features. Sorting is achieved using an asymmetric potential that is cycled on and off, whereas movement is accomplished using photophoresis. Simulations with 0.2 and 0.5 mum diameter spherical particles indicate that sorting can commence quickly from a continuous stream. Procedures to optimize the sorting scheme are also described.

Ng TW, Neild A, Heeraman P.

Optical image encryption based on the multiple-parameter fractional Fourier transform.

A novel image encryption algorithm is proposed based on the multiple-parameter fractional Fourier transform, which is a generalized fractional Fourier transform, without the use of phase keys. The image is encrypted simply by performing a multiple-parameter fractional Fourier transform with four keys. Optical implementation is suggested. The method has been compared with existing methods and shows superior robustness to blind decryption.

Tao R, Lang J, Wang Y.