Volume 89, Issue 23, 04 December 2006
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic transport and semiconductors
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
The authors introduce an emergent method to fabricate a few-nanometer-size columnar superlattice with a checkerboard pattern in inorganic spinels by harnessing the Jahn-Tellerstructural distortion. Transmission electron microscope images reveal that the fundamental building blocks are two types of long nanorods with the size, which are alternatively stacked in a way that the cross sectional and side views show checkerboard and herringbone patterns, respectively. The authors discuss that the strain induced by the Jahn-Teller distortion causes this peculiar self-assemblednanostructure in the coherent mixture of two spinel phases. This pure solid state self-assembly can be implemented to fabricate heterogeneous nanostructures with practical functionalities.
- LASERS, OPTICS, AND OPTOELECTRONICS
89(2006); http://dx.doi.org/10.1063/1.2398909View Description Hide Description
Time-resolvedKerr rotation in Voigt geometry is used to study the coherent evolution of spin states in moderately doped crystals with picosecond excitation. Due to the small spectral width of the exciting laser pulses, selective excitation of different electronic states is feasible, which allows one to extract both the dependence of the factor and the spin dephasing time on energy. The authors are able to observe coherent spin oscillations in bulk at room temperature and deduce a factor of at and a magnetic field dependent room temperature spin dephasing time of, e.g., at .
89(2006); http://dx.doi.org/10.1063/1.2400111View Description Hide Description
With a blue light-emitting diode and a phosphor layer to downconvert blue light to a second light, such as yellow, white light can be produced. The authors developed a one-dimensional model to describe the light propagating in the phosphor layer in terms of light absorption, conversion, and reflection. The parameters required for the model were determined from the data obtained by using multiple-layer phosphorfilms. The model predicts that, with a reflector between the diode and the phosphor layer that is blue-light transparent but reflects other visible light, the normalized white light intensity is above 0.9, higher than that of conventional packages (0.6–0.8).
Add-drop filters in three-dimensional layer-by-layer photonic crystals using waveguides and resonant cavities89(2006); http://dx.doi.org/10.1063/1.2400398View Description Hide Description
A three-dimensional layer-by-layer photonic crystal with a complete photonic band gap is used to experimentally and theoretically demonstrate a sharp tunable bandpass filter. The structure consists of input and output waveguide sections coupled through a nearby cavity. The authors show experimentally and verify with finite difference time domain simulations that this configuration is a bandpass filter where a particular resonant frequency of the cavity is selected from the input guide and transmitted to the output guide leaving out other input frequencies. An excellent coupling efficiency near 100% between the waveguide and the cavity is found for the drop frequencies.
89(2006); http://dx.doi.org/10.1063/1.2402263View Description Hide Description
Laser emission from optically pumped CdSequantum dots embedded in a microdisk was observed at low temperatures. Laser thresholds below and spontaneous emission coupling -factors exceeding 0.8 were determined. For different modes of the same microdisk cavity the laser threshold increases with higher mode energy, which the authors associate to a better coupling of larger quantum dots with the optical mode. Also the linewidth and energy shift of lasing modes as a function of pump power were analyzed.
89(2006); http://dx.doi.org/10.1063/1.2402897View Description Hide Description
Time evolution of the second-order nonlinearity induced in a single-hole D-shaped fiber thermally poled at and has been investigated with second-harmonic microscopy. It was found that the nonlinear layer moved towards the cathode with increasing poling time until it finally reached the cathode surface. The magnitude of the nonlinear coefficient first increased, saturated at between 5 and after poling, and then began to decrease for longer poling time. The nonlinearity remained large even when the nonlinearity layer reached the centerline between the two electrodes. In samples poled long enough , apart from the main nonlinearity layer at the cathode, weak second-order nonlinearity was also induced at the core-cladding interface sections that were nearly parallel to the poling direction.
89(2006); http://dx.doi.org/10.1063/1.2400396View Description Hide Description
The authors report the experimental investigation of pulse-shaping effects of a semiconductor saturable absorber mirror (SESAM). The induced pulse shortening and phase variation exhibit fluence dependences which follow the saturation behavior of nonlinear reflectivity. The experimental results have been compared with the prediction based on saturable population relaxation model. The study provides an unprecedented opportunity to investigate the role of SESAM in ultrafast mode-locked lasers.
89(2006); http://dx.doi.org/10.1063/1.2400400View Description Hide Description
Recent advancements in laser wakefield accelerators have resulted in the generation of low divergence, hundred MeV, quasimonoenergetic electron beams. The bremsstrahlung produced by these highly energetic electrons in heavy converters includes a large number of MeV rays that have been utilized to induce photofission in natural uranium. Analysis of the measured delayed emission demonstrates production of greater than fission events per joule of laser energy, which is more than an order of magnitude greater than that previously achieved. Monte Carlo simulations model the generated bremsstrahlung spectrum and compare photofission yields as a function of target depth and incident electron energy.
89(2006); http://dx.doi.org/10.1063/1.2402876View Description Hide Description
The one-dimensional photonic-crystal resonators at visible lights are fabricated and characterized, where may be ZnO or indium tin oxide. Owing to the absorption of ultraviolet (UV) light by the ZnO layers, the refractive index of ZnO layers is changed temporally. This fact led to a temporary shifting of the forbidden band and the resonant mode of the resonator under UVirradiation. Besides, via adjusting the thickness of the ZnO defect layer, the resonant wavelength is manipulated. These experimental data show good consistence with simulated results.
89(2006); http://dx.doi.org/10.1063/1.2402881View Description Hide Description
From resonant Raman scatteringmeasurements under hydrostaticpressure the authors were able to identify both the strain-shifted longitudinal optical phonon of CdSequantum dots with different average size embedded in a quaternary ZnCdMgSe material and a Mg-local dot mode due to interdiffusion from the barrier. They show that the large tunability of the light emission, which covers most of the visible spectrum, is not only due to a different degree of confinement but also to a size-dependent built-in strain. The frequency of the Mg mode allowed them also to roughly estimate the amount of Mg interdiffusion into the quantum dots.
Enhanced stability of the second order optical properties of high- fluorinated electro-optic copolymer89(2006); http://dx.doi.org/10.1063/1.2402886View Description Hide Description
In this work the authors investigated the second order nonlinear optical properties of a group of Disperse Red 1 based electro-optic fluorinated copolymers, synthesized with two fluorinated monomers: the -fluoroacrylate monomer bearing an adamantane side group and the -fluoroacrylate monomer bearing the Disperse Red 1 chromophore. By means of nonlinear ellipsometry at , the poling induced electro-optictensor main component was determined. The polymer’s thermostability was investigated by means of the temperature scanning and isothermal relaxation techniques, leading to the determination of the depolarization temperature, corresponding to an activation energy for the relaxation of the orientational distribution of the active chromophores . The increased thermostability of the copolymer, given by the adamantane side group, is then compared with that of other two groups of fluorinated copolymers.
Investigation of whispering gallery mode dependence on cavity geometry of quasiperiodic photonic crystal microcavity lasers89(2006); http://dx.doi.org/10.1063/1.2402894View Description Hide Description
Dodecagonal (12-fold) quasiperiodic photonic crystal (DQPC) microcavity lasers sustaining whispering gallery mode(WGM) are fabricated. Lasing characteristics of DQPC D2 microcavity lasers are obtained and compared with triangular lattice D2 photonic crystal(PC) lasers, and ultralow threshold is obtained. The strong WGM mode dependence on 12 nearest airholes of DQPC D2 microcavity and its fabrication tolerance is investigated and discussed by randomly varying the lattice of two separate regions. This mode dependence also indicates that one can enhance a WGM in different PCmicrocavities by modifying the cavity boundary to be circular.
Low threshold amplified spontaneous emission based on coumarin 151 encapsulated in mesoporous SBA-1589(2006); http://dx.doi.org/10.1063/1.2402908View Description Hide Description
Amplified spontaneous emission(ASE) characteristics of a blue dye coumarin 151 encapsulated in a highly ordered mesoporous SBA-15 were studied. The spectra narrowing was observed and found that the threshold and loss were greatly reduced, and the gain is significantly increased compared with spin-coated coumarin 151 doped poly(4-vinylphenol) film. The ASE threshold, gain, and loss, respectively, reached , , and for the coumarin 151 encapsulated in mesoporous SBA-15 film. The optimized lasing action owes much to the effects of the better spatial confinement of the molecules in the ordered mesoporous structure of the host SBA-15.
89(2006); http://dx.doi.org/10.1063/1.2403185View Description Hide Description
The authors apply the theory of photothermal lens formation and also that of pure optical nonlinearity to account for the phase modulation in a beam as it traverses a nonlinear medium. It is used to simultaneously determine the nonlinear optical refraction and the thermo-optic coefficient. They demonstrate this technique using some metal phthalocyanines dissolved in dimethyl sulfoxide, irradiated by a -switched Nd:YAG laser with repetition rate and a pulse width of . The mechanism for reverse saturable absorption in these materials is also discussed.
89(2006); http://dx.doi.org/10.1063/1.2403906View Description Hide Description
The authors study longitudinal photonic band crystal lasers with a vertical far field divergence of 9° (full width at half maximum). Differential quantum efficiency of 95% is achieved at a cavity length of . A total optical output power from broad area multimode devices was up to and a maximum continuous wave single mode power from narrow stripe devices was .
89(2006); http://dx.doi.org/10.1063/1.2403912View Description Hide Description
The authors report on the first demonstration of mode locking in an active waveguide laser manufactured by femtosecond laser writing. The active waveguide is fabricated in an Er–Yb-doped phosphate glass, and the mode locker is a fiber-pigtailed saturable absorber device based on single-wall carbon nanotubes specially designed to efficiently operate at . Transform-limited pulses were observed in a ring laser cavity configuration.
89(2006); http://dx.doi.org/10.1063/1.2404614View Description Hide Description
The authors report ultraviolet electroluminescence from ZnOnanoparticle-baseddevices prepared by the phase-segregation technique. The conditions for phase segregation are investigated using confocal microscopy. With proper parameters for phase segregation, the ZnOnanoparticles and -diphenyl--bis(3-methylphenyl)--biphenyl--diamine: poly(methyl methacrylate) can be separated into two layers upon spin-coating process. The method allows electrons and holes to recombine in the ZnOnanoparticles. The curve shows stable and excellent rectification. For the device with ZnOnanoparticles, it exhibits a very narrow spectrum with a peak at and no defect-related emission. The emission peak well corresponds to the ZnO band-gap energy.
89(2006); http://dx.doi.org/10.1063/1.2398904View Description Hide Description
Local restructuring of metallic nanorod arrays and the fabrication of metallodielectric microstructures using direct femtosecond laser writing technologies have been studied. Femtosecond laserablation has demonstrated the possibility to create sharp boundaries (down to a single nanorod) on the nanorod array. Polymer structures on the nanorods can be used to control the resonance wavelengths and thus the effective refractive index of surface plasmons supported by the array. The structurednanorods and their polymer composites are important for applications in light guiding and the development of nonlinear optical nanodevices.
89(2006); http://dx.doi.org/10.1063/1.2403905View Description Hide Description
By measuring and calculating oblique-incidence transmittance spectra of a negative-index photonicmetamaterial layer, the authors infer the in-plane dispersion relation of the magnetization wave. From the geometry and the dispersion shape, the authors conclude that coupling is predominantly via magnetic dipoles for wave propagation along the magnetic-dipole direction. These magnetization waves are the classical analog of magnon excitations of quantum-mechanical spins.
89(2006); http://dx.doi.org/10.1063/1.2403911View Description Hide Description
In this letter the authors demonstrate that the emission of plasmon/exciton polaritons in an organic semiconductor can be extracted using surface plasmon cross coupling. A -aggregated cyanine dye layer is deposited on a silverfilm and then the whole structure is periodically corrugated. As a result of this corrugation, the dispersion lines of the surface modes on both sides of the silver layer may cross. At this crossing point, an increase in the polaritonic emission is observed. The contributions of unpolarized incoherent states and of the TM polarized polaritons emission are separated.
89(2006); http://dx.doi.org/10.1063/1.2403924View Description Hide Description
Phase locking of the output beam and in-phase mode selection with the amplitude compensator at a high injection current were achieved experimentally by a quarter-Talbot cavity. The linear array used in the experiment consisted of 49 wide-aperture diodes with a diode width of and a spacing period of . Increasing output beam intensity in the far-field central lobe was demonstrated, and of output power distributed on the multilobes was concentrated on the single lobe and changed to by the amplitude compensator. The far-field divergence of the phase-locked beam was , and the spectral full width at half maximum of the output beam was suppressed from .