Volume 92, Issue 4, 28 January 2008
- 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
- organic electronics and photonics
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
For the past decade, intermetallicclathrates have been rattling their way into mainstream research in thermoelectrics. The unusual vibrations of their guest ions inside oversized cages interfere with the cage phonons while leaving the electronic flow intact, providing an exotic way to achieve a best-of-both-worlds scenario in terms of electrical and thermal transport in the same material. Here, we present the structural and thermoelectricproperties of single crystals grown in the type-I clathratestructure, showing one of the lowest recorded thermal conductivities for any bulk compound, while still behaving electronically as a heavily doped -type semiconducting crystal.
- LASERS, OPTICS, AND OPTOELECTRONICS
Determination of the glass transition temperature of photorefractive polymer composites from photoconductivity measurements92(2008); http://dx.doi.org/10.1063/1.2830707View Description Hide Description
A method to determine the glass transition temperature of photorefractive (PR) polymer composites, based on photoconductivity measurements, is proposed. The composite is identified as the temperature at which a clear change in slope in the Arrhenius plot representation of the photoconductivity is obtained. We demonstrate that for PR polymers, this method is more appropriate than the one generally used, based on differential scanning calorimetry (DSC). For PR composites based on the hole transporting polymer poly( -vinylcarbazole), the value determined from photoconductivity data is around higher than that obtained by DSC.
92(2008); http://dx.doi.org/10.1063/1.2838296View Description Hide Description
We show that both interband and intersubband transitions in quantum cascade laser structures can have sufficient gain to enable simultaneous lasing at the two transitions. High electric fieldsgenerate holes, which are necessary for interband lasing in InAs. The doping concentration in the injection parts is shown to be critical for controlling the electric fields in the device, enabling the selection of lasing at the intersubband transition alone (high doping), at the interband transition alone (low doping), and simultaneous lasing at both transitions (intermediate doping).
92(2008); http://dx.doi.org/10.1063/1.2838308View Description Hide Description
Distortions of ultrashort laser pulses propagating through turbulence are investigated both experimentally and numerically. As expected, a strong correlation is found between temporal distortions and local intensity on the speckle pattern. We suggest that the localization of distortions in low-intensity regions may favor remote control strategies based on nonlinear interactions with respect to those based on linear schemes.
92(2008); http://dx.doi.org/10.1063/1.2835707View Description Hide Description
is an attractive material for photocatalytic water splitting, but its band gap is too large to allow for efficient absorption of solar photons. We present a comprehensive first-principles investigation of band-structure modifications induced by alloying with . The band gap of alloys exhibits a dramatic decrease upon addition of even a low concentration of S. The optical absorption of visible light is attainable with sulfur concentrations less than . We have verified that the band alignment in these alloys is favorable for photocatalytic applications: in particular, the conduction band exhibits only small shifts, while the majority of the band-gap change is due to an upward shift of the valence band.
92(2008); http://dx.doi.org/10.1063/1.2838341View Description Hide Description
Fast direct writing of waveguides on polymers using low power continuous-wave lasers has been investigated. Using the cis-trans property of a functionalized sulfonated azo chromophores, we have fabricated graded index waveguides with low loss, which is due to graded index sidewalls. Fabrication is done by exposing the polymer film to a wavelength laser beam focused to about spot at the film. Losses were calculated to be at wavelength with no surface deformation. This technique enables us to fabricate integrated optical circuits including directional couplers, dividers, filters, switches, etc., as they are currently investigated in our laboratory.
Quantitative analysis of Kerr nonlinearity and Kerr-like nonlinearity induced via terahertz generation in ZnTe92(2008); http://dx.doi.org/10.1063/1.2838446View Description Hide Description
Kerr nonlinearity and Kerr-like nonlinearity induced via terahertz generation and the electro-optical effect in ZnTe crystal are investigated. In general, these nonlinear effects are concomitant and difficult to quantitatively analyze in the time domain. Even -scan technique, which is a simple and sensitive single-beam method to determine both the sign and magnitude of the nonlinear refractive index as well as the nonlinear absorption coefficient of a given material, cannot quantitatively analyze the contribution of each nonlinear effect directly. A method is proposed in the spectral domain to distinguish between Kerr nonlinearity and Kerr-like nonlinearity. Experimental results agree with theoretical analysis.
Storage and selective release of optical information based on fractional stimulated Raman adiabatic passage in a solid92(2008); http://dx.doi.org/10.1063/1.2838731View Description Hide Description
We demonstrate storage and selective release of a light pulse in a Pr:YSO crystal, which is based on atomic spin coherence created by fractional stimulated Raman adiabatic passage (STIRAP). The fractional STIRAP gives better storage fidelity compared to STIRAP. Selective release of stored optical information and better storage fidelity have practical applications in information processing and all-optical communication.
Vibrational dephasing time imaging by time-resolved broadband coherent anti-Stokes Raman scattering microscopy92(2008); http://dx.doi.org/10.1063/1.2838750View Description Hide Description
Time delay control in broadband coherent anti-Stokes Raman scattering(CARS) allows acquisition of time-resolvedCARSimages free of nonresonant background (NRB). We demonstrate that, in some cases, CARSimage contrast is not chemical contrast but simply due to differences in NRB. Time-resolvedCARS is used to rectify this by eliminating the NRB. We also construct a vibrational dephasing time image from a sequence of time-resolvedCARSimages of polystyrene beads in toluene. In doing so, we demonstrate the potential of imaging local molecular interactions between molecules and their surrounding in a structured medium.
92(2008); http://dx.doi.org/10.1063/1.2838751View Description Hide Description
Microlens array is a key element in the field of information processing, optoelectronics, and integrated optics. Many existing fabrication processes remain expensive and complicated even though relatively low-cost replication processes have been developed. Here, we demonstrate the fabrication of microlens arrays through projection photopolymerization using a digital micromirror device (DMD) as a dynamic photomask. The DMD projects grayscale images, which are designed in a computer, onto a photocurable resin. The resin is then solidified with its thickness determined by a grayscale ultraviolet light and exposure time. Therefore, various geometries can be formed in a single-step, massively parallel fashion. We present microlens arrays made of acrylate-based polymer precursor. The physical and optical characteristics of the resulting lenses suggest that this fabrication technique is potentially suitable for applications in integrated optics.
Enhanced broadband near-infrared luminescence in transparent silicate glass ceramics containing ions and -doped nanocrystals92(2008); http://dx.doi.org/10.1063/1.2839333View Description Hide Description
Spectral properties of codoped transparent silicate glass ceramics containing nanocrystals were investigated. The near-infrared emission intensity of was largely increased with codoping due to energy transfer. The qualitative calculation of the energy transfer constant and rate showed that the energy transfer was much greater than in the opposite direction. codoped glass ceramics with exhibited a near-infrared emission with full width at half maximum of and fluorescent lifetime of . The glass ceramics are promising for broadband optical amplification.
92(2008); http://dx.doi.org/10.1063/1.2838348View Description Hide Description
High-resolution angle-resolved transmission measurements are carried out through nanostructured plasmonicwaveguides made of thin gold films with very narrow slits deposited on GaAs substrates. They reveal transmission intensity modulations along the air/metal surface plasmon band, due to the presence of metal/substrate surface modes. Two propagation regimes have been identified. In this plasmonicwaveguide, the radiative emission can be enhanced or inhibited by a slight modification of the refractive index of the substrate , allowing to control the propagation regime of surface plasmon waves along the air/metal interface.
92(2008); http://dx.doi.org/10.1063/1.2838355View Description Hide Description
conductive crystal lines composed of closely compacted fine particles were patterned on the surface of (mol %) precursor glass by a continuous wave Yb fiber laserirradiation using the nickel atom heat processing method. Homogeneous and smooth crystal lines with a uniform width of were obtained with a laser power of . The ion conductivity of the laser patterned glass was about two orders of magnitude higher than that of the glass matrix due to the formation of conductive crystalline phase by Yb fiber laserirradiation.
92(2008); http://dx.doi.org/10.1063/1.2840160View Description Hide Description
A coherent absorption dip in pump-probe experiment performed on a ten layer optically thin quantum dot(QD) structure has been observed. Measurements performed for different wavelengths, polarizations, pulse widths, and temperatures allow assigning the dip to electromagnetically induced transparency (EIT). The EIT scheme is based on coupling of excitons with different spins in asymmetric QDs. Using spectrally narrow pulses, detrimental effect of the inhomogeneous broadening is reduced since only the QDs with transitions resonant with the pulse wavelength are addressed and participate in the EIT. The effect has been observed at temperatures up to .
92(2008); http://dx.doi.org/10.1063/1.2840176View Description Hide Description
Deep ultraviolet photoluminescence(PL) was employed to probe the valence bandstructure of AlN epilayers grown by metal organic chemical vapor deposition on -plane sapphire substrates. At , in addition to the dominant emission peak at polarized in the direction, we observed two additional emission peaks at 6.249 and polarized in the direction. These two emission lines are assigned to the recombination of free excitons related to the and valence bands. A more comprehensive picture of the valence bandstructure of AlN is thus directly obtained from the PL measurements.
92(2008); http://dx.doi.org/10.1063/1.2839321View Description Hide Description
We report a simple technique for the visual determination of a laser light polarization state. The technique is based on a phenomenon occurring when laser light impinges on a particular photopolymerizable mixture sandwiched between two glass plates. After a brief irradiation, some polarization dependent patterns are observed far from the incidence spot. This effect is due to light scattered by the polymerized material and waveguided by the glass. A visual inspection of the characteristic patterns allows an immediate determination of the light polarization. This technique could be conveniently exploited for qualitative and/or quantitative polarimetry, as well as for designing polarization-driven devices.
Gain and continuous-wave laser power enhancement with a secondary discharge to predissociate molecular iodine in an electric oxygen-iodine laser92(2008); http://dx.doi.org/10.1063/1.2839323View Description Hide Description
Herein the authors report on the demonstration of a 50% enhancement in gain and 38% enhancement in continuous-wave laser power on the transition of atomic iodine through the addition of a secondary discharge to predissociate the molecular iodine in an electric oxygen-iodine laser. In the primary discharge the is produced by a radio-frequency-excited electric discharge sustained in an gas mixture, and is then pumped using energy transferred from . A gain of was obtained and the total laser output power was .
92(2008); http://dx.doi.org/10.1063/1.2839324View Description Hide Description
The authors show that the incorporation of gain media in only a selected device area can annul the effect of material loss and enhance the performance of loss-limited plasmonic devices. In addition, they demonstrate that optical gain provides a mechanism for on/off switching in metal-dielectric-metal (MDM) plasmonicwaveguides. The proposed gain-assisted plasmonic switch consists of a subwavelength MDM plasmonicwaveguide side coupled to a cavity filled with semiconductor material. They show that the principle of operation of such gain-assisted plasmonic devices can be explained using a temporal coupled-mode theory.
92(2008); http://dx.doi.org/10.1063/1.2839381View Description Hide Description
We analyze the potential for the spin manipulation of vertical-cavity surface-emitting lasers(VCSELs) by operating them electrically and injecting additional spin-polarized carriers by polarized optical excitation. The output polarization of the VCSELs can be easily controlled by the spin orientation of the optically injected carriers when the injection current does not exceed the threshold current.
92(2008); http://dx.doi.org/10.1063/1.2839404View Description Hide Description
Enhancement of the light emission of ZnOfilms was observed by coupling through localized surface plasmons. By sputteringAg islands onto ZnOfilms, their band gap emission coming through the Ag island films was enhanced by threefolds, while the defect emission was quenched. The enhancement was found to be mainly dependent on the sputtering time of the Ag islands, which is related to the island size. Furthermore, the relative spectral position between the ZnO emission band and the localized surface plasmon resonance bands of Ag islands was found to be decisive for the enhancement or quenching of photoluminescence, indicating that the emission intensity of ZnOfilms can be controlled by the Ag island size and the localized surface plasmon resonance band position.
Organic-inorganic hybrid transparent materials containing chromium ion-doped forsterite crystals as gain media92(2008); http://dx.doi.org/10.1063/1.2839433View Description Hide Description
Organic-inorganichybrid transparent materials containing chromium ion-doped forsterite crystals are investigated as gain media. Thin films and bulk samples are prepared by a sol-gel technique. Starting sols for host matrices, which are prepared from diphenyldimethoxysilane, 3-glycidoxypropyltrimethoxysilane, and titanium tetra--butoxide, are mixed with flame spray made nanocrystals ultrasonically. The hybrid materials containing with high transparency are realized by matching refractive indices of the host matrices with that of by controlling starting sols’ composition. A broad emission centered at is confirmed under the excitation at .