William M. Dennis

Saturation of 1.064 μm absorption in Cr,Ca:Y3Al5O12 crystals

Saturation absorption experiments are conducted on the title compound at 1.064 μm with laser light propagating along both the [001] and [111] crystallographic axes. Rotation of the crystals about these axes during the experiments reveals that the transmission of the polarized 1.064 μm radiation is highly anisotropic in the saturation regime. The details of the anisotropy and saturation curves are explained using a simple model of subsets of optical centers along with a relevant set of rate equations. The ground and excited‐state absorption cross sections at 1.064 μm are calculated to be σgs=5.7(±2)×10−18 cm2 and σes=8(±2)×10−19 cm2, respectively, and the optimum crystallographic orientation for utilization in optical devices is discussed.

Trapping processes in CaS:Eu2+,Tm3+

CaS:Eu2+,Tm3+ is a persistent red phosphor. Thermoluminescence was measured under different excitation and thermal treatment conditions. The results reveal that the charge defects, created by substituting Tm3+ for Ca2+, serve as hole traps for the afterglow at room temperature. Tm3+ plays the role of deep electron trapping centers, capturing electrons either through the conduction band or directly from the excited Eu2+ ions. These two processes, in which two different sites of Tm3+ are involved, correspond to two traps with different depths.

Electron-phonon interaction in rare earth doped nanocrystals

Nanoscale materials exhibit properties which differ considerably from their bulk counterparts due to modifications of their phonon density of states due to finite size effects. Rare earth doped insulating nanoparticles provide an ideal model system for studying the fundamental interactions between electronic states and phonons since the narrow lines due to rare earth 4fn→4fn transitions provide a high-resolution probe, while the insulating host eliminates the additional complications of collective electronic interactions. In this paper we investigate how the size restricted geometry of the nanoparticle influences (i) electronic relaxation by the one-phonon emission process and (ii) optical dephasing by the two-phonon Raman process.

One-color sequential pumping of the 4f5d bands in Pr-doped yttrium aluminum garnet

Abstract We report on a method for populating the 4f5d bands in Pr-doped yttrium aluminum garnet (Pr 3+ :YAG) through a one-color twp-photon absorption process. Broad ultraviolet emission (between 300 and 400 nm) is obtained by pumping the crystal with a source between 470 and 490 nm. The dynamics of the two-photon process is studied by monitoring the transient rise and decay of the fluorescing populations under chopped CW excitation. The transient data show that the two photons are absorbed sequentially with either the 3 P 1 or 3 P 0 level acting as a metastable intermediate state. An excitation spectrum for the 314 nm emission from the lowest 4f5d band is compared to the excitation spectrum for the 615 nm emission from the 3 P 1 and 3 P 0 levels. Strong enhancement in UV emission occurs when the laser is resonant with transitions to the 3 P 1 or 3 P 0 levels. A model for the dynamics of the sequential two-photon pumping process is proposed. The model predicts that the population of UV fluorescing ions is indirectly controlled by energy transfer processes involving ions in the 1 D 2 state.

ONE PHONON RELAXATION PROCESSES IN Y2O3 : EU3+ NANOCRYSTALS

Abstract The phonon spectrum for isolated nanoparticles is expected to become discrete with a gap opening up at very low frequencies. In this paper we investigate the effect of the modified phonon spectrum on the relaxation of closely spaced electronic levels. We compare the results for nanoparticles with those for larger (∼μm) size particles with the same crystal structure.

Transient luminescence line shapes for Mn2+ ions in ZnS nanocrystals

Abstract Recent time-resolved luminescence spectra reported by Yu et al. from ZnS : Mn nanocrystals embedded in a polymer matrix showed the presence of two emission bands that merge into a single band as time passed following an intense laser excitation pulse. We have calculated the bandshapes as a function of time assuming a nonequilibrium initial distribution for the excited vibrational states. Our calculated bandshapes fit quite well to those observed experimentally and support the explanation given by Yu et al. This would seem to be an ideal system in which to study the approach to thermal equilibrium in nanocrystals.

Tailoring terahertz plasmons with silver nanorod arrays

Plasmonic materials that strongly interact with light are ideal candidates for designing subwavelength photonic devices. We report on direct coupling of terahertz waves in metallic nanorods by observing the resonant transmission of surface plasmon polariton waves through lithographically patterned films of silver nanorod (100 nm in diameter) micro-hole arrays. The best enhancement in surface plasmon resonant transmission is obtained when the nanorods are perfectly aligned with the electric field direction of the linearly polarized terahertz wave. This unique polarization-dependent propagation of surface plasmons in structures fabricated from nanorod films offers promising device applications. We conclude that the anisotropy of nanoscale metallic rod arrays imparts a material anisotropy relevant at the microscale that may be utilized for the fabrication of plasmonic and metamaterial based devices for operation at terahertz frequencies.

Noninertial mechanism for electronic energy relaxation in nanocrystals

The low-frequency vibrational spectrum of an isolated nanometer-scale solid differs dramatically from that of a bulk crystal, causing the decay of a localized electronic state by phonon emission to be inhibited. We show, however, that an electron can also interact with the rigid translational motion of a nanocrystal. The form of the coupling is dictated by the equivalence principle and is independent of the ordinary electron-phonon interaction. We calculate the rate of nonradiative energy relaxation provided by this mechanism and establish its experimental observability.

Atomic level stress and light emission of Ce activated SrS thin films

We find that the Ce3+ ion in polycrystalline sputtered SrS:Ce thin films resides in a distorted octahedral environment, as opposed to the cubic host environment. Using electron paramagnetic resonance and x-ray diffraction analysis, we show that the degree of axial distortion is related to the preferential growth direction of the SrS films. To first order, the blue-emission properties (emission wavelength and decay times) of the SrS:Ce films do not appear to be affected by the amount of distortion in the local Ce3+ environment.

Infrared to violet up-conversion in YLiF4 : Nd3+

Abstract We investigate ultraviolet and visible up-conversion in YLiF4 : Nd3+ (1%). The energy of the ultraviolet emission is more than twice the energy of the infrared pump light. The up-conversion is determined to be dominated by an energy transfer process. Our experimental results are compared with a rate equation analysis.