531 research outputs found
Spatial imaging of modifications to fluorescence lifetime and intensity by individual Ag nanoparticles
Highly ordered periodic arrays of silver nanoparticles have been fabricated
which exhibit surface plasmon resonances in the visible spectrum. We
demonstrate the ability of these structures to alter the fluorescence
properties of vicinal dye molecules by providing an additional radiative decay
channel. Using fluorescence lifetime imaging microscopy (FLIM), we have created
high resolution spatial maps of the molecular lifetime components; these show
an order of magnitude increase in decay rate from a localized volume around the
nanoparticles, resulting in a commensurate enhancement in the fluorescence
emission intensity.Comment: 3 pages, 2 figures, submitted Applied Physics Letter
Plasmonic engineering of metal nanoparticles for enhanced fluorescence and Raman scattering
We have investigated the effects of tuning the localized surface plasmon
resonances (LSPRs) of silver nanoparticles on the fluorescence intensity,
lifetime, and Raman signal from nearby fluorophores. The presence of a metallic
structure can alter the optical properties of a molecule by increasing the
excitation field, and by modifying radiative and non-radiative decay
mechanisms. By careful choice of experimental parameters we have been able to
decouple these effects. We observe a four-fold increase in fluorescence
enhancement and an almost 30-fold increase in decay rate from arrays of Ag
nanoparticles, when the LSPR is tuned to the emission wavelength of a locally
situated fluorophore. This is consistent with a greatly increased efficiency
for energy transfer from fluorescence to surface plasmons. Additionally,
surface enhanced Raman scattering (SERS) measurements show a maximum
enhancement occurs when both the incident laser light and the Raman signal are
near resonance with the plasmon energy. Spatial mapping of the SERS signal from
a nanoparticle array reveals highly localized differences in the excitation
field resulting from small differences in the LSPR energy.Comment: Submitted to Plasmonics (Springer
Optical characteristics of single wavelength-tunable InAs/InGaAsP/InP(100) quantum dots emitting at 1.55 um
We have studied the emission properties of individual InAs quantum dots (QDs)
grown in an InGaAsP matrix on InP(100) by metal-organic vapor-phase epitaxy.
Low-temperature microphotoluminescence spectroscopy shows emission from single
QDs around 1550 nm with characteristic exciton-biexciton behavior, and a
biexciton antibinding energy of more than 2 meV. Temperature-dependent
measurements reveal negligible optical-phonon induced broadening of the exciton
line up to 50 K, and emission from the exciton state clearly persists above 70
K. Furthermore, we find no measurable polarized fine structure splitting of the
exciton state within the experimental precision. These results are encouraging
for the development of a controllable photon source for fiber-based quantum
information and cryptography systems.Comment: 3 pages, 4 figures, submitted AP
Fine structure and magneto-optics of exciton, trion, and charged biexciton states in single InAs quantum dots emitting at 1.3 um
We present a detailed investigation into the optical characteristics of
individual InAs quantum dots (QDs) grown by metalorganic chemical vapor
deposition, with low temperature emission in the telecoms window around 1300
nm. Using micro-photoluminescence (PL) spectroscopy we have identified neutral,
positively charged, and negatively charged exciton and biexciton states.
Temperature-dependent measurements reveal dot-charging effects due to
differences in carrier diffusivity. We observe a pronounced linearly polarized
splitting of the neutral exciton and biexciton lines (~250 ueV) resulting from
asymmetry in the QD structure. This asymmetry also causes a mixing of the
excited trion states which is manifested in the fine structure and polarization
of the charged biexciton emission; from this data we obtain values for the
ratio between the anisotropic and isotropic electron-hole exchange energies of
(Delta1)/(Delta0)= 0.2--0.5. Magneto-PL spectroscopy has been used to
investigate the diamagnetic response and Zeeman splitting of the various
exciton complexes. We find a significant variation in g-factor between the
exciton, the positive biexciton, and the negative biexciton; this is also
attributed to anisotropy effects and the difference in lateral extent of the
electron and hole wavefunctions.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
Charged exciton emission at 1.3 m from single InAs quantum dots grown by metalorganic chemical vapor deposition
We have studied the emission properties of self-organized InAs quantum dots
(QDs) grown in an InGaAs quantum well by metalorganic chemical vapor
deposition. Low-temperature photoluminescence spectroscopy shows emission from
single QDs around 1300 nm; we clearly observe the formation of neutral and
charged exciton and biexciton states, and we obtain a biexciton binding energy
of 3.1 meV. The dots exhibit an s-p shell splitting of approximately 100 meV,
indicating strong confinement.Comment: 3 pages, 3 figures, submitted AP
Longitudinal photocurrent spectroscopy of a single GaAs/AlGaAs v-groove quantum wire
Modulation-doped GaAs v-groove quantum wires (QWRs) have been fabricated with
novel electrical contacts made to two-dimensional electron-gas (2DEG)
reservoirs. Here, we present longitudinal photocurrent (photoconductivity/PC)
spectroscopy measurements of a single QWR. We clearly observe conductance in
the ground-state one-dimensional subbands; in addition, a highly
temperature-dependent response is seen from other structures within the
v-groove. The latter phenomenon is attributed to the effects of structural
topography and localization on carrier relaxation. The results of
power-dependent PC measurements suggest that the QWR behaves as a series of
weakly interacting localized states, at low temperatures
Strong coupling of localized plasmons and molecular excitons in nanostructured silver films
We report on the resonant coupling between localized surface plasmon
resonances (LSPRs) in nanostructured Ag films, and an adsorbed monolayer of
Rhodamine 6G dye. Hybridization of the plasmons and molecular excitons creates
new coupled polaritonic modes, which have been tuned by varying the LSPR
wavelength. The resulting polariton dispersion curve shows an anticrossing
behavior which is very well fit by a simple coupled-oscillator Hamiltonian,
giving a giant Rabi-splitting energy of ~400 meV. The strength of this coupling
is shown to be proportional to the square root of the molecular density. The
Raman spectra of R6G on these films show an enhancement of many orders of
magnitude due to surface enhanced scattering mechanisms; we find a maximum
signal when a polariton mode lies in the middle of the Stokes shifted emission
band.Comment: 4 pages, 4 figures, submitted PR
Carrier relaxation in GaAs v-groove quantum wires and the effects of localization
Carrier relaxation processes have been investigated in GaAs/AlGaAs v-groove
quantum wires (QWRs) with a large subband separation (46 meV). Signatures of
inhibited carrier relaxation mechanisms are seen in temperature-dependent
photoluminescence (PL) and photoluminescence-excitation (PLE) measurements; we
observe strong emission from the first excited state of the QWR below ~50 K.
This is attributed to reduced inter-subband relaxation via phonon scattering
between localized states. Theoretical calculations and experimental results
indicate that the pinch-off regions, which provide additional two-dimensional
confinement for the QWR structure, have a blocking effect on relaxation
mechanisms for certain structures within the v-groove. Time-resolved PL
measurements show that efficient carrier relaxation from excited QWR states
into the ground state, occurs only at temperatures > 30 K. Values for the low
temperature radiative lifetimes of the ground- and first excited-state excitons
have been obtained (340 ps and 160 ps respectively), and their corresponding
localization lengths along the wire estimated.Comment: 9 pages, 8 figures, submitted to Phys. Rev. B Attempted to correct
corrupt figure
Optically nonlinear Langmuir–Blodgett films: Influence of the substrate on film structure and linear optical properties
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