The effect of ZnS/Ag/ZnS multiple-layer coating on the top-emitting top-cathode organic light emitting
diodes (OLED) was studied. The OLED device consisted of Ag/CuPc:F4-TCNQ/NPB/Alq3/BCP/LiF/Al
layers. All organic layers and electrodes were fabricated by thermal evaporation. F4-TCNQ was doped in
the hole-injection layer (CuPc) to enhance hole injection, since the energy barrier between Ag and CuPc was
high. ZnS layer was first deposited on the top cathode (Al) and found to enhance the light emission of the
OLED by 50% (from 10,000 cd/m2 to 15,000 cd/m2). The high-refractive index dielectric material as a
capping layer enhances light output for the semitransparent cathode. ZnS/Ag/ZnS multi-layer cathode with
photon tunneling characteristics were added on top of Al cathode, and found to further enhance the light
emission up to 20,000 cd/m2 at 13V for Al/ZnS/Ag/ZnS (17/37/8/37 nm) layers with maximum current
efficiency of 2.6 cd/A. Coupling of surface plasmon modes may occur in the ZnS/Ag/ZnS structure. By
increasing Ag layer thickness to compensate the reduction of Al layer thickness, the Al/ZnS/Ag/ZnS
(7/37/15/37 nm) cathode was used, and found to achieve the maximum brightness of 31,000cd/m2 at 15V and
a maximum current efficiency at 5.6 cd/A. The increase of luminescence efficiency is likely due to high
photon tunneling efficiency of Ag as well as its high electric conductivity improving the electron injection.
Keywords: OLED, top emission, top cathode, tunneling, surface plasmon.
The optically-induced coherent spin dynamics in a charged quantum dot (QD) is studied theoretically using
a new dynamical model for rigorous description of circularly polarized ultrashort optical pulse resonant interactions
with the electron-trion system. Generalized pseudospin master equation is derived for description of
the time evolution of spin coherences and spin populations in terms of the real state pseudospin (coherence)
vector including dissipation in the system through spin relaxation processes. The equation is solved in the time
domain self-consistently with the vector Maxwell equations for the optical wave propagation coupled to it via
macroscopic medium polarization. Using the model the long-lived electron spin coherence left behind a single
resonant ultrashort optical excitation of the electron-trion transition in a charged QD is simulated in the lowand
high-intensity Rabi oscillations regime. Signatures of the polarized photoluminescence (PL) resulting from
the numerical simulations, such as the appearance of a second echo pulse after the excitation and a characteristic
PL trace shape, specific for initial spin-up orientation, are discussed for realization of high-fidelity schemes for
coherent readout of a single spin polarization state.
The first implementation of a single photon avalanche diode (SPAD) is reported in 130nm CMOS technology. The
SPAD is fabricated as p+/nwell junction with octagonal shape. Premature edge breakdown is prevented through a guard
ring of p-well around the p+ anode. The dynamics of the new device are investigated using both active and passive
quenching methods. Single photon detection is achieved by sensing the avalanche using a fast comparator. The SPAD
exhibits a maximum photon detection probability of 41% and a typical dark count rate of 100kHz at room temperature.
Thanks to its timing resolution of 144ps (FWHM), the SPAD can be used in disparate disciplines, including medical
imaging, 3D vision, biophotonics, low-light-illumination imaging, etc.
A chemical and biological sensor based on a free-space waveguide resonant grating optical filter has been developed. Different from the conventional surface plasmon resonance sensors and the conventional waveguide mode sensors, which require either prism coupling or grating coupling, the proposed free-space optical sensor device does not require special separate coupling. The inherent 100% coupling efficiency at resonance can significantly boost probe efficiency. Both simulation and experimental results have demonstrated that the new sensor could deliver a resolution of better than 0.001 to 0.0001 for refractive index sensing, which is enough for detecting various chemical and biological materials. More importantly, under an angular detection scheme, the proposed waveguide resonant grating sensor could be one order's more sensitive than the conventional surface plasmon resonance sensor.
In this paper, we will present some studies of physics at the interfaces in the organic light emitting
devices. The paper can be separated into two parts. First part is the manipulation of interfacial energy
structures and electron transport properties of organic semiconductors. The second part is substitution
and dopant dependence of electronic structures in organic thin films
I will present an investigation of the energy structures and electrical doping mechanisms of the
organic semiconductor surface through current-voltage (I-V) characteristics and photoemission
spectroscopies. We found that both surface energy structures and transport properties can be
manipulated with mix of LiF or Cs2CO3. The I-V characteristics show that the current efficiency is
significantly improved with Cs2CO3 doped either at the surface or in the bulk Alq3. As Cs2CO3 doping
works efficiently with Al as well as other cathode metals, the interfacial chemistry and carrier injection
mechanisms of such cathode structures are compared to that of the conventional LiF thin layers.
To understand the mechanisms of the improvement on electron injection, the surface energy
levels of metal and organic materials were measured with ultraviolet photoemission spectroscopy
(UPS) and the interfacial chemistry was studied with X-ray photoemission spectroscopy (XPS). From
UPS spectra, we found that a thin layer of Cs2CO3, as thin as 0.5 A, at the metal and organic ETL
interface can bring the Fermi level of Alq3 from mid-gap to less than 0.2 eV below the lowest
unoccupied molecular orbital (LUMO), indicating that the Alq3 film at the interface is heavily n-type
doped with Cs2CO3 . The smaller gap between the Fermi level and LUMO with Cs2CO3 reduces the
electron injection barrier. Strong dipole fields are also found at the surface, which also affects the
electron injection considerably. The XPS data further show that Cs ions are dissociated at the interface
as soon as Cs2CO3 is deposited on Alq3. The result is different from the case of LiF, in which Al metal
is needed for releasing Li ions. With co-evaporation of Cs2CO3 with Alq3 in the bulk as n-doping ETL,
the current efficiency can be further improved, which is presumably attributed to the enhancement of
the electron transport in the Alq3 films.
In this paper we report the properties of photoconductive detectors fabricated on GaN and AlGaN films produced
by plasma assisted MBE. The spectral dependence of such devices shows a sharp increase over many orders of
magnitude at the gap of the semiconductor but it remains constant at shorter wavelengths consistent with absence of
surface recombination. The mobility-lifetime product, which is the intrinsic figure of merit of the photoconductive gain,
decreases monotonically with the resistivity of GaN films. This result is attributed to the existence of exponential tails
due to potential fluctuations arising from stacking faults, point defects and impurities. In the case of AlGaN alloys
similar dependence of the mobility-lifetime product on film resistivity has been observed. However, the mobility-lifetime
product for films with AlN mole fraction close to 50% is about two orders of magnitude higher than that of GaN films
with comparable resistivity. This result was accounted for by the longer lifetime of the photogenerated carriers due to the
partial atomic ordering in these alloys. The band structure of the ordered and random domains form a type-II
heterostructure and thus photogenerated electrons and holes in these detectors are physically separated, leading to an
increase in recombination lifetime.
Long-wavelength InGaAlAs-InP vertical-cavity surface-emitting lasers (LW-VCSELs), designed for applications in gas
sensing and for optical interconnects are presented. These lasers cover the wavelength-range from 1.3 to 2.3 μm. With
2.3 μm, this is the longest wavelength ever achieved with an InP-based interband laser. Fabricated with a novel highspeed
design with reduced parasitics, bandwidths in excess of 11 GHz at 1.55 μm have been achieved. To the best of our
knowledge, this is the best dynamic characteristic for a 1.55 μm VCSEL ever presented. As a proof-of-concept one- and
two-dimensional arrays have been fabricated with high yield. All devices use for current confinement a buried tunnel
junction (BTJ). This concept, together with a dielectric backside reflector with integrated electroplated gold heat sink for
thermal management enables continuous wave (CW) operation at room-temperature with typical single-mode output
powers above 1 mW. The operation voltage is around 1 V and power consumption is as low as 10 - 20 mW. Error-free
data-transmission at 10 Gbit/s over 20 km is demonstrated, which can be readily applied in uncooled Coarse Wavelength
Division Multiplex Passive Optical Networks (CWDM PONs). The functionality of tunable diode laser spectroscopy
(TDLS) systems is verified by presenting a laser hygrometer using a 1.84 μm wavelength VCSEL.
We report the recent progress of GaN-based VCSELs with two different laser structures. One is a hybrid cavity structure
comprised an epitaxial AlN/GaN DBR, an InGaN/GaN MQW active region and a top dielectric DBR. Another is a
dielectric cavity structure comprised an InGaN/GaN MQW layer sandwiched by two dielectric DBRs. Both lasers
achieved laser action under optical pumping at the room temperature with narrow linewidth. The detailed characteristics
of VCSELs will be reported. The status of the electrically pumped VCSEL will also be presented.
We analyze the previously measured performance of a set of gallium nitride based laser diodes emitting ultraviolet light
between 360 nm and 380 nm wavelength. The wavelength variation was accomplished by varying the indium content of
the InGaN quantum wells which are embedded in AlInGaN barriers. The experiments revealed a strong increase in
threshold current with shorter wavelength. Our analysis of this behavior utilizes advanced numerical laser simulation.
General models for quaternary AlInGaN material properties are developed and result in good agreement between
simulation and measurement. The measured rise in threshold current with shorter wavelength is found to have two main
reasons. The first reason is the increased absorption of ultraviolet light inside the laser, mainly within p-doped layers.
The second mechanism contributing to the performance deterioration is the leakage of carriers from the active region. In
particular, the hole leakage is found to strongly increase with lower indium mole fraction of the quantum well (shorter
wavelength), due to the reduced valence band offset.
Semiconductor optical amplifiers (SOAs) having nano-sized quantum dot (QD) particles show attractive features such as
the achievement of a steady temperature characteristic, low power consumption, and a high-speed response to the input
signal. QD active layers were designed to 15 stacks of InAs QDs, AlGaAs/GaAs double hetero structure. QD-SOAs
were fabricated for optical triode that can be used with a 1.3 μm band. Modulation results extracted by input, control and
output waveforms support the fact that cross-gain modulation and negative feedback amplification effect can be strongly
contributed to obtain essential factors for future application such as dramatical baseline suppression of output signal and
satisfying high modulation. Optical triode revealed inverted type characteristics that high output power can readily
obtained by be increased quite small amount of input power while output power rarely change though input power
increase high. Fulfillment of far more upgraded stable high-speed bit rate was completed by optical triode improved by
QD-SOA and it's containing cross-gain modulation effect. 40 Gbps performance optical triode will be the accelerator for
realization of the functions of regeneration, reshaping, multiple-wavelength processing, wavelength conversion, and
demultiplexing of high-bit rate patterned optical signals.
We propose a novel design for a guided-mode resonance (GMR) grating sensor that extends the sensitivity to a
large region of space, possibly several tens of microns away from the grating surface. This type of sensors has
high sensitivity in the half-space above the grating, close to the theoretical limit, together with a controllable -
potentially very high - quality factor. It relies on a resonance caused by a "confined" mode of a sub-wavelength
thick grating slab, a mode that is largely expelled from the grating itself into the grating environment. The small
thickness assumption allows us to derive a simple yet accurate analytical model for the sensor behavior, which
is tested numerically using a rigorous coupled-wave analysis (RCWA) method as well as in preliminary grating
In this Letter, the identification device disclosed in the present invention is comprised of: a carrier
and a plurality of pseudo-pixels; wherein each of the plural pseudo-pixels is formed on the carrier
and is further comprised of at least a light grating composed of a plurality of light grids. In a
preferred aspect, each of the plural light grids is formed on the carrier while spacing from each
other by an interval ranged between 50nm and 900nm. As the aforesaid identification device can
present specific colors and patterns while it is being viewed by naked eye with respect to a
specific viewing angle, the identification device is preferred for security and anti-counterfeit
applications since the specific colors and patterns will become invisible when it is viewed while
deviating from the specific viewing angle.
Single photon detection was realized at a telecom wavelength with quantum dot resonant tunneling diodes grown on an
InP substrate. The structure contains a AlAs/In0.53Ga0.47As/AlAs quantum well with InAs quantum dots grown on the top
AlAs barrier. The single photon detection efficiency of the device under 1310 nm illumination was measured to be about
0.35%±0.07% with a dark count rate of 1.58×10-6 ns-1. This corresponds to an internal efficiency of 6.3%.
Poly Methyl Methacrylate (PMMA) is an advantageous material than glass in oceanographic sensing applications
because of its inhospitality for marine organisms. Waveguide sensors fabricated in PMMA are often used to
measure the parameters in ocean such as PH, CO2, O2 concentrations, etc. A tightly-focused femtosecond laser
is often used to produce such a waveguide or even more complicated structures through the nonlinear effect in
the bulk of PMMA, with pulse energy at μJ or mJ level. And such a laser system requires the amplifier from
chirped-pulse amplification (CPA). The oscillator itself can produce pulse energy only at nJ level which is under
the threshold of nonlinear effect. However, in our experiment, a modification to the oscillator cavity, which
elongates the cavity length approximately 3 times and as a result, decreases the repetition rate from 93mHz to
32 mHz, can increase the pulse energy to 15 nJ. Under a tight focusing lens (100x 1.40 microscope objective),
such an intensity exceeds the nonlinear threshold of PMMA. Thus, waveguide can be fabricated in PMMA using
only a femtosecond oscillator and oceanographic sensors can be also made by this simple technique.