Photonic crystal (PhC) cavities made in broadband luminescent material offer attractive possibilities for flexible active
devices. The luminescence enables the cavity to operate as an autonomous entity. New applications of this property are
demonstrated for cavities made in the InGaAsP underetched semiconductor membrane with embedded InAs Quantum
Dots that emit in the range of 1400-1600 nm.
Planar photonic crystal membrane nanocavities were released from the parent chip by mechanical nanomanipulation.
The released cavity particle could be bonded on an arbitrary surface, which was exploited to make a novel fiber-optic tip
sensor with a PhC cavity attached to the tip.
A single mode from a short cavity is shown to couple simultaneously to at least three cavity modes of a long cavity, as
concluded from level anticrossing data when the small cavity was photothermally tuned.
Reconfigurable and movable cavities were created by locally varying the infiltration status by liquid oil near a PhC
waveguide or defect cavity. Liquid was displaced locally on a micron scale using capillary force effects or laser-induced
evaporation and condensation phenomena.
Results are presented on the use of InGaAsP photonic crystal nanobeam slot waveguides for refractive index
sensing. These sensors are read remote-optically through photoluminescence, which is generated by built-in InGaAs
quantum dots. The nanobeams are designed to maximize the electromagnetic field intensity in the slot region, which
resulted in record-high sensitivities in the order of 700 nm/RIU (refractive index unit). A cavity, created by locally
deflecting the two beams towards each other through overetching, is shown to improve the sensitivity by about 20%.
Liquid crystal (LC, Merk 5 CB) is infiltrated into active, InAs quantum dots embedded, InGaAsP membrane type
nanocavities to investigate the possible effect of the LC orientation on active cavity tuning. The tuning is demonstrated
thermally and thermo-optically. The thermal tuning showed that the cavity modes can be tuned in opposite directions and
exhibits a sudden change at the clearing temperature. The mechanism relies on the existence of both ordinary and
extraordinary refractive indices of the liquid crystal due to its molecular alignment inside the voids. It shows that the
electric field distribution of cavity modes can have a substantial component parallel to the LC director. The average
electric field orientation with respect to the LC orientation can be mode dependent, so that different modes can be
dominated by either branch of the LCs refractive index. Thermo-optic tuning of the modes is obtained when the power of
the excitation laser is increased from 40 μW to 460 μW. A large and a reversible blueshift of more than 10 nm of the
cavity modes is observed which is attributed to temperature induced liquid transport. InGaAsP type of nanocavities,
without InAs quantum dots were infiltrated with PbSe colloidal quantum dots to obtain a comparison of internal light
sources either in the semiconductor or in the holes.
Number and size control of InAs quantum dots (QDs) on truncated InP pyramids grown by selective area
Metal Organic Vapor Phase Epitaxy (MOVPE) is reported. The facet composition of the pyramid top surface
and the relative facet sizes are determined by the shape of the pyramid base and the pyramid height for a
certain base size. This allows the precise position and distribution control of the QDs due to preferential
nucleation on the {103} and {115} facets. The size of the QDs is adjusted by the growth parameters, e.g.,
InAs amount and growth rate together with the pyramid top surface size. The QD number, related to the
specific shape of the pyramid top surface, is reduced by the shrinking pyramid top surface size during growth.
Well defined positioning of four, three, two, and single QDs is realized successfully. Regrowth of a passive
InP structure around the pyramids establishes submicrometer-scale active-passive integration for efficient
microcavity QD nanolasers and single photon sources operating in the 1.55-μm telecom wavelength region
and their implementation in photonic integrated circuits.
Hexagonal symmetry InGaAsP membrane type cavities with embedded InAs quantum dots as active emitters were investigated by room temperature photoluminescence experiments at wavelengths near 1.50 µm. Cavities consisting of simple defects of just removing one or seven air holes were studied as well as modified cavities with additional holes decreased in size and shifted in position. The latter include the H0 cavity, in which only two adjacent holes were modified, but none removed. Low-Q cavity modes were observed for the simple cavities while high-Q modes were observed after modification of the surrounding holes. The resonant frequencies were varied over a large range of lithographic parameters both by changing the lattice spacing or the size of the modified holes. More than 15 nm reversible dynamic optical tuning of the resonance modes was observed by changing the applied laser power up to 5 mW. For thermo-optic tuning, this corresponds to a heating of up to 200 °C.
Photonic crystal (PC) devices in the InP/InGaAsP/InP planar waveguide system exhibiting narrow bandwidth
features were investigated for use as ultrasmall and tunable building blocks for photonic integrated circuits at
the telecom wavelength of 1.55 μm. The H1 cavity, consisting of a single PC-hole left unetched, represents
the smallest possible cavity in a dielectric material. The tuning of this cavity by temperature was investigated
under the conditions as etched and after the holes were infiltrated with liquid crystal (LC), thus separating the
contributions of host semiconductor and LC-infill. The shift and tuning by temperature of the MiniStopBand
(MSB) in a W3 waveguide, consisting of three rows of holes left unetched, was observed after infiltrating the PC
with LC. The samples finally underwent a third processing step of local wet underetching the PC to leave an
InGaAsP membrane structure, which was optically assessed through the ridge waveguides that remained after
the under etch and by SNOM-probing.
The importance of polarization manipulation is increased as optical fiber systems evolve to higher data rates. Photonic integrated circuits should be insensitive to the state of polarization of the light emanating from fibers if they are to be used as detectors, add-drop mutiplexers or cross connects. Either all the integrated components have to be polarization independent or only one polarization direction should be used. For either method, a compact polarization converter is useful. A model is developed for the mode propagation in the waveguide of the slanted side polarization converter based on double hetero structures. The model is extended to include polarization dependent absorption and mismatch. Polarization converters on different double heterostructures (with and without quantum wells) have been designed and are realized by contact optical lithography. The performance of the realized converters is well described with the model.
In this paper an overview is given of the results we have obtained at the COBRA Research Institute in our work on
passively modelocked semiconductor lasers operating in the 1.5 μm wavelength region. Most results concern
modelocked ring lasers that are realized monolithically in the InP/InGaAsP materials system as well as simulations using
lumped element and traveling wave type models. The experimental results show that the ring lasers appear as the more
stable type of lasers. The modeling results show the importance of using a symmetrical configuration in the ring laser for
stable operation. Most recent results on linear modelocked quantum dot lasers at 1.5 μm indicate the improvements
possible using these materials.
A tunable Mach-Zehnder wavelength duplexer has been realized based on P-i-n-N InGaAsP/InP. It has been
made polarization insensitive by proper wafer layer stack and proper waveguide geometry. The layer stack for
the duplexer was tested first with a waveguide phase shifter, which resulted in up to 36°/(V•mm) phase shifting
efficiency for TE polarization, which is slightly more efficient than the most efficient phase shifter reported to
date in bulk InP at 1.55 μm, and with much lower transmission loss[1]. The transmission loss was measured to
be 4 dB/cm (5 dB/cm) for TE (TM) polarized light, for 2 μm wide shallowly etched waveguides, which is rather
low compared to other reported high efficiency phase shifters for this material system. With this layerstack, we
designed a Mach-Zehnder (MZ) duplexer with narrow, 1.5 μm wide, deeply etched phase shifters that meet the
polarization insensitivity requirement. The measurement results showed that the phase shifting efficiency of this
narrow and deeply etched duplexer is up to 34°/(V•mm) for both TE and TM polarization, and the transmission
loss of this 1.5 μm wide waveguide is about 10 dB/cm for both TE and TM polarization. This is also the first
reported deeply etched narrow phase shifter with high phase shifting efficiency and relatively low loss.
Lasing and sharp line emission in the 1.55-μm wavelength region is demonstrated from ensembles and single InAs
quantum dots (QDs) embedded in InGaAsP on InP (100) by metalorganic vapor phase epitaxy (MOVPE). Wavelength tuning of the QDs is achieved through the insertion of ultra-thin (1-2 monolayers) GaAs interlayers underneath the
InAs QDs. To increase the active volume widely-stacked QD layers are identically reproduced. Closely-stacked QDs
reveal unpolarized emission from the cleaved side due to vertical electronic coupling which is important for polarization
insensitive semiconductor optical amplifiers. Fabry-Perot narrow ridge-waveguide lasers implementing five layers of
widely-stacked QDs as gain medium operate in continuous wave mode at room temperature with low threshold current,
low transparency current density of 6 A/cm2 per QD layer, and low loss of 4.2 cm-1, which are accompanied by a 80 nm
wide gain spectrum. Device performance does not suffer from sidewall recombination in deeply-etched QD lasers which
possess similar threshold currents as shallowly-etched ones and do not deteriorate with time. This allows the fabrication
of mono-mode and compact devices with small bending radii, as demonstrated by the operation of a QD ring laser with
40-GHz free spectral range. Micro-PL of single QDs exhibits sharp exciton-biexciton emission around 1.55 μm
persisting to temperatures above 70 K; the prerequisite for single photon sources working at liquid nitrogen temperature
for fiber-based quantum information and cryptography systems.
We observe an anomalous exciton lifetime in columnar InAs/GaAs quantum rods by using the time-resolved differential reflectivity (TRDR) technique. At low excitation density, the decay time of the TRDR signal show a clear excitonic type of behavior. At increased excitation, a resonant radiative lifetime develops, which seems to be dependent on the excited QD density being resonant with the probe laser spectrum. We interpret the resonance behavior as due to electromagnetic coupling between the excited QDs resonant with the probe laser.
The filling is reported of the air holes of an InP-based two-dimensional photonic crystal with solid polymer and with liquid crystal 5CB. The polymer filling is obtained by thermal polymerization of an infiltrated liquid monomer, trimethylolpropane triacrylate. The filling procedure for both the monomer and liquid crystal relies on the capillary action of the liquid inside the ~ 200 nm diameter and < 2.5 μm deep air holes. The solid polymer infiltration result was directly inspected by cross-sectional scanning electron microscopy. It was observed that the holes are fully filled to the bottom. The photonic crystals were optically characterized by transmission measurements around the 1.5 μm wavelength band both before and after infiltration. The observed high-frequency band edge shifts are consistent with close to 100% filling, for both the polymer and the liquid crystal. No differences were observed for filling under vacuum or ambient, indicating that the air diffuses efficiently through the liquid infiltrates, in agreement with estimates based on the capillary pressure rise.
Polymer filling of the air holes of indiumphosphide based two-dimensional photonic crystals is reported. The filling is
performed by infiltration with a liquid monomer and solidification of the infill in situ by thermal polymerization.
Complete hole filling is obtained with infiltration under ambient pressure. This conclusion is based both on cross-sectional
scanning electron microscope inspection of the filled samples as well as on optical transmission
measurements.
We study a set of low temperature (LT, 250°C) Stranski-Krastanow InAs/GaAs quantum dots (QDs) grown using molecular beam epitaxy (MBE). The QDs are studied by Photoluminescence (PL) and Time Resolved Differential Reflectivity (TRDR) for obtaining the carrier dynamics also. The LT-growth is expected to combine an ultrafast response time with a large QD optical nonlinearity, making it a good candidate for ultrafast all-optical switching devices. We observe a QD photoluminescence peak around 1200 nm on top of a background due to the AsGa-VAs center. We observe that the PL-efficiency is quenched above 30K. The PL-efficiency increases by a factor of 45 - 280 as a function of excitation wavelength around the GaAs bandgap, for different samples. This points towards good optical quality QDs, which are embedded in an LT-GaAs barrier with high trapping efficiency. In the TRDR measurements, we observe an initial fast decay (80ps) followed by a much slower decay of about 800ps. The strong temperature dependence of the PL-signal is not observed in the reflectivity signal. This leads us to conclude that the electrons tunnel out of the QD and are subsequently efficiently trapped by As antisite defects while the hole decay dynamics take place at a slower rate, which is monitored in TRDR. Our observations point towards QDs with good optical quality, embedded in a LT-GaAs barrier in which the carriers are efficiently trapped at anti-site defects.
We present the heterogeneous integration of InP/InGaAsP photodetectors onto ultracompact Silicon-on-Insulator (SOI) waveguide circuits using benzocyclobutene (BCB) die to wafer bonding. This technology development enables the integration of a photonic interconnection layer on top of CMOS. Fabrication processes were optimized and the transfer of a passive Silicon-on-Insulator waveguide layer using BCB was assessed.
Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B by molecular beam epitaxy (MBE). During stacking the SL template self-organizes into a highly ordered two-dimensional (In,Ga)As and, thus, strain field modulation on a mesoscopic length scale, constituting a Turing pattern in solid state. InAs QDs preferentially grow on top of the SL template nodes due to local strain recognition, forming a lattice of separated groups of closely spaced ordered QDs. The SL template and InAs QD growth conditions like number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers are studied in detail for optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy (AFM). The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution X-ray diffraction (XRD). Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature dependent PL measurements exhibit an unusual behavior of the full-width at half-maximum, indicating carrier redistribution solely within the QD molecules.
Carrier dynamics in single quasi-1D GaAs quantum wires are studied in a wide temperature range by near-field scanning optical microscopy using pico- and femtosecond pulses. Luminescence and pump-probe experiments with a spatial resolution of 250 nm and a time resolution of up to 200 fs allow for a separation of carrier transport along the quantum wire and in the embedding GaAs quantum well from local carrier relaxation phenomena. We demonstrate that local potential barriers close to the quantum wire strongly affect the lateral carrier transport into the quantum wire. This drift-diffusive motion occurs in the pico- to nanosecond regime with diffusion lengths of up to several microns. Diffusive transport along the quantum wire is characterized by carrier motion on a somewhat shorter picosecond time scale. In contrast, sub-picosecond relaxation times are found for the redistribution of carriers from high-lying to low-lying quantum wire states. This relaxation is governed by electron-electron and electron-phonon scattering.
Semiconductor surfaces develop orientation dependent morphologies during growth that can be used for fabrication of nanostructured materials. We have applied RHEED techniques to study during MBE the nanometer-scale morphologies on non-(001)-oriented GaAs surfaces and their evolution during heterogeneous deposition of AlAs. In particular, the growth behavior of the GaAs(311)Ga surface enables a controlled generation of an ordered surface corrugation. We report on two new methods for a direct synthesis of 1D structures that take advantage of an in- situ lateral patterning of the GaAs substrate: the growth of corrugated GaAs/AlAs heterostructures on the GaAs(311)Ga surface and the synthesis of doping wires by the combination lattice step growth on vicinal GaAs(001) surfaces with planar doping.
The evolution of ordered surface and interface structures on (111), (211), and (311) GaAs during molecular beam epitaxy offers the unique possibility to directly synthesize GaAs quantum wires and quantum dots in an AlAs matrix. We show that well ordered alternating thicker and thinner regions of GaAs and AlAs form symmetric and asymmetric quantum-dot structures on (111) and (211) substrates, respectively, and quantum-wire structures on (311) substrates. The observed optical properties confirm the lateral size quantization in these GaAs/AlAs multilayer structures.
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