Excitations in spatially indirect transitions feature such as excitons in type-II heterostructures or charge-transfer excitations in molecular crystals feature a permanent dipole moment which influences the transition dipole moments commonly probed by optical spectroscopy. Epitaxially grown III-V quantum heterostructures featuring suitable band alignments are ideal model systems to study the interplay between the two. Selected topics discussed include the AC Stark effect and signatures of spatially indirect coherent biexciton states as well as the potential for coherent optical current injection dynamics.
Cubic nitrides are candidate materials for next-generation optoelectronic applications as they lack internal fields and promise to cover large parts of the electromagnetic spectrum from the deep UV towards the mid infrared. This demands high-quality epitaxial growth of c-GaN as base material. We demonstrate the influence of pre-growth treatments and c- AlN buffer layers on the quality of c-GaN grown on 3C-SiC/Si substrates by molecular beam epitaxy (MBE). Optimized parameters yield extremely small surface roughness values below 1 nm of phase pure c-GaN layers with very limited stacking fault densities. Structural properties have been studied by X-ray diffraction and atomic force microscopy and surpasses the current standards, which allows for growth of more complex quantum structures for device application.
Cubic InxGa1-xN alloys are a candidate material for optoelectronic applications because they lack internal polarization fields and promise to cover a vast range of emission wavelengths. However, the large discrepancy in interatomic spacing and growth temperatures of c-GaN and c-InN hinder InxGa1-xN-growth. We report cubic InxGa1-xN layers grown by plasmaassisted MBE and achieve continuous miscibility of the indium content x(In) over the whole composition range. X-ray diffraction precisely monitors the composition, phase purity and miscibility of the thin films. Furthermore, we discuss the impact of the indium content on the crystallinity. Complementary, low-temperature photoluminescence studies elucidate the optical response of cubic InxGa1-xN layers.
The nonlinear optical response of quantum well excitons is investigated experimentally using polarization resolved four wave mixing, optical-pump optical-probe, and optical-pump Terahertz-probe spectroscopy. The four-wave mixing data reveal clear signatures of coherent biexcitons which concur with straight-forward polarization selection rules at the Γ point. The type-I samples show the well-established time-domain beating signatures in the transients as well as the corresponding spectral signatures clearly. The latter are also present in type-II samples; however, the smaller exciton and biexciton binding energies in these structures infer longer beating times which, in turn, are accompanied by faster dephasing of the type-II exciton coherences. Furthermore, the THz absorption following spectrally narrow, picosecond excitation at energies in the vicinity of the 1s exciton resonance are discussed. Here, the optical signatures yield the well-established redshifts and blueshifts for the appropriate polarization geometries in type-I quantum well samples also termed “AC Stark Effect”. The THz probe reveals intriguing spectral features which can be ascribed to coherent negative absorption following an excitation into a virtual state for an excitation below the 1s exciton resonance. Furthermore, the scattering and ionization of excitons is discussed for several excitation geometries yielding control rules for elastic and inelastic quasiparticle collisions.
The nonlinear optical response of quantum well excitons excited by optical fields is analyzed by numerical solutions of the semiconductor Bloch equations. Differential absorption spectra are computed for resonant pumping at the exciton resonance and the dependence of the absorption changes on the polarization directions of the pump and probe pulses is investigated. Coherent biexcitonic many-body correlations are included in our approach up to third-order in the optical fields. Results are presented for spatially-direct type-I and spatiallyindirect type-II quantum well systems. Due to the spatial inhomogeneity, in type-II structures a finite coupling between excitons of opposite spins exists already on the Hartree-Fock level and contributes to the absorption changes for the case of opposite circularly polarized pump and probe pulses.
Two-dimensional (2D) materials are gaining great attention due to their extraordinary thickness-dependent properties. 2D Ga-VI semiconductors have bandgaps in the UV region making them candidates for several LED concepts. The Indium-containing counterparts of Ga-chalcogenides moreover have small electron effective masses and high mobilities. Hence, 2D III-chalcogenides are promising materials for next-generation optoelectronic applications. We establish metal organic chemical vapor deposition (MOCVD) to find suitable growth routines for 2D materials. We will summarize our current understanding of the MOCVD growth of III-chalcogenides by systematic variation of the growth parameters and correlate the findings to optoelectronic properties of the layers.
Semiconductors are amongst the most efficient active laser media as they yield extreme wall-plug efficiencies. Their broad gain bandwidth also promise short-pulse operation. Yet, intrinsic charge-carrier relaxation dynamics limit the feasible repetition rates beyond constraints of cavity design and heat removal. In lieu of studying an operation device we monitor the population dynamics, i.e., the initial buildup of gain after optical excitation as well as its recovery after a stimulated emission process using multiple pump-probe spectroscopy. The first optical pulse injects hot charge carriers that eventually build up spectral gain in the sample. The energies are chosen such to mimic typical electrical injection surplus energies. Subsequently, a second laser pulse tuned to the broad spectral region in which gain is observed is used to stimulate emission and thus eliminate the gain. Analysis of the absorption spectra after stimulated emission reveals gain recovery times in the picosecond regime.
Two-dimensional layered materials like graphene pave the way to advanced (opto-) electronic devices. Their extraordinary properties can be further controlled employing plasmonic nanostructures, e.g., enhancing light focusing, increasing the absorption cross sections, and generating hot-carrier due to the excitation and decay of localized surface plasmons. However, this interplay strongly depends on the particle’s environment and geometry mandating the investigation of individual structures. Raman spectroscopy maps reveal spatially resolved information on charge transfer as well as temperature and strain distributions in graphene sheets in the vicinity of individual spherical gold nanoparticles. Hot-electrons are efficiently injected from single gold nanoparticles into graphene for resonant excitation of the localized surface plasmons of the gold nanoparticle. Additionally, heating of the graphene sheet and its intrinsic strain can be separated and quantified.
Molecular semiconductors offer intriguing electronic properties. In particular, singlet-exciton fission, the nonradiative decay of one singlet exciton into two triplet excitons effectively doubles the amount of carriers available for, e.g., photovoltaic current generation, thereby effectively surpassing the Shockley-Queisser-limit. An efficient use of singletexciton fission in actual devices, however, requires a detailed understanding of the decay dynamics in donor-acceptor heterostructures. We present a quantitative study on model single-crystalline perfluropentacene at cryogenic temperature and related heterostructures to reveal the intricate interplay between singlet-exciton fission and the nanoscopic molecular arrangement, the role of charge-transfer into and out of molecular systems and discuss the potential for functionalizing inorganic semiconductors. Finally, the potential implications in heterosystems and for functionalization of inorganic semiconductor devices are discussed.
Organic molecular solids feature various properties considered advantageous for next-generation photovoltaic devices such as mechanical flexibility and ease of fabrication by, e.g., large-scale and large volume printing. Additionally, Singlet-Exciton Fission may allow surpassing the Shockley-Queisser limit. Here, one photoexcited singlet-type exciton decays into two triplet-type excitons, effectively doubling the number of excited charge carriers. Hence, above-unity quantum efficiencies may be achieved in photovoltaics and have been reported in for example, pentacene (PEN) –C60 heterojunctions.
Here, we study the carrier dynamics at well-defined PEN-C60 interface model systems by time-resolved photoluminescence spectroscopy experiments for different excitation photon energies. Thereby, we disentangle charge transfer and excitation dynamics, i.e., injection, transport, dissociation, and extraction.
The photoluminescence spectra reveal two distinct transition energies associated with charge-transfer (CT) states expected from photoelectron spectroscopy experiments. These long-lived transitions show a clear dependence on excitation energy, corroborating the proposed CT transitions and revealing the fact that carriers need to be created in both individual constituents for CT transitions to be observable.
Additionally, the C60 photoluminescence efficiency strongly quenches for increasing PEN coverage while the lifetime is drastically enhanced yielding strong evidence for an electron transfer between the PEN ground state and C60 when only the latter is photoexcited.
Perylene-3,4,9,10-tetracarboxylic acid (PTCDA) and respective derivatives (e.g. perylene diimide - PDI) are widely used as dyes but also for device applications such as organic field effect transistors or in organic photovoltaics. Due to their intrinsically high quantum efficiencies they are also used as spectroscopic standards. One major drawback of these materials is their low solubility in organic solvents which can be addressed by long alkyl substitutions.
When introducing a tertiary amine into the molecule a mechanism known as photoinduced electron transfer (PET) can occur. Here, following an optically excited HOMO-LUMO transition of the core, an electron from the electron lone pair of the amine is transferred to the HOMO of the perylene core. Hence, radiative recombination is disallowed and photoluminescence effectively quenched.
Here, we perform a systematic study of the distance dependence of the PET by introducing alkyle groups as spacer units between PDI core and the tertiary amine. Dynamics of the PET are extracted from ultrafast time-resolved photoluminescence measurement data.
A rate equation model, simulating a three level system, reveals rate constant of the back electron transfer, otherwise not accessible with our experimental methods. Assuming a Marcus model of electron transfer, electronic coupling strength between the electronic states involved in the respective transitions can be calculated. In addition to the distance dependence, the effects of protonation and methylation of the the tertiary amine units are studied.
Aromatic molecules are among the most promising materials in the field of organic optoelectronic due to the favorable properties of the delocalized -electron system present in those molecules. One of the most studied systems in this material class is the planar molecule of pentacene. An interesting application for pentacene is the incorporation into a donor-acceptor heterojunction in combination with its perfluorinated counterpart. Such samples may be deposited as intermixed blends (molecular alloys) or as alternating layered stacks. The out-of-plane delocalized -electron systems cause significant intermolecular coupling, even enabling the formation of charge-transfer excitons across heterointerfaces. Hence, studying this model system forms the optimal platform to investigate excitation transfer and charge separation in organic solar cells.
We present a comprehensive study of the optical properties of pentacene -perfluoropentacene heterosystems. The samples are grown as crystalline thin films in different molecular configurations: either layered or as intermixed blends, both, in standing and lying molecular orientation. Time resolved luminescence and linear absorption spectroscopy are performed to obtain the carrier dynamics of the charge transfer states and response of the pure materials. The influence of different packing motifs on the optical properties is investigated, revealing a radiationless long-range energy transfer in addition to the local occupation of charge-transfer states.
Invited presentation by Prof. B. Witzingmann
The development of supercontinuum sources is advancing fast in the last decades. As do all nonlinear effects, the supercontinuum generation strongly relies on the nonlinearity of the active material. This nonlinearity may be greatly enhanced in specially designed photonic-crystal fibers, making supercontinuum sources widely available. Nevertheless, pulsed lasers are required to supply high enough field strengths to overcome the threshold for supercontinuum generation.
We study a new cluster based class of nonlinear media that exhibits ultra-low thresholds for supercontinuum generations, thus enabling the use of a low coast steady-state laser diode as the driving laser [1].
The clusters are composed of a tin sulfide based core that is surrounded by four organic ligands. The core adopts an adamantane-like architecture, [Sn4S6]. It has a tetrahedral shape and thus lacks inversion symmetry, enabling nonlinear processes. The four ligands (R = 4-(CH2=CH)-C6H4) are consolidating the structure of the core. Yet, as they are randomly oriented around the Sn-C bonds, they are also preventing any long-range order in the solid phase of the compound. As a result, the compound is obtained as a white powder with totally frustrated order.
This powder has been studied in respect to its optical properties. When irradiated with a continuous-wave infrared laser of sufficient intensity it emits a warm white spectrum that is virtually independent from the pump-wavelength in a range of 725-1050 nm. Lowering the pump intensity, however, changes the spectral weight to the red, similar to dimming of thermal emitters. The input-output characteristics, however, exclude a thermal process as the source of the observed white-light. Additionally semi-classical calculations of the white-light generation process are performed, underlining this statement.
[1] Rosemann N.W., et al.; Science, 2016, 352, 1301-1304
We present modal gain measurements in Ga(NAsP) heterostructures pseudomorphically grown on silicon substrate.
Using the variable stripe length method we analyze the modal gain performance of an unprocessed single quantum well
sample for different excitation densities. We obtain high modal gain values up to 55 cm-1 at room temperature. These
values are comparable to those of common high quality laser material. This demonstrates the high optical quality of the
new dilute nitride material Ga(NAsP) and underlines its candidacy for electrically pumped lasing on silicon substrate.
We perform ultrafast pump-probe experiments on a 50 period Ge/SiGe multiple-quantum-well structure held
at room temperature. Tunable 80 fs pulses emitted by an opto-parametric amplifier are used as a pump and a
white-light supercontinuum generated directly from a 1 kHz Ti:sapphire regenerative amplifier system is used as
a probe. The resulting spectro-temporal response shows three distinct temporal regimes. Coherent oscillations
dominate at negative times yielding a well-defined time zero across the whole detected spectral range. Dynamics
are observed within the direct conduction band valley during and shortly after the excitation while the electrons
are also scattered towards the indirect minima. After several hundreds of fs to a few ps almost all electrons
populate the L-valley states. These carriers decay out of the L-valleys on a timescale longer than several ns.
During the first ps, carrier inversion is obtained for strong enough pumping due to faster intra-valley than intervalley
scattering. The obtained gain values are similar in magnitude to those observed in typical III-V compound
semiconductors.
We present time-resolved photoluminescence measurements performed on an ensemble of InAs quantum dots
with density of 1011 dots/cm2 and ground state transition energies centered at 1.216 eV. The wavelength of the
100fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical
phonon assisted) photoluminescence (PL). The PL was detected with its polarization both parallel with and
perpendicular to the excitation polarization (along one of the crystal's cleave axes). The decay of the PL was
time-resolved with a streak camera in the interval 1.5 - 3ns to avoid scattered laser light. A strong polarization
dependence was observed. Considerable amount of the resonant fluorescence signal and even of the non-resonant
PL signatures remained linearly polarized on a nanosecond time scale. A phenomenological rate equation analysis
is made.
Design of optimized semiconductor optically-pumped semiconductor lasers (OPSLs) depends on many ingredients
starting from the quantum wells, barrier and cladding layers all the way through to the resonant-periodic gain (RPG) and
high reflectivity Bragg mirror (DBR) making up the OPSL active mirror. Accurate growth of the individual layers
making up the RPG region is critical if performance degradation due to cavity misalignment is to be avoided.
Optimization of the RPG+DBR structure requires knowledge of the heat generation and heating sinking of the active
mirror. Nonlinear Control Strategies SimuLaseTM software, based on rigorous many-body calculations of the
semiconductor optical response, allows for quantum well and barrier optimization by correlating low intensity
photoluminescence spectra computed for the design, with direct experimentally measured wafer-level edge and surface
PL spectra. Consequently, an OPSL device optimization procedure ideally requires a direct iterative interaction between
designer and grower. In this article, we discuss the application of the many-body microscopic approach to OPSL devices
lasing at 850nm, 1040nm and 2μm. The latter device involves and application of the many-body approach to mid-IR
OPSLs based on antimonide materials. Finally we will present results on based on structural modifications of the
epitaxial structure and/or novel material combinations that offer the potential to extend OPSL technology to new
wavelength ranges.
The optical emission and gain properties of Ga(AsSb) quantum-islands are investigate. These islands form during growth
in a self-organized process in a series of Ga(AsSb)/GaAs/(AlGa)As heterostructures, resulting in an additional in-plane
hole confinement of several hundreds of meV. The shape of the in-plane confinement potential is nearly parabolic and thus
yields almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum islands are 100nm
in diameter and exhibit an in-plane variation of the Sb concentration of more than 30 %. Up to seven bound hole states
are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation
density, lattice temperature, and excitation photon energy and reveal fast carrier capture into and relaxation within the
quantum islands. Furthermore, the optical gain is measured using the variable stripe-length method and the advantages of
such structures as active laser material are discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.