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.
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.
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.
GaAsBi offers the possibility to develop near-IR semiconductor lasers such that the spin-orbit-split-off energy (ΔSO) is greater than the bandgap (Eg) in the active region with lasing wavelengths in the datacom/telecom range of 1.3-1.6 μm. This promises to suppress the dominant efficiency-limiting loss processes as Auger recombination, involving the generation of “hot” holes in the spin-orbit split-off band (the so-called “CHSH” process), and inter-valence band absorption (IVBA), where emitted photons are re-absorbed in the active region, thereby increasing the internal optical losses and negatively impacting upon the laser characteristics being responsible for the main energy consumption. In addition to growth and fabrication processes refinement, a key aspect of efforts to continue the advancement of the GaAsBi material system for laser applications is to develop a quantitative understanding of the impact of Bi on key device parameters. In this work, we present the first experimental measurements of the absorption, spontaneous emission, and optical gain spectra of GaAsBi/AlGaAs QW lasers using a segmented contact method and a theoretical analysis of these devices, which shows good quantitative agreement with the experiment. Internal optical losses of 10-15 cm-1 and peak modal gain of 24 cm-1 are measured at threshold and a peak material gain is estimated to be 1500 cm-1 at current density of 2 kA/cm-2, which agrees well with the calculated value of 1560 cm-1. The theoretical calculations also enabled us to identify and quantify Bi composition variations across the wafer and Bi-induced inhomogeneous broadening of the optical spectra.
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 photoluminescence and modal gain measurements in a Ga(NAsP) single-quantum well sample
pseudomorphically grown on silicon substrate. The temperature dependence indicates that disorder induced localization
effects dominate the low temperature photoluminescence spectra. Nevertheless, using the variable stripe length method,
we observe modal gain values up to 15 cm-1 at room temperature. These values are very promising, demonstrate the high
optical quality of the new dilute nitride material Ga(NAsP) and underline its candidacy for electrically pumped lasers on
silicon substrate.
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.
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.
The GaP-based dilute nitride Ga(NAsP) reveals a direct band gap and first laser device operation based on
GaP substrate have been demonstrated recently. Since the lattice mismatch between GaP and Si is very small
and the defect free deposition of thick GaP/Ga(NP) sequences on off-oriented Si substrate have been reported
in literature, the epitaxial transfer of this novel direct band gap material Ga(NAsP) on Si substrate should
allow for the monolithic integration of laser diodes on Si microprocessors. The present study introduces a
nucleation scheme of GaP on exact oriented (001) Si substrate by metal organic vapour phase epitaxy
(MOVPE) to achieve this goal. Appling an optimized annealing procedure to (001) Si substrates with a slight
off-orientation towards (see manuscript) direction leads to a Si surface, where step-doubling has set in and bi-atomic
terraces are formed. Even though mono-atomic terraces are still present in low density, an optimized GaP
nucleation procedure ensures self-annihilation of all present antiphase domains (APDs) and reveals an
antiphase disorder free III/V film on Si after the deposition of about 50nm of GaP. This ideal nucleation layer
together with a precise strain-management allows for the deposition of Ga(NAsP)/(BGa)(AsP) multi-quantum-well (MQW) heterostructures embedded in 1μm thick (BGa)P layers on Si substrate. Structural
investigations using X-ray diffraction (XRD) and transmission electron microscopy (TEM) prove a high
crystal quality and abrupt heterointerfaces. This monolithic integration concept of the GaP-based laser
material on exact oriented (001) Si substrates enables the integration of optoelectronic devices into the
standard CMOS process.
The current status of the development of the novel dilute nitride Ga(NAsP)/GaP for the monolithic integration
of optoelectronic functionality to Si is summarized from the concept, design and epitaxial optimization to the
verification of direct energy gap and the realization of electrical injection laser devices at room temperature.
We demonstrate 0.7W cw output power at 520nm from an intracavity frequency doubled optically pumped semiconductor disk laser at room temperature. High beam quality and optical conversion efficiency of 10% has been achieved.
Optically-pumped semiconductor disk lasers offer high output power
in combination with good beam quality. By optimizing epitaxial
quality as well as thermal resistance, we have demonstrated more than 8W of continuous-wave, room-temperature emission at 1000nm. These high power-levels are tied to high optical-conversion efficiencies of more than 40%. Whereas available wavelengths for solid-state disk lasers are restricted to a set of atomic transitions, semiconductor disk lasers can be conveniently tailored to meet almost any wavelength. Building upon the high-power results at 1000nm, we have extended the emission range towards 900nm as well as 1100nm. Two prominent examples are devices realized at 920nm and 1040nm, in each case demonstrating several Watts of laser output.
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