Type-II GaInAs/GaAsSb “W” quantum well heterostructures on GaAs show strong potential for temperature-stable data communications lasers. Devices emitting at 1255 nm show promising lasing characteristics including room-temperature threshold current densities, Jth < 300 A/cm^2, pulsed output powers >1 W, and a reduced wavelength temperature dependence of 0.31 nm/C. Temperature- and pressure-dependent characterisation techniques are used to determine the roles of radiative and non-radiative recombination. Analysis of these characteristics suggest a reduced influence of non-radiative recombination on the thermal stability of type-II “W”-lasers compared to type-I devices, as will be discussed along with recommendations for future device development.
Ultra-short pulse generation with saturable-absorber-free vertical-external-cavity surface-emitting-lasers (VECSELs) has raised significant interest in recent years due to the promises it holds for further peak-power scaling and cost-efficiency as well as for the design of more flexible, compact and simpler cavities. Although demonstrated for various devices, the self-mode-locking phenomenon in VECSELs still lacks a consistent explanation. Here, nonlinear lensing in a VECSEL gain chip as a possible mode-locking mechanism, directly measured via Z-scans at laser-relevant wavelengths, and the role of the microcavity resonance on the strength and dispersion of the Kerr nonlinearity are discussed. Furthermore, the impact on self-mode-locking is considered.
Monolithic growth of III-V semiconductors on silicon is a promising path for the development of silicon-based lasers. The GaP binary has a lattice constant very close to that of silicon and can be grown defect free without anti-phase domains (APDs) or stacking faults on (001) exact orientated silicon substrates. These GaP on Si templates provide the base for growth and investigation of III-V lasers. The addition of boron can be used to partially replace Ga and further reduce the lattice constant. This can be balanced to match the lattice constant of silicon by adding As to partially replace P. The alloying also provides control of band gaps and band offsets as well as refractive index. The BxGa(1-x)P and BxGa(1-x)AsyP(1-y) alloys are being explored to provide lattice matching/ strain compensation, cladding and the Separate Confined Heterostructure (SCH). The effects of the inclusion of boron on device related alloy properties have not been studied extensively and are not well understood. We investigate the refractive index and extinction coefficient dispersion relation and the electronic band structure properties of these boron containing alloys using spectroscopic ellipsometry to provide inputs for device modelling and optimisation. Results from the spectroscopic ellipsometry are presented for a series of BGaP and BGaAsP alloy samples with boron fractions in the range 0-6.6% and arsenic fractions from 0-17% on GaP substrates and GaP/ Si templates. These results provide important information for the design of lasers with strong optical and electronic confinement, as shall be discussed.
Here we present the gain and SESAM structure design strategy employed for the demonstration of ultrashort pulses and we present a comprehensive study outlining the influence of the cavity geometry on the pulse duration and peak power achievable with a state of the art VECSEL and SESAM structure. We will discuss the physical mechanisms limiting the output power with near 100fs pulses and we will compare experimental results obtained with different cavity geometries, including a V-shaped cavity, a multi-fold cavity, and a ring cavity in a colliding pulse modelocking scheme. The experimental results are supported by numerical simulations.
Since the invention of VECSELs, vast spectral coverage has been demonstrated with emission wavelengths in the range from the UV to almost the MIR. Accordingly, a great variety of different quantum well and quantum dot gain designs have been applied so far to achieve such versatility. A novel gain design for GaAs based VECSELs emitting at wavelengths >1.2 μm employs type-II quantum wells, which exhibit spatially indirect charge-carrier recombination. The first VECSEL based on such a design has been demonstrated very recently. Our device consists of ten (GaIn)As/Ga(AsSb)/(GaIn)As heterostructures arranged as a resonant periodic gain. We summarize the development of this pioneering structure and discuss the fundamental laser characteristics, such as carrier densities, gain temperatures and slope efficiency. Remarkable output powers up to 4 W are demonstrated in multi-transverse mode operation at 1.2 μm. Also, the performance in TEM00 operation is investigated, with an M2 < 1.13. One major difference to conventional type-I gain structures is a characteristic blue shift of the material gain. Due to the importance of the detuning in quantum well based surface-emitters, the blue shift has to be considered as a critical designing parameter. Hence, we carry out a detuning study in order to determine an optimal detuning. As an important part of the optimization, the experimental results are compared with fully microscopic simulations.
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.
We present a novel Vertical External Cavity Surface Emitting Laser (VECSEL) cavity design which makes use of multiple interactions with the gain region under different angles of incidence in a single round trip. This design allows for optimization of the net, round-trip Group Delay Dispersion (GDD) by shifting the GDD of the gain via cavity fold angle while still maintaining the high gain of resonant structures. The effectiveness of this scheme is demonstrated with femtosecond-regime pulses from a resonant structure and record pulse energies for the VECSEL gain medium. In addition, we show that the interference pattern of the intracavity mode within the active region, resulting from the double-angle multifold, is advantageous for operating the laser in CW on multiple wavelengths simultaneously. Power, noise, and mode competition characterization is presented.
We present a passive and robust mode-locking scheme for a Vertical External Cavity Surface Emitting Laser (VECSEL).We placed the semiconductor gain medium and the semiconductor saturable absorber mirror (SESAM) strategically in a ring cavity to provide a stable colliding pulse operation. With this cavity geometry, the two counter propagating pulses synchronize on the SESAM to saturate the absorber together. This minimizes the energy lost and creates a transient carrier grating due to the interference of the two beams. The interaction of the two counter-propagating pulses in the SESAM is shown to extend the range of the modelocking regime and to enable higher output power when compared to the conventional VECSEL cavity geometry. In this configuration, we demonstrate a pulse duration of 195fs with an average power of 225mW per output beam at a repetition rate of 2.2GHz, giving a peak power of 460W per beam. The remarkable robustness of the modelocking regime is discussed and a rigorous pulse characterization is presented.
Ultrashort laser pulses from vertical-external-cavity surface-emitting lasers (VECSELs) have been receiving much attention in the semiconductor laser community since the first demonstration of sub-ps-pulsed devices more than a decade ago. Originally relying on semiconductor saturable-absorber mirrors for pulse formation, mode-locked operation has not only become accessible by using a variety of saturable absorbers, but also by using a saturable-absorber-free technique referred to as self-mode-locking (SML). Here, we highlight achievements in the field of SML-VECSELs with quantum-well and quantum-dot gain chips, and study the influence of a few VECSEL parameters on the assumed nonlinear lensing behavior in the system.
We present a comprehensive characterization of semiconductor gain and absorber devices utilizing novel measurement techniques. Using a 20fs probe laser, a time resolution in the few femtosecond range is achieved in traditional pump and probe measurements performed on VECSELs and SESAMs. In-situ characterizations of VECSEL samples mode-locked in the sub-500fs regime reveal the fast and longtime recoveries of the gain present in real lasing conditions. Spectrally-resolved probing gives further information about the properties of carriers in VECSEL gain media. Our results indicate that stable mode-locked operation is sustained by multiple carrier relaxation mechanisms ranging from a few femtoseconds to the pico- and nanosecond regimes.
We demonstrate a low thermal impedance hybrid mirror VECSEL. We used only 14 pairs of AlGaAs/AlAs, transparent at the pump wavelength, and we used a patterned mask to deposit pure gold on areas of the chip to be pumped, and Ti/Au on other area to circumvent the poor adhesion of gold on GaAs. A higher gain is observed on an area metallized with pure gold and an output power of 4W was obtained, showing the effectiveness of the metallic mirror and validating the bonding quality. Chip processing and laser characteristics are studied in detail and compared to simulations.
Recent development of high power femtosecond pulse modelocked VECSEL with gigahertz pulse repetition rates sparked an increased interest from the scientific community due to the broad field of applications for such sources, such as frequency metrology, high-speed optical communication systems or high-resolution optical sampling. To the best of our knowledge, we report for the first time a colliding pulse modelocked VECSEL, where the VECSEL gain medium and a semiconductor saturable absorber (SESAM) are placed inside a ring cavity. This cavity geometry provides both a practical and an efficient way to get optimum performance from a modelocked laser system. The two counter propagating pulses in our ring cavity synchronize in the SESAM because the minimum energy is lost when they saturate the absorber together. This stronger saturation of the absorber increases the stability of the modelocking and reduces the overall losses of the laser for a given intra-cavity fluence, leading to a lower modelocking threshold. This also allows the generation of fundamental modelocking at a relatively low repetition rate (<GHz) with a higher output power compared to conventional VECSEL cavity. We obtained a total output power of 2.2W with an excellent beam quality, a pulse repetition rate of 1GHz and a pulse duration ranging from 1ps to 3ps. The emitted spectrum was centered at 1007nm with a FWHM of 3.1nm, suggesting that shorter pulses can be obtained with adequate dispersion compensation. The laser characteristics such as the pulse duration and stability are studied in detail.
Since the invention of VECSELs, their great spectral coverage has been demonstrated and emission wavelengths in the range from UV to almost MIR have been achieved. However, in the infrared regime the laser performance is affected by Auger losses which become significant at large quantum defects. In order to reduce the Auger losses and to develop more efficient devices in the IR, type-II aligned QWs have been suggested as alternative gain medium for semiconductor lasers.
We present a serially-connected two-chip vertical-external-cavity surface-emitting laser design, which generates dual wavelength emission with a wavelength separation of 10 nm and over 600 W intracavity power. Intracavity type-I second-harmonic generation and sum-frequency generation have been performed in a LiNbO3 crystal. By employing different chip-combinations as well as birefringent filters, the laser is able to generate high-power emission with two wavelengths, which exhibit the same polarization and a desirable wavelength separation. Furthermore, the dependence of the emission wavelength on the cavity angle on the VECSEL chip is highlighted, which provides an additional means of wavelength tuning in VECSELs.
Vertical-external-cavity surface-emitting lasers (VECSELs) have proved to be versatile lasers which allow for various emission schemes which on the one hand include remarkably high-power multi-mode or single-frequency continuouswave operation, and on the other hand two-color as well as mode-locked emission. Particularly, the combination of semiconductor gain medium and external cavity provides a unique access to high-brightness output, a high beam quality and wavelength flexibility. Moreover, the exploitation of intra-cavity frequency conversion further extends the achievable radiation wavelength, spanning a spectral range from the UV to the THz. In this work, recent advances in the field of VECSELs are summarized and the demonstration of self-mode-locking (SML) VECSELs with sub-ps pulses is highlighted. Thereby, we present studies which were not only performed for a quantum-well-based VECSEL, but also for a quantum-dot VECSEL.
We demonstrate the highest free running single frequency power from a single chip VECSEL reported to date, with more than 15W in continuous operation at room temperature. The GaAs-based structure presents an emission wavelength of 1020nm and a tuning range <15nm, with a continuous tunability of 9GHz. The TEM00 output beam exhibits very low transverse phase fluctuations across the entire mode, leading to a beam quality M2 <1.2. To identify and reduce the different sources of noise, the relative intensity noise and frequency noise spectral density are investigated and the intensity and the frequency of the laser were independently stabilized. The laser frequency is controlled and tuned varying the cavity length using a high bandwidth piezoelectric element while intensity fluctuations are reduced by varying the pump intensity. Intrinsic and stabilized frequency and intensity noise are compared.
We report on our research in power scaling VECSEL around 1 μm to exceed 100W per chip. Recently, we have
utilized these optimized VECSEL chips to achieve a new record for a mode-locked VECSEL. The output power
of the laser was 3.4W. This corresponds to a pulse energy of 7.5nJ and a pulse peak power of 13.3kW. Both are
record values for a semiconductor laser in the femtosecond regime. These optimized structures have also been
used to demonstrate high power operation with a highly coherent TEM00 mode and to demonstrate a record
single frequency output power of 15W.
We report on our research in power scaling OPSL around 1 μm to exceed 100W per chip by combining a rigorous quantum design of an optimized MQW epitaxial structure, highly accurate and reproducible wafer growth and an efficient thermal management strategy. Recently we have utilized these state-of-the-art optimized OPSL chips to achieve a new record for a mode-locked OPSL with an intra-cavity SESAM. The average output power of the laser in the optimum operation point of mode-locked operation was 5.1W while being pumped with 25W of net pump power. This corresponds to a pulse energy of 3 nJ and a pulse peak power of 3.8 kW.
KEYWORDS: Quantum wells, Absorption, Diamond, Reflectivity, Chromium, Temperature metrology, High power lasers, Semiconducting wafers, Finite element methods, Gallium arsenide
Strategies for power scaling VECSELs, including improving thermal management, increasing the quantum well
gain/micro-cavity detuning that increases the threshold but increases roll-over temperature, and double-passing the
excess pump via reflection from a metalized reflector at the back of a transparent distributed Bragg reflector (DBR) were
studied. The influence of the heat spreader thickness and the pump profile on the temperature rise inside the active
region was investigated using commercial finite element analysis software. Improvement was observed in optical
efficiency of the VECSEL devices with a transparent DBR by double passing the pump light. Higher dissipated power at
maximum output power was found in devices with larger spectral detuning between the quantum well gain and the
micro-cavity detuning.
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.
We present an overview of the quantum design, growth and lasing operation of both IR and mid-IR OPSL
structures aimed at extracting multi-Watt powers CW and multi-kW peak power pulsed. Issues related to
power scaling are identified and discussed. The IR OPSLs based on InGaAs QW bottom emitters targeted at
wavelengths between 1015nm and 1040nm are operated in CW mode (yielding a maximum power of 64W)
and pulsed (peak power of 245W). The mid-IR top emitter OPSLs designed to lase at 2μm are based on a
novel lattice mismatched growth using InGaSb QWs and yield a maximum peak power of 350W pulsed.
An approach based on fully microscopically computed material properties like gain/absorption, radiative
and Auger recombination rates are used to design, analyze and develop optimization strategies for Vertical
External Cavity Surface Emitting Lasers for the IR and mid-IR with high quantitative accuracy. The microscopic
theory is used to determine active regions that are optimized to have minimal carrier losses and
associated heating while maintaining high optical gain. It is shown that in particular for devices in the
mid-IR wavelength range the maximum output power can be improved by more than 100% by making rather
minor changes to the quantum well design. Combining the sophisticated microscopic models with simple onedimensional
macroscopic models for optical modes, heat and carrier diffusion, it is shown how the external
efficiency can be strongly improved using surface coatings that reduce the pump reflection while retaining the
gain enhancing cavity effects at the lasing wavelength. It is shown how incomplete pump absorption can be
reduced using optimized metallization layers. This increases the efficiency, reduces heating and strongly improves
the maximum power. Applying these concepts to VECSELs operating at 1010nm has already resulted
in more than twice as high external efficiencies and maximum powers. The theory indicates that significant
further improvements are possible - especially for VECSELs in the mid-IR.
The quantum design of VECSEL structures is discusssed using a commercially available design tool. Examples
of realized structures are presented and comparisons between experimental results and modelling predictions are
shown.
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.
Semiconductor disk lasers have attracted a lot of interest in the last few years due to high output power combined
with good beam quality and possible wavelength engineering. One of the disadvantages is the need for external
optical pumping by edge-emitting semiconductor lasers that increase packaging effort and cost. Therefore,
semiconductor disk lasers with monolithically integrated pump lasers would be of high interest. We report on
a novel design and experimental realization to monolithically integrate pump lasers with a semiconductor disk
laser in a one-step epitaxial design. By careful design of integrated pump lasers and stacking sequence, it is
possible to efficiently excite vertical emitter areas with different mesa sizes. First results are shown at 1060 nm
emission wavelength with high output power out of mesa diameters of 100 μm to 400 μm. The devices can be
conveniently characterized on a wafer level using dry-etched pump laser facets. In pulsed operation 1.7W out of
a 100 μm diameter mesa and 2.5W out of a 200 μm diameter mesa are demonstrated. Additionally, more than
0.6W in cw operation using a 400 μm structure were achieved. In summary, an innovative approach for truly
monolithic integration of a semiconductor disk laser with pump lasers has been pioneered.
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.
Optically pumped semiconductor vertical-external-cavity surface-emitting laser (VECSEL) potentially provides an
innovative approach to low-cost frequency agile lasers engineered for specific applications in infrared and visible range.
In this paper, we report on the development and demonstration of a multi-Watt highly strained InGaAs/ GaAs vertical-external-
cavity surface-emitting laser (VECSEL), which can be tuned from 1147 nm to 1197 nm. Based on this tunable
InGaAs/GaAs VECSEL and intracavity frequency doubling, we develop multi-Watt frequency-doubled tunable
VECSEL in a wide yellow-orange band (579 ~595 nm). This compact high-power yellow-orange laser provides an
innovative approach to an affordable guidestar laser (~589.1 nm) solution, and has a lot of important applications in
biomedicine.
We present an overview of a novel first principles quantum approach to designing and optimizing
semiconductor QW material systems for target wavelengths. Using these microscopic inputs as basic building
blocks we predict the L-I characteristic for a low power InGaPAs ridge laser without having to use adjustable
fit parameters. Finally we employ these microscopic inputs to develop sophisticated simulation capabilities
for designing and optimizing end packaged high power laser structures. As an explicit example of the latter,
we consider the design and experimental demonstration of a vertical external cavity semiconductor laser
(VECSEL).
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.
Martin Hofmann, Nils Gerhardt, Anke Wagner, Christoph Ellmers, Falko Hoehnsdorf, Joerg Koch, Bernd Borchert, A. Egorov, Henning Riechert, Wolfgang Stolz, Wolfgang Ruehle
The emission dynamics of an optically pumped 1.3 +m (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is investigated. We achieve room-temperature operation at 1285 nm with a low optical pumping threshold and fast emission dynamics: A minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps are observed after excitation with 100 fs pulses. Laser operation with picosecond emission dynamics is demonstrated over a wide temperature range from 30 K to 388 K. We explain this extraordinarily large temperature operation range on the basis of measurements of the optical gain in (GaIn)(NAs)/GaAs. We find a gain broadening at elevated carrier densities due to contributions of higher subband transitions.
We present results of the optically excited dynamics in semiconductor quantum wells on short length and time scales. Nonlinear optical experiments are performed with high temporal and high spatial resolution. To interpret the experimental findings calculations are performed on different approximation levels. Two different time regimes are investigated: in the incoherent time regime we study the dynamics of heating, cooling, and the formation of excitons by measuring the temporal behavior of the lateral expansion rate of locally created electron-hole pairs or excitons. A monomolecular exciton formation process is found. The experimental results in this regime are well reproduced by the Boltzmann equation for incoherent exciton densities with phenomenological scattering rates. In addition we have performed a microscopic density matrix analysis for the heating scenario where we have modeled explicitly the initial transformation of coherent excitations into incoherent exciton densities. It is found that the heating due to scattering with acoustic phonons gives reasonable agreement with the observed rates. In the coherent time regime a spatio-temporal beating is observed. This unexpected non-monotonic modulation of the spatial width arises from excitonic wave-packets which modulate the detected lateral profile of the optical nonlinearity in a characteristic way. It is explained by the superposition of various signal components which are detected simultaneously due to the collinearity of our experiment. This effect is illustrated by calculations using a simplified model on the Hartree-Fock level.
Vertical-cavity surface-emitting lasers are optimized for fast intrinsic emission dynamics. The structure contains four times three quantum wells in a 2 (lambda) sin-type cavity. We have realized it using the strain-compensated (GaIn)As/Ga(PAs) material system with GaAs/AlAs Bragg mirrors. The laser emission after optical excitation with femtosecond pulses yields a pulse width of 3.2 ps and a peak delay of 4.8 ps to our knowledge the fastest values reported so far, at low temperatures. The design is successfully transferred to higher temperature operation. Picosecond dynamics is demonstrated also at room temperature with a pulse width of 13 ps and a peak delay of 9 ps. Laser operation over a broad temperature range from 140 K up to room temperature is achieved and also shows picosecond emission dynamics.
InGaAs/InAlAs quantum wells provide large barrier height for electrons accommodating numerous
confined states. Absorption and electroabsorption spectra of samples with 50 - 100 wells are
compared. Heavy hole excitons respond strongly to the electric field which for the lowest state is
in good agreement with the predicted red shift due to the quantum confined Stark effect. Higher
confined excitons show spectra of similar strength and lineshape but not because of a red shift of
the transitions but due to redistribution of oscillator strength. Superlattices with thin barriers
develop minibands whose spectra are very different from those of uncoupled wells. Features arising
from M0 and M1 singularities enable direct determination of the width of the minibands and
determination of the conduction band disontinuity. Large electric fields destroy the minibands
and quantum confined excitons reappear.
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.