PD Dr. Ulrich T. Schwarz
Research Topics
Semiconductor Optoelectronics
“Blue”
(Al,In)GaN
Laser diodes: Optical gain spectroscopy and waveguide mode dynamics
A measure for the state of knowledge
on a certain semiconductor material system is the degree to which microscopic,
parameter–free simulations are able to predict the behavior of
real–world devices. For GaInAs, InGaPAs, and GaAsSb the degree of
predictive power of the highest–level microscopic simulations is
striking. One reason for this success is the high material quality and epitaxial growth control. Inhomogeneous broadening in these
devices is typically low and nonradiative
recombination negligible. The group III–nitrides are
far from this level of understanding, because of the complexity of the system.
Quantum well (QW) fluctuations are intrinsic to this material class due to
indium segregation. Strong internal piezoelectric and spontaneous fields cause
the reduction of the electron and hole wave function
and a red shift (quantum confined Stark effect, QCSE) in all structures grown
along the crystallographic c–axis. The role of threading dislocations and
cause of other nonradiative centers in the wavelength
range from UV to green is the topic of intense discussions. The claim that
Auger processes can be neglected in this wide–bandgap
material is more a hope than a proven statement. Certainly (Al,In)GaN light emitting diodes (LED) and laser diodes (LD)
occupy a huge market segment in optoelectronic devices, which will further grow
when LEDs for general and automotive lighting and LDs
for full color laser projection become available. On the theory side, gain
spectra have been successfully compared to microscopic simulations. Yet nobody
is able at the moment to predict or optimize a full (Al,In)GaN LD or LED structure
without close feedback with experiments and at least some assumptions in form
of adjustable parameters, most notably inhomogeneous broadening.
In our laboratory we employ the Hakki-Paoli method of optical gain spectroscopy. This
method derives the optical gain with high accuracy from the modulation depth
(finesse) of the longitudinal modes below threshold. Information on the laser
diode properties can either be drawn directly from the gain spectra (internal
losses, substrate modes, degradation, gain fluctuations,…)
or via comparison to theoretical models, in particular with (mostly) parameter
free microscopic models.
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Optical gain spectra of laser diodes from Nichia Corporation. The gain
spectra of laser diodes with lasing wavelengths of375nm, 405nm, 440nm, and 470nm
from near UV to aquamarine spectral range show an increasing inhomogeneous
broadening for longer wavelengths [K. Kojima et al., Optics Express 15, 7730
(2007)] |
Optical gain is the link between
microscopic and macroscopic properties of a laser diode. On the device level we
study their dynamical behavior in spatial, temporal, and spectral dimension. (Al,In)GaN
laser diodes tend to form filaments for ridges broader than 2.5 µm. By scanning
near-field microscopy (SNOM) we are able to measure these filaments with high
spatial resolution. We can combine those spatial measurements with spectral and
temporal resolved measurements, resulting in multi-dimensional data cubes.
Questions are to what extend ridge geometry (e.g. asymmetry), thermal effects,
and fluctuations along the ridge influence the laser diode dynamics. Stable
beam patterns in near- and far-field at high optical output powers are
essential for most applications (e.g. laser TV).
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Looking
into the “eye” of an (Al,In)GaN laser diode: 405 nm (here: spontaneous) emission
emerging at the laser facet. (Bottom to top: heat spreader; laser diode on
bulk GaN substrate; bond wire).
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Near-field
measurement (inset) of the 2d waveguide mode intensity profile (left) below
and (right) above the onset of filamentation for a
narrow and broad ridge waveguide laser diode, respectively.
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The spectral or temporal
properties (lasing onset delay, decay, relaxation oscillations) are
measured and compared to rate equation simulations. In the end it should be
possible to establish a set of parameters characterizing the dynamical
properties of (Al,In)GaN laser diodes. The underlying question is, if (Al,In)GaN
laser diodes are fundamentally different (e.g. due to QCSE and/or QW
fluctuations) when compared to laser diodes in the infrared to red spectral
region.
Dislocations,
strain & optical properties of bulk GaN and (Al,In)GaN
heterostructures
Micro-photoluminescence (µPL) is
our versatile tool to study optical properties of bulk GaN
and (Al,In)GaN heterostructures. The
advantages of the method are manifold: we can choose between different
excitation wavelengths (334 nm above bandgap, 380 nm,
405 nm, 413 nm … for resonant excitation of InGaN
QWs), vary excitation density, sample temperature (down
to < 10 K), apply a bias voltage for field-dependent measurements, combine
electroluminescence, photoluminescence and photocurrent (laser beam induced
current, LBIC) modes, all with high spatial (down to 300 nm) and spectral (down
to 0.1 meV) resolution and high numerical aperture
(light gathering power).
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Single
dislocations act as centers of nonradiative
recombination. Left: near bandgap emission
(integrated D0X intensity) in bulk GaN,
the dark spots are dislocations. Middle and right: the energy shift of the
donor bound exciton line has the shape of a dipole
at every dislocation. The cause of this is a strain dipole (compressive and
tensile strain) from a single lattice plane ending at the threading
dislocation line [N. Gmeinwieser et al., Phys. Rev.
B 75, 245213 (2007)].
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The
orientation of these strain dipoles shows a six-fold symmetry, indicating the
orientation of the burgers vector in the hexagonal lattice. This is a pure optical
detection of the Burgers vector.
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With the same method we
investigate the impact of single dislocation on the properties of an InGaN QW. By resonant excitation we are able to study the
QW inside a working LED. Questions are: How are threading dislocations and fluctutations affect radiative
and nonradiative recombination, lateral and vertical
carrier transport, and in general the efficiency of an InGaN
QW LED.
Semi-
and non-polar nitrides
One of the most pressing physical
problems hindering further advances in nitride emitters is the presence of
large piezoelectric fields in these materials. Because of the hexagonal lattice
symmetry without a center of inversion the
piezoelectric coefficients for wurtzite nitrides are
non-zero. The active regions of nitride LEDs or laser
diodes are typically comprised of InGaN quantum wells
(QWs) which are under biaxial compressive stress due
to the larger lattice constant of InGaN compared to GaN. Consequently, In-GaN quantum
wells grown along the crystallographic c-axis exhibit an internal piezoelectric
field in the MV/cm range, and electrons and holes are pulled to opposite
interfaces of the QW. This spatial separation of wave functions causes a
decrease of the transition matrix element and suppresses radiative
recombination with respect to nonradiative
recombination, diminishing the efficiency drastically. The problem becomes
worse both for thicker QWs and at higher indium
content, necessary in devices designed for longer wavelength operation. In
order to overcome these problems nitride heterostructures
and QWs need to be grown along crystallographic
directions where the piezoelectric field is small or zero.
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Piezoelectric polarization of an InxGa1–xN
quantum well grown pseudomorphically between
relaxed GaN barriers. The lattice planes of
particular polar, semipolar, and nonpolar orientations are shown as insets. P′z
is the polarization perpendicular to the growth plane [U. T. Schwarz and M. Kneissl, phys. stat. sol. (RRL) 1, A45 (2007)]. |
Singular and nonlinear Optics
Optical
vortices
Imagine a beam of light
propagating in z direction with a phase front in the shape of a spiral. In the
center of the optical vortex the phase is undefined and thus the intensity must
vanish. The “threads of darkness” are the optical analogue to screw
dislocations in condensed matter physics. They are defined as topological
entities and thus stable. A perturbation of the beam will shift the position of
the vortex, but not destroy it (as it is impossible to smoothly merge a spiral
staircase).
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Spiral staircase (
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Screw
dislocation in a crystal (red: dislocation line; black: Burgers vector)..
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Phase
front of an optical vortex.
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We study the propagation and interaction
of optical vortices in Laguerre-Gaussian beams, which
allow an analytical treatment. We were the first to study linear and nonlinear
propagation of optical vortices in propagation invariant Bessel beams. The idea
was to provide a diffraction free beam as background medium for the propagating
optical vortex, which then move with constant velocity along a straight line,
in contrast to the case Gaussian background beam. Using higher order Bessel
beams, which themselves carry optical angular momentum, we demonstrated
experimentally the transfer of optical angular momentum in a nonlinear optical
interaction between pump, Stokes, and anti-Stokes beam by Raman scattering in
hydrogen gas.
Polarization
singularities
A coarse classification of
optical singularities is as follows: Caustics are the singular objects in ray
optics, optical vortices those of paraxial scalar wave optics, and polarization
singularities those of vector fields. Polarization of light is associated with birefringent crystals. So we looked into the propagation of
a scalar optical vortex upon propagation in a birefringent
crystal and discovered the most generic case of its unfolding into a bizarre,
but topologically well defined, three-dimensional structure of lines of
circular polarization (C-lines) and surfaces of linear polarization
(L-surfaces), resembling one of Riemann’s minimal surfaces.
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Experimentally determined configuration of polarization singularities
(L surface and C lines) in (x,y,L)-space[F. Flossmann
et al., Phys. Rev. Lett. 95, 253901 (2005)]. |
Ince-Gaussian modes
Laser textbooks teach about Hermite-Gaussian and Laguerre-Gaussian
modes, which live on a Cartesian and cylindrical coordinate system, respectively.
Yet, they are not the only possible stable modes of a resonator. M. A. Bandres and J. C. Gutierrez-Vega theoretically proposed Ince-Gaussian modes as solution of the paraxial wave
equation in elliptical coordinates and independent family of eigenmodes of a stable resonator. Building on their
theoretical results we were able to produce Ince-Gaussian
modes experimentally with high fidelity in a diode-pumped solid state laser.
Currently, research of Ince-Gaussian modes is
developing into an independent sub-field within physical optics.
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Beam intensity patterns of even IGMs, IGe p,m,
measured with the CCD camera and (b) their theoretical predictions [U. T.
Schwarz et al., Optics Letters 29,
1870 (2004)]. |
