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| Research news |
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Note:
the title of a newsitem links most of the time
to the relevant paper in pdf-format. Items older than
January 1st 2005 are placed in the Research
news archive.
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2010
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February 2010 - Exploiting disorder
for perfect focusing
In a Letter in Nature Photonics, COPS scientists
experimentally demonstrate that scattering can be exploited
to improve, rather than deteriorate, the focusing resolution
of a lens.
Link: http://www.nature.com/nphoton/journal/vaop/ncurrent/abs/nphoton.2010.3.html
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February 2010 – Simon Huisman graduates
Cum Laude
On the 4th of February 2010, Simon Huisman successfully
defended his masters thesis. Afterwards he received his
Master’s degree in Applied Physics, with the highest distinction
Cum Laude (special honors). The project was performed
at the Photonic Bandgaps group at the FOM Institute for
Atomic and Molecular Physics (AMOLF) in Amsterdam, and
the Complex Photonic Systems Group at the University of
Twente.
Simon has experimentally studied light propagation in
novel two-dimensional and three-dimensional silicon photonic
crystals. Surprisingly, for two-dimensional crystals a
lowest-frequency diffraction peak was observed that cannot
be explained by simple Bragg diffraction. This can have
implications for any kind of wave phenomena in periodic
structures with low-symmetry lattices. Simon has demonstrated
that the three-dimensional photonic crystals are likely
to have a broad photonic band gap. His thesis will be
coming up soon. |
2009
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| November 2009– COPS Alumnus Ivo
Vellekoop wins ‘Overijssel PhD award’ On Friday
November 27, former COPS PhD student Ivo Vellekoop received
the Overijssel PhD award for the best thesis of the University
of Twente of the last year. The award was handed out by
the commissioner of the Queen, Mr. Geert Jansen, at the
48th Dies Natalis of the university. Ivo wrote his thesis,
with the title ‘Controlling the propagation of light in
disordered scattering media’ under supervision of Prof.
Dr. Ad Lagendijk and Dr. Allard Mosk.
Ivo and co-workers developed a method to focus laser
beams through non-transparent objects, such as a sheet
of paper, a layer of paint, or the shell of an egg.
Their method, called ‘wavefront shaping’ has received
world-wide attention and was in the top 10 physics stories
of the year 2008 of the American Institute of Physics.
An animated illustration of wavefront shaping can be
found here.
The thesis itself can be downloaded here.
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October 2009 – Thesis defense of
Alex Hartsuiker
On Thursday, October 1st 2009, Alex Hartsuiker has successfully
defended his thesis on “Ultrafast all-optical switching
and optical properties of microcavities and photonic crystals”.
He has prepared his thesis under the guidance of Willem
Vos at COPS, and AMOLF in Amsterdam. It was a great pleasure
that prof. dr. Cornelia Denz, prof. dr. Jean-Michel Gérard
and prof. dr. Harm Dorren were willing to be a member
of the thesis jury.
Alex has researched storage of light using planar and
pillar microcavities. Manipulation of stored light was
succeeded. Alex and co-workers showed that light stored
in the cavity can be frequency converted. Furthermore,
Alex for the first time ever showed that a cavity can
be switched ultimately fast using the electronic Kerr
effect. The results presented show that supra-THz switching
is possible, where the switch rate is determined by the
photon storage time of the cavity. Download this thesis
here.
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September 2009 - Thesis defense
spontaneous emission of Near-Infrared Quantum Dots controlled
with Photonic Crystals by: Bart H. Husken
On Friday, May 8th 2009, Bart Husken has successfully
defended his thesis on Spontaneous emission of near-infrared
quantum dots controlled with photonic crystals. He has
prepared his thesis under the guidance of Willem Vos at
COPS as well as at AMOLF in Amsterdam. We were very pleased
to have Prof. dr. hab. T. Gregorkiewicz, Prof. dr. M.
Bayer, and dr. A.F. Koenderink as comittee members from
outside our university. In his thesis Bart demonstrates
the control of the emission of light at near-infrared
wavelengths including the range relevant for the telecom
industry. To this end he studied PbSe quantum dots inside
inverse opal photonic crystals. For the emission measurements
a new experimental setup was built to perform confocal
microscopy at near-infrared wavelengths. Bart measured
frequency- dependent escape of near-infrared quantum dot
emission and observed strong deviations from the Lambertian
emission pro le. The results are successfully described
with an escape-function model that is based on di usion
theory and extended to photonic crystals. Furthermore,
Bart demonstrated the worlds' rst control of the quantum
dots' near infrared spontaneous emission rate by changing
the Local Density of Optical States with photonic crystals.
He found an inhibition of the emission rate up to 51 %
and an enhancement up to 29 %. The experimental results
agree with earlier measurements in the visible range,
and are qualitatively explained using theory. Barts exciting
results open the road to drastically control spontaneous
emission of light in silicon 2 photonic bandgap crystals.
Furthermore an increase in absorption and emission oscillator
strength was observed with quantum dot size, giving rise
to an increase in the emission rate with size. Finally,
an experimental method was developed to recover a single,
deterministically fabricated nanostructure in various
experimental instruments without the use of arti cially
fabricated markers. This will allow us to study photonic
band gap structures with cavities. Download this thesis
here <spontaneous emission of near-infrared quantum
dots controlled with photonic crystals. (pdf
13 Mb). |
June 2009 - High impact COPS at
World of Photonics Congress LASER 2009 Munich
Scientists from COPS have presented a record number of
12 talks (and one poster, too!) at the LASER 2009 congress
in Munich. This congress encompasses the conferences CLEO/Europe,
IQEC and the European conference on Biomedical Optics.
The majority of talks were proudly presented by our enthusiastic
Ph.D. students and postdocs, and we thank our colleagues
from the Biophysical Engineering group at MESA+ and the
Max-Planck-Institute for Quantum Optics for their contributions
to this success.
We are interested to hear from Ph.D. and Postdoc candidates
to strengthen our team.
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May 2009 - 2 FOM
projects granted to COPS
COPS is pleased to announce that 2 projects have
recently been funded in the prestigious "Projectruimte"
grant scheme of the FOM, the Dutch organization that promotes
and funds physics research.
Allard Mosk and Willem Tjerkstra have proposed the
project "Breakdown of universal transparency: Is
there symmetry between absorption and gain?"
Last year, COPS reported in Physical Review Letters
on the first observation of open channels in transport
of light through disordered materials. In the course
of the newly granted project, COPS researchers will
make further precise studies of these open channels.
How do these channels behave if we introduce absorption
or gain in the system? There has been controversy in
theory about this, but no experiments so far. To answer
the questions experimentally, COPS will build a new
wavefront-shaping apparatus where fluids can be introduced
to tune absorption and gain of disordered photonic samples.
Willem Vos proposed "Zap!" Ultrafast time
control of spontaneous emission." It is well-known
that spontaneous emission by a light source is a random
process: you never know in advance when a photon is
emitted, but we do know the average probability (~ emission
rate). The emission rate can be controlled by placing
the emitter in a suitable nano-environment, as COPS
demonstrated for the first time with photonic crystals
in 2004 (see: Nature). Usually, however, the emission
rate is constant in time. Now, COPS will “zap” a cavity
(made by the Gerard group in France) with an emitter
by a short laser pulse. The cavity is temporally shifted
in frequency thereby strongly reducing or increasing
the rate. This event happens at a pre-determined moment
in time, thus introducing determinism in an otherwise
random quantum process!
In both projects, we are looking for ambitious young
researchers (Ph.D. students, postdoc, Master students).
The 2nd project will mainly be done at AMOLF, but it
will also be possible for UT students to be involved
as part of their Master project.
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March 2009 - Highly cited COPS
In a recent survey by Thomson Reuters (New York) on the
exciting research field of photonic crystals, it has appeared
that a Science paper authored by COPS’ Willem Vos and
Judith Wijnhoven is ranked no. 1 for most cited paper
in the period 1998-2008. Moreover, three of Vos’ papers
have been recognized as Highly Cited Papers in their field.
More information and an interview by science writer Gary
Taubes with Willem Vos can be found <here>
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March 2009 - Review panel praises
COPS research
As part of the 3TU Federation, a panel of eminent scientists
has recently visited the Applied Physics (APH) research
groups at the three Universities of Technology of Eindhoven,
Delft and Twente for a mid-term review. We are honored
that the panel has very highly rated the research of the
chair Complex Photonic Systems. 
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January 2009 - How many photons can quantum
dots emit?
M.D. Leistikow, J. Johansen, A.J. Kettelarij, P. Lodahl,
and W.L.
Vos
Size dependent oscillator strength and quantum
efficiency of CdSe quantum dots determined by controlling
the local density of states
Phys.Rev. B, 79 (4), 045301 (2009)
Quantum dots are small clumps of semiconductor material,
that behave like atoms with discrete transitions for spontaneous
emission. These quantum dots are studied for use in applications
ranging from LEDs and solar cells to biological characterization.
A very important property of these quantum dots is how
many photons (light particles) a dot can emit per second,
the radiative decay rate. Unfortunately, this can not
be measured directly since in experiments only the total
decay rate is measured, which also includes the loss of
energy to heat.
Researchers from the Center for Nanophotonics at the
FOM Institute AMOLF and research group COPS at the University
of Twente together with physicists from the Denmark
Technical University have now succeeded to determine
the radiative decay rate of the technologically important
CdSe quantum dots by using a trick. If the quantum dots
are placed close to a mirror, the reflection from the
mirror affects the emission. At certain positions, where
there is constructive interference, the decay rate is
increased, while at others the decay rate decreases.
The trick now is that only the radiative part is affected
by the mirror, while the nonradiative losses stay constant.
This allows to separate the radiative and nonradiative
components of the decay rate. Surprisingly, it is found
that the radiative decay rate is not strongly dependent
on quantum dot size, contrary to previous predictions.
The oscillator strength, a measure of how strongly the
quantum dot interacts with its environment, is of the
order of 0.7, much lower than expected. This is bad
news for applications of CdSe quantum dots in quantum
optics. The good news is that the behavior of these
dots can be used to test theory of condensed matter.
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2008
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December 2008 – COPS
research in AIP Top Ten of the Year 2008
The American Institute of Physics has made a list of the
ten big physics findings of the year 2008. COPS research
features in this list:
"LIGHT
PASSES THROUGH OPAQUE MATTER"
What's new---getting light to behave in a new way. When
light strikes an opaque material like milk most of the
radiation is scattered; little of it passes through the
sample. But in an experiment at the University of Twente
in the Netherlands, much more of the light can be made
to traverse the scattering material if beforehand the
wavefront of the incoming light is shaped by special filters.
Background: summary in Physics Today, Sept 2008 and an
APS essay and research paper in PRL.
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November 2008 – COPS
visits MIT
COPS scientist Allard Mosk has given several lectures
at the MIT
George R. Harrison Laboratory on the recent
research of wavefront shaping and multiple scattering
control. During this fruitful visit, many lively discussions
have taken place with scientists – including many eminent
colleagues – from this well-known Institute.
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September 2008 – Thesis
defense of Léon Woldering
On Friday, September 5th 2008, Léon Woldering has successfully
defended his thesis on “Fabrication of Photonic Crystals
and Nanocavities”. He has prepared his thesis under the
guidance of Willem Vos and co-supervisor Willem Tjerkstra
at COPS, and AMOLF in Amsterdam, and in collaboration
with ASML, the leading lithography company. It was a great
pleasure that prof. Ulrich Gösele from the Max Planck
Institute in Halle was willing to be a member of the thesis
jury.
Léon has explored the fabrication of diamond-like inverse
woodpile structures that are strongly photonic crystals.
To this end, he realized world record deep pores of 10
microns for 400 nm diameters in silicon by means of CMOS
compatible methods. Since these pores have an ordered
pattern, the arrays of pores act as high-quality 2-dimensional
photonic crystals, as confirmed by extensive optical reflectivity
measurements. By milling a second pattern of pores perpendicular
to the first set, Léon even realized prototype 3-dimensional
photonic crystals with a diamond-like symmetry, that show
photonic band gap-like behavior. In addition, Léon has
realized the first point defects on 3D self-assembled
crystals made of colloidal nanospheres (from silicon dioxide)
by means of focused-ion beam milling. Such point defects
are widely pursued since they will act as tiny cavities
to trap light. The 3D photonic crystals strongly modify
the propagation of light, especially in the near infrared
range. Therefore, applications are anticipated in optical
telecommunication, but also in sensitive miniature sensors.
Download this thesis here <Fabrication
of Photonic Crystals and Nanocavities> (pdf 32
Mb).
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August 2008 - With
a little help, light will find its way
In an upcoming paper in Physical Review Letters,
COPS researchers Ivo Vellekoop and Allard Mosk show increased
transmission of light through opaque white materials.
By carefully manipulating the wavefronts of the incoming
light, they show that, no matter how thick a scattering
layer, two thirds of the light can be transmitted.
"Threading Light through the opaque"; Science
Now website:
http://sciencenow.sciencemag.org/cgi/content/full/2008/808/1?rss=1
"Opacity to transparency"; Physics Today
Update
http://blogs.physicstoday.org/update/2008/08/opacity_to_transparency.html
Preprint of the upcoming Physical Review Letter by Ivo
Vellekoop en Allard Mosk (link: http://arxiv.org/abs/0804.2412) |
June 2008 – Thesis
defense of Philip Harding
On Friday, June 13th 2008, Philip Harding successfully
defended his thesis on “Photonic Crystals Modified by
Optically Resonant System”. He prepared his thesis under
the guidance of Willem Vos and co-supervisor Allard Mosk
at COPS, and AMOLF in Amsterdam. It was a great pleasure
that prof. Heinrich Kurz from the Technical University
of Aachen and dr. Pepijn Pinkse from the Max Planck Institute
in Garching were willing to be a member of the thesis
jury.
Philip investigated light propagation in periodic nanoscale
structures known as photonic crystals. His goal was to
gain insight into how the propagation of light can be
modified and exploited for uses in telecommunication,
regenerative energy, quantum information processing and
optical computing. To this end, he doped photonic crystals
with optically resonant systems: systems which strongly
modify light propagation. One of the systems used were
cavities, small defects which can trap light. Philip showed
how light could be released from this cavity by applying
a suitable pump pulse. He investigated the behavior of
the cavity resonance both before, during and after the
arrival of the pump pulse. Another resonant system he
probed were atoms. The extremely narrowband dispersion
of atomic resonances can strongly modify the optical properties
of photonic crystals. He showed both experimentally and
theoretically how the lineshape of the atomic transition
changes drastically for different relative positions of
the stopband to the atomic resonance.
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May 2008 - Structural
Properties of Opals Grown with Vertical Controlled Drying
Artificial opals are gemstones with a beautiful luster
known as iridescence. Opals are widely used, notably as
photonic crystals, since they are conveniently made from
periodic stackings of nanospheres. A technique that is
commonly used to fabricate artificial opals is vertical
deposition. The strong point of this technique is that
it enables to create thin opals. Surprisingly little is
known about the structural properties of the resulting
opals however.
Alex Hartsuiker and Willem Vos from the FOM institute
AMOLF in Amsterdam and the research group COPS at the
University of Twente have investigated the structural
properties of the artificial opals fabricated with the
vertical deposition technique. They found that the thickness
of the opals varied as well as the size of the crystal
domains. A linear relation was found between the domain
size and the thickness of the crystal. This linear relation
is also revealed in thin films of random stacked spheres.
The information obtained in this research can be used
to make smarter designs for photonic crystal applications.
Read more... (pdf)
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April 2008 - Ad Lagendijk
writes 'Survival Guide' for scientists
Ad Lagendijk, acclaimed physicist, veteran columnist and
amateur sociologist has compiled a life long learning
and teaching experience in a comprehensive and highly
informative 'How to' for scientists, aptly titled: "Survival
Guide for Scientists". This book is published
by the Amsterdam University Press and Chicago University
Press.
The survival kit contains guidelines for writing articles,
creating presentations, but also hard nose tips on how
to survive in the hard and competitive world of science.
You can order the book through the Amsterdam University
Press (www.aup.nl)
A blog has been initiated on www.sciencesurvivalblog.com.
For the real survival fans: A waterproof version with
conference coordinates will hit the market before the
Christmas season.
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| March 2008 - Deep stuff: Record
deep arrays of nanopores in silicon
In a recent article published in the leading British
journal Nanotechnology, Léon Woldering, Willem Tjerkstra,
Henri Jansen, Irwan Setija, and Willem Vos report on
their new method to fabricate arrays of many deep nanopores
in silicon. The Dutch team from the MESA+ Institute
for Nanotechnology at the University of Twente, the
AMOLF Institute in Amsterdam and ASML, the world's leading
provider of lithography systems for the semiconductor
industry, has fabricated many arrays of myriad nanopores.
Read more... (pdf).
Léon Woldering, Willem Tjerkstra, Henri Jansen, Irwan
Setija and Willem Vos
Periodic arrays of deep nanopores made in silicon
with reactive ion etching and deep UV lithography,
Nanotechnology 19 (pdf).
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| September
2007 – Ultra fast colour change of a nano mirror maze
Tijmen Euser (COPS and PBG group) and his colleagues have
demonstrated for the first time ultra fast changes of
the selective colours of a photonic band gap crystal that
acts as mirror maze for light. The achievement of Dr.
Tijmen Euser and his colleagues is reported in the September
issue of the Journal of Applied Physics. Read more… (pdf
 /,
158kb)
Tijmen G. Euser, Hong Wei, Jeroen Kalkman, Yoonho Jun,
Albert Polman, David J. Norris, and Willem L. Vos, Ultrafast
optical switching of three-dimensional Si inverse opal
photonic band gap crystals, Journal of Applied Physics,
volume 102 (pdf,
303 kb)
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July 2007 – National
newspaper coverage for COPS researchers Ivo Vellekoop
and Allard Mosk
We all know that it is possible to start a paper
fire using a magnifying glass to focus the sunlight. But
can you focus light with a strongly scattering material
like white paint or an eggshell? Yes! Ivo Vellekoop and
Allard Mosk proved that it is possible and published their
findings in Optics Letters. At Saturday July 14th 2007,
the Dutch national newspaper NRC Handelsblad explaines
the mechanisms of the experiment in Dutch
(pdf 1.1 Mb).
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July 2007 – Structured
laser focuses through opaque objects
From daily experience, we know that it is impossible
to see through white paint or through a glass of milk.
The reason for this is that the direction of light is
changed by numerous collisions with tiny particles in
the material. We have succeeded, as the first ever, to
direct a beam of light through a layer of white paint.
This was done by generating thousands of individually
controlled rays of light. After hundreds of collisions
in the medium, all rays interfere constructively to form
a beam of light emanating from the back side of the material.
The results are expected to have applications in medical
imaging, microscopy and telecommunication.
You can download a copy of the article here
(pdf 3.1 MB). See also the news
article
at the FOM-website
and the animation.
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June 2007 – Interview
Willem Vos and Ad Lagendijk in NRC Handelsblad
An interview with Willem Vos and Ad Lagendijk
about light in photonic crystals and random media was
published by NRC Handelsblad at Saturday June 2, 2007.
Click to view the interview
with Willem Vos (Dutch, pdf 104kb) and Ad
Lagendijk (Dutch, pdf 47kb)
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March 2007 – Spatial
Extent of Random Laser Modes
Lasers are usually sophisticated apparatus that
are kept very clean to avoid scattering of light, which
is a loss process. However, strong scattering can be advantageous:
in strongly scattering materials which amplify light lasing
can be observed. These "random lasers" have
become subject of active research. Until now, the mechanism
of random lasing was not fully understood. In literature,
two different hypotheses are put forward, leading to a
heated debate. One point of view is that random-laser
light originates from local optical cavities. The contrasting
point of view describes the origin of random-laser light
as single spontaneous emission events that follow very
long light paths.
Researchers of the University of Twente and the Amolf
Institute in The Netherlands have set up an intricate
experiment to settle this debate.
They have directly measured the size of the area of which
the light originates from. The results, that give a satisfactory
answer to the question what the origin of random lasing
is, show that random-laser light originates from small
areas inside the laser. These local cavities have a unique
emission pattern depending on the structure of the sample.
Therefore, random lasers may have an interesting application
as non-forgeable labels. You can download a preprint of
the article here
(pdf 223kb). See also the news
article
at the FOM-website.
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March 2007 – Fluorescence
decay of quantum-dot ensembles is highly non-exponential
to a degree controlled by photonic crystals
COPS researchers have investigated spontaneous
emission from many quantum dots simultaneously in photonic
crystals. The COPS’ Ivan Nikolaev (now at ASML), Peter
Lodahl (now at DTU Denmark), and Willem Vos together with
Floris van Driel (Utrecht) and Femius Koenderink (AMOLF)
observed strikingly non-exponential decay curves in time-resolved
experiments. The curves were successfully modeled with
a continuous distribution of decay rates. The widths of
the distributions reveal a large six-fold variation with
the crystals’ lattice parameter. This result agrees qualitatively
with advanced calculations of the local density of optical
states (LDOS).
The authors identify the behavior with the long anticipated
phenomenon that light sources located at different positions
in each unit cell of a crystal and with differently orientated
dipoles experience different strengths of the light field.
Therefore, the sources experience variations of the LDOS,
and thus emit light at different rates.
The calculations demonstrate that both large inhibitions
and large enhancements of the spontaneous emission can
be achieved at will, with properly positioned and oriented
light sources inside three-dimensional photonic crystals,
at room temperature and in large volumes limited only
by the crystal size. The results appear in Physical Review
B in the paper “Strongly non-exponential time-resolved
fluorescence of quantum-dot ensembles in three-dimensional
photonic crystals”; a pdf-copy can be found
here (pdf 357kb).
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February 2007
– Thesis defense of Tom Savels
On Wednesday, 14th of February 2007, Tom Savels
successfully defended his thesis on "Scattering lasers".
He worked on this thesis under the guidance of Ad Lagendijk
and co-supervisor Allard Mosk, at COPS as well as at the
University of Amsterdam and at AMOLF in Amsterdam. Tom
explored the ultimate miniaturization limit and asked
the question "how many atoms are needed to build
a laser ?" His research shows that, quite surprisingly,
only two atoms in free space suffice to exhibit laser
behavior: one atom is pumped and provides optical gain,
while the other atom acts as "mirror" and provides
feedback. The laser behavior becomes more pronounced whem
more atoms are added. Remarkably, no macroscopic mirrors
or cavity are required. The predicted laser behavior not
only occurs for atoms, but for a broad range of microscopic
objects such as quantum dots, molecules or DNA strings
and may as such lead to interesting experiments and applications.
Additionally, this research provides insight in laser
dynamics on an atomic scale, which becomes important as
laser technology enters the nanosize regime. Download
this thesis here
(pdf 2.3 Mb).
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January 2007 –
Oscar Bok graduates at COPS
On the January 31st, 2007, Oscar Bok received
his Master's degree in Applied Physics, after completing
his research project with a presentation at the University
of Twente. The project was mostly performed in the group
Photonic Bandgaps at the FOM Institute for Atomic and
Molecular Physics (AMOLF) in Amsterdam lead by Willem
Vos, in close collaboration with the COPS group. Oscar
has successfully developed a new type of samples for our
group: planar silver mirrors with spacers such that a
layer of quantum dots can be placed at a controlled distance.
This allows the study of essential properties of quantum
dots, which are an important new type of light source
that are important to photonics and in future perhaps
also in quantum information. Since the emission rate of
an emitter near a mirror is very well known, such studies
yield essential information on the quantum dots. We wish
Oscar best of success in his future career, which may
take him as far as the Far East.
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January 2007 –
What to do if Fluorescence Decay Curves are not Exponential?
Understanding the dynamics of light sources
such as semiconductor quantum dots, rare-earth ions, or
molecules is crucial for getting insight in many physical,
chemical and biological processes. In fluorescence lifetime
experiments, the number of excited sources is probed by
recording a fluorescence decay curve: a histogram of the
distribution of arrival times of single photons. Such
a curve is typically a straight line on a semi-logarithmic
plot, that is, single-exponential decay characterized
by a single decay rate. In many cases, however, the decay
curve is much more complex and strongly differs from single-exponential
decay. But surprisingly, there is little profound analysis
in literature on how to solve this problem. In collaboration
with Floris van Driel and Daniel Vanmaekelbergh (Utrecht)
and Peter Vergeer (Philips), COPS scientists Ivan Nikolaev
(now at ASML), Peter Lodahl (now at DTU Denmark) and Willem
Vos have analyzed general fluorescence decay curves from
ensembles of emitters. We find that many often-made assumptions
(common lore based on single-exponentials) are completely
wrong in the general case. Our results have recently appeared
in Physical Review B, a pdf-copy can be found here
(pdf 332kb).
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2006
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November
2006 – Charles Uju new FOM technician at COPS
We are pleased to announce that Charles Uju has started
his position as a Research Technician with our group on
November 1st. He will make new photonic samples and maintain
the COPS sample fabrication facilities. Charles' work
is part of the new project in collaboration with BPE,
prof. Subramaniam. Previously, Charles has studied in
Eindhoven and worked with Philips.
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November
2006 – Focused ion beam milling of nanocavities
Artificial opals are gemstones that are widely used, notably
as photonic crystals, since they are conveniently made
from periodic stackings of nanospheres. It would be exiting
if the shape or morphology of individual colloids could
be modified at will. This could lead, e.g., to optical
cavities in photonic crystals, to controllable features
in lithographic masks, or to chemically active substrates
with special size-selective moieties.
In a recently accepted paper for the magazine “Nanotechnology”,
Léon Woldering, Bart Husken, Willem Vos (from COPS and
the FOM Institute for Atomic and Molecular physics) and
Bert Otter (from the MESA+ cleanroom) have succeeded in
developing a technique to modify the structure of individual
targeted silicon dioxide colloidal particles on the surface
of opals. The structures milled had controllable features
with radii smaller than 40 nm and were fabricated using
focused ion beam milling (FIB). Circularly shaped material
cavities, and even arrays of cavities, were milled in
the colloids, converting the particles into donut-like
or bead-like structures.
You can download a copy of the article here
(pdf 543kb).
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November
2006 – Thesis defense of Ivan Nikolaev
On Thursday, 2nd of November 2006, Ivan Nikolaev successfully
defended his thesis on "Spontaneous-emission rates
of quantum dots and dyes controlled with photonic crystals".
Ivan worked on this thesis under the guidance of Willem
Vos, at the University of Twente and at AMOLF in Amsterdam.
COPS thanks prof. Jean-Michel Gérard (CEA, Grenoble) for
his appearance on the thesis jury. A striking result that
is a central part of Ivans thesis is the very first observation
of modified spontaneous emission rates of light sources
(e.g., quantum dots and organic dyes) in photonic crystals,
which confirms the 1987 prediction by Eli Yablonovitch
that started this exciting research field. Download this
thesis here
(pdf 3.2 Mb).
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June 2006 – FOM Projectruimte grant awarded.
A joint proposal between COPS and BPE
(prof. dr. Vinod Subramaniam) has been awarded funding
in the highly competitive FOM
Projectruimte program. The research will involve photonic
materials to modulate excited state fluorescence and energy
transfer processes in fluorescent proteins.
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June
2006 – Thesis defense of Floris van Driel
Floris van Driel has succesfully defended his Ph.D. thesis
on June 7th, 2006 in the wonderful old "Akademiegebouw"
of the University of Utrecht. Floris has worked on a successful
joint NWO project his Utrecht advisors profs. John Kelly
and Daniel Vanmaekelbergh (Utrecht), and COPS advisor
Willem Vos. It was a pleasure for COPS to have Floris
with us for an extended period, notably working on quantum
dots in photonic crystals. His thesis is entitled "Light
sources in semiconductor photonic materials". Download
his thesis here
(pdf 4.8 Mb).
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May 2006 – Adriaan Molenaar graduates Cum Laude
On the 19th of May 2006, Adriaan Molenaar received his
Master's degree in Applied Physics with the addition Cum
Laude, after completing his graduation project with a
presentation at the University of Twente. He presented
experimental results on ultra-fast optical switching of
woodpile photonic crystals. The optical properties of
the crystals were changed by free-carrier generation with
femtosecond laser pulses. The woodpile photonic crystals
that were used in the experiments are designed to have
a photonic band gap at telecom frequencies. A shift of
the stop band in perpendicular reflectivity was observed
on pumping the crystals, causing large reflectivity changes
for frequencies at the stop band edges and pointing to
considerable DOS changes in the crystal. In addition,
unexpected reflectivity changes at ultrashort time scales
were observed at temporal overlap of pump and probe pulses.
The results clearly show that the switching technique
used in the experiments is useful for further research
and future applications.
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April
2006 – Adriaan Molenaar wins "KIVI NIRIA scriptieprijs".
Adriaan Molenaar, Master's student in the Complex Photonic
Systems group, has won the first prize in a prestigious
contest organized by
"KIVI NIRIA (Dutch Engineer's Assiciation). The
prize was awarded for his internship report on "Pointing
accuracy of H.E.S.S. Cherenkov-telescopes" which was written
during a four-month internship at the Max
Planck Institute for Nuclear Physics in Heidelberg.
The prize was awarded because of the practical applicability
of the results of the internships, and because of the
enthousiastic way the results were presented on the KIVI
NIRIA "Mechatronica Valley" congress on April 18, 2006.
In the university newspaper UTnieuws,
Adriaan declared "the group where I do my Master's research
devotes a lot of attention to presentation skills. This
has helped me win the prize, for the reports of the nominees
were all very strong". The prize consists of € 1000
and free membership of KIVI NIRIA.
You can download the presentation here
(pdf 537 kb).
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March
2006 – Electrochemical formation of porous GaP in aqueous
HNO3
Gallium phosphide is transparent for light in the yellow
and red part of the visible spectrum (λ > 0.55
μm). This absence of optical absorption, together with
its very high refractive index (n = 3.3), makes
GaP a fascinating material for optical applications in
the important wavelength range, 0.55 μm < 1.1 μm,
a range where Si shows strong absorption. Because light
can be scattered strongly inside porous GaP, Anderson
localization can appear.
Porous GaP is formed by the electrochemical anodic etching
of single-crystal-line n-GaP wafers in an aqueous electrolyte.
The diameter of the pores formed depends on the current
density; the larger the current density the larger the
diameter of the pores. Until now solutions of H2SO4
have mostly been used for the formation of strongly scattering
samples of porous GaP. The range in pore size of the porous
GaP that can be made using H2SO4
as an electrolyte is limited. In an article in Electrochemical
and Solid-State Letters, Willem Tjerkstra investigated
the usability of aqueous HNO3
as an electrolyte for the formation of porous GaP with
a larger pore size, and therefore a higher scattering
strength than has been obtained before.
You can download a preprint here
(pdf 223 kb).
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March
2006 – Laser threshold of Mie resonances
The quest for small light sources or lasers is becoming
more and more important. For this quest it is important
to investigate both experimentally and theoretically small
light source. An example of such a light source that is
theoretical well understood is a small sphere. There is
an exact description for the light that scatters inside
or outside this sphere, the well-known Mie theory. In
literature an extended version of this theory is used
to describe a Mie sphere in the region of both absorption
and gain. In an article in Optics Letters, Karen van der
Molen, Peter Zijlstra, Ad Lagendijk and Allard Mosk show
with accurate measurements of the intensity and the wavelength
of the light coming from a Mie sphere that this extension
of Mie theory is only valid until a distinct value of
the gain inside the sphere. This point is identified by
us as the laser threshold. Above this threshold, the extended
Mie theory can no longer be used.
You can download a preprint here
(pdf 295 kb).
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March
2006 – Wouter Peeters graduates Cum Laude
On the 17th of March 2006, Wouter Peeters completed his
graduation project with a presentation. Afterwards he
received his Master’s degree in Applied Physics with the
addition Cum Laude.
Wouter Peeters presented measurements on the propagation
of a short pulse of light through strongly scattering
porous Gallium Phosphide. The porous GaP samples used
for the measurements are the strongest scattering non-absorbing
samples in the world. The mean free path of the GaP samples
depends strongly on the wavelength of the light. By varying
the wavelength, Wouter Peeters ‘varied’ the
mean free path over almost a factor two. He extracted
the diffusion constant by analyzing the time resolved
shape of the transmitted pulse. He showed that the energy
velocity of the light in the sample is independent of
the mean free path.
Download his thesis here
(pdf 7.0 Mb).
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2005
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November 2005 - Pushing for power
In the essay "Pushing
for power" published by Nature, Ad Lagendijk
draws a picture of the physics community dominated by
aggressive males engaged in territorial combat and believes
that these observations extend to other branches of science
as well.
Comments on the essay pointed out that Ad Lagendijk his
perception on the physics community does not seem to extend
to all branches of science. Especially biology shows a
diversity of men and women. The gross of these biologists
have been able to push their own research agenda without
being tempted or pressured to form egregiously large groups.
Nevertheless, Uriarte, Weathers and Eviner argue with
Ad Lagendijk and mention far-reaching consequences of
the current reward system. Click on the hyperlinks below
to view the comments.
Power
games cause sparks in physics, but biologists...
Power
clashes limit science and reflect archaic values
Science
is an adventure, not a battle
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November 2005 - Snellius Medal awarded to Willem Vos
On Friday 25th of November 2005 the Society
for the Advancement of Science, Medicine, and Surgery
in Amsterdam hands out the Snellius Medal to Willem
Vos of COPS.
The Society for the Advancement of Science, Medicine,
and Surgery already exists since 1790. The association
grants medals of honour to persons who, by doing original
research, achieve significant contributions at their research
area. Since 1951 the Snellius Medal is handed out once
every ten years for work in the area of Science and Technology
and Mathematics. Predecessors of Willem Vos are the Nobel
price winner prof. F. Zernike from Groningen (1955), ir.
H. Rinia from Eindhoven (1967), prof. J.H. van der Waals
from Leiden (1976), prof. N.G. de Bruijn from Eindhoven
(1985), and prof. J.W. Verhoeven from Amsterdam (1994).
Klik hier
om verder te lezen in het Nederlands.
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November
2005 - Is emission of light a 'quantum-size-effect'?
Very small particles made of semiconductor material,
also called nanocrystals or quantum dots, can emit light
with various color. Together with Floris van Driel and
Daniel Vanmaekelbergh from Utrecht University, and Christophe
Delerue and Guy Allan from CNRS (Lille, France), the COPS
Peter Lodahl and Willem Vos show how fast the various
colors are emitted. Their paper has just been accepted
by the leading journal Physical Review Letters.
Nanocrystals are important because they are used in all
kinds of devices in which light plays a crucial role,
such as lasers, light-emitting-diodes (LEDs) and solar
cells. One of the reasons for this is that many properties
of the nanocrystals, such as the emission color, are tunable
by the nanocrystals’ size. These size-dependent properties
are called ‘quantum-size effects’. Our new results show
that also the rate of emission of light is set by the
nanocrystals’ size. The small nanocrystals emit blue light
approximately 60 million times per second. The largest
nanocrystals emit red light three times slower. Excellent
agreement is obtained with the outcome of basic quantum-mechanics
theory on tiny nanoparticles. Thus the question in the
title can be answered with yes! The rate of light emission
is indeed one of the quantum size effects. This result
is important since the emission-rate determines the optical
output of nanocrystals in applications.
You can download a preprint here
(pdf 321 kb).
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November 2005
- Quantum correlations of light propagating through
white paint
Multiple scattering of light, a process that happens
in systems varying from fog and white paint to photonic
crystals, randomizes classical information such as the
direction of propagation of the light wave. Quantum correlations
are usually thought of as being very fragile, so it was
thought that no interesting quantum correlations would
survive multiple scattering. In a recent paper in Physical
Review Letters [95, 173901 (2005)], Peter Lodahl and co-workers
show that in contrast to this intuition, strong quantum
correlations do exist in light that exits from a multiple
scattering material. The multiply scattered light contains
spatial correlations: the probability to detect a photon
scattered in one direction depends on the detection of
photons in other directions. Experiments to measure such
correlations are underway. Applications of this work in
the field of quantum telecommunication, where one has
to cope with imperfect transport, seems likely.
You can download the paper here
(pdf 220 kb).
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| July
2005 -Negative Temperatures in Phys. Rev. Lett.,, July
29th 2005. Negative Kelvin temperatures can exist
in trapped atomic gases. This striking thermodynamic
observation has recently been made by Allard Mosk. A
paper on this subject will soon appear in
Physical Review Letters.
Download a preprint here
(pdf 185 kb).
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| June
2005 -Thesis defense of Boris Bret, July 14th 2005.
Boris Bret, a junior researcher at COPS, has succesfully
defended his thesis on July 14th, 2005, at 13:00 at
the University of Twente (in room 2 of building Spiegel).
His thesis is entitled 'Muliple light scattering
in porous gallium phosphide'.
Download his thesis here
(pdf 6.2 Mb).
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