Nanothermochromic diffraction gratings based on the metal-insulator transition of VO2 are fabricated by site- selective ion-beam implantation in a SiO2 matrix. The studied diffraction gratings were defined (i) directly by spatially selective ion-beam synthesis or (ii) by site-selective deactivation of the metal-insulator transition by ion-beam induced structural defects. The strongest increase of the diffraction efficient was observed at a wavelength of 1550 nm exceeding one order of magnitude for the selectively deactivated gratings. The observed pronounced thermal hysteresis extends down close to room temperature and makes these optical elements well suited for optical memory devices.
We report on recent progress towards single photon sources based on quantum dot and quantum post nanostructures
which are manipulated using surface acoustic waves. For this concept acoustic charge conveyance in a quantum well is
used to spatially separate electron and hole pairs and transport these in the plane of the quantum well. When conveyed to
the location of a quantum dot or quantum post these carriers are sequentially captured into the confined levels. Their
radiative decays gives rise to the emission of a train of single photons. Three different approaches using (i) straininduced
and (ii) self-assembled quantum dots, and (iii) self-assembled quantum posts are discussed and their application
potential is discussed. First devices and initial experiments towards the realization of such an acoustically driven single
photon source are presented and remote acoustically triggered injection into few individual emitters is demonstrated.
We study the influence of the mechanical deformation induced by a surface acoustic wave (SAW) on the resonance
frequency of a defect cavity in a 2D photonic crystal membrane. Using FDTD-simulations we determine the
resonance frequency and quality factor of a nanocavity of a GaAs based structure with embedded InAs quantum
dots. Under the influence of a SAW, we find a periodic modulation of the cavity resonance wavelength of Δλ >2
nm accompanied by only a weak < 0.5× reduction of the Q-factor. Initial experiments for a SAW wavelength of
~ 1.8μm show a pronounced broadening of the time-integrated cavity emission line corresponding to a shift of
≥ 1 nm.
A challenging topic of the lab-on-a-chip research is to implement sorting mechanisms on low cost disposable chips. In
many applications, surface acoustic waves (SAW) have recently proven to be a versatile and efficient technique for
microfluidic actuation. A SAW is excited by applying a high frequency signal to a piezoelectric substrate. When the
wave hits the solid/liquid interface it transmits its acoustic energy into the liquid and a local pressure gradient emerges,
leading to surface acoustic streaming. Experiments can be performed directly on the piezoelectric substrate or on a
separate glass slide positioned on top of the SAW source. We developed a technique for the accumulation of solid and
soft objects in SAW generated microvortices in microfluidic channels. For this purpose, the corner of a rectangular
microchannel is irradiated by a wide SAW beam. There, the SAW excites sound waves in the fluid producing a typical
acoustic streaming flow pattern which typically exhibits two vortices. Particles injected into the flow are accumulated
and dynamically trapped in one of these vortices. After the flow is stopped, the collected particles stay in the position of
the vortex. In our experiments, we use open microfluidic channels with functionalized hydrophilic-hydrophobic surfaces
on glass substrates as well as closed channels build with the elastomer PDMS via soft lithography. We find that the
accumulation efficiency for particles is strongly size dependent. Below a critical radius of 500 nm, particles tend to flow
through the vortex and are not captured in the corner. Generally, larger particles can be collected at more moderate SAW
power levels compared to smaller particles. Therefore, by adjusting the SAW power level, one is able to collect particles
above a designated size. This concept is not limited to solid particles but can also be applied to soft objects like cells.
Starting with an introduction to surface acoustic waves, their generation and detection using interdigital transducers
(IDTs) on piezoelectric materials (e.g. LiNbO3 and ZnO) will be reviewed. Then the application of surface acoustic
waves in electronic devices will be presented. Moreover, recent studies, using the technique of attaching the
material of investigation onto the sound path of the acoustic delay line between the IDTs is discussed.
The miniaturization of chemical and biological processes has made enormous progress driven mainly by genomics and proteomics. Microfluidics is the core technology to realize miniaturized laboratories with feature sizes on a submillimeter scale. Here, we report on a novel microfluidic technology which allows biochips to be programmed so that different biological assays can be performed with only one chip layout. Interdigital transducers integrated on piezoelectric substrates excite surface acoustic waves (SAW) which drive reagents on the surface of the biochip. The reagents can be placed on any desired spot on the chip’s surface, they can be merged, split and brought to reaction. SAW technology can also be used to efficiently agitate small volumes of liquids accelerating diffusion limited reactions considerably.
Remotely doped parabolic quantum wells have been grown by MBE using a digital alloy approach
to vary the Al content in the AlGai.As system. The monitoring of the beam fluxes as well as
the measured subband separations confirm the precision of our growth technique. Using a front
gate electrode we can depopulate the electrical subbands. Thus we can determine experimentally
the subband separations showing close agreement with the results of our self-consistent
calculations.
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