Arran Curran

Epitaxial liftoff of ZnSe-based heterostructures using a II-VI release layer

Epitaxial liftoff is a post-growth process by which the active part of a semiconductor heterostructure, the epitaxial layer, is removed from its original substrate and deposited onto a new substrate. This is a well established technique in GaAs-based heterostructures where epitaxial liftoff can be achieved by exploiting the contrast in the etch rates of GaAs and AlAs in hydrofluoric acid. We report here successful epitaxial liftoff of a ZnSe-based heterostructure. We find that a metastable layer of MgS acts as a perfect release layer based on the huge contrast in the etch rates of ZnSe and MgS in hydrochloric acid. Epitaxial liftoff of millimeter-sized ZnSe samples takes a fraction of the time required for GaAs liftoff. Photoluminescence experiments confirm that the liftoff layer has the same optical characteristics as the original wafer material.

Partial synchronization of stochastic oscillators through hydrodynamic coupling.

Holographic optical tweezers are used to construct a static bistable optical potential energy landscape where a brownian particle experiences restoring forces from two nearby optical traps and undergoes thermally activated transitions between the two energy minima. Hydrodynamic coupling between two such systems results in their partial synchronization. This is interpreted as an emergence of higher mobility pathways, along which it is easier to overcome barriers to structural rearrangement.

Exciton?photon coupling in a ZnSe-based microcavity fabricated using epitaxial liftoff

We report the observation of strong exciton–photon coupling in a ZnSe-based microcavity fabricated using epitaxial liftoff. Molecular beam epitaxial grown ZnSe/Zn0.9Cd0.1Se quantum wells with a one wavelength optical length at the exciton emission were transferred to a SiO2/Ta 2O5 mirror with a reflectance of 96% to form finesse matched microcavities. Analysis of our angle-resolved transmission spectra reveals key features of the strong coupling regime: anticrossing with a normal mode splitting of 23.6 meV at 20 K, composite evolution of the lower and upper polaritons and narrowing of the lower polariton linewidth near resonance. The heavy-hole exciton oscillator strength per quantum well is also deduced to be 1.78 × 10 13 cm −2 . (Some figures in this article are in colour only in the electronic version)

Segregated Ice Growth in a Suspension of Colloidal Particles.

We study the freezing of a dispersion of colloidal silica particles in water, focusing on the formation of segregated ice in the form of ice lenses. Local temperature measurements in combination with video microscopy give insight into the rich variety of factors that control ice lens formation. We observe the initiation of the lenses, their growth morphology, and their final thickness and spacing over a range of conditions, in particular the effect of the particle packing and the cooling rate. We find that increasing the particle density drastically reduces the thickness of lenses but has little effect on the lens spacing. Therefore, the fraction of segregated ice formed reduces. The effect of the cooling rate, which is the product of the temperature gradient and the pulling speed across the temperature gradient, depends on which parameter is varied. A larger temperature gradient causes ice lenses to be initiated more frequently, while a lower pulling speed allows for more time for ice lenses to grow: both increase the fraction of segregated ice. Surprisingly, we find that the growth rate of a lens does not depend on its undercooling. Finally, we have indications of pore ice in front of the warmest ice lens, which has important consequences for the interpretation of the measured trends. Our findings are relevant for ice segregation occurring in a wide range of situations, ranging from model lab experiments and theories to geological and industrial processes, like frost heave and frozen food production.

Optical shield: measuring viscosity of turbid fluids using optical tweezers

The viscosity of a fluid can be measured by tracking the motion of a suspended micron-sized particle trapped by optical tweezers. However, when the particle density is high, additional particles entering the trap compromise the tracking procedure and degrade the accuracy of the measurement. In this work we introduce an additional Laguerre-Gaussian, i.e. annular, beam surrounding the trap, acting as an optical shield to exclude contaminating particles.

Decoupled and simultaneous three-dimensional imaging and optical manipulation through a single objective

The combination of optical manipulation and three-dimensional imaging is a central technique in fields ranging from medicine to physics. Using the objective lens simultaneously for optical trapping and imaging, however, inherently confines the trapping and imaging planes to the same focal plane. Here, we combine remote refocusing microscopy and optical trapping to optically decouple the imaging and trapping planes, achieving aberration-free three-dimensional imaging and simultaneous, decoupled optical trapping without the need for feedback or aberration corrections. We demonstrate our approach by directly imaging the flow field around optically trapped spheres in three dimensions. Due to its compatibility with other imaging and optical manipulation techniques, our approach is relevant to the wide range of fields that combine imaging and optical manipulation, such as physical chemistry, cell biology, and soft matter.

Development of an epitaxial lift-off technology for II-VI nanostructures using ZnMgSSe alloys

An epitaxial lift-off technique for removing wide bandgap II-VI heterostructures from GaAs substrates has previously been demonstrated using lattice-matched MgS as the sacrificial layer. However, using MgS as an etch release layer prevents its use as a wide bandgap barrier in the rest of the structure. Here, we describe the use of the etch-resistant alloy Zn.2Mg.8S.64Se.36 which we have developed as a replacement for MgS. We demonstrate that this alloy can be grown by MBE together with MgS in heterostructures and used as a barrier for ZnSe. A ZnSe quantum well with Zn.2Mg.8S.64Se.36 barriers shows no decrease in photoluminescence intensity after the etching process but shows a shift in emission wavelength associated with the changing strain state.

Real time characterization of hydrodynamics in optically trapped networks of micro-particles

The hydrodynamic interactions of micro-silica spheres trapped in a variety of networks using holographic optical tweezers are measured and characterized in terms of their predicted eigenmodes. The characteristic eigenmodes of the networks are distinguishable within 20-40 seconds of acquisition time. Three different multi-particle networks are considered; an eight-particle linear chain, a nine-particle square grid and, finally, an eight-particle ring. The eigenmodes and their decay rates are shown to behave as predicted by the Oseen tensor and the Langevin equation, respectively. Finally, we demonstrate the potential of using our micro-ring as a non-invasive sensor to the local environmental viscosity, by showing the distortion of the eigenmode spectrum due to the proximity of a planar boundary.

Noninvasive probing of persistent conductivity in high quality ZnCdSe/ZnSe quantum wells using surface acoustic waves

Attenuation of a surface acoustic wave is used as a highly sensitive and noninvasive probe of persistent photoconductivity effects in ZnCdSe/ZnSe quantum wells. These effects are observed over long time-scales exceeding several minutes at low temperatures. By varying the optical excitation energy and power and temperature we show that these effects arise from carriers photogenerated by interband excitation which are trapped in random potential fluctuations in the quantum wells related to compositional fluctuations. Effects related to defect levels in the band gap can be excluded and a transition of the conduction mechanism with temperature from a hopping to a percolation regime is observed. The transition temperature observed for our quantum well material is strongly reduced compared to bulk crystals. This indicates a superior structural quality giving rise to only weak potential fluctuation of ≲3 meV.Temperature dependence and recombination behavior of trapped charge carriers in ZnCdSe/ZnSe multiple quantum wells are investigated employing surface acoustic waves. These weakly perturb the carrier system, but remain highly sensitive even at small conductivities. Using this non-invasive probe we are able to detect persistent photoconductivity minutes after optical excitation. Measurement of exciting photon energies, the temperature dependence and ability to quench the conductivity with energies lower than the bandgap, support the notion of spatial separation of electrons and holes in the wells, due to random local potential fluctuations possibly induced by compositional fluctuations.

Surface acoustic wave mediated exciton dissociation in a ZnCdSe/LiNbO3 hybrid

By making use of epitaxial lift-off, ZnCdSe quantum wells are transferred onto a LiNbO3 substrate in order to employ its enhanced piezoelectric properties. The photoluminescence emission of this hybrid structure is characterized and the influence of a surface acoustic wave on the free exciton and bound exciton emission is investigated. Finally, two counterpropagating surface acoustic waves are launched leading to a decrease in the acoustic wave mediated exciton dissociation.