Institute for Integrative Nanosciences (IIN)

Most recent highlights:

Electromechanical tuning of quantum dot emission energies

Elastic mechanical strain is a powerful control tool for engineering the electronic states in quantum dots. With a simple electro-mechanical device we apply in-plane biaxial stress to a 200-nm-thick GaAs membrane containing InAs quantum dots. The relative energy levels of the exciton and biexciton states can be tuned to emit photons with exactly the same color. This observation may lead to the implementation of a recently proposed concept for the generation of entangled photon pairs. The strain tuning technique adds a new degree of freedom to the field of semiconductor nanostructures, and may inspire exciting future experiments in other fields.

F. Ding et al., Physical Review Letters 104, 067405 (2010) URL PDF

Metamaterial fiber optics

A metamaterial integration for fiber optics, leading to a dual effect of surface plasmon and classical waveguiding, is presented along with experimental potentiality. We theoretically propose a metamaterial fiber in which, depending on the wavelength (from ultraviolet to infrared) and the particular metamaterial composition, one can transmit information through surface plasmon mediated or classical waveguidance. The metamaterial can be used as the core or cladding of a fiber which allows waveguidance through a subwavelength geometry.

E. J. Smith et al., Nano Letters 10, 1 (2010) DOI: 10.1021/nl900550j (2009) URL PDF

This work was highlighted in:
Nature Photonics 3, 310 (2009) URL PDF
www.technologyreview.com (April 17, 2009) URL

Epitaxial quantum dots in stretchable optical microcavities

Arrays of GaAs microring optical resonators with embedded quantum dots are placed on top of piezoelectric actuators, which allow the microcavities to be reversibly “stretched” or “squeezed” by applying relatively large biaxial stresses at low temperatures. The emission energy of both QDs and optical modes red- or blue- shift depending on the strain sign, with the QD emission shifting more rapidly than the optical mode with applied strain. Remarkably, excitonic emissions from different QDs are observed to shift at different rates, implying that this technique can be used to bring spatially separated excitons into resonance.

T. Zander et al., Optics Express 17, 22452 (2009) URL PDF

Giant persistent photoconductivity in Si nanomembranes

This paper reports the observation of giant persistent photoconductivity from rough Si nanomembranes. When exposed to light, the current in Si nanomembranes is enhanced by roughly 3 orders of magnitude and can persist for days at a high conductive state after the light is switched off. An applied gate voltage can tune the persistent photocurrent and accelerate the response to light. By analyzing the band structure of the devices and the surfaces through various coatings, we attribute the observed effect to the rough surfaces of the nanomembranes, where light activates localised charge carriers.

P. Feng et al., Nano Letters 9, 3453 (2009) URL PDF

Rolled-up hyperlens

An investigation of the material makeup and surrounding medium of an optical rolled-up hyperlens is presented. A working spectral range of the hyperlens for different material combinations is studied along with an examination of hyperlens immersion, which suppresses the diffraction of waves exiting the lens due to impedance matching, leading to a higher intensity output. This hyperlens immersion technique can be implemented into cell culture and molecular analysis.

E. J. Smith et al., Applied Physics Letters 95, 083104 (2009) URL PDF
E. J. Smith et al., Applied Physics Letters 96, 019902 (2010) URL PDF

Tuneable electronic shell structure of GaAs quantum dots

Self-assembled quantum dots (QDs) usually form on top of a thin planar wetting layer (WL), whose properties can be hardly tuned independently from those of the QDs. We now studied QDs for which the WL thickness can be arbitrarily controlled. For fixed QD shape, a systematic decrease in the energy separation between ground and excited states of QDs is observed when the WL thickness is increased. This rather surprising phenomenon can be seen as a cross talk between QD vertical and lateral confinement potential.

L. Wang et al., Physical Review B 80, 085309 (2009) URL PDF

This work is chosen as PRB Editors' suggestion.

Microtubular jet engines

We have strain-engineered microtubes traveling as self-propelled catalytic microjet engines along various trajectories with speeds up to ≈ 2 mm s^-1 (approximately 50 body lengths per second). The motion of the microjets is generated by gas bubbles thrust out of one opening of the tube. The trajectories of various geometries can be traced by long microbubble tails. A magnetic layer is integrated into the wall of the microjet engine, which allows easy control over the direction of motion by applying external magnetic fields.

A. A. Solovev et al., Small 5, 1688 (2009) URL PDF

Ultrathin AlN/GaN nanomembranes

Scanning electron micrographs of ultrathin AlN/GaN nanomembranes self-assembled into various geometries such as tubes, spirals, and curved sheets on Si(111). These freestanding structures contain nanopores with sizes from several to tens of nanometers within nanomembranes of 20−35 nm nominal thickness and can find application in molecular separation or artificial blood capillaries.

Y. F. Mei et al., ACS Nano 3, 1663 (2009) URL PDF

Self-assembled quantum dot molecules

Self-assembled semiconductor quantum dot molecules (QDMs) obtained by epitaxial growth are reviewed. A comprehensive overview of the development and current stage of the research on QDMs composed of vertically (in the growth direction) or laterally (in the growth plane) aligned QDs is provided. The cover shows a 2D photoluminescence intensity map from a self-assembled lateral QDM in an electric field applied along the molecular axis. The coupling of the two QDs is evidenced by intricate spectral line anticrossings, indicated by dotted lines.

L. Wang et al., Advanced Materials 21, 2601 (2009) URL PDF

Spectral tunability of microtube resonators on glass

The tunability of optical resonant modes of spiral microtube cavities, rolled-up from square patterned SiO/SiO₂ thin nanomembranes on glass substrates, is demonstrated experimentally by coating the tube walls by atomic layer deposition. Transverse-electric modes are observed for Al₂O₃ coatings thicker than approximately 20 nm, as revealed by linear polarization analysis of the emitted light. Such fine tunability, which is essential for realizing optical microdevices, brings a better understanding of the resonant modes in microtubular cavities, suggesting that the microtubes could be used in potential applications for on-chip components like filters and sensors.

V. A. Bolaños Quiñones et al., Optics Letters 34, 2345 (2009) URL PDF

Ferngesteuerte Mikroraketen

Wir haben winzige Mikroraketen hergestellt, die sich durch ein Magnetfeld fernsteuern lassen. Die Herstellung der Mikroraketen erfolgt durch hauchdünne Schichtsysteme, die sich auf einem Trägersubstrat von selbst zu Mikroröhrchen aufrollen. Nach dem Ablösen der Röhrchen von dem Substrat erzeugen die chemisch aktiven Innenwände eine katalytische Reaktion in einer Flüssigkeit. Die Reaktion führt zur Bildung von Sauerstoffblasen, die aus den Röhrchen ausgestoßen werden, und so für den Vortrieb der Mikrorakete sorgen. Da das aufgerollte Schichtsystem magnetische Materialien enthält, können die Mikroraketen durch ein Magnetfeld ferngesteuert werden.

Oliver G. Schmidt, Spektrum der Wissenschaft, S.16, Juli 2009 URL
Oliver G. Schmidt, Welt der Physik 16.05.2009 URL

Shaped tubular optical microcavities

We have fabricated tubular optical microcavities by releasing pre-defined stressed SiO/SiO2 bilayer nanomembranes from polymer sacrificial layers. Optical measurements at room temperature demonstrate that the resonant optical modes can be accurately tuned along the tube axes. The resonant modes shift to higher energies with decreasing number of tube wall rotations and thickness, which is well-described by simulations. Rolled-up tubular optical microcavities can be produced in large periodic arrays on arbitrary substrates and are therefore highly attractive for on-chip integration technologies.

G. S. Huang et al., Applied Physics Letters 94, 141901 (2009) URL PDF

Advanced quantum dot configurations

We have presented an overview on approaches currently employed to fabricate advanced quantum dot configurations by epitaxial growth. Absolute position control of self-assembled quantum dots, so-called 'seeded' quantum dot crystals, is achieved by the combination of bottom–up and top–down methods. A promising way to realize quantum dot crystals with controlled spatial and optical properties is described.

S. Kiravittaya et al., Reports on Progress in Physics 72, 046502 (2009) URL PDF

From wrinkling to rolling

We have explored the change-over from wrinkling to rolling for compressively strained thin solid films. For small strain gradients across the film thickness the layer wrinkles whereas for large strain gradients it rolls up into a nanotube. Our theory provides an upper limit for the maximum achievable rotations of the film and is therefore of uttermost importance for many applications such as on-chip self-wound capacitors and coils.

P. Cendula et al., Physical Review B 79, 085429 (2009) URL PDF

New planar hybrid heterostructures and superlattices

We have invented an entirely new approach to create hybrid material layer stacks, which cannot be produced by any other technology. Hybrid layers are rolled up into a multi-winding tube on a substrate surface, and subsequently pressed down into a planar geometry. This leads to a hybrid superlattice out of single crystalline semiconductors and polycrystalline metals.

T. Zander et al., Applied Physics Letters 94, 053102 (2009) URL PDF

First single photon source on silicon substrate

Single quantum emitters have become an emerging area of fundamental research during the last years, driven by the need for nonclassical light sources delivering single-photons on demand for future implementation in the field of quantum information processing. In this work, we demonstrate for the first time triggered single-photon emission from a single quantum dot grown on Si substrate. Our findings show that it is feasible to fabricate high quality indistinguishable single-photon sources aiming at compatibiltiy with Silicon ultra large scale integration technologies.

M. Benyoucef et al., Nano Letters 9, 304 (2009) DOI: 10.1021/nl802948a (2008) URL PDF

First magnetic rolled-up microtubes demonstrated

Magnetic rolled-up microtubes are fabricated for the first time, and their hysteresis curves are measured. The tubes consist of semiconductor/magnetic radial superlattices rolled up from a thin solid hybrid material film. The magnetic properties reflect the change of symmetry adopted by the film, once it forms into a cylindrical structure.

Cell culturing in single tubes integrated on a Si Chip

Transparent oxide rolled-up microtube arrays are realized by the deposition of a pre-stressed oxide layer on patterned photoresist and the subsequent removal of the photoresist. Due to the unique tubular structure and optical transparency, such rolled-up microtubes can serve as well-defined 2D confined cell culture scaffolds. Yeast cells exhibit different growth phenomena in microtubes as the diameter is scaled down. Detailed investigations of individual yeast cells in a single microtube reveal the mechanical interaction between microtubes and the 2D confined cells causing different cellular assemblies. Our appoach is fully compatible to Si technology and might lead to high speed integrated analysis systems of individual cells on a single chip.

G. S. Huang et al., Lab on a Chip 9, 263 (2009) DOI: 10.1039/b810419k (2008) URL PDF

Strain engineered micro-/nanotubes on polymers

A generic approach has been developed to engineer tubular micro-/nanostructures out of many different materials with tunable diameters and lengths by precisely releasing and rolling up functional nanomembranes on polymers. The technology spans across different scientific fields ranging from photonics to biophysics and we demonstrate optical ring resonators, magneto-fluidic sensors, remotely controlled microjets and 2D confined channels for cell growth guiding.

Y. F. Mei et al., Advanced Materials 20, 4085 (2008) URL PDF

This work was highlighted in:

P.M. Magazine (February 17, 2009) URL
Frankfurter Allgemeine Zeitung (November 11, 2008) URL
Nanowerk (October 20, 2008) URL
Pro Physik (October 20, 2008) URL
Bild (August 27, 2008) URL