Current Research Projects

Quantum light sources based on individual carbon nanotubes  

Project description: The optical emission of single wall carbon nanotubes (SWCNTs) is governed by bright exciton emission similar to self-assembled quantum dots (QDs). In contrast to self-assembled QDs, SWCNT emit efficiently at RT and their emission wavelength can be easily tuned to telecom wavelength by tuning their diameter in the CVD growth process. One goal is to explore the exciton photophysics of individual SWCNT and to come up with new ways to realize efficient quantum light sources, in particular single photon sources. Blinking and spectral diffusion are hallmarks of quantum emitters and generally considered as detrimental properties for devices applications. We have shown for the first time that the often observed quantum intermittency (blinking) and spectral diffusion effects are extrinsic phenomena and we demonstrated how to eliminate these detrimental effects by embedding SWCNTs into polymer cavities. Very recently we also demonstrated photon antibunching and prolonged exciton dephasing times, which are both promising properties towards on-chip quantum photonics. The Figure shows a recent demonstration of quantum optical signatures from individual cavity-embedded SWCNTs. The cavity device achieve about 50-fold enhanced exciton emission as compared to bare SWCNT while effectively suppressing detrimental quantum blinking.

  • “Quantum light sources based on individual carbon nanotubes”, W. Walden-Newman and S. Strauf, book chapter, to appear in “Carbon nanotubes and graphene for photonic applications”, Woodhead Publishing (2012).
  • „Quantum Light Signatures and Nanosecond Spectral Diffusion from Cavity-Embedded arbon Nanotubes“
    W. Walden-Newman, I. Sarpkaya, and S. Strauf, Nano Letters 12, 1934 (2012).
  • “Suppression of Blinking and Enhanced Light Emission from Individual Carbon Nanotubes“,
    N. Ai, W. Walden-Newman, Q. Song, S. Kalliakos, and S. Strauf, ACS Nano 5, 2664 (2011).

At the edge of graphene - how to control chirality  

Project description: Graphene edges can be either of armchair type, zigzag type, or most commonly found in form of a fractional mixture of the two. Controlling the chirality of graphenes edges is of utmost importance for electronic, photonic and spintronic device applications, since the electrical, optical, magnetic and chemical properties of graphene nanoribbons depend strongly on the type of edge terminations. Spatially selective bond rearrangement has been a long-standing goal in materials science and chemistry. We have recently demonstrated a novel technique based on Raman spectroscopy which is capable of providing the fractional degree of the edge composition in exfoliated graphene. Beyond monitoring edges we have recently demonstrated that it is also possible to directly transform and thereby purify the edges of graphene. This was possible by utilizing specific Raman selection rules in zigzag edges and polarized laser radiation for prolonged times. As a result, the measured electronic bandgap enlarges after annealing by up to 50% at constant GNR width, as illustrated in the Figure. The demonstrated technique for controlling edge chirality will lead ultimately to better optoelectronic devices and furthermore allows exploring novel ideas such as chiral heterostructures made from graphene.   
 

  • "Optical control of edge chirality in graphene",
    M. Begliarbekov, K. Sasaki, O. Sul, EH Yang, and S. Strauf, Nano Letters 11, 4874 (2011).
  • "Determination of Edge Purity in Bilayer Graphene Using micro-Raman Spectrosopy",
    M. Begliarbekov, O. Sul, S. Kalliakos, E. H. Yang, S. Strauf, Appl. Phys. Lett. 97, 031908 (2010).

Nanoplasmonic arrays made by holographic lithography  

Project Description: Holographic lithography (HL) is a versatile tool to fabricate large-scale periodic structures with high throughput at submicron scale such as photonic crystals and nanoplasmonic arrays. Standard two-beam interference is rather limited in the type of lattice and motive shape. In contrast, one can realize almost arbitrary motive shapes based on four-beam HL. In particular, we have demonstrated that feature sizes down to 20 nm can be reliably achieved based on exposure at 488 nm, i.e. 24 times smaller than the laser wavelength. The key to achieve nanoantenna arrays with 20 nm feature size and controlled motive shape (e.g. sharp tips, field orientation) is the introduction of the fourth beam in HL which splits the local motive shape into a double-peaked feature (twin motive or compound lattice), as shown in the Figure. It is this inner gap of the twin motive in the interference pattern itself that can be tuned down to sub-20 nm, or even to zero, by proper control of phase and polarization. We have fabricated nanoplasmonic templates first in polymer (Hole pattern in SU8), which were transformed into plasmonic nanogap arrays (twin-dot pattern) by metal deposition and stripping of the polymer template. Recently we have also been able to fabricate plasmonic arrays with triplet and quadruplett features and controlled field orientation, significantly enlarging the utility of HL. Currently we study the light-matter interaction of the plasmonic nanogap arrays seeking applications in SERS and characterization of graphene. 

  • “Formation of triplet and quadruplet plasmonic nanoarray templates by holographic lithography”,
    X. Zhang and S. Strauf, submitted (2012)
  • „Holographic Control of Motive Shape in Plasmonic Nanogap Arrays“,
    X. Zhang, M. Theuring, Q. Song, W. Mao, M. Begliarbekov, and S. Strauf, Nano Letters 11, 2715 (2011).

Scalable Quantum Photonic Devices  

Project Description: Quantum information science is a fast growing and highly interdisciplinary field with the potential to cause revolutionary advances in science and engineering. Semiconductor cavity quantum electrodynamics systems provide an a priori scalable platform for quantum optics experiments and allow the development of quantum photonic devices for applications in quantum information science. The objective is to demonstrate deterministic coupling of lithographically defined vertical quantum dots to photonic crystal resonators for the realization of scalable quantum photonic devices. The proposed research program aims to utilize a novel type of lithographically defined "vertical QD" etched from a 2DEG wafer which is optically active and can be precisely tuned and positioned by electron beam lithography with respect to a PC cavity mode (picture), thereby eliminating the problem of random nucleation inherent to self-assembled QDs. Quantum dots and cavities can be combined with waveguides and beam splitters in a large number on a photonic crystal platform. Since the QDs are created by etching the randomness of self-assembled growth is eliminated, paving the way for scalability.

Segmented carbon nanotubes for FET and SET applications  

Project Description: The goal of this project is to fabricate single electron transistors (SET) from single walled carbon nanotubes operating at elevated temperatures. The approach is to fabricate nanosegments along the nanotube effectively creating a CNT quantum dot with large charging energy AND large level spacing. As a first step, we have fabricated field-effect transistor (FET) structures using CNTs as the conducting channel by using chemical vapor deposition to achieve in-plane growth from catalyst tips (Figure a). Devices have an initial on/off ratio of about two in the transfer characteristic due to contribution of metallic tubes. After controlled electrical breakdown only semiconducting tubes remain and we achieve on/off ratios of semiconducting p-FETs with individual SWCNTs as high as 105 at room temperature and without top gating. At low temperatures devices display pronounced peaks in the conductivity caused by the Coulomb blockade effect  demonstrating SET operation (Figure d). Segmentation along the SWCNTs can be achieved wit local anodic oxidation (Bottom figures).
 

  • "A Study on Nanoscale Carbon Nanotube Local Oxidation Lithography using an Atomic Force Microscope", K. Kumar, O. Sul, S. Strauf, F. Fisher, D. S. Choi, M. G. Prasad, and E. H. Yang,  IEEE Trans. Nanotechnology 99, 1 (2011).
  • "Transconductance and Coulomb blockade properties of in-plane grown carbon nanotube field effect transistors", N. Ai, O. Sul, M. Begliarbekov, Q. Song, K. Kumar, D. S. Choi, E. H. Yang, S. Strauf, Nanosci. Nanotechnol. Lett. 2, 73-78 (2010).
  • "A Systematic Study of Graphite Local Oxidation Lithography Parameters Using an Atomic Force Microscope", K. Kumar, S. Strauf, and E.H. Yang, Nanosci. Nanotechnol. Lett. 2, 185-188 (2010).

 

Quantum Transport in Graphitic Nanostructures  

Project description: In contrast to most conventional materials, in which charge transport is governed by the Schrodinger equation,  charge transport in graphene obeys the Dirac equation. Consequently, the charge carrying particles are chiral Dirac fermions. This makes graphene the ideal test material for quantum electrodynamic (QED) phenomena of electrons – a regime of physics which was previously inaccessible in solid-state systems. This promises a similar degree of quantum control of electrons as compared to what is now well established in cavity-QED with photons. The goal of this project is to study the quantum transport properties in top-gated graphene filed-effect transistors and to assess their applicability to the design of nanoelectronics devices. We have recently constructed such a FET with a 100 nm top gate which defines a local tunnel barrier. Carrier transport through this barrier reveals aperiodically spaced conductivity oscillations. Besides the well known magnetic field effect, these conductance oscillations can be taken as another signature of Klein-tunneling in graphitic nanostructures.  

  • “Quantum Inductance and High Frequency Oscillators in Graphene Nanoribbons“,
    M. Begliarbekov, S. Strauf, and C.P. Search, Nanotechnology 11, 165203 (2011).
  • "Aperiodic conductivity oscillations in quasi-ballistic graphene heterojunctions",
    M. Begliarbekov, O. Sul, N. Ai, E. H. Yang, S. Strauf, Appl. Phys. Lett. 97, 122106 (2010).

 

 

Past Research Projects (As postdoc at UCSB and Bremen)  

Ultra-bright Single Photon Sources

 
 

Project description: Optoelectronic devices that provide non-classical light states on demand have a broad range of applications in quantum information science, including quantum-key-distribution (QKD) systems, quantum lithography, and quantum computing.  Single-photon sources in particular have been demonstrated to outperform QKD based on attenuated classical laser pulses. Promising single-photon source designs combine high-quality microcavities with quantum dots as active emitters. Previous results reported measured single-photon rates up to 200 kHz using etched micropillars. In this project we demonstrated a quantum-dot-based single-photon source with a measured single-photon emission rate of 4.0 MHz (31 MHz into the first lens, with an extraction efficiency of 38%) due to the suppression of exciton dark states. Devices were fabricated by integrating high-Q (50,000) GaAs/AlGaAs microcavities with embedded oxidetapered apertures, a rugged trench design, as well as buried electrical gates allowing controlled loading and Purcell tuning of individual InAs QDs. Furthermore, our microcavity design provides mechanical stability, and voltage-controlled tuning of the emitter/mode resonance and of the polarization state. This type of SPS is of direct interest for applications in quantum information science.

 

“High-frequency single photon source with polarization control”,
S. Strauf, N. G. Stoltz, M. T. Rakher, L. Coldren, P. M. Petroff, and D. Bouwmeester, Nature Photon. 1, 704-708 (2007).

"High-Q optical microcavities using oxid apertured micropillars",
N. G. Stoltz, M. T. Rakher, S. Strauf, A. Badolato, D. D. Loftgreen, P. M. Petroff, L. A. Coldren,
and D. Bouwmeester, Appl. Phys. Lett. 87, 031105 (2005).

This work was done at the University of California at Santa Barbara in Collaboration with Stevens Institute of Technology

Ultra-low Threshold Photonic Crystal Nanolaser

 
 

Project Description: Optical microcavities offer the ability to create new efficient optical sources of specified wavelength through the control of the dielectric environment. Extremely low-threshold lasers can result from the appropriate match of small mode-volume photonic-crystal cavities to optically active material such as self-assembled InAs quantum dots (QDs). We observe lasing action of only 2-4 QDs as a gain medium as demonstrated by the characteristic kink in the light-light curve, pronounced linewidth narrowing, and by the photon bunching signature in photon statistic measurements showing a transition from the thermal into the coherent light state. Lasing occurs at ultra-low thresholds of about 4 nW absorbed pump power in the temperature range from 4-80K. We have furthermore shown that lasing does not require an exciton-mode resonance, which is counter intuitive in a simple "two-level" picture of a QD. This indicates that a "self-tuned" gain mechanism is responsible for lasing. While the underlying microscopic mechanism is still debated, the findings have clear technological implications for the future design of ultra efficient nanoscale lasers.

"Self-Tuned Quantum Dot Gain in Photonic-Crystal Lasers",
S. Strauf, K. Hennessy, M. T. Rakher, J.-S. Choi, A. Badolato, L.C. Andreani, P. M. Petroff, E. L. Hu, and D. Bouwmeester, Phys. Rev. Lett. 96, 127404 (2006).

"Evolution of the onset of coherence in a family of photonic crystal nanolasers",
J.-S. Choi, M. T. Rakher, K. Hennessy, S. Strauf, A. Badolato, P. M. Petroff, E. Hu, D.
Bouwmeester, Appl. Phys. Lett. 91, 031108 (2007).

This work was done at the University of California at Santa Barbara

Tuning Photonic Crystal Cavity Modes

 
 

Project description: Photonic crystal membranes with three-dimensional optical confinement can be fabricated from GaAs wafers by a combination of ebeam lithography and HF wet etching. Photonic crystal cavities are formed by breaking the periodicity (missing air-holes) which leads to localization of the electromagnetic field. Our experiments show how one may control the dielectric environment to obtain either stable, stepped or continuous frequency tuning operation, each of which will be of interest in a variety of nanophotonic applications. We also demonstrated a new method to attach self-assembled monolayers (SAM's) to GaAs photonic crystal membrane cavities opening novel possibilities for biofunctionalized photonic devices.

"Frequency control of photonic-crystal membrane resonators by mono-layer deposition", S. Strauf, M. T. Rakher, I. Carmeli, K. Hennessy, C. Meier, A. Badolato, M.J.A. DeDood, E. G. Gwinn, P. M. Petroff, E. L. Hu, and D. Bouwmeester, Appl. Phys. Lett. 88, 043116 (2006).

This work was done at the University of California at Santa Barbara

 

Coupled Quantum Dots

 
 

Project Description: Two quantum dots (QDs) in close proximity can become coupled forming an artificial molecule. Such quantum molecules are of interest for the field of quantum information processing (QIP) since the coupled system can act as a quantum bit (CNOT gate) based on exciton-exciton interactions. Vertically stacked QDs form with high probability from self-assembled InAs/GaAs QDs due to the local strain field on top of a QD if the separation is below 20 nm (picture). We observed directional energy transfer of the exciton oscillator strength as well as quantum light signatures in photon cross correlations which both indicate that coupling is mediated by dipole-dipole interactions rather than electronic tunneling. Tunnel coupling seems suppressed, even at a separation of 4 nm, due to the nonidentical nature of stacked self-assembled QDs. While this system is of large interest for QIP based on dipole-dipole interaction, other groups have also shown that tunnel-coupling can be achieved in electrically gated structures, where the QDs can be brought into resonance.

"Photon Statistics from Coupled Quantum Dots", B. D. Gerardot, S. Strauf, M. J. A. DeDood, A. Bychkov, A. Badolato, K. Hennessy, D. Bouwmeester and P. M. Petroff, Phys. Rev. Lett. 95, 137403 (2005).

The work was done at the University of California at Santa Barbara

Quantum optics with bound excitons

 
 

Project Description:  Excitons can localize at impurity atoms forming donor- or acceptor bound exciton (BX) complexes. By confining doping to 10 nm thin ZnSe quantum wells and by etching 100 nm small mesa features an individual impurity atom in a semiconductor matrix can be addressed all optical, as was confirmed by their photon antibunching signature. Compared to quantum dots, BX emission feature better control over their emission frequencies and intrinsic lifetimes of 100ps, making them attractive for cavity-QED applications in quantum information science.

„Quantum Optical Studies on Individual Acceptor Bound Excitons in a Semiconductor", S. Strauf, P. Michler, M. Klude, D. Hommel, G. Bacher, and A. Forchel, Phys. Rev. Lett. 89, 177403 (2002).

This work was done at the Institute of Solid-State Physics, University of Bremen, Germany