Research Programme

Overall scientific concept with short-term and long-term goals

The photon concept, introduced by A. Einstein in 1905, has led to momentous advances in experimental and theoretical physics. One of the most exciting inventions based on Einstein's discovery was the laser (1960), which revolutionized basic research as well as engineering and started to turn optics into photonics.

Recent developments in nonlinear optics, spectroscopy, imaging and telecommunications (Physics Nobel Prize 2009: fibers and CCDs) attest to this fact. Due to the manifold opportunities and the future potential of optics and photonics the 21st century is expected to develop into "the century of the photon".

Periodic structures and the combination of novel materials in photonics enable the control of light creation, propagation, and light-matter and light carrier interaction from collective excitations down to single atom manipulation.

Unforeseen novel photonic concepts based on the interplay of technology, theory, and experiments will strongly influence gas phase, cluster, and condensed matter research during the next decade and will form a symbiosis of basic and applied research.

 

The SFB NextLite generates strong momentum in the following areas:

Light Synthesis

  • High-intensity coherent femtosecond/sub-femtosecond light sources
  • Multicolor phase-locked infrared optical parametric amplification
  • Quantized transitions in nanostructures (Microwaves, MIR & THz)
  • Plasmon emission from nano-particles
  • Coherent exciton-polarition emission from micro-cavities
  • Single photon emission

Light-Matter Interaction

  • Sub-cycle tunneling dynamics in solids
  • Dynamics of quantized transitions in nanostructures
  • Strong coupling (excitons, intersubband transitions, nitrogen vacancies) in cavities
  • Plasmonic control and enhancement of light-matter interaction

Light Cavities

  • Micro-cavities (visible, NIR, MIR, THz, MW)
  • Ring-cavities for directional control
  • Cavity coupling for light control
  • Optical bottle microresonators
  • Photonic molecule/crystal cavities
  • Plasmonic guiding and coupling

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Project Overview


Project 01 -
Coordination Project

Speaker of the SFB: Gottfried Strasser


Administrative Assistant: Alexandra Linster

(Former project assistant: Ingrid Unger)

 

Project 02 -
THz Synthesis and Interaction

Project Leader: Karl Unterrainer

 

Project 03 -
Strong-field Spectroscopy with Multicolor Pulses

Project Leader: Andrius Baltuška

 

Project 04 -
Interactions of Ultrashort Field with Solid Surfaces and Nanostructures

Project Leader: Joachim Burgdörfer

 

Project 05 -
Plasmonic Sources and Interaction

Project Leader: Joachim Krenn

 

Project 06 -
Simulation of Plasmonic Nanoparticles

Project Leader: Ulrich Hohenester

 

Project 08 -
Strong Coupling of Remote Ultra-high Q Microresonators

Project Leader: Arno Rauschenbeutel

 

Project 09 -
Coupling and Cavity Interaction in Quantum Cascade Lasers and Detectors

Project Leader: Gottfried Strasser

 

Project 10 -
Nonlinear Phenomena in Complex Photonic Structures

Project Leader: Stefan Rotter

 

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Project Coordination - Project 01

Speaker of the SFB NextLite: Gottfried Strasser
Administrative Assistant: Alexandra Linster

The Coordination Project provides the administrative infrastructure for the research and organizational work of the SFB NextLite.

The major administrative tasks are coordinating and performing all financial transactions between the participating institutions and the FWF, collecting all publications and conference contributions, to organize the internal SFB events, scientific and technological meetings to enable a strong and efficient interaction of the SFB participants and coworkers, organizing scientific meetings with international participation, providing a central information office for all SFB members, creating and continuously updating of the NextLite Homepage, coordinating SFB "outreach" activities (for high school students and undergraduate students), and managing the public relations work and interaction with press and media.

The NextLite workshops are organized in Vienna and Graz, and Next-Lite symposia are organized on a bi-annual basis with invited international speakers. In addition public lectures are offered during the workshops and symposia for general public.

These tasks are performed by the speaker of the SFB, Gottfried Strasser, with the support of an administrative assistant, Alexandra Linster.

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THz Synthesis and Interaction - Project 02

Project leader: Karl Unterrainer

The region of the electromagnetic spectrum between visible (infrared) photonics and high frequency electronics is called the Terahertz range. It can be defined from 0.3 to 30 THz (1 mm – 10 μm wavelength) and is a frontier research area.

In this project part we will search for novel THz sources and investigate THz light matter interaction. We will study THz sources based on intersubband transitions with new cavity designs (photonics crystal cavities), coupling between cavities, and strong cavity – intersubband coupling. We will also use femtosecond laser pulses to generate few cycle using elementary excitation in solids with non-linear frequency mixing.

The development of more intensive femtosecond laser pulses in the SFB is a requirement for the generation of high-intensity THz transients. With these high-intensity pulses we will be able to enter the strong THz field physics by studying intersubband transitions. A unique aspect is that we will employ a new phase-resolved THz time domain method. This allows to study all strong field effects phase resolved i.e. we will be able to control the phase of the driving pulses as well as to detect the phase of the response. From these experiments we will gain new insight into the fundamentals of non-linear interaction and into the dephasing and scattering processes of nanostructures. The later will be very usefull for the understanding of intersubband THz sources. In particular, for the cavities investigated strong confinement is expected which increases the coupling between THz radiation and intersubband transitions. Thus, the timeresolved study of these cavities is a further important goal within this project part.

The long term goal is to be able to combine the advantages of THz emission from intersubband transitions with the phase-control of femto-second laser pulse generated THz pulses.

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Strong-field Spectroscopy with Multicolor Pulses - Project 03

Project Leader: Andrius Baltuska

Electric field strengths required to free electrons from a binding potential in atoms, molecules and solids are easily achieved in femtosecond light pulses emitted by modern laser amplifiers and optical parametric amplifiers. The strong-field regime provides straightforward access to attosecond control over electron emission because electron liberation is temporally localized to a narrow fraction of an optical cycle, particularly when the ionization mechanism is dominated by tunneling. The best-known example of sub-cycle electron emission control is the generation of attosecond bursts of coherent XUV radiation - high-order harmonic generation (HHG) - occurring upon the electron recombination with the parent ion. Intra-cycle control of the generation of free charge carriers in transparent bulk solids opens an intriguing avenue toward creating new transient properties in materials and, therefore, presents a fascinating exploration route toward much faster - potentially 1014 Hz - optoelectronic devices. In this project within the SFB, we will experimentally study the possibility to create a transient metallic response in bulk dielectrics and in nanometer-scale dielectrics.

One aim is to exploit the optical signature of the tunneling current that exhibits twice-per-cycle periodicity and, when probed by optical light, creates a transient optical response. In 2010, we conducted a proof-of-principle experiment to confirm the existence of this phenomenon using a single-color excitation. Within the proposed SFB, we will take advantage of state-of-the art few-cycle IR parametric amplifiers that, on the one hand, permit us to reach a predominantly tunneling ionization regime and, on the other hand, provide a platform for multicolor optical pulse shaping which should result in a much better resolved temporal and spatial localization of the effect.

The other major aim of the project is to exploit the possibility to enhance a strong-field interaction - such as HHG and THz wave emission of plasma micro-currents - by introducing nanoparticles into a gaseous target. The near-term objective within this project is to explore the applicability limits of nano-localized attosecod currents created in bulk solids and gases by tailored optical strong fields. The longterm perspective is to apply such sophisticated attosecond control to increasingly more complex targets, such as various nano-objects and large polyatomic molecules with strongly correlated electrons.

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Interactions of Ultrashort Field with Solid Surfaces and Nanostructures - Project 04

Project Leader: Joachim Burgdörfer

The focus of this project part is on the development and application of wavepacket-propagation methods to explore ultrashort light-matter interactions spanning the time scales from attosecond to nanoseconds in nanostructures, surfaces, and in the condensed phase.

Following light-driven dynamics of electronic motion is key to both the microscopic understanding of photonic processes in matter and to develop novel protocols for light synthesis and characterization. The interaction of intense few-cycle pulses requires a non-perturbative treatment well beyond a single or few-photon picture as the instantaneous field amplitude rather than the cycle-averaged spectral intensity controls the ensuing electronic dynamics. In turn, the non-linear response of matter modifies the electromagnetic field resulting in pronounced changes in phase, amplitude, and spectral content. Following the evolution of the pulse-driven coherent excitation in the many-body environment of the solid requires in many cases an open-quantum system approach to account for dephasing and decoherence.

The methods employed lend themselves to a multitude of applications within this SFB. Envisioned applications during the first four-year period include the simulation of sub-cycle ionization dynamics in bulk and nanoscale dielectrics exploring transient metallization, field enhancement and dielectric breakdown, above-threshold photoemission and harmonic generation from nanostructures, controlling dephasing by optimal shaping of pulses strongly coupled to NV color centers in diamond, and few-cycle THz pulse driven interband transitions in semiconducting quantum-well heterostructures.

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Plasmonic Sources and Interaction - Project 05

Project Leader: Joachim Krenn

Plasmons modes in metal nanostructures bridge the scales of optical and electronic processes by generating highly confined and resonantly enhanced local fields. Plasmonics is accordingly a topic of high current interest for the nanoscale concentration of light, the emission control of elementary emitters, research in optical metamaterials and applications in, e.g., sensor devices or photovoltaics. In this project we explore the properties of complex plasmonic nanostructures, and their combination with elementary emitters to build hybrid nanophotonic systems.

We will look into metal nanoparticles and –wires with well-defined plasmonic resonance, mode volume and quality, and field pattern. These properties shall be tailored by nanoscale lithographic fabrication for, on one hand, the spectrally and polarization-selective coupling to propagating far-field light. On the other hand, the plasmonic fields and their mode densities will be tailored for optical near-field coupling, for the coupling to (single) elementary emitters such as quantum dots or color centers. We will include so-called dark modes, i.e., plasmon modes with zero net dipole moment which are high quality modes as they are not radiation damped. On such a platform the nanostructure-induced modifications of emitter lifetimes, their emission intensities and radiation patterns can be probed with unprecedented precision.

This part of the work program prepares the ground for tackling, first, the important question of loss in plasmonic structures and, second, photodetection based on plasmonically enhanced quantum dots. Overcoming loss addresses the major roadblock in the application of plasmonic components which are limited due to the rather low mode quality and propagation lengths. Based on a profound understanding of plasmon/emitter interaction we will explore plasmon amplification by stimulated emission and evaluate the possibility of plasmon-based lasing. In addition, we will aim at the application of plasmonically enhanced quantum dots for highly miniaturized photodetection, ultimately mediated by a single quantum dot.

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Simulation of Plasmonic Nanoparticles - Project 06

Project Leader: Ulrich Hohenester

This project extends concepts of cavity-QED to hybrid systems consisting of plasmonic nanoparticles and quantum emitters, such as semiconductor nanocrystals or NV centers in nanodiamands (together with the Krenn and Majer groups), and explores possible routes towards lightwave electronics through ultrafast photoexcitation of electrons in the strong, evanescent SP fields (together with the Burgdörfer group). Plasmonic nanoparticles modify the photonic environment, and allow for tailoring the decay and energy transfer properties of quantum emitters [Nie97, Andrew04, Anger06, Akimov07].

Contrary to natural atoms, which can be modeled as generic two-level systems, artificial atoms possess a more complex level structure and are embedded in a solid state environment, which might be of importance for SP-based quantum control, hybrid nanostructures, or active plasmonics. The SP-induced field enhancement in presence of femtosecond laser pulses can be exploited for hot-electron generation, as recently demonstrated for metal tips [Krüger11, Herink12]. Expanding this scheme to more complex, lithographically grown objects allows to tailor the evanescent SP-fields and the ponderomotive acceleration of hot electrons, which might beneficial for lightwave electronics applications, as well as generation of terahertz and higher harmonic radiation.

Within the SFB there will be a direct cooperation with the Krenn group on the coupling between quantum dots and plasmonic nanoparticle. The investigation of strong-field effects of electrons in presence of SP-enhanced fields of an ultrafast laser pulse will be carried out in collaboration with the Burgdörfer group.

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Strong Coupling of Remote Ultra-high Q Microresonators - Project 08

Project Leader: Arno Rauschenbeutel

We aim to achieve a long-standing goal of photonics: the strong coupling of two remote optical microresonators, compatible with the requirements of cavity quantum electrodynamics. This objective is motivated by exciting applications in the context of optical (quantum) information processing as well as for the implementation of strong light–light interaction at the level of single photons.

The project will employ a novel type of ultra-high Q whispering-gallery-mode resonator – the so-called bottle microresonator. Bottle microresonators combine characteristic advantages of whispering-gallery-mode resonators, like ultra-high quality factors, small mode volumes, and nearly lossless in- and out-coupling of light via tapered fiber couplers, with a customizable and fully tunable mode structure which is similar to Fabry-Pérot microresonators. This unique combination of properties makes bottle microresonators ideal candidates for CQED experiments. In a second experiment, we plan to trap and to optically interface laser-cooled neutral atoms with the evanescent field surrounding the nanofiber waist of a tapered optical fiber. The trap relies on the combined effect of the attractive dipole force exerted on the atoms by the evanescent field and the repulsive centrifugal barrier that arises when the atoms orbit around the nanofiber.

This system realizes a two-dimensional “artificial atom” and is predicted to exhibit genuine quantum dynamical effects like collapse and revival of the spinning atomic wave packet. Light that propagates through the nanofiber and that interacts with the orbiting atoms will carry an imprint of this dynamics and will thus allow one to probe and to test the quantum nature of the atomic motion with several hundred quanta of angular momentum.

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Coupling and Cavity Interaction in Quantum Cascade Lasers and Detectors - Project 09

Project Leader: Gottfried Strasser

This project part of of Next-Lite uses novel intersubband device concepts by combining epitaxial growth and processing of III-V materials to realize new cavity designs, coupling between cavities, and strong cavity–intersubband interaction. Coupling between cavities via evanescent field or direct coupling will be investigated, fabricating various coupling scenarios of active materials in these cavities in the MIR and, together with Unterrainer (P02), at THz frequencies.

The growth of novel material combinations will not only used to increase the performance of the proposed devices, but also to study the influence of interfaces, alloy scattering, and doping profiles on light matter interactions. Shaping of beam profiles in QCLs will be done utilizing surface plasmons [Yu08], photonic crystals [Xu09, Chassagneux09], and ring DFBs [Mujagic10]. On a longer time scale we want to be able to change or completely switch the coupling strength between cavities. With Rotter (P10) sophisticated cavity designs will be proposed and fabricated. Intersubband detectors using QWIPs or quantum cascade designs will be combined with photonic crystals to circumvent polarization constrictions [Kalchmair11].

Short-term goals are optimization of detector performance by integrating PhCs, QWIPs, and free standing slabs, long-term goals are novel resonator structures for active source materials,the combination of mechanical and optical cavities, and quantum structures allowing emission and detection at the same wavelength to get more insight into light matter interactions. Polaritons can probe strong light-matter coupling in THz micro-cavities and nanostructures [Todorov10]. This strong coupling utilizing intersubband transitions with polaritons is an alternative approach to the work we plan with Unterrainer (P02), using a timedomain THz method.

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Nonlinear Phenomena in Complex Photonic Structures - Project 10

Project Leader: Stefan Rotter

This theoretical project of the SFB NextLite is dedicated to three topics, all of which are strongly connected to the experimental efforts within the SFB:

Nonlinear effects in highly tunable laser devices

We have recently demonstrated on a theoretical level how the coupling between two microcavity lasers can lead to very interesting behavior in the pump-dependence of such a coupled laser device [1]. When pumping the two laser components separately of each other, so-called “exceptional points” can be induced in this system. These points correspond to singularities of the non-Hermitian operator, which describes the lasing behavior. In the vicinity of these points the counter-intuitive situation is realized that the laser may turn off even when the overall pump power is increased. In collaboration with the groups of Unterrainer (P02) and Strasser (P09) we were now able to observe this phenomenon also in experiments employing quantum cascade lasers [2]. In a next step we will build on this exciting progress to investigate exceptional points in multiple coupled lasers, as well as the many fascinating effects that they give rise to.

Light scattering in optical resonators

In a second line of research, we are interested in optical micro-resonators and the physics involved in the coupling to them - be it the evanescent in- and out-coupling of light by means of tapered fiber couplers or the quantum optical aspects involved in an emitter which couples to the resonator field. For this project we are collaborating with the Rauschenbeutel group (P08), which has acquired extensive expertise with so-called “bottle resonators” that can be used to strongly couple light to atomic emitters. For this strong-coupling regime we could recently demonstrate that emitters coupled to multiple resonator modes can feature periodic quantum revivals in their excitation probability on a time scale associated with the resonator round-trip time [3].

Quantum optics with spin ensembles

In a joint work with the group of Majer we have recently demonstrated that a superconducting transmission line cavity can be used to strongly couple the cavity-enhanced microwave field to a macroscopic ensemble of nitrogen-vacancy spins in a synthetic diamond [4]. Investigations of this kind are motivated by the desire to store the information encoded in a photon (“flying qubit”) in a solid-state device (“stationary qubit”) with comparatively long coherence times. To describe the dynamics during the storage and retrieval of information in such a quantum memory, we developed a powerful approach that fully captures the strongly non-Markovian dynamical features appearing in such a context. On this basis we could now show both theoretically and experimentally, that the decoherence induced by the inhomogeneous broadening of the considered spin ensemble can be suppressed in the strong-coupling limit – an effect known as “cavity protection” [5]. This effect is key to promising coherent control schemes that can now be employed to perform quantum operations with the spin ensemble without the detrimental influence of decoherence.

[1] M. Liertzer, L. Ge, A. Cerjan, A. D. Stone, H. E. Türeci, and S. Rotter, “Pump-Induced Exceptional Points in Lasers”, Phys. Rev. Lett., vol. 108, no. 17, p. 173901, Apr. 2012.
[2] M. Brandstetter, M. Liertzer, C. Deutsch, P. Klang, J. Schöberl, H. E. Türeci, G. Strasser, K. Unterrainer, and S. Rotter, “Reversing the pump dependence of a laser at an exceptional point”, Nat. Commun., vol. 5, p. 4034, Jun. 2014.
[3] D. O. Krimer, M. Liertzer, S. Rotter, and H. E. Türeci, “Route from spontaneous decay to complex multimode dynamics in cavity QED”, Phys. Rev. A, vol. 89, no. 3, p. 033820, Mar. 2014.
[4] R. Amsüss, C. Koller, T. Nöbauer, S. Putz, S. Rotter, K. Sandner, S. Schneider, M. Schramböck, G. Steinhauser, H. Ritsch, J. Schmiedmayer, and J. Majer, “Cavity QED with Magnetically Coupled Collective Spin States”, Phys. Rev. Lett., vol. 107, no. 6, p. 060502, 2011.
[5] S. Putz, D. O. Krimer, R. Amsüss, A. Valookaran, T. Nöbauer, J. Schmiedmayer, S. Rotter, and J. Majer, “Protecting a Spin Ensemble against Decoherence in the Strong-Coupling Regime of Cavity QED”, Nature Physics 10, 720 (2014).

Sketch of a PT-symmetric system with balanced gain (G) and and loss (L).
We show that the PT-symmetry breaking transition in such structures is independent of the presence of external mirrors
[see Phys. Rev. X 3, 41030 (2013)]

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