The Nano & Quantum Optics group at the Institute of Applied Physics targets to explore and understand the properties of nanoscale matter and objects and their interaction with photons. Some of the most recent research highlights are:

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  Scalable Functionalization of Optical Fibers Using Atomically Thin Semiconductors

Integrating atomically thin transition metal dichalcogenides into optical fibers yields novel opportunities in optical communication, remote sensing, and all‐fiber optoelectronics. However, the scalable and reproducible deposition of high‐quality monolayers on optical fibers was so far an unsolved challenge towards monolayer-functionalized integrated optical systems. Here we demonstrate chemical vapor deposition of monolayer MoS2 and WS2 crystals on the core of microstructured exposed‐core optical fibers. Their interaction with the fiber modes is reported, yielding the first true demonstration of monolayer-functionalized guided wave optics. Two distinct experiments demonstrate their application possibilities: first, the excitonic 2D material photoluminescence is simultaneously excited and collected with the fiber modes, opening a novel route to remote sensing. Then it is shown that third‐harmonic generation is modified by the highly localized nonlinear polarization of the monolayers, yielding a new avenue to tailor nonlinear optical processes in fibers. It is anticipated that the results may lead to significant advances in optical‐fiber‐based technologies.

Published in Advanced Materials DOI: 10.1002/adma.202003826 (2020) >>> more

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Tunable visible light metasurface displays

In recent years, tunable metasurfaces and metadevices have attracted extensive research efforts, aiming at pushing optical metasurfaces for practical applications. Infiltrating metasurfaces with nematic liquid crystals is an attractive approach due to its high compatibility with the existing industrial technologies and optical devices. Here, we demonstrated a LC-infiltrated metasurface with large transmittance modulation at red wavelengths for display applications. Different from conventional LC displays, the transmittance modulation takes place inside the subwavelength metasurface layer, rather than usually the bulk LC layer. Additionally, for the first time we use the technique of photoalignment to define and improve the LC alignment. Our demonstration shows the potential for largely reducing the LC layer thickness, display pixel size, and thus significantly reduce the device response time, power consumption and improve the display resolution.

Published in ACS Photonics doi: 10.1021/acsphotonics.9b00301 (2019) >>> more

 Teaser_web250x250   COPRA meets Python

Femtosecond laser pulses are among the shortest man-made events. To measure their temporal profile several techniques have been developed - most of which are known by their rather creative acronyms. Probably the most influential are frequency-resolved optical gating (FROG) and spectral phase interferometry for direct electric field reconstruction (SPIDER). In this work we developed a common pulse retrieval algorithm (COPRA) that can be applied to most of these measurements [1], independent of the specific experimental implementation. Besides its universality it has the advantage of being fast and robust against noise commonly encountered in experiments. We have published a Python package that implements pulse retrieval algorithms including COPRA for the measurement schemes mentioned above. It is available under an open-source license on github.

Published in Optica 6, 495-505 (2019) >>> more

 dual_tip_web250x250   Automated dual-tip SNOM for localized near-field investigations

Near-field optical microscopes with two independent tips for simultaneous excitation and detection can be essential tools for studying localized optical phenomena on the subwavelength scale, such as in plasmonic nanostructures, dielectric metasurfaces, or disordered photonic systems. In this work, we report on the implementation of a fully automated and robust dual-tip scanning near-field optical microscope (SNOM), in which the excitation tip is stationary, while the detection tip automatically scans the surrounding area. We have used this technique to map the near-field dipolar emission pattern of the surface plasmon-polariton on a gold film.

Published in Review of Scientific Instruments 90, 053705 (2019) >>> more

 Dielectric Resonators   Dielectric resonators for imaging beyond conventional filters

Digital color imaging relies on spectral filters on top of a pixelated sensor, such as a CMOS image sensor. An important parameter of imaging devices is their resolution, which depends on the size of the pixels. For many applications, a high resolution is desirable, consequently requiring small spectral filters. Dielectric nanostructures, due to their resonant behavior and its tunability, offer the possibility to be assembled into flexible and miniature spectral filters, which could potentially replace conventional pigmented and dye-based color filters. We demonstrate submicrometer RGB filter arrays for a pixel size down to 0.5 μm.

Published in ACS Photonics 6, 1018-1025 (2019) >>> more

 chi_rot_wire_greenish_web250x250   Let's twist again: controlling the chirality of light

Both circularly and elliptically polarized light have a rotation direction in which its field vector is turning, giving light a handedness comparable to that of the DNA helix. This so called chirality of light can be tuned to different applications from sensing of chiral molecules to encoding in optical computers.
We experimentally demonstrated that, interestingly, a combination of non-chiral metasurfaces in a stack can yield chiral behavior while being easy to fabricate and understand. Moreover, our findings prove that chirality depends in part on Fabry-Perot-resonances from far-field interactions between the layers. This knowledge is very useful in future endeavors when chirality or other polarization properties have to be controlled from the design up.

Published in Opt. Express 27, 1236-1248 (2019) >>> more

  Factorizable counterpropagating photon pairs from periodic waveguides

Spectrally unentangled photon pairs, aka factorizable pairs, are highly desired for implementation of heralded single-photon sources. In our work, we propose the use of photonic crystal waveguides (PCWs) for the realization of such sources. Using a design in lithium niobate, we numerically demonstrate that PCWs are capable of phase-matching a counterpropagating spontaneous parametric down-conversion process, which combined with PCWs' dispersion engineering property allows for reaching factorizability with a high degree of tunability. Such a source is uniquely suited for heralding applications, as the pair is separated directly at the source.

Published in Optics Letters 44, 69 (2019) >>> more

 PhaseTransition_web250x250   Disorder-induced phase transitions in dielectric metasurfaces

Metasurfaces made from dielectric nanoparticles typically possess a rather far-reaching inter-particle interaction, which is why their collective optical response is also quite sensitive to the particular arrangement of neighboring particles. We theoretically and experimentally studied the influence of disorder in the particle positions on the phase angle spectrum of light transmitted through such metasurfaces and found, for example, that the phase angle remains almost completely unaltered with increasing positional disorder, but only up to a certain critical threshold, at which the phase angle abruptly changes from normal to anomalous dispersion.

Published in Phys. Rev. Lett. 122, 015702 (2019) >>> more

 Magnetic_dipole_emission_web250x250   Manipulation of magnetic dipole emission

Mie-resonant dielectric metasurfaces are a viable platform for enhancing both electric and magnetic dipole transitions of fluorescent emitters. We study the enhancement of emission of Eu ions, featuring both electric and magnetic-dominated dipole transitions, by dielectric metasurfaces. We observe an enhancement of the Eu3+ emission associated with the electric (at 610 nm) and magnetic (at 590 nm) dipole transitions. The enhancement factor depends on the spectral proximity of the atomic transitions to the Mie resonances and the nature of the transition. Our results open new opportunities for bright nanoscale light sources based on magnetic transitions.

Published in Nano Lett. 19, 1015-1022 (2019) >>> more

 Light_emmision_web250x250   Directional and spectral shaping of light emission

We study how to manipulate the light emission with Mie-resonant silicon nanoantenna arrays on a glass substrate. When the spectral position of the silicon nanoantennas' magnetic dipole resonance overlaps with the intrinsic emission from the glass, the emission is selectively enhanced for certain spectral and spatial frequencies detemined by the design of the nanoantenna array. Furthermore, we observe that the nanoantenna array induces a reshaping of the emission pattern in the air half-space into a narrow lobe directed out of the substrate plane. This control of emission spectra and directionality is required for advances in multifunctional and "smart lighting".

Published in ACS Photonics 5, 1359-1364 (2018) >>> more

 GaAs_sketch_web250x250   Second-harmonic diffraction from a GaAs metasurface

Nonlinear photonic metasurfaces are an increasingly investigated field. In our group, we study the control of second-harmonic generation as the lowest order nonlinear frequency conversion in structured nonlinear materials. GaAs is a prominent candidate because it shows a strong second-order nonlinearity. But due to its crystal symmetry, no second-harmonic generation leaves the (100)-cut crystal surface when excited in normal incidence. The structured metasurface, however, is able to shape the second-harmonic radiation into the first diffraction orders of the periodic array.

Published in ACS Photonics 5, 1786-1793 (2018) >>> more

 Dirorder-enabled chirality_web250x250   Disorder Induced Pure Circular Dichrosim

Many molecules in nature show different absorption properties for left and right handed circular polarized light. This so called circular dichroism is used to study the higher order structures of e.g. macromolecules. By introduction of rotational disorder to chiral wire pairs we were experimentally able to reproduce this effect and fabricate a metasurface with a pure circular dichroism several orders of magnitude stronger as in every natural molecule.

Published in ACS Photonics 5, 1773-1778 (2018) >>> more

 SPDC_spectroscopy_250x250   Waveguides for integrated SPDC spectroscopy

Spontaneous parametric down-conversion (SPDC) spectroscopy using photon pairs is a promising avenue towards affordable mid-infrared (MIR) spectroscopy. Here, we experimentally investigate the feasibility of using periodically poled waveguides in lithium niobate for SPDC spectroscopy applications. We find the waveguides suitable to generate wavelength non-degenerate photon pairs with one photon in the MIR spectral range with high fluence. We use this to determine the cutoff wavelengths of the waveguide mode in the MIR by performing only measurements in the near-infrared spectral range.

Published in APL Photonics 3, 021301 (2018) >>> more
Featured in Nature Photonics 12, 189 (2018) >>> more

 Atom_mediated_web250x250   Atom-mediated pair generation using photonic bandgap modes

We propose the concept of atom-mediated pair generation, in which pair generation can take place only in the presence of a single 2-level emitter, relying on the bandgap evanescent modes of a nonlinear periodic waveguide. We show analytically, that the zero density-of-states of the bandgap modes prohibits pair generation in the nonlinear system, while adding an emitter to its vicinity mediates the process, in which a single photon is generated and its pair manifests itself as the emitter's excitation. This configuration can be the basis for new schemes in quantum spectroscopy and for the realization of hybrid sources of quantum light.

Published in Optics Letters 42, 4724 (2017) >>> more

  Routing of telecom signals with waveguide-integrated nanoantennas

Optical nanoantennas are uniquely suited to control the propagation of light. Here we show together with collaborators from the Australian National University, that a single gold nanoantenna integrated on a silicon waveguide can be used to sort optical signals into different output modes and propagation directions based on the input polarization. We demonstrate, that this is even possible for modulated signals used in telecom applications without measurable impairment of the signal quality.

Published in Science Advances 3, e1700007 (2017) >>> more

 SPDC_backward_Bell   Path-entangled photon pairs from periodic waveguides

Photon pairs entangled in the path degree of freedom are uniquely suited for qubit encoding in integrated optical platforms. In our work, using the unique properties of Bloch modes, we propose a general scheme for direct and fully integrated generation of counterpropagating photon pairs in a single periodic waveguide, entangled in the propagation direction. We numerically demonstrate the Bell state generation with a design in a lithium niobate photonic crystal slab waveguide.

Published in Physical Review Letters 118, 183603 (2017) >>> more

 SPDC_coupler_web250x250    Tunable source of entangled photon states

For the practical implementation of on-chip integrated quantum light sources for quantum information applications, it is crucial to develop sources delivering entangled quantum photon states with on-demand tunability. Together witch co-workers from Australia we developed and experimentally demonstrated an integrated all-optically tunable source based on spontaneous parametric down-conversion in two coupled waveguides. We experimentally demonstrated the control of entanglement and spatial shape of the generated two-photon states.

Published in Laser & Photonics Review 10, 131 (2016) >>> more


Nonlinear coupling in optical waveguide arrays

We experimentally demonstrate nonlinear coupling between optical waveguides, where the rate of energy exchange between neighboring waveguides depends nonlinearly on the optical power. Using discrete waveguide arrays, i.e. regular chains of weakly coupled waveguides, we show the profound impact of such nonlinear coupling on second-harmonic generation. In particular, we find a peculiar dependence of the generated second-harmonic power on the symmetry and the propagation direction of the exciting beam.

Published in Phys. Rev. A 92, 043832 (2015) >>> more

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Superfocusing of radially polarized conical surface plasmon polaritons

By employing superfocusing of surface plasmon polaritons (SPP) a sharp metallic tip enables efficient detection of optical nearfields and nano-objects with ultrahigh spatial resolution. We demonstrated experimentally the excitation and superfocusing of radially polarized SPP in a fully metal-coated conically tapered M-profile fiber (concentric double-core fiber) tip. In this structure, the radially polarized waveguide mode, propagating inside the fiber, resonantly excites the radially polarized SPP on the metal surface that consequently superfocuses at the apex generating strong field oscillating longitudinally along the tip axis.

Published in ACS Photon. 2, 1468 (2015) >>> more


Enhanced second-harmonic generation from gold nanorings filled with lithium niobate

Lightsources localized in nanometer volumes are important for several applications ranging from fluorescence spectroscopy to quantum communications. A flexible way to generate light at almost any desired frequency is by parametric nonlinear effects as e.g. second-harmonic generation. We experimentally demonstrate enhanced second-harmonic generation in gold nanorings filled with lithium niobate. We make use of the plasmonic resonances of the gold, which concentrate the fields inside the nanorings where it efficiently interacts with lithium niobate and achieve enhancement factors around 20 with respect to unpatterned lithium niobate.

Published in Nano Lett. 15, 1025 (2015) >>> more

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All-dielectric Huygens' nanosurfaces

Resonant optical nanosurfaces tailored to impose a spatially variant phase shift onto an incident wavefront have developed as a breakthrough concept for advanced wave-front engineering. However, reflection and/or absorption losses as well as low polarization-conversion efficiencies pose a fundamental obstacle for achieving the high transmission efficiencies that are required for practical applications. We showed that all-dielectric nanosurfaces with high efficiency and full phase coverage in transmission can be realized at NIR frequencies using arrays of silicon nanodisks with crossed electric and magnetic dipole resonances. If these resonances are brought into spectral overlap, the nanodisks emulate the behavior of the forward-propagating elementary wavelets known from Huygens' principle. All-dielectric Huygens' nanosurfaces offer unique opportunities for flat optical devices, including beam-steering, beam-shaping, and focusing, as well as holography and dispersion control.

Published in Adv. Mater. 24, 813 (2015) >>> more


Realization of lithium niobate nanowaveguides

Nanoscale nonlinear waveguides are fundamental building blocks for high density integrated optics. For applications making use of frequency conversion, lithium niobate is an ideal substrate material. We fabricated mm-long waveguides with cross-sections below 1 µm² in lithium niobate using the potassium hydroxide-assisted ion-beam enhanced etching technique. Waveguide widths as low as 200 nm were realized, demonstrating precise control over the geometrical parameters of the waveguide, which is crucial to achieve modal phase-matching. We experimentally demonstrated efficient second-harmonic generation for a phase-matched waveguide.

Published in Opt. Lett. 40, 2715 (2015) >>> more


Nonclassical biphoton generation via quantum walks in a nonlinear waveguide array

Nonclassical quantum states of several photons with strong entanglement are a key to the optical realization of quantum phenomena which can be used for unbreakable cryptography and powerful computational algorithms. The practical implementation of such complex applications requires stable quantum interference as well as the minimization of coupling losses. We experimentally demonstrated a nonlinear optical chip based on an array of coupled waveguides as a flexible integrated platform that achieves these requirements. Using nonlinearly driven quantum walks our approach allows for the generation of entangled photon pairs with reconfigurable nonclassical correlations.

Published in Phys. Rev. X 4, 031007 (2014) >>> more


Plasmonic near-fields explored by two SNOM tips

A long-standing issue of nanooptics lies in the fact that the electromagnetic near-fields around subwavelength structures are fully vectorial in nature. In consequence, the classical notion of polarization of electromagnetic radiation is generally not applicable in the near-field. In our most recent work we have studied the polarization characteristics of light emission and collection in the near field by a unique setup consisting of two scanning near-field optical microscopes (SNOMs). For the first time, the mapping of different polarization components of a plasmonic dipole pattern emitted by an aperture probe was demonstrated.

Published in Nano Lett. 14, 5010 (2014) >>> more


Order-disorder-transitions in photonic metasurfaces

Understanding the impact of order and disorder is of fundamental importance to perceive and to appreciate the functionality of modern photonic metasurfaces. Together with coworkers from Europe and Australia, we have investigated photonic metasurfaces with different lattice arrangements and uncovered the influence of lattice disorder on their electromagnetic properties at the full angular spectrum.

Published in Sci. Rep. 4, 4484 (2014) >>> more


Vortex Light Bullets - Self-trapped Light with a Twist

In a recent series of breakthrough experiments researchers from Jena have observed the most complex spatiotemporally self-confined solitary wave. In their experiments light, which propagates through a fiber array, was found to withstand linear broadening due to dispersion and diffraction and at the same time rotate around a center of symmetry in a cohesive manner. These nonlinear wavepackets are hence termed Vortex Light Bullets. They exhibit a striking level of internal dynamics and interact in a complex manner with asymmetries present in their surroundings and pave the way for a better understanding of spatiotemporal dynamics of nonlinear waves.

Published in Phys. Rev. X 3, 041031 (2013) >>> more
Featured as a highlight in Nature Photon. 8, 169 (2014) >>> more


Cavity Optical Pulse Extraction - Building Lasers with Hawking Radiation

The interaction of ultrashort, high-power pulses with slow light stored in resonant structures leads to the study of non-stationary optics. Researchers from Jena, in collaboration with researchers from Australia, have shown that non-stationary optics has the potential to lead to the development of sources of pulsed radiation without the need for pulsed laser sources. Structurally non-stationary optics is related to wave mechanics in warped space-time and therefore opens a path towards laboratory cosmology and gravito-quantum-electrodynamics.

Published in Sci. Rep. 3, 2607 (2013) >>> more


Metamaterials for spatial and spectral light shaping

Plasmonic metamaterials open up new possibilities for the design of diffractive elements and holograms. These materials strongly interact with light and already an ultra-thin layer allows for comprehensive modulation of light amplitude and phase. Variations in the geometry of the metamaterial are used to adjust the local transmission and dispersion properties in order to shape light spatially and spectrally. Based on the fishnet metamaterial we constructed a computer-generated hologram which projects different images at two distinct wavelengths in the near infrared.

Published in Adv. Mater. 24, 6300 (2012) >>> more
Featured on the front cover of Adv. Mater. >>> more
Featured as a highlight of Advances in Engineering >>> more
Featured as a highlight of Renewable Energy Global Innovations >>> more


Nanowires shine bright

Optically excited nanostructures usually elude the observability of their characteristic light distribution patterns unless involved and time-consuming near-field microscopy is applied. In our recent work we present a novel method which permits sensing the infrared near-field in GaAs nanowires in the far-field. Collecting their second harmonic generated signal, guided wave patterns along the nanowires are precisely mapped. Our approach opens promising paths for the characterization and advancement of nanoscale optical devices, like e.g. nanolasers.

Published in Nano Lett. 12, 5412 (2012) >>> more


Giant optical activity of loop-wire metamaterials

A metamaterial with a three dimensional chiral unit cell shows an optical activity in the VIS/NIR spectral range which exceeds by far any natural material. Based on a combined spectroscopic and interferometric characterization, the entire complex transmission response in terms of a Jones matrix is disclosed. The polarization output state of light after propagation through the nanostructures can be decoded for any excitation configuration. The rotation of the polarization azimuth of linearly polarized light exceeds 50° at wavelengths around 1.08 μm.

Published in Nano Lett. 11, 4400 (2011) >>> more
Read German reprint article in: Photonik 2, 40 (2012) >>> more
or Photonik International 1, 21 (2012) >>> more


Observation of non-diffracting Airy plasmons

Even though diffraction is a ubiquitous process in nature, non-diffracting 2D beams, i.e. Airy plasmons, have been generated at nanostructured gold surfaces. These self accelerating Airy plasmons are excited by an engineered nanoscale phase grating. The Airy plasmon demonstrates significant beam bending and was observed by scanning nearfield optical microscopy. Their self-healing properties, suggest novel applications in plasmonic circuitry and surface optical manipulation.

Published in Phys. Rev. Lett. 107, 116802 (2011) >>> more
Read viewpoint by Salandrino and Christodoulides >>> more


At last! The observation of the light bullet

The long quest for the light bullet finds its end by their unambiguous experimental observation in microstructured fibers. Light Bullets are wave packets that are localized in all three dimensions (3D) as they propagate in space or evolve in time. Stringent evidence of the excitation of light bullets is based on time-gated images and spectra which perfectly match numerical simulations. A novel adiabatic evolution mechanism is revealed which eventually is also responsible for their decay.

Published in Phys. Rev. Lett. 105, 263901 (2010) >>> more
Read viewpoint by Frank Wise >>> more