06-06-2017 \\ Hotspot interaction length

Superconducting single photon detectors detect single photons efficiently, but are much less sensitive to multiple photons. This is particularly true for longer nanowires when biased at low currents. In this regime multiple photons need to be absorbed at the same position along the wire to create a detection event. Our paper, published today in Applied Physics Letters, uses this effect and quanifies the reducation in multiphoton efficiency in short NbN nanowires. These data can be used to quantify the finite hotspot interaction length resulting in a value of 23 +/- 2 nm.

07-12-2016 \\ Surface plasmons

Surface plasmon lasers require gain and feedback. In our metal hole arrays the gain is provided by an optically pumped InGaAs semiconductor while feedback is generated by in-plane scattering of plasmons on a periodic array of holes in the gold layer. We describe the surface plasmon dispersion relation in terms of scattering coefficients for a complete collection of hexagonal, honeycomb and Kagome lattices. Unlike square lattices, the hexagonal family of lattices provides opportuities of complete two-dimensional feedback which may lead to new lasing behavior in metal hole arrays.

18-07-2016 \\ WSi single photon detectors

Together with our collaborators at TU Eindhoven and NIST Boulder we investigate superconducting single phot detectors made out of WSi using detector tomography on nanofabricated constrictions. WSi meandering wires have a higher reported efficiency as compared to similar device made out of NbN and a question arises if this is due to the morphology of the material and other technical issues or becuase the physics of photon detection is fundamentally different. Given our current understanding of photon detection in NbN we would expect a non-linear depends of the threshold current on photon energy because of the significantly smaller superconducting gap of WSi. Our experiments show a convincing linear dependence with an onset of non-linear behavior for energies below 0.8 eV.

16-05-2016 \\ Understanding plasmon lasers

Lasers emit coherent radiation and can exists if gain and feedback are present in a medium. Plasmons, travelling on a metal hole array on top of a gain medium can thus be used to create plasmonic lasing. The properties of these lasers were not well understood and the amount of experimental data available to improve models of plasmonic lasing was limited. Our latest results published in ACS photonics show that it is possible to retrieve the complete amplitude and phase profile of the lasing mode shedding new light on the operation of nanoscale lasers.

25-04-2016 \\ Yb doped ring resonators

Rare earth ions implanted in ultra-high Q ring resonators show strongly enhanced light-matter intetraction when cooled down to millikelvin temperatures. We achieve Purcell factors up to 10 for non-optimized structures with the prospect to increase this further. The increased interaction between light and Ytterbium ions occurs when the influence of the fluctuating environment is reduced, i.e. when the linewidth of the atom becomes smaller than the linewidth of the cavity. The enhanced interaction is interesting for quantum memmory applications and few photon interactions with small ensembles of Yb ions. Online publication in Nature Photonics

16-02-2016 \\ 4-photon OAM entanglement

We report in Phys. Rev. Lett. the first experimental demonstration of higher-dimensional entanglement between more than two particles. The experiment led by Dr. Wolfgang Loeffler uses the discrete orbital angular momentum of light as a degree of freedom to create spatially entangled photons. The experiments on a carefully tuned and highly efficient source of photon pairs allows to create a reasonable fraction of 4-photon states in the down-conversion. These 4-photon states are selected in the experiment to demonstrate genuine high-dimensional 4-photon entanglement.

24-11-2015 \\ Soft-nanoimprint lithography

Our results on soft-nanoimprint lithography are published in MRS Communications. Soft-nanoimprint lithography is a technique where a polymer stamp is used to create nanoscale patterns. First author Marc Verschuuren has succesfully applied this technique to create nanoholes arrays. These plasmonic structures are known for extra-ordinary light transmission. In this article we show how they can be printed and packaged into a solid-matrix surrounding the gold metal with silica on both sides.

27-10-2015 \\ Photon detection at subwavelength scales

Congratulations Qiang Wang for defending his PhD thesis titled 'Photon detection at subwavelength scales'. In this thesis Qiang Wang describes the method of detector tomogrpahy to characterize and optimize NbN based superconducting single photon detectors. A second part of the thesis describes simulation on novel scanning probe techniques that either probe a nanodetector, or that use the nanodetector as a probe.

01-10-2015 \\ Accuracy of detector tomography

Quantum detector tomogrpahy is an important tool to obtain the probability of multi-photon detection events. The purpose of tomography is to obtian these probabilities in an agnostic way so that understanding of the physics of photon detection is shifted towards the interpretation of these detection probabilities. In a real experiment that is affected by noise it is not clear how accurately these probabilities can be determined. In a recent publication we address how this accuracy can be determined and discuss the various factors that affect tomography on NbN detectors.

14-07-2015 \\ An investigation of NbN detectors with scanning probe techinques

A publication in Optics Express shows how the recent concept of a local detection efficiency can be used to simulate the response of a superconducting single photon detector in a scattering near-field optical microscope. The aim of this simulation is to show that light scattered by a small metal tip can be used to probe these detectors with a resolution of ~20 nm. An experimental realization of this proposal comprises a direct obeservation of the local detection efficiency.

09-07-2015 \\ Nanoscale response of NbN single photon detectors

To unravel the detection mechanism of superconducting single photon detectors we used the fact that light is non-uniformly absorbed for different polarizations of the incoming field. Microscopic models of the detection mechanism predict that the response of these detectors depends on the position where the photon is absorbed. With the aid of tomographic methods we are the first to measure this nanoscale response of the detector and are able to quantitatively explain the response of technoligcally relevant devices. This research, recently published in Nano Letters, reveals the physical mechanism of photon-assited vortex entry as being responsible for the less than one internal detection efficiency of state-of-the-art detectors.

06-03-2015 \\ NbN single photon detectors in magnetic field

When a superconducting single photon detector is placed in a magnetic field the response of these devices is altered. We have performed an experiment on the simplest possible detector that consists of a single constriction. This rules out geometric effects that may affect the response and reveals that an increase in magentic field does not improve detector performance. More importantly these results report an exchange between magentic field and current and show that the mechanism of generating dark counts is fundamentally different from detecting single photons. These results are published in Applied Physics Letters.


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