Niels Gregersen

Modulation response of nanoLEDs and nanolasers exploiting Purcell enhanced spontaneous emission

The modulation bandwidth of quantum well nanoLED and nanolaser devices is calculated from the laser rate equations using a detailed model for the Purcell enhanced spontaneous emission. It is found that the Purcell enhancement saturates when the cavity quality-factor is increased, which limits the maximum achievable spontaneous recombination rate. The modulation bandwidth is thereby limited to a few tens of GHz for realistic devices.

Designs for high-efficiency electrically pumped photonic nanowire single-photon sources

We propose and analyze three electrically-pumped nanowire single-photon source structures, which achieve output efficiencies of more than 80%. These structures are based on a quantum dot embedded in a photonic nanowire with carefully tailored ends and optimized contact electrodes. Contrary to conventional cavity-based sources, this non-resonant approach provides broadband spontaneous emission control and features an improved fabrication tolerance towards surface roughness and imperfections. Using an element-splitting approach, we analyze the various building blocks of the designs with respect to realistic variations of the experimental fabrication parameters.

Controlling the emission profile of a nanowire with a conical taper

The influence of a tapering on nanowire light-emission profiles is studied. We show that, for nanowires with divergent output beams, the introduction of a conical tapering with a small opening angle reduces the beam divergence and increases transmission. This results in a dramatic increase in the collection efficiency of the detection optics. For a realistic tapering and a modest NA, the collection efficiency is enhanced by more than a factor of 2. This improvement is ensured by the adiabatic expansion of the guided mode in the tapering.

Highly indistinguishable on-demand resonance fluorescence photons from a deterministic quantum dot micropillar device with 74% extraction efficiency

The implementation and engineering of bright and coherent solid state quantum light sources is key for the realization of both on chip and remote quantum networks. Despite tremendous efforts for more than 15 years, the combination of these two key prerequisites in a single, potentially scalable device is a major challenge. Here, we report on the observation of bright single photon emission generated via pulsed, resonance fluorescence conditions from a single quantum dot (QD) deterministically centered in a micropillar cavity device via cryogenic optical lithography. The brightness of the QD fluorescence is greatly enhanced on resonance with the fundamental mode of the pillar, leading to an overall device efficiency of η = (74 ± 4) % for a single photon emission as pure as g(2)(0) = 0.0092 ± 0.0004. The combination of large Purcell enhancement and resonant pumping conditions allows us to observe a two-photon wave packet overlap up to ν = (88 ± 3) %.

Bright single photon source based on self-aligned quantum dot-cavity systems

We report on a quasi-planar quantum-dot-based single-photon source that shows an unprecedented high extraction efficiency of 42% without complex photonic resonator geometries or post-growth nanofabrication. This very high efficiency originates from the coupling of the photons emitted by a quantum dot to a Gaussian shaped nanohill defect that naturally arises during epitaxial growth in a self-aligned manner. We investigate the morphology of these defects and characterize the photonic operation mechanism. Our results show that these naturally arising coupled quantum dot-defects provide a new avenue for efficient (up to 42% demonstrated) and pure (g(2)(0) value of 0.023) single-photon emission.

Harnessing light with photonic nanowires: fundamentals and applications to quantum optics.

The efficient feeding of spontaneous emission (SE) into a controlled optical mode lies at the heart of a new generation of advanced optoelectronic devices, such as low-threshold microlasers and bright sources of quantum light. In the solid state, single-mode emission was first demonstrated by using the Purcell effect that arises in a resonant microcavity. Recently, the need to relax the constraints inherent to such a narrow-band approach has motivated large effort to develop structures ensuring broadband and efficient SE control. This minireview deals with fiber-like photonic nanowires, a class of high-index waveguides that features key assets in this context. Combining theoretical predictions and experimental results, the paper details the SE dynamics in such tiny wires. In addition, it shows how the far-field emission of a single wire can be tailored through proper engineering of the two wire ends. As an application in the field of quantum optics, we review the realization of an ultrabright single-photon source. This first device was based on a self-assembled quantum dot embedded in a wire antenna realized with a top-down fabrication process. Considering recent advances in the direct growth of tapered photonic wires, we also propose a bottom-up fabrication route to realize a complete device. In particular, this proposal ensures the optimal 3D positioning of a single emitter inside the antenna. Finally, future research and application prospects are also reviewed.

Oscillatory variations in the Q factors of high quality micropillar cavities

We report on the observation of oscillatory variations in the quality (Q) factor of quantum dot-micropillar cavities based on planar Bragg reflectors. The oscillatory behavior in the Q versus diameter dependence appears in the diameter range between 1.0 and 4.0 μm, has a characteristic period of a few hundred nanometers and increases in amplitude with increasing reflectivity of the planar microcavity structures. The experimental results are well reproduced by numerical calculations which support the interpretation that the Q oscillations are caused by coupling of propagating Bloch modes of different orders at the mirror interfaces.

Modulation response of quantum dot nanolight-emitting-diodes exploiting purcell-enhanced spontaneous emission

The modulation bandwidth for a quantum dot light-emitting device is calculated using a detailed model for the spontaneous emission including the optical and electronic density-of-states. We show that the Purcell enhancement of the spontaneous emission rate depends critically on the degree of inhomogeneous broadening relative to the cavity linewidth and can improve the modulation speed only within certain parameter regimes.

Calculation, normalization, and perturbation of quasinormal modes in coupled cavity-waveguide systems

We show how one can use a nonlocal boundary condition, which is compatible with standard frequency domain methods, for numerical calculation of quasinormal modes in optical cavities coupled to waveguides. In addition, we extend the definition of the quasinormal mode norm by use of the theory of divergent series to provide a framework for modeling of optical phenomena in such coupled cavity-waveguide systems. As example applications, we calculate the Purcell factor and study perturbative changes in the complex resonance frequency of a photonic crystal cavity coupled to a defect waveguide.

A fiber-coupled quantum-dot on a photonic tip

We present the experimental realization of a quantum fiber-pigtail. The device consists of a semiconductor quantum-dot embedded into a conical photonic wire that is directly connected to the core of a fiber-pigtail. We demonstrate a photon collection efficiency at the output of the fiber of 5.8% and suggest realistic improvements for the implementation of a useful device in the context of quantum information. We also discuss potential applications in scanning probe microscopy. The approach is generic and transferable to other materials including diamond and silicon.Davide Cadeddu,1 Jean Teissier,1 Floris Braakman,1 Niels Gregersen,2 Petr Stepanov,3, 4 Jean-Michel Gérard,3, 4 Julien Claudon,3, 4 Richard J. Warburton,1 Martino Poggio,1 and Mathieu Munsch1 Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, Building 343, DK-2800 Kongens Lyngby, Denmark Universit Grenoble Alpes, F-38100 Grenoble, France CEA, INAC-SP2M, 17 rue des Martyrs, F-38054 Grenoble, France (Dated: June 23, 2015)