Scientific Orientation
The central goal of my research is to experimentally research on the ultimate limits of single emitter and single photon spectroscopy. This includes their detection, their microscopic localization, and the sensing of single emitters and their environment. In some experiments I coupled single molecules to external structures, such as silver nanowires. The combination of single solid-state emitters and the narrow-band features of an atomic vapor opens up a novel field of quantum-hybrid systems. My research on single molecules has been very influential to other areas, such as the research on quantum dots and single atoms.
Chemical Sensors
In my diploma thesis I developed a new microscope for impedance microscopy. The technique is similar to the light addressable potentiometric sensor (LAPS). This new method is called scanning photo-induced impedance microscopy (SPIM). Using this technique, the impedance of thin films can be measured with lateral resolution.
The optical setup is based on a CD-ROM player optic. A focus of 2.6 µm was achieved using this cheap and easy to handle device. We performed extensive studies to the optical resolution and carrier diffusion in the semiconductor bulk. The lateral resolution of LAPS measurements can be improved by using GaAs as the semiconductor material instead of Si. The diffusion length of the minority charge carriers was determined to be smaller than 3.1 µm.
Single Molecules & other Single Emitters
The invention of the laser in the 1960s was a key evolution in the path towards optical single molecule detection. Already in 1976 Hirschfeld performed an experiment which was an important step towards this goal. Using a laser to excite a fluorescently doped sample and detecting the spectrally filtered light on a photomultiplier, he was able to see the fluorescent fingerprint of a cluster of molecules. In the 1980s Moerner and Kador succeeded in the first optical detection of single pentacene molecules, but this new technique only became important for biology and sensing in the 1990s when the experiments were extended to work at room temperature. These experiments rely on efficient discrimination of the excitation laser light from the molecule's red shifted fluorescence. Since then, single molecule spectroscopy has become a valuable tool to overcome ensemble averaging over many emitters and to perform microscopy at a sub-diffraction limited scale. In terms of sensitivity the detection of a single molecule of a certain compound represents the ultimate limit.
I researched on several species of single molecules at room and cryogenic temperatures.
Strong Focusing, Microscopy & Imaging
In the past years I am very interested in strong focusing and microscopy. The usual technique to monitor single molecules is confocal microscopy with beam scanning. I did several analytic calculations on the electric field in the proximity to an optical focus. This is meant to optimize the interaction of a single emitter and an optical field.
Own developments in the field of microscopy and localization include the localization of a single molecule, based on a location dependent Stark shift, the first experimental implementation of a semi-conductor based impedance microscopy (SPIM) and the application of coherent properties (Rabi oscillations) for locating a single emitter.
Also research on light in confined geometries (e.g. photonic crystal fibers) was performed.
Near-field Optics
As part of my PhD work I set up and operated a cryogenic scanning near-field optical microscope (SNOM). In a SNOM a metal coated glass fiber probe guides laser light to a sub-wavelength sized aperture which is positioned a few nanometers above a sample. The light exiting this aperture has an evanescent component, which is decreasing exponentially with distance, but (is scattered off the sample and) enables optical microscopy with a resolution on the order of the aperture diameter. Each fiber probe was manufactured by first coating a heat-pulled glass fiber with aluminum and then cutting the end off using a focused ion beam.
Absorption & Extinction
I was able to show the first experiments where a single quantum system strongly affects the amplitude and phase of a laser field emerging from a sub-wavelength aperture. Since then the topic of light absorption and extinction kept me busy in several fields.
Quantum Optics
Single dye molecules at cryogenic temperatures exhibit many spectroscopic phenomena known from the study of free atoms and are thus promising candidates for experiments in fundamental quantum optics. However, the past techniques for their detection have either sacrificed information on the coherence of the excited state or have been inefficient. I was able to show that these problems can be addressed by focusing the excitation light near to the extinction cross-section of a molecule. The detection scheme enabled us to explore resonance fluorescence over nine orders of magnitude of excitation intensity and to separate its coherent and incoherent parts. In the strong excitation regime, we were able to demonstrate the first direct observation of the Mollow fluorescence triplet from a single solid-state emitter. Under weak excitation, we report the detection of a single molecule with an incident power as faint as 600aW, paving the way for studying nonlinear effects with only a few photons.
In short-pulse experiments I was able to excite single molecules via narrow zero-phonon transitions. By monitoring the Stokes-shifted fluorescence, we studied the excited state population as a function of the delay time, laser intensity, and frequency detuning. A Pi-pulse excitation was demonstrated with merely 500 photons, and 5 Rabi cycles were achieved at higher excitation powers. Our findings are in good agreement with theoretical calculations and provide a first step toward coherent manipulation of the electronic states of single molecules with few photons.
I am specialized in time-resolved single photon counting.
Atomic Spectroscopy
In this work an experiment was set up to study nonlinear optical processes in a Rubidium vapor cell with the aim of developing a new technique which allows to create correlated narrow-band photon pairs.
The options of implementing a narrow-band Faraday anomalous dispersion optical filter (FADOF) with Rb for efficient filtering of single photons were evaluated.
Via an S-P-D transition directed blue emission was observed.
Please also note the sodium page for our efforts on atomic sodium.
Quantum Key Distribution & Cryptography
In Singapore I was working with the free-space quantum key distribution (QKD) implementation. Here a free space transmission path is used. We were able to extend the experiments to daylight operation by implementing spectral, spatial and temporal filtering techniques. It is possible to establish a secure key continuously over several days under varying light and weather conditions.
Digging more into the techniques of single photon detection I researched on a van-Enk phreaking attack on the polarization detector, but an optical attack was shown to be more successful. Here a physical imperfection (detector vulnerability) was used. We have successfully built an intercept-resend attack and demonstrated eavesdropping under realistic conditions on an installed quantum key distribution line.
Quantum cryptography, as being based on the laws of physics, was claimed to be much more secure than all classical cryptography schemes. (Un)fortunately physical hardware is not beyond of an evil control: We present a successful attack of an existing quantum key distribution system exploiting a photon detector vulnerability which is probably present in all existing devices. Without Alice and Bob losing their faith in their secure communication, we recorded 100% of the supposedly secret key. To test the randomness of bitstreams in quantum key distribution, I implemented the NIST random number testsuite in Mathematica and Python. For further details on random number testing, see here.
