Jaesuk Hwang

A single-molecule optical transistor

The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms. Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nanometre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams. However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities or waveguides. Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second ‘gating’ beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.

Efficient coupling of photons to a single molecule and the observation of its resonance fluorescence

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 existing techniques for their detection have either sacrificed information on the coherence of the excited state or have been inefficient. Here, we show that these problems can be addressed by focusing the excitation light near to the extinction cross-section of a molecule. Our detection scheme enables 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 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 600 aW, paving the way for studying nonlinear effects with only a few photons.

Exploring the limits of single emitter detection in fluorescence and extinction

We show experimentally that the signal-to-noise ratio of extinction detection can be advantageous to fluorescence measurements of a single molecule. We discuss the prospects of detecting weak emitters such as rare earth ions.

Phase control of a laser beam by a single molecule

Measurements involving the phase of a light beam are employed in a variety of fields such as interferometry, phase contrast imaging, holography, coherent signal processing, quantum nondemolition measurements and quantum gates. It is, thus, of fundamental concern to examine the influence of a single emitter on the phase of a light beam. Usually this is performed using a high-finesse cavity coupled to the single emitter in order to greatly enhance the inherently small effect. However, recently we showed that a single naked molecule can attenuate and amplify a focused laser beam substantially [1–3]. Considering that coherent scattering of light by an oscillator is accompanied by a phase shift, these observations imply that a single emitter should also be able to shift the phase of a laser beam. Indeed, a phase shift measurement of 1° on a single atom in a trap has been recently achieved [4]. Here we report on a phase shift measurement of 3° imprinted on a laser beam by a single molecule in the solid state. We also show the realization of a single-molecule electro-optic modulator and discuss applications of our work in microscopy and signal processing [5]. Utilizing a direct focusing geometry (i.e. without a cavity) can be attractive both in terms of its fundamental simplicity as well as in terms of practical integration and application.

Efficient coupling of light to a single molecule, observation of its fluorescence Mollow triplet, and more

Efficient coupling of laser to single quantum emitters is achieved via near- and far-field techniques. Direct detection of 11.5% extinction and the first direct measurement of the Mollow triplet in solid state system are demonstrated.

Strong light extinction by a single molecule

We present cryogenic experiments where the direct signature of a single molecule on an incident laser beam is demonstrated. Strong extinction larger than 10% is achieved in near and farfield geometries.

Quantum Optical Experiments with Single Molecules

We introduce a cryogenic experiment where the direct signature of a single molecule on an incident laser beam becomes evident We use both near-field and far-field optics to achieve strong extinction effects larger than 10%.

Strong Light Extinction by a Single Molecule

In this paper direct measurements of light extinction by single molecules are reported. We use both near-field aperture tips and confocal microscopy at cryogenic temperature to tightly confine the excitation light to an area comparable to the absorption cross section of a single molecule, which results in directly observable 10% extinction of the laser beam. It is concluded that the access to both the incoherent and coherent emission of single molecules opens up a broad range of studies on the light-matter interaction of single solid state emitters.