Marc Achermann

Bio: Marc Achermann is an academic researcher from University of Massachusetts Amherst. The author has contributed to research in topics: Femtosecond & Quantum dot. The author has an hindex of 27, co-authored 57 publications receiving 5411 citations. Previous affiliations of Marc Achermann include École Polytechnique Fédérale de Lausanne & Lucerne University of Applied Sciences and Arts.

Single-exciton optical gain in semiconductor nanocrystals

Abstract: Nanocrystal quantum dots have favourable light-emitting properties. They show photoluminescence with high quantum yields, and their emission colours depend on the nanocrystal size—owing to the quantum-confinement effect—and are therefore tunable. However, nanocrystals are difficult to use in optical amplification and lasing. Because of an almost exact balance between absorption and stimulated emission in nanoparticles excited with single electron–hole pairs (excitons), optical gain can only occur in nanocrystals that contain at least two excitons. A complication associated with this multiexcitonic nature of light amplification is fast optical-gain decay induced by non-radiative Auger recombination, a process in which one exciton recombines by transferring its energy to another. Here we demonstrate a practical approach for obtaining optical gain in the single-exciton regime that eliminates the problem of Auger decay. Specifically, we develop core/shell hetero-nanocrystals engineered in such a way as to spatially separate electrons and holes between the core and the shell (type-II heterostructures). The resulting imbalance between negative and positive charges produces a strong local electric field, which induces a giant (∼100 meV or greater) transient Stark shift of the absorption spectrum with respect to the luminescence line of singly excited nanocrystals. This effect breaks the exact balance between absorption and stimulated emission, and allows us to demonstrate optical amplification due to single excitons. Semiconductor nanocrystals have very good light-emitting properties, so have potential as optical amplification media that can be easily processed with solution-based techniques: possible applications include optical interconnects in microelectronics, lab-on-a-chip technologies and quantum information processing. The problem with these structures is that at least two excitons (bound electron–hole pairs) need to be present in a nanocrystal before optical gain can be achieved, and this limits performance. In effect, the excitons annihilate each other before optical amplification can occur. This obstacle has now been overcome using nanocrystals with cores and shells made from different semiconductor materials, constructed in such a way that electrons and holes are separated from each other. This makes optical gain based on single excitons possible, significantly enhancing their promise as a practical optical material for laser applications. Semiconductor nanocrystals seem good candidates for 'soft' optical gain media, but optical gain and lasing is hard to achieve owing to a fundamental optical effect, which involves the problem that at least two excitons need to be present in a nanocrystal to achieve gain, and this limits performance. Here the problem is circumvented by designing nanocrystals with cores and shells made from different semiconductor materials, and in such a way that electrons and holes are separated from each other: this makes possible optical gain based on single excitons, thereby significantly enhancing the promise of semiconductor nanocrystals as practical optical materials for a wide range of lasing applications.

Energy-transfer pumping of semiconductor nanocrystals using an epitaxial quantum well.

Abstract: As a result of quantum-confinement effects, the emission colour of semiconductor nanocrystals can be modified dramatically by simply changing their size1,2. Such spectral tunability, together with large photoluminescence quantum yields and high photostability, make nanocrystals attractive for use in a variety of light-emitting technologies—for example, displays, fluorescence tagging3, solid-state lighting and lasers4. An important limitation for such applications, however, is the difficulty of achieving electrical pumping, largely due to the presence of an insulating organic capping layer on the nanocrystals. Here, we describe an approach for indirect injection of electron–hole pairs (the electron–hole radiative recombination gives rise to light emission) into nanocrystals by non-contact, non-radiative energy transfer from a proximal quantum well that can in principle be pumped either electrically or optically. Our theoretical and experimental results indicate that this transfer is fast enough to compete with electron–hole recombination in the quantum well, and results in greater than 50 per cent energy-transfer efficiencies in the tested structures. Furthermore, the measured energy-transfer rates are sufficiently large to provide pumping in the stimulated emission regime, indicating the feasibility of nanocrystal-based optical amplifiers and lasers based on this approach.

Synthesis and Characterization of Co/CdSe Core/Shell Nanocomposites: Bifunctional Magnetic-Optical Nanocrystals

Abstract: Nanocomposite materials provide the possibility for multifunctional properties in contrast with their more-limited single-component counterparts. Here, we report the synthesis and characterization of the first all-inorganic core/shell hybrid magnetic-optical nanoparticle, cobalt/cadmium selenide. The core/shell nanocrystals are prepared in a facile one-pot reaction, and their microstructure is analyzed using low- and high-resolution transmission electron microscopy. Using magnetic and optical characterization, we demonstrate bifunctional behavior, whereby the core retains the magnetic properties of the starting Co nanoparticle, and the shell emits similarly to a single-component CdSe nanoparticle. Interestingly, while the coercivity was found to be unchanged by shell formation, the blocking temperature for the composite structure was observed to be substantially lower (Co: >350 K; Co/CdSe: 240 K). In addition, we observed that at low temperatures (20 K) shell CdSe photoluminescence (PL) decay was very rapid ( 50 ns). Finally, we propose possible explanations for the unusual magnetic and optical behavior of the core/shell hybrid nanostructures.

Nonequilibrium electron dynamics in noble metals

Abstract: Electron-electron and electron-lattice interactions in noble metals are discussed in the light of two-color femtosecond pump-probe measurements in silver films. The internal thermalization of a nonequilibrium electron distribution created by intraband absorption of a pump pulse is followed by probing the induced optical property changes in the vicinity of the frequency threshold for the d band to Fermi surface transitions. This is shown to take place with a characteristic time constant of 350 fs, significantly shorter than previously reported in gold. This difference is ascribed to a weaker screening of the electron-electron interaction by the d-band electrons in silver than in gold. These results are in quantitative agreement with numerical simulations of the electron relaxation dynamics using a reduced static screening of the electron-electron Coulomb interaction, and including bound electron screening. Electron-lattice thermalization has been studied using a probe frequency out of resonance with the interband transitions. In both materials, the transient nonthermal nature of the electron distribution leads to the observation of a short-time delay reduction of the energy-loss rate of the electron gas to the lattice, in very good agreement with our theoretical model.

Multicolor light-emitting diodes based on semiconductor nanocrystals encapsulated in GaN charge injection layers.

Abstract: Numerous technologies including solid-state lighting, displays, and traffic signals can benefit from efficient, color-selectable light sources that are driven electrically. Semiconductor nanocrystals are attractive types of chromophores that combine size-controlled emission colors and high emission efficiencies with excellent photostability and chemical flexibility. Applications of nanocrystals in light-emitting technologies, however, have been significantly hindered by difficulties in achieving direct electrical injection of carriers. Here we report the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture in which semiconductor nanocrystals are incorporated into a p-n junction formed from GaN injection layers. The critical step in the fabrication of these nanocrystal/GaN hybrid structures is the use of a novel deposition technique, energetic neutral atom beam lithography/epitaxy, that allows for the encapsulation of nanocrystals within a GaN matrix without adversely affecting either the nanocrystal integrity or its luminescence properties. We demonstrate electroluminescence (injection efficiencies of at least 1%) in both single- and two-color regimes using structures comprising either a single monolayer or a bilayer of nanocrystals.

Plasmonics: Fundamentals and Applications

Abstract: Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Abstract: A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

Abstract: Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

Plasmonics beyond the diffraction limit

Abstract: Recent years have seen a rapid expansion of research into nanophotonics based on surface plasmon–polaritons. These electromagnetic waves propagate along metal–dielectric interfaces and can be guided by metallic nanostructures beyond the diffraction limit. This remarkable capability has unique prospects for the design of highly integrated photonic signal-processing systems, nanoresolution optical imaging techniques and sensors. This Review summarizes the basic principles and major achievements of plasmon guiding, and details the current state-of-the-art in subwavelength plasmonic waveguides, passive and active nanoplasmonic components for the generation, manipulation and detection of radiation, and configurations for the nanofocusing of light. Potential future developments and applications of nanophotonic devices and circuits are also discussed, such as in optical signals processing, nanoscale optical devices and near-field microscopy with nanoscale resolution.