Fluctuations in the Photoluminescence Excitation Spectra of Individual Semiconductor Nanocrystals.

Most single quantum emitters display non-steady emission properties. Models that explain this effect have primarily relied on photoluminescence measurements that reveal variations in intensity, wavelength, and excited-state lifetime. While photoluminescence excitation spectroscopy could provide complementary information, existing experimental methods cannot collect spectra before individual emitters change in intensity (blink) or wavelength (spectrally diffuse). Here, we present an experimental approach that circumvents such issues, allowing the collection of excitation spectra from individual emitters. Using rapid modulation of the excitation wavelength, we collect and classify excitation spectra from individual CdSe/CdS/ZnS core/shell/shell quantum dots. The spectra, along with simultaneous time-correlated single-photon counting, reveal two separate emission-reduction mechanisms caused by charging and trapping, respectively. During bright emission periods, we also observe a correlation between emission red-shifts and the increased oscillator strength of higher excited states. Quantum-mechanical modeling indicates that diffusion of charges in the vicinity of an emitter polarizes the exciton and transfers the oscillator strength to higher-energy transitions.

Active Mode Switching in Plasmonic Microlasers by Spatial Control of Optical Gain.

The pursuit of miniaturized optical sources for on-chip applications has led to the development of surface plasmon polariton lasers (plasmonic lasers). While applications in spectroscopy and information technology would greatly benefit from the facile and active tuning of the output wavelength from such devices, this topic remains underexplored. Here, we demonstrate optically controlled switching between predefined wavelengths within a plasmonic microlaser. After fabricating Fabry-Pérot plasmonic cavities that consist of two curved block reflectors on an ultrasmooth flat Ag surface, we deposit a thin film of CdSe/CdxZn1-xS/ZnS colloidal core/shell/shell nanoplatelets (NPLs) as the gain medium. Our cavity geometry allows the spatial and energetic separation of transverse modes. By spatially modulating the gain profile within this device, we demonstrate active selection and switching between four transverse modes within a single plasmonic laser. The fast buildup and decay of the plasmonic modes promises picosecond switching times, given sufficiently rapid changes in the structured illumination.

Measurement of Raman Optical Activity with High-Frequency Polarization Modulation.

Many chiroptical spectroscopic techniques have been developed to detect chirality in molecular species and probe its role in biological processes. Raman optical activity (ROA) should be one of the most powerful methods, as ROA yields vibrational and chirality information simultaneously and can measure analytes in aqueous and biologically relevant solvents. However, despite its promise, the use of ROA has been limited, largely due to challenges in instrumentation. Here, we report a new approach to ROA that exploits high-frequency polarization modulation. High-frequency polarization modulation, usually implemented with a photoelastic modulator (PEM), has long been the standard technique in other chiroptical spectroscopies. Unfortunately, the need for simultaneous spectral and polarization resolution has precluded the use of PEMs in ROA instruments. We combine a specialized camera system (the Zurich imaging polarimeter, or ZIMPOL) with PEM modulation to perform ROA measurements. We demonstrate performance similar to the current standard in ROA instrumentation while reducing complexity and polarization artifacts. This development should aid researchers in exploiting the full potential of ROA for chemical and biological analysis.

Oxidation of Reduced Peat Particulate Organic Matter by Dissolved Oxygen: Quantification of Apparent Rate Constants in the Field.

Peat particulate organic matter (POM) is an important terminal electron acceptor for anaerobic respiration in northern peatlands provided that the electron-accepting capacity of POM is periodically restored by oxidation with O2 during peat oxygenation events. We employed push-pull tests with dissolved O2 as reactant to determine pseudo-first-order rate constants of O2 consumption ( kobs) in anoxic peat soil of an unperturbed Swedish ombrotrophic bog. Dissolved O2 was rapidly consumed in anoxic peat with a mean kobs of 2.91 ± 0.60 h-1, corresponding to an O2 half-life of ∼14 min. POM dominated O2 consumption, as evidenced from approximately 50-fold smaller kobs in POM-free control tests. Inhibiting microbial activity with formaldehyde did not appreciably slow O2 consumption, supporting abiotic O2 reduction by POM moieties, not aerobic respiration, as the primary route of O2 consumption. Peat preoxygenation with dissolved O2 lowered kobs in subsequent oxygen consumption tests, consistent with depletion of reduced moieties in POM. Finally, repeated oxygen consumption tests demonstrated that anoxic peat POM has a high reduction capacity, in excess to 20 µmol electrons donated per gram POM. This work demonstrates rapid abiotic oxidation of reduced POM by O2, supporting that short-term oxygenation events can restore the capacity of POM to accept electrons from anaerobic respiration in temporarily anoxic parts of peatlands.

Combined SPR and SERS microscopy in the Kretschmann configuration.

A novel hybrid spectroscopic technique is proposed, combining surface plasmon resonance (SPR) with surface-enhanced Raman scattering (SERS) microscopy. A standard Raman microscope is modified to accommodate the excitation of surface plasmon-polaritons (SPPs) on flat metallic surfaces in the Kretschmann configuration, while retaining the capabilities of Raman microscopy. The excitation of SPPs is performed as in standard SPR-microscopy; namely, a beam with TM-polarization traverses off-axis a high numerical aperture oil immersion objective, illuminating at an angle the metallic film from the (glass) substrate side. The same objective is used to collect the full Kretschmann cone containing the SERS emission on the substrate side. The angular dispersion of the plasmon resonance is measured in reflectivity for different coupling conditions and, simultaneously, SERS spectra are recorded from Nile Blue (NB) molecules adsorbed onto the surface. A trade-off is identified between the conditions of optimum coupling to SPPs and the spot size (which is related to the spatial resolution). This technique opens new horizons for SERS microscopy with uniform enhancement on flat surfaces.

Combining surface plasmon resonance (SPR) spectroscopy with surface-enhanced Raman scattering (SERS).

The simultaneous measurement of surface plasmon resonance (SPR) spectroscopy and surface-enhanced Raman scattering (SERS) on flat metallic surfaces is demonstrated on a relatively simple experimental setup based on the Kretschmann configuration. This setup requires only minor modifications to standard Raman microscopes, and we show that it can be applied successfully to the most common conditions of SPR spectroscopy, i.e., water-based solutions on gold films. Our results emphasize the peculiar properties of the Kretschmann configuration for spectroscopy in general and SERS measurements in particular, especially in terms of the asymmetry between excitation and collection requirements. The combination of simultaneous SPR-SERS spectroscopy opens up interesting prospects in analytical science to study, for example, reaction kinetics at surfaces under conditions which are already available in commercial SPR instruments.

Quantifying resonant Raman cross sections with SERS.

We propose a method based on surface-enhanced Raman scattering (SERS) to estimate the resonance Raman cross sections of dyes. The latter are notoriously difficult (or impossible) to obtain by normal (spontaneous) constant wave Raman spectroscopy when the fluorescence quantum yield of the molecules is good and the overwhelming effect of fluorescence masks the Raman spectrum. We propose here to use the fluorescence quenching occurring in SERS conditions to overcome simply this problem. The principles of the method are described and its limitations discussed in detail. The method is demonstrated by estimating the resonance Raman differential cross sections for Rhodamine 6G for seven different excitation wavelengths across the visible range.