Understanding plasmonic lasers: Fabrication, characterization, and design of plasmonic Fabry–Pérot lasers

Plasmonic lasers are the plasmonic analog to conventional lasers. They coherently amplify surface plasmon polaritons (shortened to surface plasmons) instead of photons. Surface plasmons are electromagnetic waves that propagate at the interface between a metal and a dielectric. The free electrons in the metal contribute to this wave giving it partial electronic character. This allows surface plasmons to be confined to much smaller volumes than the minimum size of conventional light as dictated by the diffraction limit. Therefore, plasmonic lasers can be much smaller than their photonic counterparts. Moreover, they provide sources of coherent surface plasmons that can be used to feed optical circuitry. Consequently, optical components can be miniaturized, rendering them more efficient and cost-effective. In addition, due to their high field intensity, coherently amplified surface plasmons can be used to improve sensors and applications that rely on nonlinear effects. In this thesis, we thoroughly study the fabrication, characterization, and design of plasmonic lasers, enabling an in-depth understanding of these devices. First, we experimentally investigate a plasmonic laser that is based on a metallic cavity inside which a gain medium is deposited. The open-cavity design allows us to characterize the lasing behavior. We find that the thickness of the gain medium largely determines whether the metallic cavity lases in the plasmonic or photonic modes. A theoretical model gives insight into the underlying physics of these findings and allows us to make predictions for improved laser designs. Second, we examine the confinement factor, a metric for describing how geometrical aspects of waveguides that include a gain medium influence the amplification of waveguide modes. This discussion is particularly important, as ambiguous interpretations of the confinement factor are common in the literature, hindering optimization of optical gain in waveguides. We clarify these ambiguities and provide the necessary understanding to correctly employ the confinement factor for optimization of designs in nanophotonics and plasmonics. Third, we optimize the geometry of plasmonic Fabry–Pérot lasers to minimize their threshold gain. A plasmonic laser combines a lossy metal and a medium that exhibits gain. By tailoring the geometry of the waveguide inside the Fabry–Pérot cavity, the material contributions to the amplification of the waveguide mode can be tuned. Therefore, the gain required to reach the lasing threshold can be minimized by clever design choices. We find that the magnitude of the reflection losses significantly influences the optimal geometry and identify suitable design guidelines. In summary, this thesis provides an in-depth understanding of various aspects of plasmonic lasers. Besides the experimental study that involves fabrication and characterization of plasmonic lasers, it also gives physical insights through theoretical models. This knowledge can be used to improve the design of plasmonic lasers for practical applications

Bi2O3 boosts brightness, biocompatibility and stability of Mn-doped Ba3(VO4)2 as NIR-II contrast agent

Deep-tissue fluorescence imaging remains a major challenge as there is limited availability of bright biocompatible materials with high photo- and chemical stability. Contrast agents with emission wavelengths above 1000 nm are most favorable for deep tissue imaging, offering deeper penetration and less scattering than those operating at shorter wavelengths. Organic fluorophores suffer from low stability while inorganic nanomaterials (e.g. quantum dots) are based typically on heavy metals raising toxicity concerns. Here, we report scalable flame aerosol synthesis of water-dispersible Ba3(VO4)2 nanoparticles doped with Mn5+ which exhibit a narrow emission band at 1180 nm upon near-infrared excitation. Their co-synthesis with Bi2O3 results in even higher absorption and ten-fold increased emission intensity. The addition of Bi2O3 also improved both chemical stability and cytocompatibility by an order of magnitude enabling imaging deep within tissue. Taken together, these bright particles offer excellent photo-, chemical and colloidal stability in various media with cytocompatibility to HeLa cells superior to existing commercial contrast agents.ISSN:2050-7518ISSN:2050-750

Absorption by water increases fluorescence image contrast of biological tissue in the shortwave infrared

© 2018 National Academy of Sciences. All Rights Reserved. Recent technology developments have expanded the wavelength window for biological fluorescence imaging into the shortwave infrared. We show here a mechanistic understanding of how drastic changes in fluorescence imaging contrast can arise from slight changes of imaging wavelength in the shortwave infrared. We demonstrate, in 3D tissue phantoms and in vivo in mice, that light absorption by water within biological tissue increases image contrast due to attenuation of background and highly scattered light. Wavelengths of strong tissue absorption have conventionally been avoided in fluorescence imaging to maximize photon penetration depth and photon collection, yet we demonstrate that imaging at the peak absorbance of water (near 1,450 nm) results in the highest image contrast in the shortwave infrared. Furthermore, we show, through microscopy of highly labeled ex vivo biological tissue, that the contrast improvement from water absorption enables resolution of deeper structures, resulting in a higher imaging penetration depth. We then illustrate these findings in a theoretical model. Our results suggest that the wavelength-dependent absorptivity of water is the dominant optical property contributing to image contrast, and is therefore crucial for determining the optimal imaging window in the infrared

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.ISSN:1530-6984ISSN:1530-699

Compositional Grading for Efficient and Narrowband Emission in CdSe-Based Core/Shell Nanoplatelets

ISSN:0897-475

High-Temperature Synthesis of CdSe-Based Core/Shell, Core/Shell/Shell, and Core/Graded-Shell Nanoplatelets for Stable and Efficient Narrowband Emitters

Colloidal semiconductor nanoplatelets exhibit exceptionally narrow photoluminescence spectra. This occurs because samples can be synthesized in which all nanoplatelets share the same atomic-scale thickness. As this dimension sets the emission wavelength, inhomogeneous linewidth broadening due to size variation, which is always present in samples of quasi-spherical nanocrystals (quantum dots), is essentially eliminated. Nanoplatelets thus offer improved, spectrally pure emitters for various applications. Unfortunately, due to their non-equilibrium shape, nanoplatelets also suffer from low photo-, chemical, and thermal stability, which limits their use. Moreover, their poor stability hampers the development of efficient synthesis protocols for adding high-quality protective inorganic shells, which are well known to improve the performance of quantum dots. Herein, we report a general synthesis approach to highly emissive and stable core/shell nanoplatelets with various shell compositions, including CdSe/ZnS, CdSe/CdS/ZnS, CdSe/CdxZn1–xS, and CdSe/ZnSe. Motivated by previous work on quantum dots, we find that slow, high-temperature growth of shells containing a compositional gradient reduces strain-induced crystal defects and minimizes the emission linewidth while maintaining good surface passivation and nanocrystal uniformity. Indeed, our best core/shell nanoplatelets (CdSe/CdxZn1–xS) show photoluminescence quantum yields of 90% with linewidths as low as 56 meV (19.5 nm at 655 nm). To confirm the high quality of our different core/shell nanoplatelets for a specific application, we demonstrate their use as gain media in low-threshold ring lasers. More generally, the ability of our synthesis protocol to engineer high-quality shells can help further improve nanoplatelets for optoelectronic devices.</div

High precision, localized proton gradients and fluxes generated by a microelectrode device induce differential growth behaviors of pollen tubes

Pollen tubes are tip-growing plant cells that deliver the sperm cells to the ovules for double fertilization of the egg cell and the endosperm. Various directional cues can trigger the reorientation of pollen tube growth direction on their passage through the female tissues. Among the external stimuli, protons serve an important, regulatory role in the control of pollen tube growth. The generation of local guidance cues has been challenging when investigating the mechanisms of perception and processing of such directional triggers in pollen tubes. Here, we developed and characterized a microelectrode device to generate a local proton gradient and proton flux through water electrolysis. We confirmed that the cytoplasmic pH of pollen tubes varied with environmental pH change. Depending on the position of the pollen tube tip relative to the proton gradient, we observed alterations in the growth behavior, such as bursting at the tip, change in growth direction, or complete growth arrest. Bursting and growth arrest support the hypothesis that changes in the extracellular H+ concentration may interfere with cell wall integrity and actin polymerization at the growing tip. A change in growth direction for some pollen tubes implies that they can perceive the local proton gradient and respond to it. We also showed that the growth rate is directly correlated with the extracellular pH in the tip region. Our microelectrode approach provides a simple method to generate protons and investigate their effect on plant cell growth

Compact Plasmonic Distributed-Feedback Lasers as Dark Sources of Surface Plasmon Polaritons

Plasmonic modes in optical cavities can be amplified through stimulated emission. Using this effect, plasmonic lasers can potentially provide chip-integrated sources of coherent surface plasmon polaritons (SPPs). However, while plasmonic lasers have been experimentally demonstrated, they have not generated propagating plasmons as their primary output signal. Instead, plasmonic lasers typically involve significant emission of free-space photons that are intentionally outcoupled from the cavity by Bragg diffraction or that leak from reflector edges due to uncontrolled scattering. Here, we report a simple cavity design that allows for straightforward extraction of the lasing mode as SPPs while minimizing photon leakage. We achieve plasmonic lasing in 10-μm-long distributed-feedback cavities consisting of a Ag surface periodically patterned with ridges coated by a thin layer of colloidal semiconductor nanoplatelets as the gain material. The diffraction to free-space photons from cavities designed with second-order feedback allows a direct experimental examination of the lasing-mode profile in real- and momentum-space, in good agreement with coupled-wave theory. In contrast, we demonstrate that first-order-feedback cavities remain “dark” above the lasing threshold and the output signal leaves the cavity as propagating SPPs, highlighting the potential of such lasers as on-chip sources of plasmons.ISSN:1936-0851ISSN:1936-086

Compact Plasmonic Distributed-Feedback Lasers as Dark Sources of Surface Plasmon Polaritons

Plasmonic modes in optical cavities can be amplified through stimulated emission. Using this effect, plasmonic lasers can potentially provide chip-integrated sources of coherent surface plasmon polaritons (SPPs). However, while plasmonic lasers have been experimentally demonstrated, they have not generated propagating plasmons as their primary output signal. Instead, plasmonic lasers typically involve significant emission of free-space photons that are intentionally outcoupled from the cavity by Bragg diffraction or that leak from reflector edges due to uncontrolled scattering. Here, we report a simple cavity design that allows for straightforward extraction of the lasing mode as SPPs while minimizing photon leakage. We achieve plasmonic lasing in 10-μm-long distributed-feedback cavities consisting of a Ag surface periodically patterned with ridges coated by a thin layer of colloidal semiconductor nanoplatelets as the gain material. The diffraction to free-space photons from cavities designed with second-order feedback allows a direct experimental examination of the lasing-mode profile in real- and momentum-space, in good agreement with coupled-wave theory. In contrast, we demonstrate that first-order-feedback cavities remain "dark"above the lasing threshold and the output signal leaves the cavity as propagating SPPs, highlighting the potential of such lasers as on-chip sources of plasmons