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Optical beams with helical phase fronts carry orbital angular momentum (OAM). To exploit this property in integrated photonics, micrometer-scale devices that generate beams with well-defined OAM are needed. Consequently, lasers based on microring resonators decorated with azimuthal grating elements have been investigated. However, future development of such devices requires better methods to determine their OAM, as current approaches are challenging to implement and interpret. If a simple and more sensitive technique were available, OAM microring lasers could be better understood and further improved. In particular, despite most devices being pulsed, their OAM output has been assumed to be constant. OAM fluctuations, which are detrimental for applications, need to be quantified. Here, we fabricate quantum-dot microring lasers and demonstrate a simple measurement method that can straightforwardly determine the magnitude and sign of the OAM down to the level of individual laser pulses. We exploit a Fourier microscope with a cylindrical lens and then investigate three types of microring lasers: with circular symmetry, with “blazed” grating elements, and with unidirectional rotational modes. Our results confirm that previous measurement techniques obscured key details about the OAM generation. For example, while time-averaged OAM from our unidirectional laser is very similar to our blazed grating device, single-pulse measurements show that detrimental effects of mode competition are almost entirely suppressed in the former. Nevertheless, even in this case, the OAM output exhibits shot-to-shot fluctuations. Thus, our approach reveals important details in the underlying device operation that can aid in the improvement of micrometer-scale sources with pure OAM output. ISSN:1936-0851 ISSN:1936-086X

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:1530-6984 ISSN:1530-6992

To improve the photophysical performance of colloidal quantum dots for laser applications, sophisticated core/shell geometries have been developed. Typically, a wider bandgap semiconductor is added as a shell to enhance the gain from the quantum-dot core. This shell is designed to electronically isolate the core, funnel excitons to it, and reduce nonradiative Auger recombination. However, the shell could also potentially provide a secondary source of gain, leading to further versatility in these materials. Here we develop high-quality quantum-dot ring lasers that not only exhibit lasing from both the core and the shell but also the ability to switch between them. We fabricate ring resonators (with quality factors up to ∼2500) consisting only of CdSe/CdS/ZnS core/shell/shell quantum dots using a simple template-stripping process. We then examine lasing as a function of the optical excitation power and ring radius. In resonators with quality factors >1000, excitons in the CdSe cores lead to red lasing with thresholds at ∼25 μJ/cm2. With increasing power, green lasing from the CdS shell emerges (>100 μJ/cm2) and then the red lasing begins to disappear (>250 μJ/cm2). We present a rate-equation model that can explain this color switching as a competition between exciton localization into the core and stimulated emission from excitons in the shell. Moreover, by lowering the quality factor of the cavity we can engineer the device to exhibit only green lasing. The mechanism demonstrated here provides a potential route toward color-switchable quantum-dot lasers., Nano Letters, 18 (2), ISSN:1530-6984, ISSN:1530-6992

Gratings1 and holograms2 are patterned surfaces that tailor optical signals by diffraction. Despite their long history, variants with remarkable functionalities continue to be discovered3,4. Further advances could exploit Fourier optics5, which specifies the surface pattern that generates a desired diffracted output through its Fourier transform. To shape the optical wavefront, the ideal surface profile should contain a precise sum of sinusoidal waves, each with a well-defined amplitude, spatial frequency, and phase. However, because fabrication techniques typically yield profiles with at most a few depth levels, complex ‘wavy’ surfaces cannot be obtained, limiting the straightforward mathematical design and implementation of sophisticated diffractive optics. Here we present a simple yet powerful approach to eliminate this design–fabrication mismatch by demonstrating optical surfaces that contain an arbitrary number of specified sinusoids. We combine thermal scanning-probe lithography6–8 and templating9 to create periodic and aperiodic surface patterns with continuous depth control and subwavelength spatial resolution. Multicomponent linear gratings allow precise manipulation of electromagnetic signals through Fourier-spectrum engineering10. Consequently, we overcome a previous limitation in photonics by creating an ultrathin grating that simultaneously couples red, green, and blue light at the same angle of incidence. More broadly, we analytically design and accurately replicate intricate twodimensional moiré patterns11,12, quasicrystals13,14, and holograms15,16, demonstrating a variety of previously impossible diffractive surfaces. Therefore, this approach can provide benefit for optical devices (biosensors17, lasers18,19, metasurfaces4, and modulators20) and emerging topics in photonics (topological structures21, transformation optics22, and valleytronics23).

In the initial proposal of the spaser – a source of coherent, intense, and narrow-band surface plasmons – colloidal quantum dots were envisioned as an ideal gain medium for compensation of the significant losses intrinsic to plasmonics. However, many spasers shown to date have required a single material to both serve as a gain medium and define the plasmonic cavity, a design that prevents the use of quantum dots or other colloidal nanomaterials. In addition, these concepts are inherently challenging for integration in a larger plasmonic circuit.

The study of light fields near nanophotonic structures continually reveals new fundamental features of light–matter interactions on the nanoscale, driving advances in fields ranging from nonlinear and quantum optics to biosensing. Here, we have succeeded in separately mapping the electric and magnetic fields. This allows us to present the first fully three-dimensional maps of the in-plane electromagnetic near fields of a photonic crystal waveguide, in experiment and theory. In these fields, we identify and study the spatial evolution of infinitesimally small optical entities: optical singularities. We discuss the topological properties of the local light fields in the vicinity of the singularities and show that the trajectory traced by each singularity through three-dimensional space is distinct. These results are an important step toward understanding the behavior of light at the nanoscale, opening up new avenues for on-chip control and detection of nanoscopic or quantum objects with structured light fields.

Combining the ability to localize electromagnetic fields at the nanoscale with a directional response, plasmonic antennas offer an effective strategy to shape the far-field pattern of coupled emitters. Here, we introduce a family of directional multiresonant antennas that allows for polarization-resolved spectral identification of fluorescent emission. The geometry consists of a central aperture surrounded by concentric polygonal corrugations. By varying the periodicity of each axis of the polygon individually, this structure can support multiple resonances that provide independent control over emission directionality for multiple wavelengths. Moreover, since each resonant wavelength is directly mapped to a specific polarization orientation, spectral information can be encoded in the polarization state of the out-scattered beam. To demonstrate the potential of such structures in enabling simplified detection schemes and additional functionalities in sensing and imaging applications, we use the central subwavelength aperture as a built-in nanocuvette and manipulate the fluorescent response of colloidal-quantum-dot emitters coupled to the multiresonant antenna., ACS Nano, 11 (12), ISSN:1936-0851, ISSN:1936-086X