Multi-frequency coherent emission from superstructure thermal emitters

Coherent light sources have been demonstrated based on a wide range of nanostructures, however, little effort has been devoted to probing their underlying coherence properties. Here, we report long-range spatial coherence of lattice plasmon lasers constructed from a periodic array of gold nanoparticles and a liquid gain medium at room temperature. By combining spatial and temporal interferometry, we demonstrate millimeter-scale (∼1 mm) spatial coherence and picosecond (∼2 ps) temporal coherence. The long-range spatial coherence occurs even without the presence of strong coupling with the lattice plasmon mode extending over macroscopic distances in the lasing regime. This plasmonic lasing system thus provides a platform for understanding the emergence of long-range coherence from collections of nanoscale resonators and points toward novel types of distributed lasing sources.

The degree of spatial coherence, as basic characteristics of the radiation, becomes an important guide to evaluate the performance of X-rays from newly introduced advanced light sources including the X-ray free electron laser (XFEL). Often the modification of the X-ray wavefronts to fulfill various applications is necessary, but also there is the need to preserve its coherence property. However, experimental investigation directly comparing the coherence property of focused X-ray radiations with the unmodified ones has not been available. We have performed Young's double-slit experiments by recording diffraction patterns both from slit apertures for unfocused XFEL radiation and from pairs of Au nanoparticles for one-micron focused XFEL radiations. The results confirm that the degree of spatial coherence is preserved for well-built K-B focusing mirrors.

The spatial coherence function |Γ(s)| of ruby laser pulses has been measured by self-diffraction at a laser-induced grating depending on the distances between two points in the beam profile. The spatial coherence of the laser beam was changed by operating the laser in the fundamental transverse mode, in a transverse multimode and by applying different pump powers. The degree of spatial coherence was experimentally found to be a Gaussian, as described by the Collett-Wolf model. The new method for measuring the spatial coherence function |Γ(s)| is described and possible improvements are discussed.

Spatial coherences of SR beam at visible light region were measured both /spl sigma/ and /spl pi/-polarized components at Photon Factory. A wave-front division type polarized interferometer was designed and constructed for this experiment. Interferograms were observed clearly in the vertical direction. /spl pi/ phase difference of the interference fringe was observed between interferograms corresponding to /spl sigma/ and /spl pi/-polarized components. Degree of spatial coherence was measured and by means of the van Citterut-Zernike's theorem, a vertical beam profile was obtained by the Fourier transform of the degree of spatial coherence.

The accidental Dirac cone of a photonic crystal allows the realization of a low loss dielectric metamaterial with simultaneous near-zero effective permittivity and permeability. The resulting zero refractive index allowed applications that require unique spatial coherency. While most thermal light sources were considered highly incoherent, structurally engineered thermal emitters have achieved relatively high spatial coherency. Here, we propose an epsilon-and-mu-near-zero metamaterial as a spatial coherency converter for thermal emissions, and experimentally demonstrate surface-normal directional emissions with an angular width of 20 degrees.

Kirchhoff's law shows that reciprocal materials have equal spectral emissivity at two symmetric polar angles, which is a fundamental limit for a thermal emitter to achieve a small angular divergence in the normal direction. Nonreciprocal materials allow violation of Kirchhoff's law as the emissivity at the two symmetric polar angles can be different. However, achieving strong nonreciprocal thermal radiation near zero angle is challenging. In this work, to reduce the power consumption of a light source for e.g. gas sensing, an ultra‐high‐directional nonreciprocal thermal vertical emitter is proposed, with a periodic structure of magneto‐optical material. When B = 3 T or 1.5 T, magneto‐optical lattice resonances enable the near‐perfect emissivity at 22.36 µm or 22.99 µm at zero angle. The strong nonreciprocity contributed by the collective modes allows for a near‐complete violation of Kirchhoff's law at small angles of ±1°. The nonreciprocal emitters have a very small angular divergence (≈1°), which is better than that of the state‐of‐the‐art thermal emitters. The highly directional nonreciprocal thermal emission is robust despite ±25% change in material loss and ±5% fluctuation in structural parameters. This work should inspire the design of high‐directional nonreciprocal thermal emitters and their applications in high‐resolution thermal imaging, infrared gas sensing, biomedical breath monitoring, and so on.

Beams generated by Gaussian quasi-homogeneous sources

The coherence properties of white light emission of a tungsten incandescent lamp additionally excited with laser radiation are investigated. The degree of spatial coherence is determined from measurements of the contrast of interference fringe patterns obtained for a tungsten bulb as the light source. It is demonstrated that the degree of spatial coherence of an incandescent lamp is increased by additional excitation of the filament with focused laser radiation. Furthermore, the degree of spatial coherence can be adjusted by selecting the voltage applied to the filament. Such light sources can find applications in imaging techniques where precise control of spatial coherence of illumination is required.

Spatial coherence is an impactful source parameter in many applications ranging from atomic and molecular physics to metrology or imaging. In lensless imaging, for example, it can strongly affect the image formation, especially when the source exhibits shot-to-shot variations. Single-shot characterization of the spatial coherence length of a source is thus crucial. However, current techniques require either parallel intensity measurements or the use of several masks. Based on the method proposed by González et al. [J. Opt. Soc. Am. A28, 1107 (2011)JOAOD60740-323210.1364/JOSAA.28.001107], we designed a specific arrangement of a two-dimensional non-redundant array of apertures, which allows, through its far field interference pattern, for a single-shot measurement of the spatial coherence, while being robust against beam-pointing instabilities. The strategic configuration of the pinholes allows us to disentangle the degree of spatial coherence from the intensity distribution, thus removing the need for parallel measurement of the beam intensity. An experimental validation is performed using a high-harmonic source. A statistical study in different regimes shows the robustness of the method.

The propagation of radiation from a source embedded in an inhomogeneous refractive medium is studied, using both mode-analysis and ray-tracing methods. Scaling laws for excitation strengths of the modes, the degree of spatial coherence, and the shift of the spectral line are derived.

Research in microfabrication is important to many areas of science and technology. An emerging set of microlithographic techniques, known as soft lithography, uses elastomeric elements for microcontact printing (μCP), molding, or near field photolithography. We describe the use of two soft lithographic techniques to build DFB lasers with narrow emission linewidths. This demonstrates the suitability of soft lithography for building structures that have (i) feature sizes (∼300 nm) significantly less than 1 μm, and (ii) long-range (∼1 mm) spatial coherence. The lasers use optically pumped gain material deposited on a DFB resonator formed from periodic relief on the surface of a transparent substrate. The required relief was generated on glass and polymer surfaces with μCP and replica molding respectively. μCP produced lines of gold on a glass slide; RIE using the gold as a resist generated periodic relief (600 nm period, 50 nm depth) on the glass surface. Polymer gratings with similar dimensions were formed by curing a polyurethane film against an elastomeric mold. The good performance of the lasers suggest the potential utility of soft lithographic techniques for low-cost fabrication of high-performance optoelectronic devices. Other types of resonators, including distributed Bragg reflector and quasi two-dimensional photonic bandgap based resonators can also be fabricated with the techniques demonstrated here.

Abstract. We have examined, for the first time, the spatial coherence of oceanic parameters, namely, the ocean surface wind and significant wave height (SWH), using near-instantaneous along-track sampling over vast oceanic tracks provided by Poseidon-2 Radar Altimeter onboard Jason-1. The spatial coherence length scales over the Arabian Sea and the Bay of Bengal regions, derived using Auto correlation analysis, display interesting characteristics in relation to onset and evolution of SW Monsoon over India. The estimated spatial coherence scales are found to be of the order of 100–500 km. Generally, SWH coherence lengths over Arabian Sea are observed to increase from June to July during good monsoon year (2012). This trend is not followed for the poor monsoon year (2002). Similar trend is observed for spatial coherence scales for wind speed. The temporal evolution of analyzed spatial coherence scales of winds and waves over Arabian Sea clearly brings out the distinction between a "good" (2012) and a "poor" (2002) monsoon year. The spatial coherence lengths of these parameters over the Bay of Bengal are found to be shorter and do not show any systematic relation to onset and evolution of SW monsoon. Anisotropy in coherence lengths is also analyzed by treating ascending and descending tracks separately. Generally, the descending tracks measurements show higher spatial coherence lengths than the ascending ones. Like the spatial coherence scales, the anisotropy of the coherence scales over the Arabian Sea also mimics the behaviour of the strength of SW Monsoon.

The evolutions of the spectrum and spatial-coherence of light in optical resonators are investigated theoretically in detail on the basis of coherence theory in the space-frequency domain. The initial light in the resonator is assumed to be polychromatic and spatially incoherent with a certain spectral width. By considering the effects of the edge diffraction and phase modulation, the evolutions of the spectrum and coherence are clearly demonstrated for different types of resonator. The results show that the spectral evolution is affected not only by the g-factor of the resonator (the type of resonator), but also by the edge diffraction and phase modulation. We introduce a new parameter, called the global degree of spatial coherence, to characterize the spatial coherence on each mirror. The spatial-coherence evolution is also affected by g-factor of the resonator, edge diffraction, and phase modulation. The physical mechanism of these phenomena is explained and discussed.

We report on the experimental determination of the complete two coordinate spatial coherence function of light emitted by a quasi-random laser, implemented on recently introduced dye-doped transparent wood. The spatial coherence was measured by means of a double grating interferometer, which has some advantages over the standard Young's interferometer. Analysis of the spatial coherence reveals that emission from such a material can be considered as a superposition of several spatial modes produced by individual emitters within semi-ordered scattering medium. The overall degree of coherence, γ¯, for this quasi-random laser was found to be 0.16 ± 0.01, having possible applications in speckle free laser imaging and illumination.

Although only a handful of organic materials have shown polariton condensation, their study is rapidly becoming more accessible. The spontaneous appearance of long-range spatial coherence is often recognized as a defining feature of such condensates. In this Letter, we study the emergence of spatial coherence in an organic microcavity and demonstrate a number of unique features stemming from the peculiarities of this material set. Despite its disordered nature, we find that correlations extend over the entire spot size, and we measure g(1)(r,r') values of nearly unity at short distances and of 50% for points separated by nearly 10 μm . We show that for large spots, strong shot-to-shot fluctuations emerge as varying phase gradients and defects, including the spontaneous formation of vortices. These are consistent with the presence of modulation instabilities. Furthermore, we find that measurements with flat-top spots are significantly influenced by disorder and can, in some cases, lead to the formation of mutually incoherent localized condensates.