We experimentally study surface plasmon lasing in a series of metal hole arrays on a gold-semiconductor interface. The sub-wavelength holes are arranged in square arrays of which we systematically vary the lattice constant and hole size. The semiconductor medium is optically pumped and operates at telecom wavelengths (λ ~ 1.5 μm). For all 9 studied arrays, we observe surface plasmon (SP) lasing close to normal incidence, where different lasers operate in different plasmonic bands and at different wavelengths. Angle- and frequency-resolved measurements of the spontaneous emission visualizes these bands over the relevant (ω, k||) range. The observed bands are accurately described by a simple coupled-wave model, which enables us to quantify the backwards and right-angle scattering of SPs at the holes in the metal film.

ABSTRACT:We study the near-and far-field radiation patterns of surface plasmon (SP) lasers in metal hole arrays and observe radially polarized vortex-vector laser beams in both near and far field. Besides the intensity profile, also the complementary phase profile is obtained with a beam block experiment, where we block part of the beam in the near field, measure the resulting changes in the far field, and retrieve the phase using an iterative algorithm. This phase profile provides valuable information on the feedback mechanisms and coherence of the laser and shows that our SP laser operates in a phaseslip mode instead of a pure dark mode. To explain our observations, we extend the standard model for distributed feedback lasers by introducing a position dependence in the optical gain and refractive index. KEYWORDS: surface plasmon laser, distributed feedback, phase retrieval, metal hole array, plasmonic crystal laser, active mirror O ptically coherent laser radiation can be generated if both gain and optical feedback are present in a medium. Our physical understanding of these phenomena originates from comparisons between measured intensity distributions and models of both the amplitude and the phase of the radiation. The optical phase is typically discarded because it evolves too fast to resolve directly with an optical detector or a camera. The inability to measure both amplitude and phase of the emitted laser radiation presents a recurring challenge in optics and limits progress in the field.More ingenious schemes are needed to observe the phase using slow detectors. One of the simplest schemes uses the mixing of the amplitude and phase information on the light field upon propagation. At the laser exit the amplitude contains information where the light is emitted, while the phase profile contains information about the propagation direction. Recording the intensity distribution on different positions allows retrieval of the phase information by an iterative algorithm. 1−3The ability to resolve both amplitude and phase is particularly relevant for lasers that emit nonstandard beam profiles that are not yet fully understood. Examples of such lasers are surface-emitting distributed feedback lasers, such as photonic and plasmonic crystal lasers. Two-dimensional surface-emitting photonic-crystal lasers often emit donut beams with azimuthal polarization, 4 while surface plasmon lasers create radially polarized vector-vortex beams. 5 Devices can be tailored to emit other beam shapes, 6 but information about the phase and amplitude profile is scarce and either has low resolution 7 or an electrical contact blocks the view. 8 A better understanding of gain and feedback in plasmonic systems is important for improving photonics applications that use the strong confinement and light−matter interaction provided by plasmons. These applications include ultrasensitive molecule sensors (SERS), 9 anticounterfeiting measures, 10 perfect absorbers, 11 ultrafast optical modulators, 12 and future metal−dielectric metamaterials consi...

Lensless imaging is an approach to microscopy in which a high-resolution image of an object is reconstructed from one or more measured diffraction patterns, providing a solution in situations where the use of imaging optics is not possible. However, current lensless imaging methods are typically limited by the need for a light source with a narrow, stable and accurately known spectrum. We have developed a general approach to lensless imaging without spectral bandwidth limitations or sample requirements. We use two time-delayed coherent light pulses and show that scanning the pulse-to-pulse time delay allows the reconstruction of diffraction-limited images for all the spectral components in the pulse. In addition, we introduce an iterative phase retrieval algorithm that uses these spectrally resolved Fresnel diffraction patterns to obtain high-resolution images of complex extended objects. We demonstrate this two-pulse imaging method with octave-spanning visible light sources, in both transmission and reflection geometries, and with broadband extreme-ultraviolet radiation from a high-harmonic generation source. Our approach enables effective use of low-flux ultra-broadband sources, such as table-top high-harmonic generation systems, for high-resolution imaging.

We study surface plasmons on two-dimensional square arrays of sub-wavelength holes in a gold film deposited on an optically-excited semiconductor. We observe four resonances of which we measure the resonance frequencies, the spectral widths, and the relative intensities. The spectral widths allow us to quantify various loss processes, including ohmic loss, optical absorption/gain and radiative scattering loss. Prominent kinks in the plasmon dispersion relation occur around the Rayleigh anomaly. A coupled mode model that includes a frequency dependent gain of the semiconductor reproduces the main features in the experimental data.

In semiconductor device manufacturing, optical overlay metrology measures pattern placement between two layers in a chip with sub-nm precision. Continuous improvements in overlay metrology are needed to keep up with shrinking device dimensions in modern chips. We present first overlay metrology results using a novel off-axis dark-field digital holographic microscopy concept that acquires multiple holograms in parallel by angular multiplexing. We show that this concept reduces the impact of source intensity fluctuations on the noise in the measured overlay. With our setup we achieved an overlay reproducibility of 0.13 nm and measurements on overlay targets with known programmed overlay values showed good linearity of R2= 0.9993. Our data show potential for significant improvement and that digital holographic microscopy is a promising technique for future overlay metrology tools.

We demonstrate surface-plasmon lasing in hexagonal metal hole arrays with a semiconductor gain medium. The device can be tuned between two laser modes, with distinct wavelengths, spatial distributions, and polarization patterns, by changing the size of the optically pumped area. One of the modes exhibits a six-fold polarization pattern, while the mode observed for larger pump spots has a rotationally symmetric polarization pattern. We explain the mode tuning by the differences of in-plane and radiative out-of-plane losses of the modes. The spatial and polarization properties of the modes are conveniently described by a sum of vectorial orbital angular momentum beams with orbital, spin, and total angular momentum j=ℓ+s.

We demonstrate a technique that enables lensless holographic imaging with extended reference structures, using ultra-broadband radiation sources for illumination. We show that this 'two-pulse imaging' approach works with one- and two-dimensional HERALDO reference structures, and demonstrate that the obtained spectrally resolved data can be used to improve the signal-to-noise ratio in the final image. Intensity stitching of multiple exposures is applied to increase the detected dynamic range, leading to an improved image reconstruction. Furthermore, we show that a combination of holography and iterative phase retrieval can be used to obtain high-quality images quickly and reliably, by using the HERALDO reconstruction as the initial support constraint in the iterative phase retrieval algorithm. A signal-to-noise improvement of two orders of magnitude is achieved compared to the basic HERALDO result.

We present a systematic experimental study on the optical properties of plasmonic crystals (PlC) with hexagonal symmetry. We compare the dispersion and avoided crossings of surface plasmon modes around the Γ-point of Au-metal hole arrays with a hexagonal, honeycomb and kagome lattice. Symmetry arguments and group theory are used to label the six modes and understand their radiative and dispersive properties. Plasmon-plasmon interaction are accurately described by a coupled mode model, that contains effective scattering amplitudes of surface plasmons on a lattice of air holes under 60• , 120• , and 180• . We determine these rates in the experiment and find that they are dominated by the hole-density and not on the complexity of the unit-cell. Our analysis shows that the observed angle-dependent scattering can be explained by a single-hole model based on electric and magnetic dipoles.