A monopole antenna at optical frequencies: single-molecule near-field measurements

We present a monopole antenna for optical frequencies (~600 THz) and discuss near-field measurements with single fluorescent molecules as a technique to characterize such antennas. The similarities and differences between near-field antenna measurements at optical and radio frequencies are discussed in detail. Two typical antenna properties, polarization selectivity and resonances, are studied for the optical monopole by direct near-field measurements and finite integration technique calculations. The antenna is driven by the local field of a sub-wavelength aperture. This gives rise to a dependence of the antenna response on the orientation of the local field vector, in an analogous way to the polarization selectivity of linear wire antennas. The antenna resonances are studied by varying the antenna length. Typical monopole resonances are demonstrated. The finite conductivity of metals at optical frequencies (in combination with the antenna radius) causes the wavelength of the surface charge density oscillation (surface plasmon polariton) along the antenna to be shortened in comparison to the free space wavelength. As a result, resonances for the optical monopole antenna occur at much shorter relative lengths than for conventional radio monopole antennas\ud \u

High index contrast passive potassium double tungstate waveguides

High-refractive-index-contrast potassium double tungstate waveguides have been experimentally demonstrated. A bulk KY(WO4)2 layer was successfully bonded onto a lower refractive index carrier using a UV curable optical adhesive and polished down to the thickness of 2.4 μm. A set of rib waveguides with ~2 μm width and 0.85 μm slab thickness were fabricated on the thin transferred KY(WO4)2 layer by focused-ion-beam milling. The upper-limit of the propagation losses of the fabricated waveguides is estimated to be 1.5 dB/cm at the wavelength of 1.55 μm using the Fabry-Perot method

Video: Freezing supersonic flow by LED based Schlieren imaging

Benefiting from the development of increasingly advanced high speed cameras, flow visualization and analysis nowadays yield detailed data of the flow field in many applications. Notwithstanding this progress, for high speed and supersonic flows it is still not trivial to capture high quality images. In this study we present a Schlieren setup that uses pulsed LEDs with high currents (up to 18 Ampere) to increase the optical output to sufficient levels. The bright and short pulses, down to 130 nanoseconds, allow detailed and sharp imaging of the flow with a high spatial resolution adequate for supersonic flow. The pulse circuit and pulse width determination are explained in detail. As a test case we studied the near field of a 2 mm diameter sonic jet injected transversely into a supersonic cross flow. This is a model flow for fuel injection in scramjet engines, which is not yet fully understood. Owing to the high resolution and accuracy of the images produced by the newly developed system we prove the existence of a large (density) gradient wave traveling in the windward subsonic region between the Mach barrel and the bowshock, which hitherto was observed only in some numerical studies but not yet shown in experiments. Furthermore, we demonstrate with this Schlieren setup that time-correlated images can be obtained, with an interframe time of 2 microseconds, so that also flow unsteadiness can be studied such as the movement of shock waves and trajectories of vortices. The obtained results of the jet penetration height are presented as a power law correlation. The results of this study show that the designed setup using a low-cost LED and low-cost control system is a high potential option for application in visualization studies of high speed flows

Near-field driving of a optical monopole antenna \ud

Nanosized optical antennas have the potential to confine and enhance optical electromagnetic fields, making nano-antennas essential tools for applications in integrated nano-optical devices and high-resolution microscopy. The size, shape and material of the nano-antenna, together with the optical frequency, determine the antenna response and its resonances. Here, we discuss a λ/4 long optical nano-antenna, analogous to the radio frequency monopole antenna. The antenna is fabricated at the end of a near-field aperture-type fibre probe by focused-ion-beam milling in two sequential steps. Illumination through the fibre creates a localized evanescent excitation source, with the advantage of a lower background compared to 'apertureless' techniques, which require far-field excitation. Previously, we have studied the field localization, antenna excitation conditions and antenna resonances, both in experiment, by near-field single-molecule detection experiments, and in theory, by finite integration technique simulations. In this study we investigate the importance of both polarization conditions and antenna position in creating an efficient local driving field for the monopole antenna. It is shown that the antenna is driven by the field component along the antenna axis. Next we show the advantage of the antenna over the aperture: upon reduction of the diameter the antenna gains local field intensity, while the aperture field decreases rapidly. Finally, the highly localized field near the antenna apex is probed by single molecules and detected molecular emission features below 30 nm FWHM are presented.\u

Optical antennas with sinusoidal modulation in width

Small metal structures sustaining plasmon resonances in the optical regime are of great interest due to their large scattering cross sections and ability to concentrate light to subwavelength volumes. In this paper, we study the dipolar plasmon resonances of optical antennas with a constant volume and a sinusoidal modulation in width. We experimentally show that by changing the phase of the width-modulation, with a small 10 nm modulation amplitude, the resonance shifts over 160 nm. Using simulations we show how this simple design can create resonance shifts greater than 600 nm. The versatility of this design is further shown by creating asymmetric structures with two different modulation amplitudes, which we experimentally and numerically show to give rise to two resonances. Our results on both the symmetric and asymmetric antennas show the capability to control the localization of the fields outside the antenna, while still maintaining the freedom to change the antenna resonance wavelength. The antenna design we tested combines a large spectral tunability with a small footprint: all the antenna dimensions are factor 7 to 13 smaller than the wavelength, and hold potential as a design element in meta-surfaces for beam shaping

Coherent imaging of local fields in photonic crystals

Summary form only given.\ud In the past years, much research has been focused on so called photonic crystals. Different ways of fabricating and simulating these structures have been introduced. Characterization is performed mostly with black box experiments, where reflected or transmitted light is detected. We demonstrate a different approach, which allows us to take a direct look inside such structures. With a heterodyne interferometric photon scanning tunneling microscope (PSTM), we are able to visualize not only the amplitude of the local optical field, but also the phase information of light propagating inside the crystal. Heterodyne interferometric photon scanning tunneling microscopy gives detailed insight in reflected and transmitted waves as they develop through periodic structures. Ultimately, this method will allow us to visualize the opening of a stop gap in one or two-dimensional photonic crystals

Atomic force microscope with integrated optical microscope for biological applications

Since atomic force microscopy (AFM) is capable of imaging nonconducting surfaces, the technique holds great promises for high‐resolution imaging of biological specimens. A disadvantage of most AFMs is the fact that the relatively large sample surface has to be scanned multiple times to pinpoint a specific biological object of interest. Here an AFM is presented which has an incorporated inverted optical microscope. The optical image from the optical microscope is not obscured by the cantilever. Using a XY stage to move the sample, an object is selected with the optical microscope and an AFM image of the selected object can be obtained. AFM images of chromosomes and K562 cells show the potential of the microscope. The microscope further enables a direct comparison between optically observed features and topological information obtained from AFM images