We present an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam. This modulation is attributed to an interference phenomenon at each of the slits, instead of at the detector. The interference arises as a consequence of the excitation of surface plasmons propagating from one slit to the other. DOI: 10.1103/PhysRevLett.94.053901 PACS numbers: 42.79.Dj, 73.20.Mf, 78.66.Bz Recently, there has been a surge of interest in the phenomenon of light transmission through subwavelength apertures in metal plates. This followed the observation by Ebbesen et al. [1] that the transmission through a twodimensional hole array can be much larger than predicted by conventional diffraction theory [2]. This discovery has rekindled the interest in a similar but simpler problem, viz., the transmission of a one-dimensional array of subwavelength slits in a metal film, i.e., of a metal grating [1,[3][4][5][6][7][8][9][10][11][12][13][14][15][16]. In many cases the enhanced transmission of hole or slit arrays has been explained in terms of the excitation of (coupled) surface plasmons on the metal film [3][4][5][6], an explanation that has recently been challenged [16]. It has been shown that, for slit arrays, Fabry-Pérot-type waveguide resonances can also give rise to a considerably enhanced transmission [5,7,9,10,12].In the present Letter we study an even more fundamental system than the metallic grating, namely, a thin metal layer perforated by just two parallel subwavelength slits. In contrast to the systems that have recently attracted so much attention, our slits are separated by many optical wavelengths. Thus we study the light transmission of a setup that lies at the heart of wave physics, namely, that of Thomas Young. We do, however, not focus on the wellknown interference pattern named after him, but on the angle-integrated power transmission coefficient of the perforated screen, i.e., the transmission integrated over many interference orders. We show that this transmission coefficient is strongly modulated as a function of the wavelength of the incident light for the case that that light is TMpolarized, i.e., with the electric field aligned perpendicular to the slits. In contrast, there is no such modulation when the incident light is TE-polarized, or when the ''wrong'' metal is chosen. All our observations can be explained in terms of a model involving the coherent transport of electromagnetic energy between the slits by surface plasmons.Our samples consist of a 200 nm thick gold film, evaporated on top of a 0.5 mm thick fused quartz substrate with a 10 nm thick titanium adhesion layer between the gold and the glass. In such a sample a two-slit pattern is written using a focussed ion beam, each slit being 50 m long and 0:2 m wide. The slits are separated by a dis...

In brain tumor surgery, recognition of tumor boundaries is key. However, intraoperative assessment of tumor boundaries by the neurosurgeon is difficult. Therefore, there is an urgent need for tools that provide the neurosurgeon with pathological information during the operation. We show that third harmonic generation (THG) microscopy provides label-free, real-time images of histopathological quality; increased cellularity, nuclear pleomorphism, and rarefaction of neuropil in fresh, unstained human brain tissue could be clearly recognized. We further demonstrate THG images taken with a GRIN objective, as a step toward in situ THG microendoscopy of tumor boundaries. THG imaging is thus a promising tool for optical biopsies.

Scaffold contraction is a common but underestimated problem in the field of tissue engineering. It becomes particularly problematic when creating anatomically complex shapes such as the ear. The aim of this study was to develop a contraction-free biocompatible scaffold construct for ear cartilage tissue engineering. To address this aim, we used three constructs: (i) a fibrin/hyaluronic acid (FB/HA) hydrogel, (ii) a FB/HA hydrogel combined with a collagen I/III scaffold, and (iii) a cage construct containing (ii) surrounded by a 3D-printed poly-ɛ-caprolactone mold. A wide range of different cell types were tested within these constructs, including chondrocytes, perichondrocytes, adipose-derived mesenchymal stem cells, and their combinations. After in vitro culturing for 1, 14, and 28 days, all constructs were analyzed. Macroscopic observation showed severe contraction of the cell-seeded hydrogel (i). This could be prevented, in part, by combining the hydrogel with the collagen scaffold (ii) and prevented in total using the 3D-printed cage construct (iii). (Immuno)histological analysis, multiphoton laser scanning microscopy, and biomechanical analysis showed extracellular matrix deposition and increased Young's modulus and thereby the feasibility of ear cartilage engineering. These results demonstrated that the 3D-printed cage construct is an adequate model for contraction-free ear cartilage engineering using a range of cell combinations.

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We report on a method to generate a stationary interference pattern from two independent optical sources, each illuminating a single slit in Young's interference experiment. The pattern arises as a result of the action of surface plasmons traveling between subwavelength slits milled in a metal film. with E the complex amplitude of the field, assumed here to be scalar; P 1 and P 2 denote the positions of the slits, a delay time, and the brackets a time average. For our purpose it is useful to employ the normalized mutual coherence function (the so-called complex degree of coherence), defined aswhere I͑P i ͒ is the averaged intensity at slit i. Under typical circumstances, the visibility V of the interference fringes near a point P in the far zone is equal to the modulus of the complex degree of coherence, i.e.,with equal to the time difference ͑P 1 P − P 2 P͒ / c, c being the speed of light in air. If one slit is illuminated by a light source radiating at frequency 1 while the other slit is illuminated by a separate source radiating at frequency 2 , it is easily seen that then ␥͑P 1 , P 2 , ͒ = 0. Under these illumination conditions the fringe visibility should thus be zero across the entire interference pattern for sufficiently long integration times. In this line of reasoning it is assumed that the radiative field emerging from a slit is simply, up to some factor, equal to the radiative field incident on that slit. When surface plasmons propagate between the two slits this assumption is no longer valid. 4,5 Consequently, a stationary interference pattern should be observed even if the frequencies of the lasers illuminating the individual slits are very different. Here we confirm this idea in an experiment where the two lasers run at frequencies differing by as much as 1.8 THz. Furthermore, we show that an interference pattern is also observed when only one slit is illuminated. When the polarization of the incident light is chosen such that no surface plasmons can be excited, the stationary interference pattern is observed to be absent.The experimental setup is shown in Fig. 1. Two separate lasers, a tunable narrowband Ti:sapphire laser and a semiconductor diode laser operating at 812 nm, each illuminate a single subwavelength slit in a 200 nm thick gold film. Each laser is focused to a spot of approximately 5 m FWHM. The two parallel slits, ϳ25 m apart, are 50 m long and 0.2 m wide. The gold film is evaporated on top of a 0.5 mm thick fused-quartz substrate with a 10 nm thick titanium adhesion layer between the gold and the quartz. A CCD camera is used to record the far-field pattern. The outputs of a fiber-coupled diode and a Ti:sapphire laser are individually focused on one of a pair of 200 nm wide slits, separated by Ϸ25 m, in a thin gold film. The light diffracted at the two parallel slits is imaged onto a CCD camera. A, attenuator; M, mirror; BS, beam splitter; /2, half-wave plate; P, polarizer; L, lens; S, gold sample. The inset shows the illumination of the double slit.

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Abstract:We have experimentally studied the polarization-dependent transmission properties of a nanoslit in a gold film as a function of its width. The slit exhibits strong birefringence and dichroism. We find, surprisingly, that the transmission of the polarization parallel to the slit only disappears when the slit is much narrower than half a wavelength, while the transmission of the perpendicular component is reduced by the excitation of surface plasmons. We exploit the slit's dichroism and birefringence to realize a quarter-wave retarder.

Real‐time assessment of excised tissue may help to improve surgical results in breast tumor surgeries. Here, as a step towards this purpose, the potential of second and third harmonic generation (SHG, THG) microscopy is explored. SHG and THG are nonlinear optical microscopic techniques that do not require labeling of tissue to generate 3D images with intrinsic depth‐sectioning at sub‐cellular resolution. Until now, this technique had been applied on fixated breast tissue or to visualize the stroma only, whereas most tumors start in the lobules and ducts. Here, SHG/THG images of freshly excised unprocessed healthy human tissue are shown to reveal key breast components—lobules, ducts, fat tissue, connective tissue and blood vessels, in good agreement with hematoxylin and eosin histology. DNA staining of fresh unprocessed mouse breast tissue was performed to aid in the identification of cell nuclei in label‐free THG images. Furthermore, 2‐ and 3‐photon excited auto‐fluorescence images of mouse and human tissue are collected for comparison. The SHG/THG imaging modalities generate high quality images of freshly excised tissue in less than a minute with an information content comparable to that of the gold standard, histopathology. Therefore, SHG/THG microscopy is a promising tool for real‐time assessment of excised tissue during surgery.