Martin M. Greve

Focusing of a neutral helium beam below one micron

In 2008 we presented the first images obtained with a new type of matter wave microscope: NEutral Helium Atom MIcroscopy (NEMI). The main features in NEMI are the low energy of the atoms (<0.1 eV) and the fact that they are neutral. This means that fragile and/or insulating samples can be imaged without surface damage and charging effects. The ultimate resolution limit is given by the de Broglie wavelength (about 0.06 nm for a room-temperature beam), but reaching a small focus spot is still a major challenge. The best result previously was about 2 μm. The main result of this paper is the focusing of a helium atom beam to a diameter below 1 μm. A particular challenge for neutral helium microscopy is the optical element for focusing. The most promising option is to manipulate neutral helium via its de Broglie wavelength, which requires optical elements structured to nanometre precision. Here we present an investigation of the helium focusing properties of nanostructured Fresnel zone-plates. Experiments were performed by varying the illuminated area and measuring the corresponding focused spot sizes and focused beam intensities. The results were fitted to a theoretical model. There is a deviation in the efficiency of the larger zone plate, which indicates a distortion in the zone-plate pattern, but nevertheless there is good agreement between model and experiments for the focus size. This together with the demonstration of focusing to below 1 μm is an important step towards nanometre resolution neutral helium microscopy.

Finite-size limitations on Quality Factor of guided resonance modes in 2D Photonic Crystals

High-Q guided resonance modes in two-dimensional photonic crystals, enable high field intensity in small volumes that can be exploited to realize high performance sensors. We show through simulations and experiments how the Q-factor of guided resonance modes varies with the size of the photonic crystal, and that this variation is due to loss caused by scattering of in-plane propagating modes at the lattice boundary and coupling of incident light to fully guided modes that exist in the homogeneous slab outside the lattice boundary. A photonic crystal with reflecting boundaries, realized by Bragg mirrors with a band gap for in-plane propagating modes, has been designed to suppress these edge effects. The new design represents a way around the fundamental limitation on Q-factors for guided resonances in finite photonic crystals. Results are presented for both simulated and fabricated structures.

Optimization of an electron beam lithography instrument for fast, large area writing at 10 kV acceleration voltage

Electron beam lithography (EBL) is a maskless lithography technique used in numerous applications for fabrication of ultrahigh-resolution photolithography masks. The main disadvantage of EBL is that it is time-consuming, requiring the pattern to be written in a successive fashion. Various approaches are used to lower the write time. Throughput-oriented EBL instruments used in industrial applications typically apply a very high acceleration voltage (≥50 kV). However, in many research environments, more cost-effective instruments are used. These tools are usually optimized for high-resolution writing and are not very fast. Hence, they are normally not considered very suitable for writing large-scale structures with high pattern densities, even for limited resolution applications. In this paper, the authors show that a carefully considered optimization of the writing parameters in an EBL instrument (Raith e_LiNE) can improve the writing time to more than 40 times faster than commonly used instrument settings...

The Beynon Gabor zone plate: a new tool for de Broglie matter waves and hard X-rays? An off axis and focus intensity investigation

Optical elements based on Fresnel zones are used in a range of applications, from X-ray telescopy to microscopy and recently also in the manipulation of de Broglie matter waves. In 1992 Beynon and co-workers presented a binary Gabor type zone plate (henceforth referred to as the Beynon Gabor zone plate). Because this zone plate has no higher order foci, it is in principle a very attractive candidate for focusing of de Broglie matter waves and in some cases X-rays. So far the Beynon Gabor zone plate investigations presented in the literature have concentrated on the intensity distribution along the optical axis and in the focal plane. Here we present a detailed numerical investigation of the Beynon Gabor zone plate, including an investigation of the off-optical axis, off focal plane intensity distribution for point source illumination. We show that at integer fractions of the focal length, the beam becomes nearly toroidal (doughnut-shaped). This offers potentially interesting new possibilities for de Broglie matter wave and X-ray optics, for example in STED-like applications. We further show that the increased intensity at the focal point predicted in the literature for a particular Beynon Gabor zone plate transmission function configuration is an artifact due to the lack of sampling nodes. We support our calculations with experimental measurements in the visible light range, using a Beynon Gabor zone plate fabricated with electron beam lithography.

Nanostructuring of free-standing, dielectric membranes using electron-beam lithography

Nanostructured dielectric membranes are used in several applications ranging from de Broglie matter-wave optical elements to photonic crystals. Precise pattern transfer and high aspect ratio structures are crucial for many applications. The authors present an improved method for direct patterning on free-standing, dielectric membranes using electron-beam (e-beam) lithography. The method is based on an advanced etchmask that both reduces charging and allows for tuning of the etch mask thickness to support high aspect ratios even for small structures. The authors etched structures as small as 50 nm radius holes in a 150 nm thick membrane and achieved aspect ratios of up to 1.3 for this structure size range. The etch mask thickness can be tuned to achieve the required aspect ratio. The etchmask is composed of a three layer stack consisting of poly(methyl methacrylate), SiO2 and an antireflective coating polymer. Scanning-electron micrographs of membranes produced with the fabrication method are presented.

A systematic investigation of the charging effect in scanning electron microscopy for metal nanostructures on insulating substrates.

Scanning electron microscopy is perhaps the most important method for investigating and characterizing nanostructures. A well‐known challenge in scanning electron microscopy is the investigation of insulating materials. As insulating materials do not provide a path to ground they accumulate charge, evident as image drift and image distortions. In previous work, we have seen that sample charging in arrays of metal nanoparticles on glass substrates leads to a shrinkage effect, resulting in a measurement error in the nanoparticle dimension of up to 15% at 10 kV and a probe current of 80 ± 10 pA. In order to investigate this effect in detail, we have fabricated metal nanostructures on insulating borosilicate glass using electron beam lithography. Electron beam lithography allows us to tailor the design of our metal nanostructures and the area coverage. The measurements are carried out using two commonly available secondary electron detectors in scanning electron microscopes, namely, an InLens‐ and an Everhart–Thornley detector. We identify and discriminate several contributions to the effect by varying microscope settings, including the size of the aperture, the beam current, the working distance and the acceleration voltage. We image metal nanostructures of various sizes and geometries, investigating the influence of scan‐direction of the electron beam and secondary electron detector used for imaging. The relative measurement error, which we measure as high as 20% for some settings, is found to depend on the acceleration voltage and the type of secondary electron detector used for imaging. In particular, the Everhart–Thornley detectors lower sensitivity to SE1 electrons increase the magnitude of the shrinkage of up to 10% relative to the InLens measurements. Finally, a method for estimating charge balance in insulating samples is presented.

Measuring the localized surface plasmon resonance effect on large arrays (5 mm × 5 mm) of gold and aluminum nanoparticles on borosilicate glass substrates, fabricated by electron beam lithography

Metal nanoparticles have interesting optical properties, in particular, the localized surface plasmon resonance effect (LSPR), which can be used in a range of applications such as photon detectors, photovoltaics, and biosensors. The theory of LSPR is complex, and hence, it is very important to develop reliable model systems, where the properties of the particles can be investigated under controlled conditions. Important parameters, which influence the frequency and intensity of the LSPR, are size, shape, and spacing of the individual nanoparticles, as well as the metal and the surrounding medium. For applications related to photovoltaics, in particular, it is important to investigate the effect of nanoparticles, covering large areas to mimic realistic operating conditions. Here, the authors present a preparation method and optical investigations of LSPR for large arrays (5 mm × 5 mm) of gold and aluminum nanoparticles ranging in diameter from 44 to 140 nm (corresponding to around 120 million particles). T...

Atom sieve for nanometer resolution neutral helium microscopy

Neutral helium microscopy is a new tool for imaging fragile and/or insulating structures as well as structures with large aspect ratios. In one configuration of the microscope, neutral helium atoms are focused as de Broglie matter waves using a Fresnel zone plate. The ultimate resolution is determined by the width of the outermost zone. Due to the low-energy beam (typically less than 0.1 eV), the neutral helium atoms do not penetrate solid materials and the Fresnel zone plate therefore has to be a free-standing structure. This creates particular fabrication challenges. The so-called Fresnel photon sieve structure is especially attractive in this context, as it consists merely of holes. Holes are easier to fabricate than the free-standing rings required in a standard Fresnel zone plate for helium microscopy, and the diameter of the outermost holes can be larger than the width of the zone that they cover. Recently, a photon sieve structure was used for the first time, as an atom sieve, to focus a beam of he...

Temperature induced color change in gold nanoparticle arrays: Investigating the annealing effect on the localized surface plasmon resonance

The localized surface plasmon resonance (LSPR) effect in metal nanoparticles is important for a range of applications, including photovoltaics and sensors. The actual LSPR effect is difficult to predict, because it can vary strongly with the size, shape, surface structure, and surrounding media of the nanoparticles. In order to understand this better, more experimental data are needed. Here, the authors present a study of the LSPR effect in macroscopic two-dimensional square arrays of gold nanoparticles, 50–80 nm in diameter with a pitch of approximately 160 nm, fabricated on borosilicate substrates. The arrays were exposed to different annealing temperatures in steps of 50 up to 600 °C. The authors observe an irreversible blue-shift of the LSPR extinction peak, from around 580 to around 520 nm at annealing temperatures of only 450 °C, an effect clearly visible to the naked eye. The authors also present measurements of the shape of the nanoparticles at the different annealing steps. These measurements wer...

Lab-on-a-chip device for fabrication of therapeutic microbubbles on demand

Electron beam lithography (EBL) was used to fabricate microchannels to produce microbubbles with highly homogeneous size distributions. Using the EBL technique, microchannels can be prototyped at a fast and cost effective rate allowing for evaluation of various mi- crobubble shell materials.