attosnom I: Topography and Force Images NANOSCOPY APPLICATION NOTE M06 RELATED PRODUCTS G

Transcription

1 APPLICATION NOTE M06 attosnom I: Topography and Force Images Scanning near-field optical microscopy is the outstanding technique to simultaneously measure the topography and the optical contrast of a sample. A new low coherence, optical system has been implemented in force detection schemes for scanning near-field optical microscopy applications. The experimental setup of the low coherence inter-ferometric system is presented in Fig. 1. A laser beam is coupled into a single mode fiber which guides the light through a 50/50 coupler directly to a specially prepared SNOM cantilever (Witec system*).the laser beam is partially reflected on the back side of that cantilever and coupled again into the control fiber. The resulting interference fringes are measured by the detector 1, thus providing the measurement of the tip vibration amplitude. The distance between the SNOM tip and the control fiber is typically 2 to 20 microns. As the tip approaches the sample surface, the cantilever amplitude drops sharply with decreasing tip-sample distance in the nanometer range. The vibration amplitude serves as the input of a feedback loop, which maintains the tip oscillations constant, and consequently the tip-sample forces interactions at a certain level. The output of the feedback loop is recorded during the scan, providing a topography image simultaneously with the near-field optical image. The fiber based low coherence system allows characterizing the tip oscillation modes and amplitudes on the one hand, and, on the other hand, performing topographical measurements with a high precision. Fig. 1: Schematic drawing of the experimental setup of the sensor system based on low coherence interferometry. R1 (=4%) and R2 (=96%) are the reflection coefficients at the end of the control fiber and the SNOM tip. Fig. 2: Optical transmission measurement of a detector glass plate (9x9 microns). Local defects are resolved by the SNOM technique. Fig. 4: The attosnom I microscope sensor head. Fig. 3: Topography and optical measurement of a gold grating (4x4 microns). Different local defects are resolved. attosnom I cantilever based scanning nearfield

2 APPLICATION NOTE M07 attosnom II: Optical Inspection of Waveguides Scanning near-field optical microscopy (SNOM) allows the measurements of both the topography and the optical contrast of a sample with subwavelength resolution. attosnom II works by scanning a sub-wavelength sized probe in the near field of a sample surface. The probe consists of a glass tip that can be covered with an opaque metal layer, with a clear nanometric aperture at the tip apex. The near-field fiber based probe acts simultaneously as a topographic sensor and a nanometric optical aperture that records an optical signal. The attosnom has a novel force detection scheme based on an all fiber low-coherence interferometer. The probe-sample distance control mechanism works by detecting the damping of the oscillating tip by sub-pn lateral forces (the so called shear force ) close to the surface. The probe oscillation amplitude is measured with a precision better than 100 fm/hz 1/2. The oscillation is damped when the probe approaches the sample in the nanometric range due to the shear force. Stable feedback in air and fluids is obtained for tip-sample interaction forces below 1 pn. The instrument records simultaneously this force and the optical signal while scanning the probe on the sample surface. Fig. 3: Principle of measurement: the optical fiber tip (red beam) is scanned very close to the surface of the waveguide, so that the collection of the emitted evanescent field is possible. The upper fiber (yellow beam) serves as a sensor of the tip vibration (distance control). Fig. 1: SEM micrograph of an attocube systems SNOM tip: a 100 nm thin coating guides the laser light to the end of the tip. Fig. 2: The attosnom II microscope sensor head. Fig. 4: Measurement of the evanescent field decay length on the surface of a waveguide. This decay is a function of the effective index of the propagating mode in the waveguide. This method brings the key information when investigating optical waveguides in telecom applications. attosnom II all fiber based scanning nearfield

3 APPLICATION NOTE M08 attosnom II: High Optical Resolution Imaging Scanning near-field s (SNOM) are designed to measure the optical contrast of a sample with sub-wavelength resolution. The attosnom II works by scanning a sharp probe in the near field of a sample surface. The probe consists of a glass tip that can be covered with an opaque metal layer, with a clear nanometric aperture dimension at the tip apex that records an optical signal. The research and development of new probe concepts are gaining increasing interest in near-field optical microscopy. The aim is a better control of the light distribution at the probe aperture, a higher light transmission and an increase in the achievable resolution in the nearfield. One possible solution is the use of a radiating gold nanostructure as a probe for near-field optical microscopy. The gold protruding nanostructure provides the facility to locally graft at the end of the tip a chemical linker to a specific molecule. The probe consequently consists of a topographical, optical and chemical sensor at the same time. Related article: O. Sqalli, M. Bernal, P. Hoffmann and F. Marquis-Weible, Improved tip performance for Scanning Near-Field Optical Microscopy by the attachment of a single gold nano-particle, Appl. Phys. Lett. 76, 2134 (2000). Fig. 1: SEM (Scanning Electron Microscopy) picture of a SNOM tip where a single gold particle is attached at the apex of the probe. Fig. 3: The attosnom II microscope sensor head. Fig. 2: Light transmission over a chromium on glass grating with a period of 372 nm before and after the attachment of a single gold nanoparticle at the tip apex: (a) no particle (b) with particle. attosnom II fiber based scanning nearfield

4 APPLICATION NOTE M09 attosnom III: Topography and Force Imaging Scanning near-field optical microscopy (SNOM) has drawn considerable research interest in recent years since it allows the measurements of both the topography and the optical contrast of a sample with subwavelength resolution. The attosnom III has a piezo-based force detection sensor for scanning near-field optical microscopy applications. The probe-sample distance control works by detecting the damping of the oscillating tip as the tip approaches the sample in the nanometer range. Two different detection methods can be used: 1. PI feedback loop: The force between the tip and the sample serves the input of the PI feedback electronic that keeps the force value constant and equal to a value defined by the user. A Lock-In amplifier is used to measure very small signals. Therefore, the user can plot two images: the so called topography (that corresponds to the piezo voltage of the z- stage) and also the phase between the dither piezo (that makes the tip vibrating) and the measured oscillation. The dephasing of these signals gives additional information. 2. PLL (Phase Locked Loop): The PLL has the same functionalities as the PI electronics, but some more advantages. Unlike the PI electronics, the PLL makes the tip vibrate at its resonance frequency. During the scan, the mechanical resonance frequency of the tip changes, due to interaction forces between the tip and the sample. The PLL can either record an image that is the resonance frequency of the tip, or plot an image that corresponds to the shift of the resonance frequency due to interactions between the tip and the sample. Additional information, such as elasticity of the sample, etc... is then available. Fig. 1: Topography measurement of a chess board with 2 microns in period. Top: data taken with a PI analog feedback; Bottom: phase measurement with a PLL analog feedback loop. Related article: K. Karrai et al., Interfacial Shear-Force Micros-copy, Phys. Rev B 62, (2000) Fig. 1: Left: topographic image of quantum dots performed in vacuum at 4K; Right: the noise in that measurement is well below 0.1 nm. Fig. 3: Line cut of the images shown in Fig. 2. attosnom III scanning nearfield optical microscope

5 Cantilever based Low Temperature Scanning Near-Field Optical Microscope Laser L Detector 1 Detail Interferometer Scanning near-field s (SNOMs) are designed to measure the optical contrast of a sample with sub-wavelength resolution. The attosnom I works by scanning a cantilever in the optical near-field of a sample surface. This microfabricated SNOM sensor, distributed by WITec, consists of a Silicon cantilever with a hollow Aluminum pyramid as tip. At its apex the pyramid has a small aperture of approx. 100 nm in diameter. The near-field probe in this configuration acts simultaneously as a topographic sensor in contact or modulation mode, hence enabling also force measurements, and as an optical aperture. One major advantage of these robust, mass-produced tips is the ease of probe handling unique in SNOM technology. The well known tip-sample distance control using the interferometric detection scheme as also applied in attoafms is another key feature of this system. The schematic drawing in Figure 1 describes this setup. A laser beam coupled into a single mode fiber (port 1) is used to illuminate the SNOM cantilever via a fiber coupler. Hereby, the backside of the cantilver and the fiber end form a Fabry-Perot interferometer and monitoring the intensity of the interference fringes allows to measure the tip vibration amplitude. The non-reflected part of the light is guided through the hollow cantilever and illuminates the sample through the aperture. The cantilever is scanned across the sample in a proximity smaller than the wavelength and the scattered light can be detected in transmission or reflection. Detector 2 (optional) Coupler z-axis y-axis x-axis Scanner x-axis y-axis z-axis liquid He Vacuum Window Thin-Walled Stainless Steel Vacuum Tube Single Mode Fiber for Deflection Detection Detail Interferometer SNOM Cantilever Sample R1 R2 Superconducting Magnet (optional) Ultra Stable Titanium Housing attocube systems Positioning Stages Liquid He Dewar (optional) Figure 1: Schematic drawing of the low temperature attosnom I system. Figure 2 shows the results of simultanously conducted measurements on a Vanadium rhomb-structure on glass substrate with the attosnom I. Whereas the left picture illustrates the topography of the sample, the right one depicts the results of a near-field measurement in transmission. The image shown in Figure 2 was acquired at room temperature and ambient conditions, but ongoing studies are currently establishing these measurements at cryogenic temperatures around 4 K. The attosnom I is an easy-to-use system that combines the advantages of SNOM and Atomic Force Microscopy in a single instrument which is highly suitable for applications under extreme environments such as low temperature and high magnetic fields. attocube systems explore your nanoworld attosnom I ANPxyz101/LT /LT ASC500 cantilever based scanning nearfield high precision, highly stable piezo electric, inertial positioner electronic stepper controller scanning probe microscopy controller attocube systems AG, Königinstrasse 11 A (Rgb), D München, Germany, Tel. +49 (0) , Fax. +49 (0) mm 5 mm Figure 2: Topography measurement (left) and simultaneously obtained near-field measurement in transmission (right) using the attosnom I. Sample: Vanadium rhomb-structure on glass substrate with a layer thickness of 10 nm and a period of 5 µm. Distance control: interferometric sensor. (attocube application labs, 2007). attocube systems AG All rights reserved.