Near-field imaging probes electromagnetic waves
Nanoscale optical imaging is key to practical developments in nanophotonics and plasmonics, and recent breakthroughs leading to multiprobe near-field optical systems foreshadow advances in these and other areas, from semiconductor optical physics to photonic-bandgap devices.
Abraham Israel, Michael Mrejen, Yulia Lovsky, Mila Polhan, Stefan Maier, and Aaron Lewis
Near-field optics has become an important tool for material research, nanoscale physics, state-of-the-art photonics, and biological studies. Although developed more than 20 years ago, the need and demand for nanoscale optical imaging has been rising as devices have moved beyond the micron scale.1 A clear example is the intensive efforts that are leading to chip-based integration of photonic devices and circuits. Such applications have made near-field imaging an important tool, along with scanning- and transmission-electron micrography (SEM/TEM), Raman spectroscopy, and atomic-force microscopy (AFM) and its scanned-probe cousins, scanning-probe, far-field, and confocal microscopy. The rich history and theory behind near-field imaging has been a springboard to the most popular imaging modalities and to the latest developments in near-field imaging instrumentation and its contribution to contemporary research.
Far-field optics-in which the optical element is many wavelengths from the object to be imaged-is dominated by Fresnel and Fraunhofer optics, in which the dominant criterion of resolution is based on Lord Rayleigh’s definition of optical resolution as being at best half the wavelength of the irradiation being used. Thus, as the wavelength becomes smaller the resolution becomes greater. One ultimate example of this effect is that the de Broglie wavelength of electrons is close to atomic in dimension and so an electron microscope can image very small features with nanometric dimensionality. Such nanometric imaging has generally not been possible with optical radiation from the blue to the near-infrared (IR) and beyond.
These far-field limitations are problematic, considering the growing interest in the behavior of samples as they interact with visible or near-IR light. In such cases, the optical characteristics (whether absorption, fluorescence, or other phenomena) in nanometric structures (such as quantum dots or nanowires) must be understood both spectrally and spatially. Near-field optics is the only optical technique that allows for the investigations that are increasingly required, for example, in silicon waveguides with dimensions as small as a few hundred nanometers, or in plasmonic devices in which optical fields in nanometric structures must be imaged to within 100 nm of a surface to understand evanescent fields.
Near-field scanning optical microscopy
Near-field scanning optical microscopy (NSOM, or SNOM in the European version) can be classified as a scanning-probe-microscopy (SPM) technique in which the measuring optical element or probe is brought to a distance of just a few nanometers from the sample’s surface and remains close to the surface by a feedback mechanism, allowing topographic imaging of the sample, along with additional channels of information obtained by the probe. The NSOM probe adds optical data to the topography and the topography feedback keeps the NSOM probe intact (see table).
There are two major categories for NSOM probes: apertured and apertureless. The basic NSOM operation modes include transmission, reflection, fluorescence, collection, and apertureless. For the first three, the AFM/NSOM probe is a source of coherent light illuminating areas through apertures as small as 50 nm. Nanonics Imaging and other research teams showed that excellent NSOM images could be obtained with pulled, coated, and cantilevered optical fiber together with topography imaging.2, 3, 4 Such a probe with a sharp plasmonic antenna of gold can be fabricated with a focused ion beam, together with electron-beam deposition (EBD; see Fig. 1).5 This EBD NSOM probe combines apertured and apertureless probe qualities and the sensitivity of normal force sensing, which is critical to maintain the integrity of the antenna.
Another way to fabricate small apertures in a metal uses coated silicon nitride cantilevers.6 These probes have no waveguide properties and have to be illuminated with a dedicated lens so the probe cannot be moved. They are essentially limited to transmission NSOM because the cantilever obscures the reflection, and the lack of waveguiding causes other limitations.
In general, there must be relative motion of the sample and the probe, while the light that comes out of the nanometric aperture in transmission or reflection mode interacts with the surface and is subsequently collected by standard far-field microscopic techniques. Relatively sensitive photon counting is then used for pixel-by-pixel detection. The small spot coming out of the aperture is nanometric in size (in all three axes) as the intensity of the beam that is ejected from the probe drops dramatically due to diffraction from such a small aperture. As a result, the sample penetration depth for the photons is small and allows experiments in the third dimension as well for different sample-probe distances. Near-field optics is the only optical technique that has solved the problem of out-of-focus light.
For collecting the light, an upright microscope configuration is used for reflection NSOM, while transparent samples can also be mounted on an inverted microscope for transmission measurements.7 As in standard far-field microscopy, image contrast can indicate changes in index of refraction, transmittance, reflectivity, or the existence of absorptive or fluorescent molecules for the laser wavelength used (see Fig. 2).
Collection-mode NSOM uses the same apertured fiber probes as NSOM, but the probes are used to collect light from the sample rather than to introduce light. This technique is making a tremendous contribution in the study of photonic devices and structures especially in view of its capability to image both evanescent and propagating light from laser diodes, waveguides, lensed fibers, fiber lasers, and other photonic devices with much greater resolution than any other available method, and in three dimensions.8 It is the only technique available that can make a true M2 measurement in which the probe with AFM feedback can measure the true near field and its relationship to the topography of the surface without out-of-focus contributions. Thus, near-field optics is the only technique that can characterize a beam from, say, a fiber laser as being truly Gaussian by accurately recording how close M2 is to one.
These capabilities can also be exploited in the imaging of evanescent waves and modes of light that propagate in a photonic-crystal waveguide and in plasmonic structures (see Fig. 3). These fields do not propagate in the direction of the far-field imaging system and therefore cannot be seen otherwise. Measuring the light with a heterodyne detection system also adds important phase information, together with unprecedented capabilities in correlating light distribution to topography.9 In fact, near-field optics is the only optical technique that can give pixel-by-pixel correlation of optical contrast with topographic correlation.
In terms of apertureless near-field optics, an important examples are nonlinear optical phenomena and tip-enhanced Raman scattering (TERS), in which the light is projected and collected using far-field techniques onto a gold or silver nanoparticle at the end of a cantilevered nanopipette causing a local field enhancement of four orders of magnitude that increases the Raman scattering or the nonlinear optical signal to the single molecule level.10, 11 Comparing a far-field measurement and the TERS data isolates the tip effect and therefore increases the lateral resolution in comparison to far-field imaging. Like in aperture NSOM, the local enhanced field does not penetrate the sample deeply and so it can be used for subsurface characterization. As device dimensions shrink in all three dimensions, these two advantages add apertureless NSOM as a tool for micro/nano fabrication analysis.12
A significant advance has been the development and introduction by Nanonics of a multiprobe scanning-probe microscope with near-field optical capabilities. The development of the system followed multiple innovations, including a variety of exposed-tip glass probes pioneered by Nanonics. The system can have up to four probes in contact with the surface simultaneously, each with individual feedback and scanning capabilities. The probes can be separated and the distance between two or more probes can be measured with nanometer-scale accuracy. As in all our scanning-microscope heads, this system has a completely free optical axis from below and above, so it can be placed in most optical microscopes (upright or inverted) or micro-Raman spectroscopic systems. This open geometry allows for ease of approach of optical and lensed fibers, direct or focused laser beams, or electrical and mechanical connections from all directions. The system is designed to work in sample-scanning and tip-scanning modes to allow maximum flexibility in such photonic experiments as plasmonic waveguide analysis and beam profiling of diode lasers, lensed fiber, and other output devices.
Besides the more conventional AFM, NSOM, and TERS experiments, this multiprobe system enables experimentation in online nanolithography with either nanoindentation or nanoprinting probes. Thus, light could be injected with one probe and collected by another probe, while a nanoparticle is injected with a third probe at, say, a topographically specified location with a cantilevered nanopipette Fountain Pen probe.13 The system permits for the first time classic pump-probe experiments in the near-field with nanometric spatial separation (see Fig. 4). In addition, an important nonoptical application of this system is the measurement of electrical currents between any two points in a sample with high-resolution exposed-tip cantilevered glass insulated nanowires. Such a scanning-probe microscopy system permits experiments in near-field optics and imaging, and scanned-probe microscopy in general, that simply could not be conceived before its development.
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ABRAHAM ISRAEL and MICHAEL MREJEN are applications scientist and AARON LEWIS is is a professor of applied physics at the Hebrew University and the president and founder of Nanonics Imaging, Manhat Technology Park, Malcha, Jerusalem, Israel 91487; e-mail: firstname.lastname@example.org; www.nanonics.co.il. YULIA LOVSKY is a graduate student at the Hebrew University of Jerusalem, Edmond Safra Campus-Givat Ram, Jerusalem, Israel 91904. STEFAN MAIER is a reader in physics in the Experimental Solid State Group, Physics Department, Imperial College, London SW7 2AZ, UK; e-mail: S.Maier@imperial.a.