Scanning Near-Field Optical Microscopy: Characterizing Nanostructures

Andreas Frank

As design rules for technologies used in applications as diverse as materials research and advanced data storage rapidly approach nanometer size, the hunt is on for simple and effective methods to characterize nanostructures. The ideal would be a high-resolution microscopy technique offering easy-to-interpret optical images; however, light’s diffraction limit presents a stumbling block to enhanced resolution. Even confocal microscopy is unable to overcome this limit, which imposes effective resolution in the 150-nm range.

One possible solution is scanning near-field optical microscopy. Even working with long-wavelength illumination such as near-IR light, the technique’s expected resolution is less than 100 nm, and it offers several other benefits. The results correlate closely with images provided by conventional microscopy, making their evaluation straightforward (Figure 1). The process takes place under ambient conditions, and sample preparation is fairly simple. In addition, several operational modes make the technique flexible enough for a variety of applications, including luminescence analysis of quantum dots and the study of the optical properties of waveguides or photonic crystals.

Figure 1. Different microscopy imaging modes can complement each other. At left is an atomic force microscopy shear-force topography image of bacteria bred to produce green fluorescent protein. At right is an image captured with transmission-mode scanning near-field optical microscopy that shows that the fluorescence emanates from a small number of discrete areas within the sample. Sample courtesy of EMBL Heidelberg in Germany.

Lithium fluoride that contains color centers, for example, finds use in optically pumped tunable solid-state lasers exhibiting broadband output and high efficiency. Scientists write the color center stripes, some 100 μm long with a typical width from 100 nm to 5 μm, into single-crystal lithium fluoride using electron-beam lithography (Figure 2). Scanning near-field optical microscopy allows imaging the detail to subwavelength resolution.

Figure 2. Fluorescence scanning near-field optical microscopy allows the imaging of color centers in single-crystal LiF to subwavelength resolution. Data courtesy of NanOpTec in Lyon, France.

In the near field

To achieve high optical resolution, scanning near-field optical microscopy, such as is possible with Omicron’s TwinSNOM system, relies on evanescent waves to carry optical information from structures much smaller than the wavelength of the light used. Because the intensity of these waves decreases rapidly as a function of distance, conventional imaging is no longer possible even a few tens of nanometers away from the structures.

To gain access to this information, the device uses a tapered fiber optic probe coated with an opaque material everywhere except at a roughly 50-nm transparent aperture at the tip. The system scans this probe across the sample a few nanometers from the surface. Light emerging from the aperture forms the evanescent waves required to image structures.

Imaging in both reflection and transmission modes is possible because the device encompasses a conventional upright microscope, which allows reflection-mode data collection, as well as a second inverted optical microscope for light collection in transmission mode. If required, the probe can work in reverse, collecting and processing near-field light emitted by the specimen.

Whether the sample is opaque or transparent, the TwinSNOM will collect reflected or transmitted light and focus it directly onto a detector for imaging or feed it through an optical fiber to an external device, such as a spectrometer.

For emissive samples, users can collect the light with the tip and guide it to a variety of detectors. Different detector types allow the user to adapt the experimental setup to the expected light intensity and wavelength.

During operation, it is vital to keep the tip of the probe in the near-field regime of the sample. The reason is simple. Limit the separation between aperture and sample to a distance roughly half the diameter of the aperture, and the source has no opportunity to diffract before interacting with the sample. As a result, the aperture diameter, not the wavelength of light used, determines system resolution.

To monitor the distance between the aperture and the sample, the microscope relies on shear-force detection, in which a piezoelectric element measures the resonant-frequency vibration of the fiber tip. As the tip approaches the sample, wave interaction with the surface damps the vibration, producing a phase shift between the excitation and detection frequencies. Because this phase shift is a function of distance, the TwinSNOM device can monitor the distance between probe and surface simultaneously and use the regulating signal to form a high-resolution topographical image of the scanned surface.

In principle, the microscope can use any straight fiber-based tip; however, the refractive index and the shape of the tip will directly influence imaging performance. As a result, microfabricated types work best for applications requiring optimum performance in either transmission or collection mode, because transmission capability is about 100 times higher than with conventional pulled fibers.

Modes of operation

As with conventional microscopy, reflection is the most common operating mode (Figure 3, center). The method of light transmission is simple and applicable to virtually any sample. For this mode, we have developed a mirror objective that can collect 80 percent of the usable light.

Figure 3. In collection mode (left) the tip collects light emitted by a sample and feeds it to the detector for optical characterization. Imaging in both reflection (center) and transmission (right) modes is possible with a conventional microscope in upright position and a second, inverted, microscope.

The other major mode of operation is transmission (Figure 3, right). The microscopy system can easily image transparent samples using a standard, high-numerical-aperture (e.g., oil immersion) objective attached to the lower inverted optical microscope. Light transmits directly to a detector for image formation or to an external spectrometer via fiber for analysis of the sample’s optical properties. Because the light source and scanning action are the same ones used for reflection scanning near-field optical microscopy, the sample can be observed in both modes simultaneously.

Of particular interest for photonic and semiconductor applications is the collection mode (Figure 3, left). Here, the tip collects light emitted from a self-luminant sample, such as the surface of a laser diode, and feeds it directly into the detector for optical characterization of the luminous area of the sample.

The system also is useful for characterization of samples using fluorescence and luminescence techniques. These can work effectively in reflection, transmission or collection mode, with users observing different fluorescent species simply by changing the filters in the light path using conventional sliding carriers. Similarly, a wide variety of materials analysis applications can benefit from performing the technique with polarized light.

The configuration of this microscopy system — in particular the easy access to the illumination and detection paths via fiber optic couplings — also facilitates its use with optical spectroscopy techniques, including Raman spectroscopy. Because the technique is closely related to scanning force microscopy, it also offers this method of surface analysis. In addition, the scanning force microscopy technique is independent of the size of the optical aperture of the tip, so the lateral and vertical resolutions of the resulting image (typically 10 and 1 nm, respectively) are considerably higher than those of the scanning near-field optical microscopy image. To fully exploit the near-field microscope’s performance in this mode, users can add an optional sensor to facilitate atomic force microscopy (Figure 4).

Figure 4. Users can add an optional sensor to facilitate atomic force microscopy with the TwinSNOM system, as illustrated by this topography image of a submonolayer oligomer film on GaAs. Courtesy of N. Koch, Joanneum Research GmbH in Austria.

One advantage of the scanning near-field optical microscopy system is its ability to image a wide range of sample sizes. The coarse adjustment provides a maximum field of examination of 30 x 30 mm without repositioning the specimen; however, the worktable is big enough to accommodate even larger samples, such as DVDs. Samples also can be relatively thick. The lateral scan range is 100 x 100 μm. The vertical range is 20 μm, which compensates for sample tilt and large topographical features.

The TwinSNOM device combines two high-resolution conventional microscopes, each mechanically isolated from the scanning table. Conventional optical microscopy is in no way affected by the use of this technique. Instead, options such as video and confocal microscopy are ideal complements. The modular design concept also facilitates the integration of more capabilities as needed.

Meet the author

Andreas Frank is marketing manager for Omicron NanoTechnology GmbH in Taunusstein, Germany.