Near-field scanning optical microscope (NSOM) probe designs consisting of a subwavelength aperture offset of either a metallic or metal-coated dielectric cantilevered tip are investigated using finite-difference time-domain calculations. The offset aperture and metal-coated dielectric tip couple surface plasmons that illuminate the tip apex, which results in a single-lobed probing optical spot having a full-width half maximum (FWHM) similar to the apex diameter. Since the surface plasmons converge at the apex, an offset-apertured probe promises significantly higher throughput light intensities than an apertured NSOM having a comparable spot FWHM.
©2007 Optical Society of America
Originally proposed by Synge in 1928  and demonstrated independently by Pohl et al.  and Lewis et al.  in 1984, apertured near-field scanning optical microscopy (NSOM) has become a commonplace tool for sub-100-nm spatial resolution optical imaging. This is accomplished by placing a near-field optical probe, having a subwavelength-sized opening, within a distance comparable to the aperture’s radius, r, to a sample. When this near-field probe is operated in collection mode, evanescent fields localized to subwavelength features are partially scattered by the aperture into propagating waves. Alternatively when the same near-field probe is operated in transmission mode, the subwavelength aperture illuminates the sample in a localized sub-diffraction spot, where the incident light is scattered by subwavelength features into both propagating waves and evanescent fields. In either operating mode, a far-field detector collects the scattered propagating waves. Thus, the near-field probe functions either as a localized source or collector, depending on its mode of operation, whose maximum spatial resolution is comparable to its aperture diameter. Accordingly, a fundamental drawback of apertured near-field probes is the fact that the transmitted light intensity, I, is strongly attenuated when the aperture diameter is reduced [4,5]. In practice, it is challenging to detect the low intensity of light from probes having sub-30 nm apertures. Equally challenging is the fabrication of probes with sufficiently small apertures to resolve such small features. To overcome such challenges, the throughput light intensity has been enhanced by a factor of 103 when employing a hybrid near-field solid-immersion probe without sacrificing spatial resolution . Another specialized near-field probe having a triangular aperture has been demonstrated, providing 30–40 nm resolution while improving light intensity throughput . However, the triangular aperture configuration had a flattened apex of ~300-nm width, which would limit its capability to image optical features located in deep and narrow features.
More recent development in near-field optical microscopy involves apertureless (or scattering-type) probes, where a sharp tip apex scatters the evanescent fields associated with a sample’s subwavelength features into the far field. These probes have demonstrated spatial resolutions of ~10 nm [10,11], limited only by the apex’s diameter. However, it should be noted that the process of reconstructing an actual optical image is complex as a result of the presence of a large background signal. The large background light signal is the result of scattering from the tip and reflection of the diffraction-limited illumination spot from the sample. The contribution from the background scattered light can be relatively combated by oscillating the tip, using lock-in detection, and interfering the scattered near-field signal with a reference signal . Despite such efforts, the interferometric effects resulting from the background light signal cannot be fully suppressed, which leads to image artifacts that do not directly correspond to optical features of the sample. Furthermore, the large illuminated area may be detrimental in certain applications where it is desirable to illuminate only the sub-diffraction area of interest and not its surroundings, such as fluorescence microscopy or nanometer-scale ablation.
Another type of near-field probes has gained wide interest as a result of promising investigations. These probes bring together the distinct features of both apertured and apertureless probes. Such hybrid probes are capable of operating in a fashion similar to normal apertured probes in transmission mode while providing the high spatial resolution offered by apertureless probes. Here, light is strongly scattered by the open aperture edges and consequently couples into evanescent waves (surface plasmon polaritons). Spatial localization of high electric field density at the apex of a sharp protrusion provides the means for nanometer scale imaging. Generally, these hybrid near-field configurations provide good image discrimination against the background-scattered photons since the probing electric field originates internally from the probe itself. Complex probe designs, such as circular apertures with coaxial protruding tips [13,14], circular apertures with edge-attached conical tips [15–17] and I-shaped apertures on asymmetric pyramidal tips  have been either fabricated [13–17] or proposed  for near-field microscopy applications.
Recently, Frey et al. demonstrated a near-field probe in the form of a dielectric apertureless tip coated with a very thin (~20 nm) silver layer . This near-field probe acts as a spatially localizing source where surface plasmons couple onto the surface of the tip and propagate on the tip, “focusing” the electric fields to yield localized high electromagnetic energy density at the tip’s apex. When illuminating a sample with the fields localized to the apex of this probe, the aforementioned large background scattered signal associated with typical apertureless near-field probes is mostly eliminated.
In this paper, cantilevered hybrid NSOM probe designs, each having a large subwavelength aperture offset from a metal-coated dielectric tip as shown in Fig. 1(a–b), are presented and studied through numerical calculations. The overall geometry of both probe designs represents an offset-apertured NSOM probe. This near-field probe can be fabricated using conventional fabrication techniques. Cantilevered solid [19–22] and hollow [23–25] silicon dioxide tips having various conical angles have been fabricated using a variety of process flows. Furthermore, it is possible to fabricate an aperture in a cantilever, making use of electron beam or projection lithography . Metal layers of varying thicknesses can be deposited onto the cantilevered probes using physical vapor deposition techniques.
One advantage of the proposed offset-apertured probe is its long and narrow tip, which is able to provide optical and topographical imaging of samples having deep and narrow topographical features. Furthermore, the relatively large spatial separation between the aperture and the probing apex allows for a wider range of surface plasmon coupling geometries, potentially allowing for much improved light throughput efficiency. As presented in this paper, our probe design enhances light throughput by taking advantage of a large aperture combined with a Kretschmann-like geometry for coupling surface plasmons to the apex. By placing a large aperture in an offset position relative to the tip, the waveguide cut-off limitations inherent in conventional apertured near-field probes are easily circumvented. Interestingly, the resultant light throughput enhancement is essentially background-free, thus enabling a maximum spatial resolution similar to the diameter of the apex.
2. Modeling an offset-apertured near-field scanning optical microscope probe
A typical structure of the proposed offset-apertured NSOM consists of an atomic force microscope-type cantilevered silicon dioxide tip, where both the cantilever arm and probing tip are coated with silver film. In the cantilever arm and directly adjacent to the base of the tip, a large aperture of diameter d≈λ/2 is placed, where λ is the light wavelength. As the offset-apertured probe is illuminated with laser radiation, light is coupled through the aperture and the open base of the tip cone into surface plasmons on the tip surface. It should be noted that the light that enters through the opening at the base of the cone is coupled into surface plasmons on the tip surface due to the Kretschmann-like geometry  of the silver-coated dielectric tip. These surface plasmons propagate down the length of the tip’s surface L, where the energy is “focused” toward the apex. The advantage of coupling light through the large aperture and Kretschmann-like geometry is the fact that the resolution is limited here to the apex diameter and not the aperture size. This is contrary to a typical apertured NSOM probe, shown in Fig. 1(c), of similar dimensions.
The calculations are performed using the three-dimensional finite-difference time-domain (FDTD) method . The particular advantages of the FDTD method are the abilities to input complex geometries and directly extract field values from the calculations. The FDTD algorithm is used to solve for displacement, D, electric, E, and magnetic, H, field values using Maxwell’s equations at all temporal and spatial points. In order to model the plasmonic response of the tip, the Drude model for permittivity is implemented through the auxillary differential equation formalism . Here, the permittivity is defined as
where ω is the light frequency, ω p is the plasma frequency, ν is the damping frequency, ε 0 is the permittivity of free space, and ε ∞ is the dc dielectric constant. Relating the displacement and electric fields using the equation D=ε(ω)E, the auxillary differential equation is given as:
The metal layers used are silver, having a plasma frequency ωp=1.3×1016 rad/s and a damping frequency ν=4.6×1013 rad/s. At the illuminating wavelength of λ=800 nm used in the calculations, silver and silicon dioxide have relative permittivities of ε=-29.5+i0.6 [28,29] and ε=2.25 respectively. The computational window consisted of 150×300×150 cells. Perfectly-matched layers are used to minimize non-physical reflections from the calculation space boundaries [27,30].
In order to perform a complete investigation of the offset-apertured NSOM tip and achieve optimal performance, we study the following geometrical parameters in great detail: the supporting cantilever silver layer thickness, t, the dielectric tip metal layer thickness through which surface plasmons are coupled, T, the surface wave propagation length, L, and the aperture diameter, d. In keeping the design in line with experimentally easily attainable values with conventional fabrication techniques, the apex diameter is kept at 40 nm. In order to maximize the field strength enhancement at the apex of the tip, the tip conical half-angle is kept small at θ=11.3°. To highlight the advantages of the offset-apertured NSOM probe, all calculated results are compared with those for the silicon dioxide-filled single-apertured probe shown in Fig. 1(c). This single-apertured probe consists of a silver-coated cantilevered silicon dioxide tip and an aperture in the silver layer at the apex of the tip. The tip’s conical half-angle is chosen to be 45°, wider than the offset-apertured probe. However, unlike with the offset-apertured NSOM probe, the increased conical half-angle of the single-apertured NSOM probe results in a corresponding increase in energy throughput. This increase is due to the larger number of allowed conic waveguide modes and shorter distance between the waveguide cut-off location and the aperture . Additionally, the thick silver coating of 155 nm on top of the dielectric prevents light leakage and guides light toward the aperture.
Fig. 2(a) and 2(b) respectively illustrate the steady-state and the dynamical field distributions of an offset-apertured probe when illuminated by a light wave whose electric field polarization direction is represented by an arrow. Surface plasmons are coupled onto the silver-air interface from the offset aperture and the base aperture. These surface plasmon waves propagate on both the inner and the outer surfaces of the offset-apertured NSOM probe toward the apex, where the sharp apex enhances the intensity. An important parameter in this study is the high degree of light coupling into surface plasmon waves via the Kretschmann-like geometry. This coupling mechanism determines not only the light throughput intensity but also the level of background radiation. Initially, we consider the contribution from the offset aperture by replacing the probe’s dielectric silicon dioxide core with a metallic silver core such that surface plasmons are allowed only to form on the outer surface of the solid silver tip as shown in Fig. 1(a). In this approach, the offset aperture is isolated from the influence of the base aperture, thus permitting us to gain greater insight into the overall performance of this probe. Henceforth, we refer to the offset-apertured probe having a solid silver tip as an Offset Apertured – Metallic Apertureless (OAMA) probe. Likewise, we will refer to the offset apertureless-apertured probe having a silver-coated silicon dioxide tip, shown in Fig. 1(b), as an Offset Apertured – Metal-coated Dielectric Apertureless (OAMDA) probe.
3. An offset apertured – metallic apertureless (OAMA) probe
A major difference between an offset-apertured probe and the typical apertured NSOM probe is the spatial distribution of light intensity in their illumination spots. As shown in Fig. 3, while an OAMA probe produces a single-lobed intensity distribution spot, the intensity distribution spot is double-lobed for the apertured probe when illuminated with linearly-polarized light. This can be understood as a result of charge density accumulation at sharp edges during charge oscillations (confined surface plasmons), which yields electric field enhancement at opposite edges of the aperture. In contrast, the electric field from the offset apertured probe is locally enhanced only at the apex of the tip and thus a spatially uniform ‘focus’ is expected. Note that a single-lobed spot is produced by an offset-apertured probe regardless of whether the probe has a solid silver or silver-coated dielectric tip, consistent with the findings of Novotny et al. .
The main detriment of the double-lobed energy distribution is the likelihood of imaging artifacts while imaging a single sharp scattering edge where the resultant captured image would yield two closely-spaced edges. Although the double-lobed spot can be remedied by distancing the apertured probe from the sample, both the spatial resolution and the light intensity would be significantly reduced due to strong diffraction and the reduced contribution of the fields localized on the edges of the aperture, respectively. Such limitations are easily overcome by both OAMA and OAMDA NSOM probes.
One key aspect of an OAMA probe’s geometry is the distance, L, on the tip’s surface over which the surface plasmons propagate from the aperture to the apex. This parameter is varied while the other parameters are kept constant at an apex diameter of 40 nm, θ=11.3°, t=100 nm, and d=100 nm. As shown in Fig. 4(a), the total intensity, Itot, exhibits a strong dependence on L, where maxima occur at L=3.61, 3.99, and 4.36 µm. The attenuation of the maximal total intensities with increasing L is due to resistive losses of the silver film. The λ/2 spatial periodicity of the maximal values of Itot is indicative of interference between outgoing and reflected surface plasmon waves. Here, the OAMA probe’s cantilever aperture can be considered to be a source that launches outgoing surface plasmon waves along the probe tip’s surface toward its apex. Due to the silver surface discontinuity at the tip’s apex, these surface plasmon waves encounter an abrupt impedance discontinuity at the apex’s silver-air interface. At the tip’s apex, surface plasmon waves reflect and interfere with outgoing waves. As such, the location of the intensity maxima can be predicted using
where λSP is the surface plasmon wavelength and n is an integer. Given the above Lmax values, the surface plasmon wavelength is calculated to be λSP=760 nm, similar to the expected value of 780 nm for a semi-infinite silver slab having a planar silver-air interface.
Notably, the full-width half-maximum (FWHM), defined as the width that encompasses half of the total power under the intensity distribution, varies sinusoidally with L as shown in Fig. 4(b). This definition for the FWHM is adopted in realization of the fact that probing is performed with most of the energy contained within the NSOM’s probe spot. The minimal FWHM of 68±4 nm occurs at L=3.61, 3.99, and 4.36 µm, coinciding with the maximal total intensities. Contrarily, at non-optimal values of L, the intensity of the apex-localized fields is significantly reduced, resulting in a probing optical spot that is defined primarily by the background light. In practice, a minimum tip length is necessary for probing samples having large topographical variations, thus the choice of L=4 µm is representative of a typical small tip.
The cantilever silver layer thickness, t, is of critical importance. Due to the fact that the offset aperture diameter is <λ/2, the offset aperture in the cantilever silver layer is analogous to a cylindrical waveguide having a diameter below cut-off. Since the electromagnetic fields need to tunnel through the thickness of the offset aperture, an exponential dependence of light throughput intensity is expected and shown in Fig. 5(a). In order to prevent light from being transmitted through the cantilever arm, t was chosen to be ≥3δ(≥80 nm), where δ is the radiation skin depth of silver . Interestingly, calculated using t=80 nm, the total intensity is 200 % relative to that from an apertured NSOM probe.
It is essential for any NSOM probe to have high light intensity throughput. However, increasing intensity implies either employing a larger scattering tip apex for an apertureless probe or having a larger aperture opening for an apertured probe. Both situations compromise the spatial resolution of the NSOM probe. Thus, it is essential to decouple the intensity throughput from the spatial spot’s FWHM. Remarkably, the OAMA probe offers such an advantage since the FWHM is limited by the tip apex diameter and not by the offset aperture size. Since the offset aperture is not used for imaging but rather as a light (or surface plasmon) coupler, one can optimize the intensity throughput while still maintaining high spatial resolution as imaging is performed by tip apex. To minimize background radiation, however, the offset aperture diameter must be less than the cut-off for propagating modes (d ~λ/2), thus a maximum diameter of d=320 nm is chosen. Using d=320 nm, the total intensity is 6.6×103 % greater than those of the apertured probe. As shown in Fig. 5(b), the calculated total intensity, obtained 20 nm from the tip apex, is proportional to d 3.84, highlighting the strong dependence on the light throughput from the offset aperture. Moreover, complicated coupling geometries can further increase the throughput through the aperture without affecting the probe’s ability to image deep and narrow topographical features. Although this paper does not study such coupling geometries, various researchers have observed increased throughput through circular apertures in thin metal films by employing concentric ring grooves in metals and dielectrics . However, despite the simple geometry of the offset aperture, 20 % of the power incident on the offset aperture of d=320 nm reaches the tip apex.
The polarization of the incident light waves will have a major influence on the degree of light-surface plasmon coupling as a result of the aperture being adjacent to the base of the OAMA probe’s solid tip. Since only electric fields having components normal to a metal surface can excite surface plasmons, it is expected that the tip will be polarization selective to the incident radiation. As depicted in Fig. 6(a), when the incident light polarization is tangential to the interface between the base of the tip’s cone and the aperture, surface plasmons are weakly coupled at two regions where small components of electric field (perpendicular to the tip surface) exist. These regions are evident on the right half of the tip as shown in Fig. 6(b). In contrast, as illustrated in Fig. 6(c), when the incident light polarization is perpendicular to the interface between the base of the tip’s cone and the aperture, surface plasmons are strongly coupled to the tip surface. As shown in Fig. 6(d), surface plasmons couple to the region on the tip surface directly beyond the aperture. It must be noted that the fields on the left halves of the tips in Fig. 6(b, d) are the result of surface plasmon propagation to the side opposite to the aperture. The total intensity measured 20 nm from the solid tip’s apex for the case of polarization parallel to the interface is only 2.07 % of the value for the case of perpendicular polarization.
4. An offset apertured – metal-coated dielectric apertureless (OAMDA) probe
As shown previously, the OAMA probe offers a significant enhancement in intensity of the probing optical spot with respect to an apertured NSOM. Furthermore, the OAMA probe has a single-lobed probing spot rather than the double-lobed spot of an apertured NSOM. However, despite these advantages, the offset-apertured probe performance can be further improved by replacing the solid silver tip with a silver-coated silicon dioxide tip, resulting in an OAMDA probe. As aforementioned, the silver layer on the silicon dioxide forms an additional surface where surface plasmon coupling can occur.
Light is coupled to surface plasmons through the silver layer, thus the thickness of the silicon dioxide tip’s silver coating, T, is a crucial parameter for efficient coupling. Fig. 7 shows that the total intensity is notably higher for T=60 nm than for larger values of T. In comparison with a solid silver tip having the same t, L, d and θ, the OAMDA probe having T=60 nm enhances Itot by a factor of 300 %. Notably, this indicates that 1/3 of the throughput intensity originates from the offset aperture while 2/3 of the throughput originates from the silver-coated silicon dioxide tip. Remarkably, even greater enhancement (2×104 %) of Itot is realized when the OAMDA probe is compared with the apertured NSOM probe of Fig. 1(b). Further study on perfecting the Kretschmann-like geometry promises to yield even greater light throughput since the current geometry delivers only 3 % of the power incident on the offset aperture and tip base opening to the tip apex.
An important fact to consider is that the dependences characterized for a probe having a solid silver tip may not be well suited for the OAMDA probe since the characteristics of surface plasmon coupling through a metal are different from aperture coupling mechanism. As can be seen, the curves in Fig. 8(a) representing the throughput light intensity and Fig. 8(b) corresponding to the probe’s spot FWHM significantly differ from those of Fig. 4(a) and 4(b), respectively. Interestingly, the highest Itot (2×104 %) and lowest spot FWHM (68±4 nm) values occur at L=4 µm, corresponding to the chosen optimal L value for the solid silver tip. Interestingly, in comparison with the OAMA probe, the OAMDA probe is less affected by changes in incident polarization as this conical tip maintains the same surface plasmon coupling efficiency irrespective of the direction of incident polarization. However, the strength of surface plasmon coupling from its associated offset aperture still varies with the direction of polarization as discussed previously.
5. Offset-apertured probes compared with other near-field microscope probes
Offset-apertured probes’ probing optical spots have been calculated to be significantly more intense than the spot of a typical apertured probe having similar spot FWHM. Considering the geometries studied in this manuscript, the highest optical spot intensity calculated for an offset-apertured probe is 2×104 % greater than the spot of the apertured probe shown in Fig. 1(c). However, as aforementioned, other NSOM probe designs that employ surface plasmons exist, and it is relevant to compare the offset-apertured probe with other such probe designs.
Frey et al. present and study “tip-on-aperture” (TOA) NSOM probes having circular apertures with edge-attached protrusions in [14–15], and Taminau et al. perform further characterization of such TOA probes in . Data from  can be used to compare the optical spot intensity of an optimal TOA probe and the optical spot intensity of an aperture in a 10-nm thick perfect electrical conductor (PEC) for λ=514 nm. A probe having a 100 nmdiameter aperture and a 100 nm-long protrusion having a 40 nm-diameter apex offers 2×104 % greater intensity than an aperture that yields an optical spot of ~100-nm FWHM. It is a certainty that the intensity enhancement offered by a TOA probe would be >2×104 % of the spot intensity yielded by the smaller 60 nm-diameter aperture of the probe in Fig. 1(c). Hence, the TOA probe will have greater intensity than the offset-apertured probes presented in this paper. Similarly, the I-shaped-apertured pyramidal NSOM probe as presented by Tanaka et al. in  yields a calculated probing optical spot that is 280 % more intense than the optimized OAMDA probe presented in this paper.
The offset-apertured NSOM probe geometries presented in this paper have similar, albeit slightly inferior, optical spot intensities with respect to other recent surface plasmon-based NSOM probes. The advantage of the offset-apertured probe over the other probes, as aforementioned, is the long and narrow probing tip that permits the imaging of deep and narrow topographical features. Such topographical features would be difficult to access using the TOA probe’s primarily flat tip apex or the I-shaped-apertured probe’s wide-tapered pyramidal tip. Furthermore, as aforementioned, the offset-apertured probe designs presented in this paper can be improved on by increasing surface plasmon coupling efficiency. To enhance throughput through an offset-apertured probe, various cone angles and more complex geometries surrounding the offset aperture will be studied.
In this paper, two designs of offset-apertured near-field scanning optical microscope (NSOM) probes are presented and studied in detail. Both designs take advantage of light-surface plasmon coupling onto a cantilevered tip, on which they are focused toward the apex of the tip. As a result, they yield probing optical spots of orders-of-magnitude higher throughput light intensity than a typical apertured NSOM having similar spot full-width half-maximum. As an additional advantage over apertured NSOM, the offset-apertured NSOM probe’s optical spot is single-lobed rather than double-lobed. Furthermore, the light throughput has negligible background, circumventing the interference effects that afflict apertureless NSOM operation. The first design incorporates a long and narrow solid metallic tip adjacent to a cantilever arm aperture that couples surface plasmons to illuminate the tip. By placing the large aperture offset from the tip, waveguide cut-off limitations inherent to apertured NSOM probes are avoided. The second design replaces the metallic tip with a metal-coated dielectric tip. Light in the dielectric tip is coupled through the metal coating to surface plasmons on the tip’s external surface in addition to the coupling from the offset aperture. The increased surface plasmon coupling efficiency triples the throughput light intensity while maintaining the spot full-width half-maximum. Further studies involving cone angle and complex geometries surrounding the offset aperture will be performed, potentially improving on the throughput of the presented offset-apertured probes.
Additionally, both probes offer advantages beyond the generation of probing optical spots. Firstly, the probes can be easily fabricated using conventional microfabrication techniques and simple modifications to existing fabrication process flows. Furthermore, the sharpness of the designs’ tips will provide researchers with means to perform optical imaging in deep and narrow topographical features that have otherwise proven inaccessible. Finally, the offset-apertured probes can be used for non-imaging purposes as well. The intense localized illumination of a sample is ideal for precise and accurate manipulation and alteration of the sample that is difficult to achieve using an apertured NSOM’s low light throughput intensity or an apertureless NSOM’s expansive external illumination.
The authors acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would also like to thank V. Kohli of the Ultrafast Optics and Nanophotonics Laboratory, Department of Electrical and Computer Engineering, at the University of Alberta for his suggestions.
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