To measure the transmission properties of nano‐apertures, two instruments are implemented. A transmission‐mode near‐field scanning optical microscope (NSOM) is used to obtain the near‐field intensity distribution from apertures. A far‐field optical system is also set up to measure the transmission through apertures. Comparison of the intensity distribution and the transmission among different apertures can not only demonstrate the characteristics of the proposed design and theory but also provide more information to analyze mechanisms behind the phenomena.
4.2.1 Near-field Scanning Optical Microscope
To resolve an image of an object, optical components are necessary to collect the field of each image point which carries spatial frequency information.
However, because the propagating light diffracts into the far field and the aperture of the component is not infinitely large enough to collect all the diffraction light, the resolving capability of an optical component is limited.
Evanescent or non‐propagating waves that exist only near the object carry more high‐frequency information of the object but have intensities that decay exponentially with distance from the object. Therefore, the spatial resolution beyond the diffraction limit can be obtained if a detector is placed close to the object to detect and make use of evanescent waves with high‐frequency information. This theory gives birth to near‐field scanning optical microscopes (b) collection, (c) illumination collection, (d) reflection, and (e) reflection collection
NSOM can be classified into two types, apertured or apertureless type. An apertured NSOM utilizes a tapered and metal‐coated fiber probe with an aperture having a diameter less than a wavelength on the tip. The fiber tip
(a) (b)
functions as a near‐field light source to illuminate the sample or a near‐field detector to collect and transfer evanescent fields into detectable signals in far field. Since the aperture has a finite size, the signal actually is the convolution of the field of the object and the aperture. An apertureless NSOM uses a metallic tip to scatter high‐frequency evanescent fields to be converted into low‐frequency propagating fields that can be detected by a far‐field detector.
The far‐field signals are modulated by perturbations in near field. For an apertured NSOM, there are five primary operation modes, illumination, collection, illumination collection, reflection, and reflection collection, as shown in Fig. . The first two are transmission modes while the remaining three are reflection modes. Therefore, the choice of operation modes depends on sample characteristics, e.g. opaque or transparent, and total amount of light on the sample.
The essential factor to ensure the success of NSOM is the scanning system that drives a fiber tip to fly over sample surfaces at a height of a few nanometers above the surface. Two basic functions are required: capability of precise positioning on the sample surface and accurate servo control for maintaining a constant gap between the tip and the sample surface. A common way to maintain the gap is the shear force feedback method. A fiber probe is attached to one arm of a quartz crystal tuning fork and the other arm of the tuning fork is attached to a piezo‐ceramics oscillator which can oscillate the tuning fork at its resonant frequency. Because of the piezoelectric effect of the tuning fork, i.e. an electrical field generated under pressure and conversely dimensions changed when an electrical field is applied, the oscillation induces an AC signal which can be monitored. When the fiber tip is approaching to the sample surface, the shear force between the tip and the surface damps the oscillation and causes a change in the induced signal amplitude. The dependence of the amplitude change on the distance then is used as a feedback servo signal to maintain the
gap. Full‐range moving and positioning of the scanning head consisting of the fiber probe, the tuning fork, and the piezo oscillator are accomplished by employing a 3‐axis piezoelectric tube.
The extremely low transmission through the aperture of the fiber probe results in a low signal‐to‐noise ratio. Thus, the illuminating light on the sample is modulated by a chopper at a fixed frequency and the optical signal detected by a photomultiplier tube (PMT) is amplified by a lock‐in amplifier at the same frequency to filter out background noise. Moreover, compared to the image obtained at once from conventional far‐field microscopes, the data from NSOM is built point by point. It means that only local image information is taken in a small step and the measured signal is an integral of the collected signal for a finite time period. Therefore, we can only obtain a relative near‐field intensity distribution rather than an image of an absolute illumination distribution from NSOM. Furthermore, the topography also induces a significant influence on optical images because the probe cannot completely follow the contour of sample surfaces and a nonlinear effect of the boundary conditions occurs as the topography changes. It implies that the near‐field intensity distribution from NSOM is not exactly the same as the real distribution of the sample.
Consequently, optical and topographic images must be correlated to prevent misconstruing the data.
The NSOM used in our measurement belongs to Nano‐Photonics Laboratory at Research Center for Applied Science, Academia Sinica. The measurement is conducted in collection mode. The membrane perforated with apertures is installed on a 3‐axis stage and illuminated by a focused beam with a wavelength of 633 nm. The fiber tip scans over the surface of the sample to obtain the near‐field intensity distribution through the subwavelength aperture in the metal film.
633-nm Laser
Collimating Lens
Detector Objective
Lens C-aperture
Substrate
Metal Film
4.2.2 Far-field Transmission Measurement System
A measurement system is designed and carried out to measure the far‐field power throughput. The system configuration is illustrated in Fig. 4‐4. A linearly polarized laser beam with a wavelength of 633 nm is focused on apertures by an objective lens. The substrate that supports the apertures is attached to a holder on a 3‐axis stage so that the apertures can be finely positioned to the focused beam. A collimating lens behind the aperture is utilized to collect and collimate the transmitted light. In the optical path lies a CCD camera to capture optical images through apertures, or an optical power sensor to measure the transmitted power. In the case of oblique illumination measurement, the laser diode and the objective lens are installed on a rotation stage that can rotate with respect to the aperture.
Fig. 4‐4 Configuration of far‐field measurement system
To align the aperture to the focal point of the incident beam, a microscope is necessary to zoom in the illuminated area of the substrate. With the aid of alignment keys around the aperture on the substrate, the position of the focused
spot can be ensured to be on the aperture area. However, the aperture is too small to be seen under a microscope so that we are not sure whether the aperture is exactly in the optical axis and the focal plane of the incident beam.
The real‐time images captured by the CCD camera provide sufficient information to finely tune the position. With respect to the center of the aperture, the misalignment of the incident beam causes a symmetric change in the image pattern. It means that there are two mirror images in opposite positions relative to the center of the aperture. Consequently, by moving the aperture back and forth until two mirror images occur, the middle position of two mirror images will be the target central position.
A laser beam at the focal plane is supposed to have a Gaussian distribution.
However, within the effective area around the aperture, the incident field is assumed to be a plane wave because the focused spot size is much larger than the dimensions of the aperture. In addition, the ratio of the measured transmitted power to the total incident power represents the overall power transmission. To coincide with the power throughput used in the simulation, the central peak of the Gaussian beam is assumed as the incident amplitude at the aperture. The transmitted power is measured and then the power throughput is calculated and compared to the simulation results accordingly.