12/17/1998

The topic of my Ph.D. research involves investigating a near-infrared Near-Field Scanning Optical Microscope (NSOM). The purpose of this research is to broaden the knowledge base of Near-Field Scanning Optical Microscopy both experimentally and computationally. This goal will be achieved through the construction of a constant height near-IR NSOM instrument. This instrument will consequently be utilized to demonstrate the feasibility of spectrally imaging LDL receptors. Furthermore, the development and use of electromagnetic models will be utilized in conjunction with experimental results to explain interesting near-field imaging effects.

The first months of my IGERT fellowship tenure were spent preparing for and taking my Ph.D. qualifying exam. This examination included a written examination from each committee member. These written exams were administered over a two-week period. With the completion of the written examinations, an oral examination was administered. In addition to answering a variety of questions based upon the previous written examinations, a presentation was also made describing my Ph.D. topic and the relevant research involved.

The current NSOM being developed for this dissertation utilizes a fiber-optic probe that is slowly stretched while heating in a fiber pulling apparatus to form a subwavelength tip. The probe is then coated in metal. Once the fiber is made, it is placed over a sample and raster-scanned while a detector beneath the sample collects the transmitted signal. The resolution of the system is dependent on the specific aperture size, source-sample separation distance, and scanning step increment. The basic NSOM system can be visualized in Figure 1 below.

Figure 1: Basic NSOM Setup.

The actual instrument was recently completed as depicted in Figure 2. This NSOM instrument utilizes a Melles Griot Nano-Block M piezo positioning stage in conjunction with a near-IR tunable laser diode source. In order to complete the functionality of the near-IR NSOM, control and data collection/analysis software was written as seen in Figure 3 below. Specifically, Figure 3 demonstrates a program run during the testing of the software where the NSOM probe was left disconnected. This software allows the number of image pixels (steps) and the number of wavelengths to be entered. Once these parameters are entered, the program is executed and one image is produced per wavelength and is plotted as an intensity graph in the upper right-hand portion of the program interface window. Additionally, the data collection can be monitored point by point in the line graph seen in the lower right-hand portion of the program interface window.

Figure 2: Actual Near-IR NSOM Instrument.

Figure 3: Near-IR NSOM Software Interface.

With the completion of both the NSOM instrument and its software, preparations are underway to collect image spectra of both polystyrene and cholesterol samples. Once the spectral images are obtained, the spectra can then be compared to the spectra of the same or similar samples obtained using conventional instrumentation. In this manner, the ability to obtain spectra of LDL receptors will be demonstrated.

Figure 4: Coarse Near-IR Polystyrene Spectrum.

Before the completion of the NSOM software, the near-IR NSOM instrument was utilized to obtain a coarse polystyrene spectrum corresponding to a single image point as seen in Figure 4. This spectrum can be compared to the complete near-IR spectrum in Figure 5, which was collected by a Bran and Lubbe spectrometer. By comparing these two figures, it can be seen that the NSOM instrument reproduced the falling edge of the main 1680nm peak along with the rise of the shoulder to the right of this peak. With the completion of the NSOM program, a new and improved polystyrene spectrum will soon be collected.

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Figure 5: Complete Near-IR Polystyrene Spectrum.

Figure 6: Typical Moment Method Thin-Wire Image.

Although the majority of time during my IGERT fellowship tenure has involved completing the physical near-IR NSOM instrument, the electromagnetic computational modeling has also progressed. Prior to my IGERT fellowship tenure, computational electromagnetic models were employed in order to gain a better understanding of the effects inherent in NSOM instruments. The basic problem in understanding near-field optics involves the electromagnetic scattering characteristics of sample objects illuminated within the near field of a subwavelength aperture. To this end both Moment Method (MM) and Finite Difference Time Domain (FDTD) models were developed to analyze a transmission mode NSOM. Consequently, these models have been utilized to analyze such situations as aperture size, aperture/sample separation distance, raster scan step size, as well as various incident wave polarization. In particular, thin-wire samples were analyzed as seen in Figure 6 below. These thin-wire samples produced an interesting result whereby the transmitted power incident upon the detector is actually greater when the probe is directly behind the wire. This effect will be studied further using both the near-IR NSOM and an X-band NSOM setup. The IGERT fellowship has proven extremely valuable for this purpose since it will allow access to conductive nanotubes that will serve as near-IR thin-wires.

While the current versions of these models work well to produce simulated, the FDTD model is currently being improved. Specifically, the current model utilizes a second-order Higdon absorbing boundary conditions (ABC) to absorb the outward propagating waves thereby simulating an infinite free-space environment. The Higdon ABCs work well for small scattering objects but can result in asymmetric currents and patterns when the edges of large objects approach the problem space's boundary. In order to alleviate this difficulty, a better ABC, known as a perfectly matched layer (PML), is currently being implemented. Once the FDTD code is updated, it will be used in conjunction with the MM model to simulate a variety of NSOM samples. These simulations can then be compared to actual NSOM images.

In conclusion, the first semester of my IGERT fellowship tenure has proven to be extremely productive. In short, I completed my Ph.D. qualifying examination, completed the NSOM instrument hardware, completed the NSOM instrument software, and began to enhance the current FDTD model. At this point, the research is on the verge of producing results. These results will include obtaining relevant spectra and spectral images. In this manner, the feasibility of spectrally imaging LDL receptors will be demonstrated. Furthermore, the implementation of the aforementioned computational models can then be utilized to obtain results that, when compared to specific experimental results, can and will provide greater insight into explaining the near-field effects evident in NSOM instruments, particularly those seen for thin-wire samples. It is anticipated that, once these results are obtained, several papers, which are currently being written, will be submitted for publication during the spring of '99.