Overview

Our efforts are focused on developing a novel sensor, the Surface Plasmon Band Gap sensor (SPRBG), which is based on the properties of the propagation of surface plasmon waves through periodic nano-structures. This sensor offers the possibility of bringing the very powerful and versatile surface plasmon sensing technology on the field for demanding sensing environments such as ground water nitrate monitoring or algae plumes detection in seawater. Furthermore, since surface plasmon sensors can be functionalized, this sensor, once developed, can become a powerful center-wide sensing platform that can be used for a wide range of chemicals and biological markers. The possibility of batch fabrication makes it also a very attractive candidate for use in large scale sensor networks.

Systems/Experiments

Our thrust has been divided into two complementary components, a numerical and an experimental one.

Numerical effort: The purpose of the numerical part is to prove the concept of the sensor and to optimize its characteristics without having to resort to systematic experimentation, difficult because of the challenge of nanostructure fabrication. We implemented two codes, one simulating the behavior of electromagnetic fields on arbitrary interface (C method), the other simulating a stack of periodically modified layers (RCWA, Figure 1). This year, we considerably developed the latter code, which has now become a very powerful analysis tool. It can now calculate the intensity of the reflection and transmission of light on an arbitrary periodic structure, thanks to a redesigned stair-case approximation algorithm. Another critical feature that has been implemented this year is the ability to calculate the field inside the grating, which is primordial in understanding the intimate physics of the system.

Figure 1: Numerical Simulations. Left: Schematic of the numerical experiment. Our code can now simulate an arbitrary structure by approximating it with a stair case algorithm designed to minimize the number of layer, critical for fast calculation. Right: Example of the results for the simulation of a 40 nm thick silver slab on top of a glass prism in air. The gray shading represents the intensity of the reflected beam versus the angle of incidence (x-axis) and the wavelength (y-axis).

 

Experimental effort: We entirely redesigned our experimental system. When the first generation was focused on qualitatively understanding the propagation of surface plasmon through nanostructures, this last generation was designed from the bottom up with a prospective of quantitative data collection. Restarting from tabula rasa, we overcame the limitations that we encountered in the first generation by completely interfacing the experiment with computer controls, and by introducing some better optical components. For example, the fiber optic based spectrometer that is now used has considerably improved the quality of our qualitative measurements without compromising the portability of the system.

 

Problems Encountered

Numerical effort: The numerical part is extremely satisfactory. The only problem we faced was due to the nature of the code, which emulates an infinite surface when experiment shows that edge effects might be important. Also, another inherent difficulty with our simulation algorithm is that the stair case approximation used to simulate an arbitrary smooth surface introduces some sharp angles in the system that can yield to divergences in the field. This is a great limitation that impedes greatly the convergence speed of our code.

 

Experimental effort: The grating is a critical part of the system and its fabrication is maybe the main challenge of this project. It requires the controlled fabrication of features at the 100 nm scale, which is impossible using conventional optical lithography. Therefore, we experimented with four techniques, e-beam lithography, nanoimprinting, nanostamping and holographic lithography.

 

Accomplishments

Numerical effort: The numerical simulations have been extremely promising and they proved not only the concept of our sensor but also enabled the comparison of the performances of our sensor versus other existing sensors. We have also been able to compare the performance of different geometries and configurations. Figure 3 is an example of the results obtained in the simulation of the optical field intensity in the nanostructure. Figure 4 is an example of the comparison of the sensitivity of our sensor versus a traditional SPR sensor, in the case of a perfectly collimated optic and in the case of a more realistic optical situation of a slightly divergent beam.

Figure 3: Numerical Simulations in the surface plasmon band gap situation. (a): Reflectivity of light versus angle of incidence and wavelength. The surface plasmon band gap is clearly visible; it separates two modes of different energy. (b): Field intensity inside the nanostructure for the high energy mode: the field is concentrated in the air. (c) Field intensity inside the nanostructure in the low energy mode: the field is concentrated in the dielectric.

Figure 4: Numerical Comparison of the sensitivity of our sensor (SPBG) and of the traditional Surface Plasmon Resonance (SPR) sensor for (a) a perfectly collimated beam and (b) for a beam with 3 degrees of divergence, likely to be the case in an field implementation.

 

Experimental effort: Our second generation setup can directly measure the propagation of surface plasmon through nanostructures and is extremely close to the very specific situation that is modeled in the numerical experiments. Figure 5 are experimental results that are to be compared with the numerical results of Figure 4. We can clearly see the presence of the surface plasmon band gap, which validates the quality of the nanofabrication. Furthermore, we included in the system a fluidic cell which allows us to vary the optical index of the medium to be detected. At the time of this report, some measurements in fluid have been made and the first experiments on optical index measurements will be done shortly.

Figure 5: Experimental measurement of the reflectivity of (a) a metallic slab and (b) a periodic nanostructure. These results are to be compared with the numerical calculation presented in Figure VI-4.

Figure 6: Nanostamping technique. (a) Stretching setup enabling the control of the period of the nanostructures. (b) Process flow. (c) AFM picture of the nanostructure.

Nanofabrication: Out of the four technologies investigated, two provided excellent results. The nanoimprinting technique can reproduce existing grating structures with the possibility of changing the period of the structure in an extremely easy and low-cost way, suitable to mass fabrication of the sensor in a large scale network. The holographic lithography technique is more time consuming but allows the fabrication of these nanostructures ab-initio. The combination of these two techniques yields to a very attractive nanofabrication platform, where the holographic lithography is used for the fabrication of the masters and prototypes and the nanoimprinting is used for massive replication.

Some numerical results have been presented at the Bio-Nano-Info Fusion conference (July 2004 in Marina Del Rey, CA). The concept of the sensor has been presented at the IEEE NEMS conference in Zhuhai, China. A paper on one of the fabrication technique (nanoimprinting) has been submitted for publication. A patent is pending for the SPBG ("Plasmonic Band Gap Sensor" 60/709,676 filed on August 18, 2005).

Future Directions

The last challenge for the demonstration of the sensor concept is to experimentally measure the sensitivity using a variation of optical index in the bulk medium. Then, we will functionalize the surface and demonstrate specific detection.

People

Chih-Ming Ho, Professor, UCLA
Arnaud Benahmed, GSR, UCLA
Nikolaus Rechner, Undergrad, UCLA
Robert Lam, Undergrad, UCLA