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Prof. Xiang Zhang's Laboratory at UC Berkeley |
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Surface plasmon interference Objective To investigate the physical properties for surface plasmon interference and to develop a new method for nanolithography. Background The surface plasmons (SPs) are electromagnetic waves which are resonantly oscillating of free electrons on the metal dielectric interface. Since the momentum of SP is larger than that of the light in the surrounding dielectric, SP is nonradiative and bounded to the interface with smaller wavelength than the excitation light. The SP field is exponentially decaying on the distance from the interface but is free to propagate along the metal surface with relatively long distance which is finally determined by the surface roughness and Ohmic losses. This property has led to the realization of 2D plasmonic component such as mirrors, waveguides, and lenses and so on.
Owing to the collective excitations of conduction electrons in metals such as Au, Ag, and Al, εm for these metals is strongly wavelength dependent and negative at optical frequencies. The wave vector of surface plasmons can thus become significantly larger than that of the free space light at the same frequency when the real part of εm approaches εd. The frequency at which Re(εm) =εd is called the resonant surface plasmon frequency ωsp. At frequencies close to ωsp, surface plasmons possess “an optical frequency, but a down to X-ray wavelength”. As a result, utilizing surface plasmon waves for lithography may dramatically increase the pattern resolution. Results We have numerically demonstrated the surface plasmon interference nanolithography (SPIN). Multiple 1D gratings are used to convert free-space light into surface plasmon waves, and those waves propagating outside the grating area form an interference pattern when they encounter each other. By using a different number of gratings or surface plasmon waves, various interference patterns such as periodic lines and 2D dot arrays can be obtained. The numerical results show that the resolution can go far beyond the free-space diffraction limit by tuning the excitation light frequency close to the resonant surface plasmon frequency. This SPIN technique promises various practical fabrication applications since it only requires UV photoresists.
Two parallel gratings are used to excite two counter propagating surface plasmon waves. The electrical field distribution (Figure 1) clearly shows that a standing wave or an interference pattern with a ~100 nm periodicity. Under the exposure of this spatially modulated electromagnetic field (Figure 1d), parallel lines with approximately sinusoidal profile and 100-nm periodicity can be developed in the photoresist. If the half peak width is taken as the feature size, features about 1/7 of the excitation wavelength could be reached. If two gratings are considered as partially reflective mirrors similar to those in a laser cavity, it can be expected that the interference contrast will be affected by the grating separation. The simulation results show that the L change will only slightly change the contrast of the interference pattern. This stable performance mainly comes from low reflectivity from the grating and exponentially decaying property of surface plasmon.
The resolution of SPIN can be tuned by varying the excitation frequency. When the incident light frequency is tuned to be close to the plasmon resonant frequency ω sp, a slight increase in the excitation frequency could lead to significant decrease in the SP wavelengths, which significantly increase the SPIN resolution. However, it comes with the cost of reduction of the plasmon wave decay length. In addition to the 1D interference fringes, 2D patterns such as dot arrays can be obtained by crossing more than two surface plasmon waves (Figure 3). Reference: Zhao-Wei Liu, Qi-Huo Wei, and Xiang Zhang, "Surface Plasmon Interference Nanolithography", Nano Letters, Vol 5, No. 5, pp957-961, 2005
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