Manipulating light at the nanoscale

Plasmonics is an enabling technology that incorporates noble metals to guide and manipulate light at optical frequencies on the surface of the metals. It allows the nanoscale confinement of light in gaps between two metal surfaces. This tight confinement is not possible by conventional waveguiding techniques. Plasmonics provides a new paradigm for miniaturizing photonic devices to nanoscale dimensions and possibly making tiny devices as small as transistors. This miniaturization is not possible with conventional materials. Nanoscale plasmonics would allow us to have optical and electronic components on the same chip. Optical components can give fast transmission and data processing at the speed of light. In fact, using optical interconnects to replace the electrical ones would solve the problem of limited bus transmission speed between processor cores due to parasitic losses in the copper wires. Nanophotonics also supports a high density of integration (large number of functional devices over a very small area) for a wide range of on-chip applications such as optical interconnects, multisensor chips, and lab-on-a-chip devices.

Over the last few years, we have developed a good coupling mechanism between a silicon waveguide with a 500nm core size, and a plasmonic slot with a 50nm slot size.^{1, 2} This coupling scheme allows novel nanoscale devices such as filters, sensors, polarization rotators, tunable filters (see Figure 1), power splitters, and demultiplexers to be made with plasmonic technology.^3–12 We have also devised a simple analytical modeling tool that has good accuracy and allows for fast simulations of these devices.^3–13

Figure 1. Schematic diagram of the plasmonic tunable filter. This filter is based on Fabry-Pérot resonance between the metal stubs. Changing the applied voltage should change the refractive index of the electro-optic dielectric material inside the filter, and hence the peak wavelength.

Plasmonic waves propagate on metals with negative permittivity, which enables tight confinement and unique wave properties. This effect is used in plasmonic lens design where double nanoholes are surrounded by optimized diffraction gratings. The nanoholes couple part of the incident light to a surface plasmon wave traveling on the grating surface. These lightwaves interfere constructively with each other and with the light passing directly through the hole to create a very high intensity spot of nanoscale size. We have designed transit gratings that last for picoseconds, allowing ultrafast focusing.¹⁴ The size of these spots can be much smaller than the wavelength, and this creates a superfocusing effect (it only occurs when the spot size is smaller than half the incident wavelength of the light).

More recently, we developed a superlens design that is simple and easy to fabricate (see Figure 2).¹⁵ As this superfocusing effect can be exploited to create a high field gradient that would generate a Lorentzian force on any particle of nanosize within the light spot, we can use these nanoscale optical tweezers to trap, manipulate, and move nanoscale particles. Drug particles, for example, could be delivered to only the infected area using our lens. This selective drug delivery is of prime importance in cancer treatment.

Figure 2. (a) Schematic of the novel plasmonic superlens proposed. Incident light (black arrows) interferes with surface plasmons (blue arrows) due to the double nanohole and diffraction grating arrangement, creating a superfocused light spot. (b) Superfocusing produced by the device in the deep UV at a wavelength of 186.716nm with full width at half-maximum of 64nm.

The strong field localization created by plasmonic waves can also increase solar cell efficiency. Sunlight creates a localized plasmonic field inside the absorption layer. Conventional noble metals are expensive to use in such an application, and increase the cost per watt of the solar cell. Thus, the search for new, cheap materials that can create plasmonic effects is in progress. We recently proposed the use of titanium nitride (TiN) as a plasmonic material for solar cell applications. We show that the plasmonic solar cell created using TiN is as efficient as those created using silver, but much cheaper in cost and much less sensitive to temperature changes (see Figure 3).¹⁶

Figure 3. Cross section of a plasmonic solar cell. Dotted lines show the unit cell. The silver (Ag), titanium nitride (TiN), zinc oxide (ZnO), amorphous silica (a-Si), and indium tin oxide (ITO) layers can be conformally deposited in turn on top of the silica substrate. To find the best solar cell, several sizes and symmetries of the shaded unit cell were studied.¹⁶

The negative permittivity of noble metals such as gold, silver, aluminum, and copper in the visible and near-IR ranges allows these metals to support plasmonic waves. We recently proposed a new platform based on using a highly doped semiconductor (see Figure 4) that can easily create a mid-IR plasmonic wave.¹⁷ An increase in sensitivity due to plasmonic waves is useful, because this range contains absorption peaks for a wide variety of gases and biomedical substances. Thus, using highly doped silicon, for example, we could make tiny, mid-IR sensors for health and environmental applications.

Figure 4. Slot waveguide structure with gap width, W_gap, rib width, W_rib, and height, H. Inset shows mid-IR plasmonic wave.¹⁷ Waveguides like this could have applications in biodmedical sensing.

Nanoplasmonics is a growing field with many interesting applications, such as miniaturizing photonic devices, speeding up computer processors, drug delivery via optical tweezers, novel biomedical sensors operating in the IR range, and increasing solar cell efficiency. Our next step is to increase the functionality of our proposed nanosystems by integrating more functional devices (sensors, microfluidics) on the same chip. We also aim to reduce both the cost and size of our nanosystems, as well as increase their performance and efficiency.