ALEXANDRA BOLTASSEVA
Purdue University and Nano-Meta Technologies, Inc.
URCAN GULER
Nano-Meta Technologies, Inc.
The influence of optical technologies on the development of modern society cannot be overestimated. From conventional mirrors, lenses, microscopes, telescopes, optical sensors, and high-precision measurement systems to lasers, fiber-optic communications, and optical data storage systems, optical instruments have enabled revolutionary advances and novel concepts in many disciplines, including astronomy, manufacturing, chemistry, biology (particularly bio- and chemical sensors), medicine (particularly ophthalmology and optometry), various engineering fields, and information technology.
INTRODUCTION
The rise of a new generation of optical technologies is fueled by the emergence of nanophotonics, the study of the behavior of light at the nanometer scale and the interaction of nanometer-scale objects with light. A major focus of the field is development of a new class of plasmonic structures and “metamaterials” as potential building blocks for advanced optical technologies.
Plasmonics involves metallic components that can focus and manipulate light at the nanometer scale via excitation of surface plasmons, collective oscillations of free electron clouds (found in metallic materials) coupled to light (Maier 2007). Plasmonics can “squeeze” light into tiny volumes much smaller than the wavelength of light, thus offering light confinement beyond the usual diffraction limit as well as extreme light enhancement in nanometer-scale areas known as plasmonic “hot spots” (Lindquist et al. 2013; Schuller et al. 2010).
Plasmonic devices are expected to transform optoelectronics, microelectronics, on-chip optical communication, and data transmission by enabling low-power,
nanometer-scale photodetectors; fast light modulators; and nanoscale, power-efficient lasers and light sources. Plasmonics is paving the way for optical microscopy and photolithography with nanometer-scale resolution, novel concepts for data recording and storage, improved energy harvesting through optimized light-capturing techniques, single-molecule sensing, and advanced spectroscopy.
PLASMONIC CONSTITUENT MATERIALS: CHALLENGES
It is now possible to design plasmonic structures and metamaterials—artificial composite surfaces or materials that use plasmonic building blocks as their functional unit cells—with versatile properties that can be tailored to fit almost any practical need. But new plasmonic technologies will require the resolution of significant limitations associated with the use of metals as constituent materials. There are difficulties in the fabrication and integration of metal nanostructures with existing semiconductor technology, and the materials need to be more precisely tuned to have the optical properties needed for the required functionality.
Current Materials
In the devices demonstrated so far, too much light is absorbed in the metals (e.g., silver and gold) commonly used in plasmonic structures and metamaterials. Moreover, such metals are soft materials with relatively low melting points. Thus plasmonic devices cannot meet the challenges that real industry applications face, particularly those characterized by high operational temperatures (e.g., energy harvesting or data recording applications, as explained below), high pressure, and harsh chemical environments. For these reasons, plasmonics is a “what happens in the lab stays in the lab” area of research.
The development of practical devices utilizing plasmonic concepts will depend on new constituent materials that both exhibit the plasmonic properties required to capture and manipulate light at the nanoscale and provide durable, chemically, mechanically, and thermally stable solutions for the realization of rugged optical instruments.
New Materials: Metal Nitrides
Plasmonic ceramic materials have recently been proposed as the basis for practical, low-loss, complementary metal-oxide semiconductor (CMOS)–compatible plasmonic devices, an important advance for the field (Naik et al. 2013).
Transition metal nitrides such as titanium nitride (TiN) and zirconium nitride (ZrN) have been suggested as refractory plasmonic materials that are capable of sustaining high temperatures (the TiN melting point is 2930°C) and exhibit good optical properties along with bio- and CMOS compatibility, robustness, chemi-
cal stability, corrosion resistance, mechanical strength, and durability (Guler et al. 2014a, 2015b). The attractiveness of TiN for practical device applications is illustrated by its extensive use in semiconductor manufacturing (as both a gate layer and a diffusion barrier), large-scale integrated microelectronics, microelectromechanical systems (MEMS), and biotechnology.
APPLICATIONS OF PLASMONICS
Energy Conversion
Improved efficiency of light harvesting is among the major engineering challenges for the upcoming decade. Photovoltaics (PV) is considered an important potential energy source, but its development is hampered by problems connected to efficiency and stability degradation when the device heats during the absorption of solar radiation.
Various techniques based on plasmonic effects have been proposed to improve solar cell efficiencies via field concentration and hot electron generation (Polman and Atwater 2012). Plasmonic metamaterials have been investigated as broadband absorbers and spectrally engineered emitters for solar thermophotovoltaic (STPV) systems (Li et al. 2014; Molesky et al. 2013). The STPV concept involves a perfect absorber designed for broad absorption of solar radiation while a selective emitter (designed to emit light in a narrow energy band just above the semiconductor bandgap in the PV cell) can be heated by the absorber through an intermediate layer or via chemical, nuclear, or waste heat sources (Bauer 2011; Fan 2014).
The beauty of the STPV approach is that the system can be used in a hybrid mode (hence the name), but high-temperature operation again introduces the problem of material degradation (Guler et al. 2015a). High operational temperatures (well above 800°C) have hindered STPV progress because of low melting points for noble metals, poor optical performance, and lattice imperfections for refractory metals.
Refractory plasmonic ceramics such as TiN represent a unique platform for realizing STPV as an energy conversion concept that promises efficiencies of up to 85 percent (Guler et al. 2015a). TiN absorbers have already been shown to provide high optical absorption (about 95 percent) over a broad range while being extremely durable under exposure to heat and strong illumination (Li et al. 2014).
Figure 1(a) gives a schematic representation of an STPV system and exemplary absorber and emitter metamaterial designs. Figure 1(b) shows the absorption spectra measurements from identical plasmonic metamaterials made of TiN and gold (Au). TiN metamaterial provides broader absorption and retains its optical properties after 8 hours of annealing at 800°C, whereas Au metamaterial has narrower resonance peaks and its absorption properties degrade after just 15 minutes annealing at the same temperature (Li et al. 2014).
FIGURE 1 (a) A solar thermophotovoltaic system consists of a broadband solar absorber (top left) and a spectrally selective emitter (bottom left) engineered to match the bandgap of a photovoltaic cell. Adapted with permission from Guler et al. (2015a). © Elsevier (2015). (b) Titanium nitride (TiN) metamaterial provides better absorption compared to an identical gold (Au) absorber and retains its properties after exposure to high temperatures (800°C). Adapted with permission from Li et al. (2014). © John Wiley & Sons, Inc. (2014).
Durable, refractory TiN also holds great promise to enable efficient, TPV-based waste heat recovery (Bauer 2011). Efficient heat energy harvesting could have a transformative effect on a number of industries (e.g., metal casting, aerospace, and gas and oil) and lead to the development of fossil fuel–based power generation (including diesel and gas engines), radioisotope-based cells, fuel-fired cells, and portable power generators for civil and military needs. TiN properties are also well suited for solar thermoelectric generators (Kraemer et al. 2011), plasmon-mediated photocatalysis (Clavero 2014), and plasmon-assisted chemical vapor deposition (Boyd et al. 2006).
MEDICAL TREATMENT
Other heat-generating applications of plasmonic nanoparticles are in health care. Photothermal therapy utilizes a unique property of metallic nanoparticles to concentrate light and efficiently heat a confined nanoscale volume around the plasmonic structure (Loo et al. 2004). Nanoparticles thus delivered to a tumor can be heated via laser illumination at near-infrared wavelength in the biological transparency window. Hyperthermia is known to induce cell death in diseased and other tissues and has been shown to increase both local control of treatment and overall survival in combination with radiotherapy and chemotherapy in randomized clinical trials.
Gold nanoparticles are emerging as promising agents for cancer therapy and are being investigated as drug carriers, photothermal agents, contrast agents, and radiosensitizers. But gold nanoparticles resonate at light wavelengths that lie
outside the biological transparency window and therefore require larger dimensions and complex geometries such as nanoshells and nanorods (Huang et al. 2008). The larger sizes in turn affect the nanoparticles’ pharmacokinetics, bio-distribution, and in vivo toxicity.
TiN-nanofabricated particles have been shown to exhibit both plasmonic resonance in the biological transparency window and higher heating efficiencies than gold (Guler et al. 2013). More importantly, TiN obviates the need for complex geometries and provides simple, small-size particles that optimize cellular uptake and clearance from the body after treatment (Guler et al. 2014b).
Medical Imaging
Figure 2(a) shows a high-resolution transmission electron microscope image and optical transmittance data from a colloidal single crystalline TiN sample. Lattice parameters of the nanoparticle closely match the tabulated single crystalline bulk values of TiN samples, and the optical transmittance data show the plasmonic extinction dip at the biological transparency window.
Figure 2(b) shows a comparison between the calculated absorption efficiencies of TiN (left-hand graph) and Au (right-hand graph) nanodisks. The dipolar resonance peak of Au is located around 520 nm, where the excitation light is strongly attenuated in biological samples. The TiN dipolar resonance peak, at about 800 nm, allows the use of small particles.
TiN is a very contamination-safe material (and therefore widely used in surgical tools, food-contact applications, and medical implants), so TiN colloidal particles could become a next-generation solution for tumor-selective photothermal therapy, medical imaging, and other biomedical applications.
Data Recording
Refractory plasmonic materials are considered the best candidates for applications that require nanometer-scale field enhancement and local heating. An example of such an application is heat-assisted magnetic recording (HAMR; Challener et al. 2009), a nanophotonic next-generation data recording technology that will significantly increase the amount of data on a magnetic disk by using a laser light tightly focused on a magnetic material. The tight focusing to a subwavelength spot is achieved via a plasmonic nanoantenna.
In contrast to noble metals that are prone to deformations such as melting and creep because of material softness and melting point depression in nanostructures, any degradation of refractory plasmonic materials can be avoided with the proper material integration (Guler et al. 2015a; Li et al. 2014). TiN antennae have recently been shown to satisfy the stringent requirements for an optically efficient, durable HAMR near-field transducer, paving the way for next-generation data recording systems (Guler et al. 2015a).
FIGURE 2 (a) Actual measures of lattice parameters of a single crystalline titanium nitride (TiN) nanoparticle closely match calculated values from bulk TiN samples. The graph below shows the transmittance data from a colloidal TiN sample with a plasmonic extinction dip at the biological transmittance window. a. u. = arbitrary unit. Adapted with permission from Guler et al. (2014b). (b) Absorption efficiencies calculated for TiN (left) and gold (Au; right) nanodisks according to the design at the top of the figure. Small TiN nanodisks provide enhanced absorption at 800 nm (dashed vertical line) while large nanodisks of Au are required at the same wavelength because of spectral mismatch. Qabs = absorption efficiency (optical cross-section normalized by the geometric cross-section). Adapted with permission from Guler et al. (2013). ©American Chemical Society (2013).
More generally, the use of refractory plasmonic ceramics can greatly expand the realm of tip-based applications, including near-field scanning optical microscopes and other local field–enhanced signal measures, opening up measurement capacities in previously unavailable frequency ranges and operational regimes (Boltasseva and Shalaev 2015; Guler et al. 2014a).
Other Applications
The durability and refractory properties of TiN and ZrN could also make them the only material building block for high-temperature, harsh environment optical sensors, flat photonic components such as ultrathin lenses, and spatial light modulators using the concepts of the emerging field of metasurfaces. Refractory flat optical components last longer in harsh environments, provide more reliable
data, and offer ultracompactness combined with a planar fabrication process that is large-scale, robust, and low-cost. In oil and gas industries, ultracompact, extremely durable plasmonic sensors could replace electrical sensors and enable novel measures for pressure, flow, drill bit temperature, and breakage detection.
The thermal, mechanical, and chemical stability of TiN, together with its high conductivity and corrosion resistance, make it an ideal material for nanofabrication. TiN can be used for making ultradurable imprint stamps with unparalleled hardness and resistance to wet chemistry processes. When combined with emerging plasmonic nanolithography schemes, TiN films can be used to create durable multiple-use master molds and novel fabrication concepts for large-scale sub-10-nanometer resolution patterning.
Finally, CMOS-compatible refractory plasmonic materials are considered a platform for next-generation on-chip hybrid photonic-electronic devices such as subwavelength photodetectors, optical interconnects, and modulators with unprecedented compactness, speed, and efficiency (Kinsey et al. 2015).
CONCLUSION
With an excellent combination of hardware performance properties, appealing optical properties, durability, and contamination safety, plasmonic ceramics in general and TiN in particular have the capacity to enable highly robust, ultracompact, CMOS-compatible optical devices capable of addressing numerous application-specific challenges. As such, they are promising building blocks for advanced optical technologies, including data processing, exchange, and storage; new concepts for energy conversion, including improved solar cells; nanoscale-resolution imaging techniques; a new generation of cheap, enhanced-sensitivity sensors; and novel types of light sources.
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