Coherent control of nonlinear and ultrafast plasmon-polariton mediated interactions has attracted wide attention for its potential for enhancing functionality in nano-scale photonic devices and applications. Contemporary research in ultrafast and nonlinear plasmonics primarily utilizes noble metals, such as gold and silver, as material platforms because of their high performance both in linear and nonlinear optical properties. Unfortunately, noble metals possess numerous drawbacks including low melting points, chemical instabilities, and an incompatibility with standard CMOS processing techniques, all of which hamper their incorporation into functional plasmonic devices. Here we investigate the mid-infrared ultrafast and nonlinear properties of the alternative plasmonic material, aluminum-doped zinc oxide (AZO). By performing time-resolved pump-probe spectroscopy, we observe an unprecedentedly large and ultrafast (sub-picosecond) response in AZO thin films for both intra- and inter-band pumping frequencies. These two nonlinearities arise from distinct electron excitation dynamics and, as such, can be controlled simultaneously and independently to provide a novel method of dynamic tunability. We demonstrate this phenomenon with two-color excitation and find our AZO films exhibit a THz modulation bandwidth. We also probed the nonlinear response of AZO films at the epsilon-near-zero (ENZ) frequency and observed a dramatic increase in the Kerr nonlinearity with an induced refractive index change on the order of unity. In summary, our ultrafast and nonlinear studies strongly support AZO as an alternative plasmonic material with qualities pertinent to the development and realization of practical plasmonic technologies.
Plasmonic interconnects as well as metasurfaces manipulating the phase and amplitude of reflected and/or transmitted light, have attracted significant attention in the fields of planar optics and on-chip nanophotonics. The application space of plasmonics has recently been expanded by new materials classes including transparent conducting oxides (TCOs) and refractory transition metal nitrides (TMNs). These materials offer superior thermal stability, robustness, tailorability and CMOS compatibility, thus outperforming the conventional plasmonic components (e.g. gold and silver) in application-specific requirements. Here, we discuss recent progress in the areas of plasmonic interconnects, modulators and metasurfaces realized using TMNs and TCOs.
Ultrafast nonlinear responses near the epsilon near zero (ENZ) region have been recently demonstrated for Al doped zinc oxide (AZO) (TCO). This has spurred the development of ultrafast, on-chip modulators with TCOs as an active material. Building from on our previous work on LRSPP waveguides with ultrathin Titanium Nitride (TiN) (5.5mm propagation length) and Zirconium Nitride (ZrN), we will report solid-state hybrid mode waveguides using TiN as well as a modulator based on this waveguide which provides all-optical tunability, low optical losses at the telecommunication window, and CMOS-compatibility.
Successful integration of these alternative plasmonic components into the phase gradient metasurfaces platform without loss of performance have also been demonstrated. A metasurface employing ZrN brick antennas shows the photonic spin Hall Effect (PSHE), by reflecting the two circular polarizations in different directions. In another device, a metasurface based on nanostructures of Ga doped ZnO (TCO), functioning as a quarter-wave plate, have been realized.
Graphene has been demonstrated to be a promising photodetection material because of its atomic-thin nature, broadband and uniform optical absorption, etc. Photovoltaic and photothermoelectric, which are considered to be the main contributors to photo current/voltage generation in graphene, enable photodetection through driving electrons via built-in electric field and thermoelectric power, respectively. Graphene photovoltaic/photothermoelectric detectors are ideal for ultrafast photodetection applications due to the high carrier mobilities in graphene and ultrashort time the electrons need to give away heat. Despite all the advantages for graphene photovoltaic/photothermoelectric detectors, the sensitivity in such detectors is relatively low, owing to the low optical absorption in the single atomic layer. In the past, our research group has used delicately designed snowflake-like fractal metasurface to realize broadband photovoltage enhancement in the visible spectral range, on SiO2 thin film backed by Si substrates. We have also demonstrated that the enhancement from the proposed fractal metasurface is insensitive to the polarization of the incident light. In this current work, we have carried out experiments of the same fractal metasurface on transparent SiO2 substrates, and obtained higher enhancement factor on the fractal metasurface than that achieved on SiO2/Si substrates. Moreover, the device allows more than 70% of the incident light to transmit during the detection, enabling photodetection in the optical path without any significant distortion. Another possibility to make use of the large portion of transmitted light is to stack multiple such devices along the optical path to linearly scale up the sensitivity.
Graphene has been demonstrated to be a promising photo-detection material because of its ultra-broadband absorption, compatibility with CMOS technology, and dynamic tunability. There are multiple known photo-detection mechanisms in graphene, among which the photovoltaic effect has the fastest response time thus is the prioritized candidate for ultrafast photodetector. There have been numerous efforts to enhance the intrinsically low sensitivity in graphene photovoltaic detectors using metallic plasmonic structures, but such plasmonic enhancements are mostly narrowband and polarization dependent. In this work, we propose a gold Cayley-tree fractal metasurface design that has a multi-band resonance, to realize broadband and polarization-insensitive plasmonic enhancement in graphene photovoltaic detectors. When illuminated with visible light, the fractal metasurface exhibits multiple hotspots at the metal-graphene interface, where the electric field of the incident electromagnetic wave is enhanced and contributes to generating excess electron-hole pairs in graphene. The large metal-graphene interface length in the fractal metasurface also helps to harvest at a higher efficiency the electron-hole pairs by built-in electric field due to metal-graphene potential gradient. To demonstrate the concept, we carried out experiment using Ar-Kr CW laser, an optical chopper, and lock-in amplifier. We obtained experimentally an almost constant ten-fold enhancement of photocurrent generated on the fractal metasurface compared to that generated on the plain gold-graphene edge, at all tested wavelengths (488 nm, 514 nm, 568 nm, and 647 nm). We also observed an unchanged photoresponse with respect to incident light polarization angles, which is a result of the highly symmetric geometry of the fractal metasurface.
Transparent conducting oxides (TCOs) have long been used in optics and electronics for their unique combination of both high transmission and high electrical conductivity. In recent years, the impact of such TCOs has been felt in the subgenre of nanophotonics and plasmonics.1-3 Specifically, the TCOs provide plasmonic response in the near infrared and infrared region,4 epsilon-near-zero (ENZ) properties in the telecom band, tunable static optical properties through deposition/annealing control,5 and the potential for dynamic control of their properties under electrical or optical biasing.6-8 Due to the combination of these interesting properties, TCOs such as In:SnO (ITO), Al:ZnO (AZO), and Ga:ZnO (GZO) have become leaders in the drive to produce high-performance dynamic and alternative nanophotonic devices and metamaterials. In our work, we have studied the potential for optical control of AZO thin films using both above bandgap and below bandgap excitation, noting strong changes in reflection/transmission with enhancement due to the ENZ as well as ultrafast response times less than 1 ps. Using a photo-modified carrier density and recombination to model above bandgap excitation, we demonstrated 40%/30% change in the reflection/transmission of a 350 nm AZO film with an 88 fs recombination time, corresponding to a modification of the carrier density by 10%.6 Below bandgap excitation has experimentally shown the potential for similar variations in the reflection and transmission under increased fluences with a factor of ~8x increase in the normalized ΔR at ENZ. Current efforts are focused to model the material response as well as to investigate electrical modulation of AZO films. In summary, our work has demonstrated the potential for optical control of AZO films both above and below bandgap on an ultrafast timescale which can be enhanced through ENZ. Combining this with traditional nanophotonic and metamaterial devices opens a broad range of high impact studies such as tunable optical components, on-chip photonic elements, and controllable nonlinear enhancement.
As a result of the significant attention in searching for alternative plasmonic materials for real-life nanophotonic devices, transparent conducting oxides (TCOs) have been proposed as promising constituent building blocks for telecommunication wavelengths. They are eminently practical materials because they are CMOS-compatible, can be grown on many different types of substrates, patterned by standard fabrication procedures, and integrated with many other standard technologies. Due to the ability of TCO nanostructures to support strong plasmonic resonance in the near infrared (NIR), metasurface devices, such as a quarter wave plate, have been demonstrated whose properties can be easily adjustable with post processing such as thermal annealing. Additionally, TCOs can be used as epsilon near zero (ENZ) materials in the NIR. From our recent study of the behavior of nanoantennae sitting upon a TCO substrate, we found that TCOs serve as an optical insulating media due to the high impedance of TCOs at the ENZ
frequency, enabling emission shaping. Finally, the optical properties of TCOs can be varied by optical or electrical means. Current research is focused on studying the ultrafast carrier dynamics in doped zinc oxide films using pump-probe spectroscopy. We have shown that aluminum doped zinc oxide films can achieve a 40% change in reflection with ultrafast dynamics (<1ps) under a small fluence of 3mJ/cm2. Consequently, TCOs are shown to be extremely flexible materials, enabling fascinating physics and unique devices for applications in the NIR regime.
Plasmonics has long been seen as a promising technology for integrated optical devices for many fundamental applications such as telecommunications, chemistry, quantum science, and medicine. However, for these devices to be realized in a large scale, they should be CMOS-compatible – a problem for plasmonic devices which have generally relied on noble metals. Recently, CMOS compatible materials titanium nitride and transparent conducting oxides (such as doped zinc oxide) have been proposed as the most promising materials for telecommunication applications. TiN is a gold-like ceramic material with a permittivity cross-over near 500 nm. In addition, TiN can attain ultra-thin, ultra-smooth epitaxial films on substrates such as c-sapphire, MgO, and silicon. Partnering TiN with CMOS-compatible silicon nitride enables a fully solid state waveguide which is able to achieve a propagation length greater than 1 cm for a ~8 μm mode size at 1.55 μm. In fact, similar designs using TiN have outperformed gold waveguides due in large part to the reduced scattering loss of epitaxial quality films. Utilizing highly doped zinc oxide films as a dynamic photonic material, high performance modulators can also be realized. Together, these alternative materials form the base of a fully integrated nanophotonic system, capable of exceptional performance with speeds greater than 1 THz, in large part due to the development of alternative materials. Consequently, nanophotonic technologies are reaching a critical point where many applications including telecom, medicine, and quantum science can see practical systems which provide new functionalities.
There is a continual need to explore new and promising dynamic materials to power next-generation switchable devices. In recent years, transparent conducting oxides have been shown to be vital materials for such systems, allowing for both optical and electrical tunability. Using a pump-probe technique, we investigate the optical tunability of CMOS-compatible, highly aluminum doped zinc oxide (AZO) thin films. The sample was pumped at 325 nm and probed with a weak beam at 1.3 μm to determine the timescale and magnitude of the changes by altering the temporal delay between the pulses with a delay line.
For an incident fluence of 3.9 mJ/cm2 a change of 40% in reflection and 30% (max 6.3dB/μm modulation depth) in transmission is observed which is fully recovered within 1ps. Using a computational model, the experimental results were fitted for the given fluence allowing the recombination time and induced carrier density to be extracted. For a fluence of 3.9 mJ/cm2 the average excess carrier density within the material is 0.7×10^20cm-3 and the recombination time is 88fs. The ultrafast temporal response is the result of Auger recombination due to the extremely high carrier concentration present in our films, ~10^21 cm-3. By measuring and fitting the results at several incident fluence levels, the recombination time versus carrier density was determined and fitted with an Auger model resulting in an Auger coefficient of C = 1.03×10^20cm6/sec. Consequently, AZO is shown to be a unique, promising, and CMOS-compatible material for high performance dynamic devices in the near future.
Recently, there has been a flurry of research in the field of alternative plasmonic materials, but for telecommunication applications, CMOS compatible materials titanium nitride and doped zinc oxides are among the most promising materials currently available. TiN is a gold-like ceramic with a permittivity cross-over near 500nm. In addition, TiN can attain ultra-thin, ultra-smooth epitaxial films on substrates such as c-sapphire, MgO, and silicon. Partnering TiN with CMOS compatible silicon nitride enables a fully solid state waveguide which is able to achieve a propagation length greater than 1cm for a ~8μm mode size at 1.55μm.
Utilizing doped zinc oxide films as a dynamic material, high performance modulators can also be realized due to the low-loss achieved by the TiN/Si3N4 waveguide. Simply by placing a thin layer of aluminum doped zinc oxide (AZO) on top of the waveguide structure, a modulator with very low insertion loss is achieved. Our recent work has investigated optical tuning of AZO films by the pump-probe method, demonstrating a change in the refractive index of -0.17+0.25i at 1.3μm with an ultrafast response of 1ps. Assuming this change in the refractive index for the AZO film, a modulation of ~0.7dB/μm is possible in the structure with ~0.5dB insertion loss and an operational speed of 1THz. Further optimization of the design is expected to lead to an increased modulation depth without sacrificing insertion loss or speed.
Consequently, nanophotonic technologies are reaching a critical point where many applications including telecom, medicine, and quantum science can see practical systems which provide new functionalities.
The development of new plasmonic materials enables novel optical devices, and they in turn assist in the progress of optical communications. As a result of the significant attention in searching for alternative materials, transparent conducting oxides (TCOs) have been proposed as promising plasmonic compounds at telecommunication wavelengths . They are eminently practical materials because they are CMOS-compatible, can be grown on many different types of substrates, patterned by standard fabrication procedures, and integrated with many other standard technologies. Due to the ability of TCO nanostructures to support strong plasmonic resonance in the NIR, metasurface devices, such as a quarter wave plate, have been demonstrated whose properties can be easily adjustable with post processing such as thermal annealing [2,3]. Additionally, TCOs can be used as epsilon near zero (ENZ) materials in the NIR. From our recent study of the behavior of nanoantennae sitting upon a TCO substrate, we found that TCOs serve as an optical insulating media due to the high impedance of TCOs at the ENZ frequency, enabling emission shaping. Finally, the optical properties of TCOs can be varied by optical or electrical means. Current research is focused on studying the ultrafast carrier dynamics in doped zinc oxide films using pump-probe spectroscopy. We have shown that aluminum doped zinc oxide films can achieve a 40% change in reflection with ultrafast dynamics (<1ps) under a small fluence of 3mJ/cm2. Consequently, TCOs are shown to be extremely flexible materials, enabling fascinating physics and unique devices for applications in the NIR regime. References  A. Boltasseva and H. Atwater, Science 331(6015), 290-291, 2011.  J. Kim et al, Selected Topics in Quantum Electronics, IEEE Journal of, 19, 4601907-4601907, 2013.  J. Kim et al, CLEO: QELS_Fundamental Science. Optical Society of America, 2014. This work was supported by ONR MURI N00014-10-1-0942