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Publications
Junghyun Park; Ju-Hyung Kang; Xiaoge Liu; Scott J. Maddox; Kechao Tang; Paul C. McIntyre; Seth R. Bank; Mark L. Brongersma
Dynamic thermal emission control with InAs-based plasmonic metasurfaces Journal Article
In: Science Advances, vol. 4, no. 12, pp. eaat3163, 2018.
@article{park2018dynamic,
title = {Dynamic thermal emission control with InAs-based plasmonic metasurfaces},
author = {Junghyun Park and Ju-Hyung Kang and Xiaoge Liu and Scott J. Maddox and Kechao Tang and Paul C. McIntyre and Seth R. Bank and Mark L. Brongersma},
doi = {10.1126/sciadv.aat3163},
year = {2018},
date = {2018-12-07},
journal = {Science Advances},
volume = {4},
number = {12},
pages = {eaat3163},
abstract = {Thermal emission from objects tends to be spectrally broadband, unpolarized, and temporally invariant. These common notions are now challenged with the emergence of new nanophotonic structures and concepts that afford on-demand, active manipulation of the thermal emission process. This opens a myriad of new applications in chemistry, health care, thermal management, imaging, sensing, and spectroscopy. Here, we theoretically propose and experimentally demonstrate a new approach to actively tailor thermal emission with a reflective, plasmonic metasurface in which the active material and reflector element are epitaxially grown, high-carrier-mobility InAs layers. Electrical gating induces changes in the charge carrier density of the active InAs layer that are translated into large changes in the optical absorption and thermal emission from metasurface. We demonstrate polarization-dependent and electrically controlled emissivity changes of 3.6%P (6.5% in relative scale) in the mid-infrared spectral range.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Thermal emission from objects tends to be spectrally broadband, unpolarized, and temporally invariant. These common notions are now challenged with the emergence of new nanophotonic structures and concepts that afford on-demand, active manipulation of the thermal emission process. This opens a myriad of new applications in chemistry, health care, thermal management, imaging, sensing, and spectroscopy. Here, we theoretically propose and experimentally demonstrate a new approach to actively tailor thermal emission with a reflective, plasmonic metasurface in which the active material and reflector element are epitaxially grown, high-carrier-mobility InAs layers. Electrical gating induces changes in the charge carrier density of the active InAs layer that are translated into large changes in the optical absorption and thermal emission from metasurface. We demonstrate polarization-dependent and electrically controlled emissivity changes of 3.6%P (6.5% in relative scale) in the mid-infrared spectral range.
Xiaoge Liu; Kai Zang; Ju-Hyung Kang; Junghyun Park; James S Harris; Pieter G Kik; Mark L Brongersma
Epsilon-Near-Zero Si Slot-Waveguide Modulator Journal Article
In: ACS Photonics, vol. 5, no. 11, pp. 4484–4490, 2018.
@article{liu2018epsilon,
title = {Epsilon-Near-Zero Si Slot-Waveguide Modulator},
author = {Xiaoge Liu and Kai Zang and Ju-Hyung Kang and Junghyun Park and James S Harris and Pieter G Kik and Mark L Brongersma},
doi = {10.1021/acsphotonics.8b00945},
year = {2018},
date = {2018-10-04},
journal = {ACS Photonics},
volume = {5},
number = {11},
pages = {4484\textendash4490},
abstract = {We experimentally demonstrate a broadband electro-absorption modulator exploiting indium tin oxide (ITO) as the active switching material. Si strip waveguides are fabricated and covered with 8 nm of HfO2 and 15 nm of ITO to form metal-oxide-semiconductor capacitor (MOS-C) based modulators. The mobile carrier density in the ITO film is controlled using a postanneal treatment to tune its permittivity ε to a near-zero value at the operation wavelength of 1550 nm. Using simulations and experiments, we demonstrate that realizing an epsilon-near-zero (ENZ) can enhance the modulation performance as it increases the overlap of the guided mode with the active ITO layer. We then show even greater benefits of this approach with Si waveguides featuring a central slot filled with ITO. Leveraging the ENZ effect, we achieve a notable 3 dB modulation depth of optical signals in a nonresonant waveguide structure with a length of 20 μm. The results provide insight into the design of very compact modulators for chip-scale optical links.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We experimentally demonstrate a broadband electro-absorption modulator exploiting indium tin oxide (ITO) as the active switching material. Si strip waveguides are fabricated and covered with 8 nm of HfO2 and 15 nm of ITO to form metal-oxide-semiconductor capacitor (MOS-C) based modulators. The mobile carrier density in the ITO film is controlled using a postanneal treatment to tune its permittivity ε to a near-zero value at the operation wavelength of 1550 nm. Using simulations and experiments, we demonstrate that realizing an epsilon-near-zero (ENZ) can enhance the modulation performance as it increases the overlap of the guided mode with the active ITO layer. We then show even greater benefits of this approach with Si waveguides featuring a central slot filled with ITO. Leveraging the ENZ effect, we achieve a notable 3 dB modulation depth of optical signals in a nonresonant waveguide structure with a length of 20 μm. The results provide insight into the design of very compact modulators for chip-scale optical links.
Xiaoge Liu; Ju-Hyung Kang; Hongtao Yuan; Junghyun Park; Yi Cui; Harold Y. Hwang; Mark L. Brongersma
Tuning of Plasmons in Transparent Conductive Oxides by Carrier Accumulation Journal Article
In: ACS Photonics, vol. 5, no. 4, pp. 1493–1498, 2018.
@article{liu2018tuning,
title = {Tuning of Plasmons in Transparent Conductive Oxides by Carrier Accumulation},
author = {Xiaoge Liu and Ju-Hyung Kang and Hongtao Yuan and Junghyun Park and Yi Cui and Harold Y. Hwang and Mark L. Brongersma},
doi = {10.1021/acsphotonics.7b01517},
year = {2018},
date = {2018-02-28},
journal = {ACS Photonics},
volume = {5},
number = {4},
pages = {1493\textendash1498},
abstract = {A metal naturally displays dramatic changes in its optical properties near the plasma frequency where the permittivity changes from a negative to a positive value, and the material turns from highly reflective to transparent. For many applications, it is desirable to achieve such large optical changes by electrical gating. However, this is challenging given the high carrier density of most metals, which causes them to effectively screen externally applied electrical fields. Indium tin oxide (ITO) is a low-electron-density metal that does afford electric tuning of its permittivity in the infrared spectral range. Here, we experimentally show the tunability of the plasma frequency of an ITO thin film by changing its sheet carrier density via gating with an ionic liquid. By applying moderate gate bias values up to 1.4 V, the electron density increases in a thin (∼3 nm) accumulation layer at the surface of the 15-nm-thick ITO film. This results in notable blue shifts in the plasma frequency. These optical and electrical changes are monitored simultaneously, which facilitates construction of a model that provides a consistent picture for the dc electrical and infrared optical properties. It can be used to quantitatively predict the optical changes in the ITO layer with applied bias. This work builds our understanding of electrically tunable plasmonic materials and aids the design of ultracompact, active nanophotonic elements.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A metal naturally displays dramatic changes in its optical properties near the plasma frequency where the permittivity changes from a negative to a positive value, and the material turns from highly reflective to transparent. For many applications, it is desirable to achieve such large optical changes by electrical gating. However, this is challenging given the high carrier density of most metals, which causes them to effectively screen externally applied electrical fields. Indium tin oxide (ITO) is a low-electron-density metal that does afford electric tuning of its permittivity in the infrared spectral range. Here, we experimentally show the tunability of the plasma frequency of an ITO thin film by changing its sheet carrier density via gating with an ionic liquid. By applying moderate gate bias values up to 1.4 V, the electron density increases in a thin (∼3 nm) accumulation layer at the surface of the 15-nm-thick ITO film. This results in notable blue shifts in the plasma frequency. These optical and electrical changes are monitored simultaneously, which facilitates construction of a model that provides a consistent picture for the dc electrical and infrared optical properties. It can be used to quantitatively predict the optical changes in the ITO layer with applied bias. This work builds our understanding of electrically tunable plasmonic materials and aids the design of ultracompact, active nanophotonic elements.
Xiaoge Liu
Transparent oxides for active nanophotonics PhD Thesis
Stanford University, 2017.
@phdthesis{xiaogeliuthesis,
title = {Transparent oxides for active nanophotonics},
author = {Xiaoge Liu},
url = {http://purl.stanford.edu/yb195qf2632},
year = {2017},
date = {2017-08-01},
address = {Stanford, CA, US},
school = {Stanford University},
abstract = {In this thesis, we will apply indium tin oxide (ITO), a widely used transparent conductive oxide (TCO) materials in the touchscreen and solar cell industry, to design and experimentally demonstrate active nanophotonics device. ITO behaves as a Drude metal with a plasma frequency that is controlled by its free carrier density. Attenuated total reflection (ATR) measurement can be used to excite and study the surface plasmon polaritons (SPP) on ITO film. We systematically tune the plasma frequency across the infrared (IR) range by annealing treatments in a reducing environment that produce high electron concentrations (~1021cm-3). These optical measurements are complemented by Hall measurements to obtain a comprehensive picture of the Drude response of the ITO films. It was found that a complete description of the optical properties at very high carrier densities needs to account for the nonparabolicity of the conduction band of ITO and a reduced carrier mobility. Electrical tuning of metals' optical behavior stands very challenging, because noble metals such as gold can effectively screen electrical field due to their high electron densities. ITO, as a low-electron-density metallic material, exhibits electrical-tuneable permittivities in IR regime. We experimentally show the tunability of the plasma frequency of ITO thin film by changing the sheet carrier density through electrical gating with an ionic liquid (IL). The surface plasmon resonance wavelength of ITO accumulation layer is substantially shifted from 3.9 μm to 2.4 μm with gate bias increased to 1.5 V. Quantitative analysis confirms a concomitant relation between the optical behavior and electrical properties of electron density and mobility. Generally, the SP resonances supported by noble metal nanostructures are well-explained by classical models, at least until the nanostructure size is decreased to a few nanometers, approaching the Fermi wavelength λF of the electrons. In particular, observation of quantum size effect in metallic films and its tuning with thickness has been particularly challenging. We experimentally demonstrate active tuning of quantum size effects seen in the SP resonances supported by a 20-nm-thick metallic film of ITO. The IL is used to electrically gate and partially deplete the ITO layer. The experiment shows results in a controllable and reversible blue-shift in the SP resonance wavelength above a critical voltage. A quantum mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red-shift. We believe this work opens up new approaches to investigate quantum plasmonic phenomena and achieve tunable plasmonic devices and circuits that will operate robustly at the quantum level Lastly, we experimentally demonstrate a broadband, ultra-compact, waveguide-integrated modulator exploiting the epsilon-near-zero (ENZ) effect in ITO material. Si waveguide strip is covered with HfO2 and ITO film to form a metal-oxide-semiconductor (MOS) capacitor. Electrical gate bias is applied to accumulate electrons in ITO film to induce an ENZ layer in the spectral region near the important telecommunications wavelength of λ = 1.55µm. When the ENZ layer occurs, this modulator leverages the combination of a local electric field enhancement and increased absorption in the ITO. This leads to large changes in modal absorption upon gating. A 3 dB modulation depth is achieved in a non-resonant structure with a length under 30 µm. The results provide insight into the design of ultra-compact, nanoscale modulators for future integrated nanophotonic circuits. The ITO materials show great potential in the application of active nanophotonics. It opens up a myriad of opportunities in the fields of optical communications, integrated optical devices, advanced imaging and display systems and so on.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
In this thesis, we will apply indium tin oxide (ITO), a widely used transparent conductive oxide (TCO) materials in the touchscreen and solar cell industry, to design and experimentally demonstrate active nanophotonics device. ITO behaves as a Drude metal with a plasma frequency that is controlled by its free carrier density. Attenuated total reflection (ATR) measurement can be used to excite and study the surface plasmon polaritons (SPP) on ITO film. We systematically tune the plasma frequency across the infrared (IR) range by annealing treatments in a reducing environment that produce high electron concentrations (~1021cm-3). These optical measurements are complemented by Hall measurements to obtain a comprehensive picture of the Drude response of the ITO films. It was found that a complete description of the optical properties at very high carrier densities needs to account for the nonparabolicity of the conduction band of ITO and a reduced carrier mobility. Electrical tuning of metals' optical behavior stands very challenging, because noble metals such as gold can effectively screen electrical field due to their high electron densities. ITO, as a low-electron-density metallic material, exhibits electrical-tuneable permittivities in IR regime. We experimentally show the tunability of the plasma frequency of ITO thin film by changing the sheet carrier density through electrical gating with an ionic liquid (IL). The surface plasmon resonance wavelength of ITO accumulation layer is substantially shifted from 3.9 μm to 2.4 μm with gate bias increased to 1.5 V. Quantitative analysis confirms a concomitant relation between the optical behavior and electrical properties of electron density and mobility. Generally, the SP resonances supported by noble metal nanostructures are well-explained by classical models, at least until the nanostructure size is decreased to a few nanometers, approaching the Fermi wavelength λF of the electrons. In particular, observation of quantum size effect in metallic films and its tuning with thickness has been particularly challenging. We experimentally demonstrate active tuning of quantum size effects seen in the SP resonances supported by a 20-nm-thick metallic film of ITO. The IL is used to electrically gate and partially deplete the ITO layer. The experiment shows results in a controllable and reversible blue-shift in the SP resonance wavelength above a critical voltage. A quantum mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red-shift. We believe this work opens up new approaches to investigate quantum plasmonic phenomena and achieve tunable plasmonic devices and circuits that will operate robustly at the quantum level Lastly, we experimentally demonstrate a broadband, ultra-compact, waveguide-integrated modulator exploiting the epsilon-near-zero (ENZ) effect in ITO material. Si waveguide strip is covered with HfO2 and ITO film to form a metal-oxide-semiconductor (MOS) capacitor. Electrical gate bias is applied to accumulate electrons in ITO film to induce an ENZ layer in the spectral region near the important telecommunications wavelength of λ = 1.55µm. When the ENZ layer occurs, this modulator leverages the combination of a local electric field enhancement and increased absorption in the ITO. This leads to large changes in modal absorption upon gating. A 3 dB modulation depth is achieved in a non-resonant structure with a length under 30 µm. The results provide insight into the design of ultra-compact, nanoscale modulators for future integrated nanophotonic circuits. The ITO materials show great potential in the application of active nanophotonics. It opens up a myriad of opportunities in the fields of optical communications, integrated optical devices, advanced imaging and display systems and so on.
Xiaoge Liu; Ju-Hyung Kang; Hongtao Yuan; Junghyun Park; Soo Jin Kim; Yi Cui; Harold Y. Hwang; Mark L. Brongersma
Electrical tuning of a quantum plasmonic resonance Journal Article
In: Nature Nanotechnology, vol. 12, pp. 866–870, 2017.
@article{liu2017electrical,
title = {Electrical tuning of a quantum plasmonic resonance},
author = {Xiaoge Liu and Ju-Hyung Kang and Hongtao Yuan and Junghyun Park and Soo Jin Kim and Yi Cui and Harold Y. Hwang and Mark L. Brongersma},
doi = {10.1038/nnano.2017.103},
year = {2017},
date = {2017-06-12},
journal = {Nature Nanotechnology},
volume = {12},
pages = {866\textendash870},
abstract = {Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light\textendashmatter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λF of the electrons. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements, as well as imperfect control over size, shape, faceting, surface reconstructions, contamination, charging effects and surface roughness in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal. An ionic liquid (IL) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Surface plasmon (SP) excitations in metals facilitate confinement of light into deep-subwavelength volumes and can induce strong light–matter interaction. Generally, the SP resonances supported by noble metal nanostructures are explained well by classical models, at least until the nanostructure size is decreased to a few nanometres, approaching the Fermi wavelength λF of the electrons. Although there is a long history of reports on quantum size effects in the plasmonic response of nanometre-sized metal particles, systematic experimental studies have been hindered by inhomogeneous broadening in ensemble measurements, as well as imperfect control over size, shape, faceting, surface reconstructions, contamination, charging effects and surface roughness in single-particle measurements. In particular, observation of the quantum size effect in metallic films and its tuning with thickness has been challenging as they only confine carriers in one direction. Here, we show active tuning of quantum size effects in SP resonances supported by a 20-nm-thick metallic film of indium tin oxide (ITO), a plasmonic material serving as a low-carrier-density Drude metal. An ionic liquid (IL) is used to electrically gate and partially deplete the ITO layer. The experiment shows a controllable and reversible blue-shift in the SP resonance above a critical voltage. A quantum-mechanical model including the quantum size effect reproduces the experimental results, whereas a classical model only predicts a red shift.
Junghyun Park; Ju-Hyung Kang; Soo Jin Kim; Xiaoge Liu; Mark L. Brongersma
Dynamic reflection phase and polarization control in metasurfaces Journal Article
In: Nano Lett., vol. 17, pp. 407, 2017.
@article{Park:2017,
title = {Dynamic reflection phase and polarization control in metasurfaces},
author = {Junghyun Park and Ju-Hyung Kang and Soo Jin Kim and Xiaoge Liu and Mark L. Brongersma},
doi = {10.1021/acs.nanolett.6b04378},
year = {2017},
date = {2017-01-02},
journal = {Nano Lett.},
volume = {17},
pages = {407},
abstract = {Optical metasurfaces are two-dimensional optical elements composed of dense arrays of subwavelength optical antennas and afford on-demand manipulation of the basic properties of light waves. Following the pioneering works on active metasurfaces capable of modulating wave amplitude, there is now a growing interest to dynamically control other fundamental properties of light. Here, we present metasurfaces that facilitate electrical tuning of the reflection phase and polarization properties. To realize these devices, we leverage the properties of actively controlled plasmonic antennas and fundamental insights provided by coupled mode theory. Indium\textendashtin\textendashoxide is embedded into gap-plasmon resonator-antennas as it offers electrically tunable optical properties. By judiciously controlling the resonant properties of the antennas from under- to overcoupling regimes, we experimentally demonstrate tuning of the reflection phase over 180°. This work opens up new design strategies for active metasurfaces for displacement measurements and tunable waveplates.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Optical metasurfaces are two-dimensional optical elements composed of dense arrays of subwavelength optical antennas and afford on-demand manipulation of the basic properties of light waves. Following the pioneering works on active metasurfaces capable of modulating wave amplitude, there is now a growing interest to dynamically control other fundamental properties of light. Here, we present metasurfaces that facilitate electrical tuning of the reflection phase and polarization properties. To realize these devices, we leverage the properties of actively controlled plasmonic antennas and fundamental insights provided by coupled mode theory. Indium–tin–oxide is embedded into gap-plasmon resonator-antennas as it offers electrically tunable optical properties. By judiciously controlling the resonant properties of the antennas from under- to overcoupling regimes, we experimentally demonstrate tuning of the reflection phase over 180°. This work opens up new design strategies for active metasurfaces for displacement measurements and tunable waveplates.
