Christopher Gladden - Academia.edu (original) (raw)
Papers by Christopher Gladden
Metamaterials, Metadevices, and Metasystems 2016, 2016
Nano Letters, 2011
We report an invisibility carpet cloak device, which is capable of making an object undetectable ... more We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.
Advanced materials (Deerfield Beach, Fla.), 2015
Photoelectrochemical etching of silicon can be used to form lateral refractive index gradients fo... more Photoelectrochemical etching of silicon can be used to form lateral refractive index gradients for transformation optical devices. This technique allows the fabrication of macroscale devices with large refractive index gradients. Patterned porous layers can also be lifted from the substrate and transferred to other materials, creating more possibilities for novel devices.
2011 37th IEEE Photovoltaic Specialists Conference, 2011
Abstract Exceeding the Shockley-Queisser efficiency limit for a single junction solar cell has be... more Abstract Exceeding the Shockley-Queisser efficiency limit for a single junction solar cell has been theorized using various means. Specifically, up-and down-conversion, carrier multiplication and intermediate band transitions have been posited as methods of improving the efficiency. Here, we compare these methods using a thermodynamic approach with a newly devised pseudo-linear system model. This method allows a schematic interpretation of the internal processes of efficiency enhancement techniques. In particular, we ...
Frontiers in Optics 2011/Laser Science XXVII, 2011
We report an invisibility carpet cloak device, which is capable of making an object undetectable ... more We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nano-porous silicon oxide substrate with a very low refractive index. The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequency.
Frontiers in Optics 2009/Laser Science XXV/Fall 2009 OSA Optics & Photonics Technical Digest, 2009
ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These n... more ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These nanowire-based plasmonic lasers are not subjected to diffraction limitations, hence can operate below the photonic mode cut-off diameter of purely dielectric nanowire lasers.
Frontiers in Optics 2009/Laser Science XXV/Fall 2009 OSA Optics & Photonics Technical Digest, 2009
Integrated Photonics Research, Silicon and Nanophotonics and Photonics in Switching, 2010
ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These n... more ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These nanowire-based plasmonic lasers are not subjected to diffraction limitations, hence can operate below the photonic mode cut-off diameter of purely dielectric nanowire lasers.
ABSTRACT Laser science has tackled physical limitations to achieve higher power, faster and small... more ABSTRACT Laser science has tackled physical limitations to achieve higher power, faster and smaller light sources. The quest for ultra-compact laser that can directly generate coherent optical fields at the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge. Microscopic lasers based on photonic crystals3, metal clad cavities4 and nanowires can now reach the diffraction limit, which restricts both the optical mode size and physical device dimension to be larger than half a wavelength. While surface plasmons are capable of tightly localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale lasers. Recent theory has proposed a way to significantly reduce plasmonic loss while maintaining ultra-small modes by using a hybrid plasmonic waveguide. Using this approach, we report an experimental demonstration of nano-scale plasmonic lasers producing optical modes 100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct measurements of emission lifetime reveal a broad-band enhancement of the nanowire's exciton spontaneous emission rate up to 6 times due to the strong mode confinement and the signature of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show downscaling of the lateral dimensions of both device and optical mode. As these optical coherent sources approach molecular and electronics length scales, plasmonic lasers offer the possibility to explore extreme interactions between light and matter, opening new avenues in active photonic circuits, bio-sensing and quantum information technology.
Solar Energy Materials and Solar Cells, 2012
Methods of exceeding the detailed balance limit for a single junction solar cell have included do... more Methods of exceeding the detailed balance limit for a single junction solar cell have included downconverting high energy photons to produce two photons; and carrier multiplication, whereby high energy photons produce more than one electron-hole pair. Both of the methods obey the conservation of energy in similar ways, and effectively produce a higher current in the solar cell. Due to this similarity, it has been assumed that there is no thermodynamic difference between the two methods. Here, we compare the two methods using a generalized approach based on Kirchhoff's law of radiation and develop a new model for carrier multiplication. We demonstrate that there is an entropic penalty to be paid for attempting to accomplish all-in-one splitting in carrier multiplication systems, giving a small thermodynamic -and therefore efficiency -advantage to spectral splitting prior to reaching the solar cell. We show this analytically using a derivation of basic thermodynamic identities; numerically by solving for the maximal efficiency; and generally using heat-generation arguments. Our result modifies the existing literature on entropy generation limits in solar cells, and creates a new distinction among 3 rd generation photovoltaic technologies.
Physical Review Letters, 2012
Current methods for evaluating solar cell efficiencies cannot be applied to low-dimensional struc... more Current methods for evaluating solar cell efficiencies cannot be applied to low-dimensional structures where phenomena from the realm of near-field optics prevail. We present a theoretical approach to analyze solar cell performance by allowing rigorous electromagnetic calculations of the emission rate using the fluctuation-dissipation theorem. Our approach shows the direct quantification of the voltage, current, and efficiency of low-dimensional solar cells. This approach is demonstrated by calculating the voltage and the efficiency of a GaAs slab solar cell for thicknesses from several microns down to a few nanometers. This example highlights the ability of the proposed approach to capture the role of optical near-field effects in solar cell performance.
Physical Chemistry Chemical Physics, 2013
Nanoporous silicon (Si) networks with controllable porosity and thickness are fabricated by a sim... more Nanoporous silicon (Si) networks with controllable porosity and thickness are fabricated by a simple and scalable electrochemical process, and then released from Si wafers and transferred to flexible and conductive substrates. These nanoporous Si networks serve as high performance Li-ion battery electrodes, with an initial discharge capacity of 2570 mA h g(-1), above 1000 mA h g(-1) after 200 cycles without any electrolyte additives.
Nature, 2011
When light illuminates a rough metallic surface, hotspots can appear, where the light is concentr... more When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect 1,2 , has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes 3,4 , caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1-2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200-300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy 5 , electron energy-loss spectroscopy 6 , cathode luminescence imaging 7 and two-photon photoemission imaging 8 have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect 9-11 , how the local field is distributed remains unknown.
Nature, 2009
Laser science has been successful in producing increasingly highpowered, faster and smaller coher... more Laser science has been successful in producing increasingly highpowered, faster and smaller coherent light sources . Examples of recent advances are microscopic lasers that can reach the diffraction limit, based on photonic crystals 3 , metal-clad cavities 4 and nanowires . However, such lasers are restricted, both in optical mode size and physical device dimension, to being larger than half the wavelength of the optical field, and it remains a key fundamental challenge to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit 10,11 . A way of addressing this issue is to make use of surface plasmons 12,13 , which are capable of tightly localizing light, but so far ohmic losses at optical frequencies have inhibited the realization of truly nanometre-scale lasers based on such approaches 14,15 . A recent theoretical work predicted that such losses could be significantly reduced while maintaining ultrasmall modes in a hybrid plasmonic waveguide 16 . Here we report the experimental demonstration of nanometre-scale plasmonic lasers, generating optical modes a hundred times smaller than the diffraction limit. We realize such lasers using a hybrid plasmonic waveguide consisting of a high-gain cadmium sulphide semiconductor nanowire, separated from a silver surface by a 5-nmthick insulating gap. Direct measurements of the emission lifetime reveal a broad-band enhancement of the nanowire's exciton spontaneous emission rate by up to six times owing to the strong mode confinement 17 and the signature of apparently threshold-less lasing. Because plasmonic modes have no cutoff, we are able to demonstrate downscaling of the lateral dimensions of both the device and the optical mode. Plasmonic lasers thus offer the possibility of exploring extreme interactions between light and matter, opening up new avenues in the fields of active photonic circuits 18 , bio-sensing 19 and quantum information technology 20 .
Nano Letters, 2011
We report an invisibility carpet cloak device, which is capable of making an object undetectable ... more We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.
Biophysical Journal, 2012
ABSTRACT
Applied Physics Letters, 2012
Imaging and Applied Optics Congress, 2010
Laser science has tackled physical limitations to achieve higher power, faster and smaller light ... more Laser science has tackled physical limitations to achieve higher power, faster and smaller light sources [1-8]. The quest for ultra-compact laser that can directly generate coherent optical fields at the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge [9, 10]. Microscopic lasers based on photonic crystals [3], micro-disks [4], metal clad cavities [5] and nanowires [6-8] can now reach the diffraction limit, which restricts both the optical mode size and physical device dimension to be larger than half a wavelength. While surface plasmons [11-13] are capable of tightly localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale lasers [14, 15]. Recent theory has proposed a way to significantly reduce plasmonic loss while maintaining ultra-small modes by using a hybrid plasmonic waveguide [16]. Using this approach, we report an experimental demonstration of nano-scale plasmonic lasers producing optical modes 100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct measurements of emission lifetime reveal a broadband enhancement of the nanowire's spontaneous emission rate by up to 6 times due to the strong mode confinement [17] and the signature of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show down-scaling of the lateral dimensions of both device and optical mode. As these optical coherent sources approaches molecular and electronics length scales, plasmonic lasers offer the
Metamaterials, Metadevices, and Metasystems 2016, 2016
Nano Letters, 2011
We report an invisibility carpet cloak device, which is capable of making an object undetectable ... more We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.
Advanced materials (Deerfield Beach, Fla.), 2015
Photoelectrochemical etching of silicon can be used to form lateral refractive index gradients fo... more Photoelectrochemical etching of silicon can be used to form lateral refractive index gradients for transformation optical devices. This technique allows the fabrication of macroscale devices with large refractive index gradients. Patterned porous layers can also be lifted from the substrate and transferred to other materials, creating more possibilities for novel devices.
2011 37th IEEE Photovoltaic Specialists Conference, 2011
Abstract Exceeding the Shockley-Queisser efficiency limit for a single junction solar cell has be... more Abstract Exceeding the Shockley-Queisser efficiency limit for a single junction solar cell has been theorized using various means. Specifically, up-and down-conversion, carrier multiplication and intermediate band transitions have been posited as methods of improving the efficiency. Here, we compare these methods using a thermodynamic approach with a newly devised pseudo-linear system model. This method allows a schematic interpretation of the internal processes of efficiency enhancement techniques. In particular, we ...
Frontiers in Optics 2011/Laser Science XXVII, 2011
We report an invisibility carpet cloak device, which is capable of making an object undetectable ... more We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nano-porous silicon oxide substrate with a very low refractive index. The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequency.
Frontiers in Optics 2009/Laser Science XXV/Fall 2009 OSA Optics & Photonics Technical Digest, 2009
ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These n... more ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These nanowire-based plasmonic lasers are not subjected to diffraction limitations, hence can operate below the photonic mode cut-off diameter of purely dielectric nanowire lasers.
Frontiers in Optics 2009/Laser Science XXV/Fall 2009 OSA Optics & Photonics Technical Digest, 2009
Integrated Photonics Research, Silicon and Nanophotonics and Photonics in Switching, 2010
ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These n... more ABSTRACT We report a plasmonic laser device exhibiting strong sub-wavelength confinement. These nanowire-based plasmonic lasers are not subjected to diffraction limitations, hence can operate below the photonic mode cut-off diameter of purely dielectric nanowire lasers.
ABSTRACT Laser science has tackled physical limitations to achieve higher power, faster and small... more ABSTRACT Laser science has tackled physical limitations to achieve higher power, faster and smaller light sources. The quest for ultra-compact laser that can directly generate coherent optical fields at the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge. Microscopic lasers based on photonic crystals3, metal clad cavities4 and nanowires can now reach the diffraction limit, which restricts both the optical mode size and physical device dimension to be larger than half a wavelength. While surface plasmons are capable of tightly localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale lasers. Recent theory has proposed a way to significantly reduce plasmonic loss while maintaining ultra-small modes by using a hybrid plasmonic waveguide. Using this approach, we report an experimental demonstration of nano-scale plasmonic lasers producing optical modes 100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct measurements of emission lifetime reveal a broad-band enhancement of the nanowire's exciton spontaneous emission rate up to 6 times due to the strong mode confinement and the signature of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show downscaling of the lateral dimensions of both device and optical mode. As these optical coherent sources approach molecular and electronics length scales, plasmonic lasers offer the possibility to explore extreme interactions between light and matter, opening new avenues in active photonic circuits, bio-sensing and quantum information technology.
Solar Energy Materials and Solar Cells, 2012
Methods of exceeding the detailed balance limit for a single junction solar cell have included do... more Methods of exceeding the detailed balance limit for a single junction solar cell have included downconverting high energy photons to produce two photons; and carrier multiplication, whereby high energy photons produce more than one electron-hole pair. Both of the methods obey the conservation of energy in similar ways, and effectively produce a higher current in the solar cell. Due to this similarity, it has been assumed that there is no thermodynamic difference between the two methods. Here, we compare the two methods using a generalized approach based on Kirchhoff's law of radiation and develop a new model for carrier multiplication. We demonstrate that there is an entropic penalty to be paid for attempting to accomplish all-in-one splitting in carrier multiplication systems, giving a small thermodynamic -and therefore efficiency -advantage to spectral splitting prior to reaching the solar cell. We show this analytically using a derivation of basic thermodynamic identities; numerically by solving for the maximal efficiency; and generally using heat-generation arguments. Our result modifies the existing literature on entropy generation limits in solar cells, and creates a new distinction among 3 rd generation photovoltaic technologies.
Physical Review Letters, 2012
Current methods for evaluating solar cell efficiencies cannot be applied to low-dimensional struc... more Current methods for evaluating solar cell efficiencies cannot be applied to low-dimensional structures where phenomena from the realm of near-field optics prevail. We present a theoretical approach to analyze solar cell performance by allowing rigorous electromagnetic calculations of the emission rate using the fluctuation-dissipation theorem. Our approach shows the direct quantification of the voltage, current, and efficiency of low-dimensional solar cells. This approach is demonstrated by calculating the voltage and the efficiency of a GaAs slab solar cell for thicknesses from several microns down to a few nanometers. This example highlights the ability of the proposed approach to capture the role of optical near-field effects in solar cell performance.
Physical Chemistry Chemical Physics, 2013
Nanoporous silicon (Si) networks with controllable porosity and thickness are fabricated by a sim... more Nanoporous silicon (Si) networks with controllable porosity and thickness are fabricated by a simple and scalable electrochemical process, and then released from Si wafers and transferred to flexible and conductive substrates. These nanoporous Si networks serve as high performance Li-ion battery electrodes, with an initial discharge capacity of 2570 mA h g(-1), above 1000 mA h g(-1) after 200 cycles without any electrolyte additives.
Nature, 2011
When light illuminates a rough metallic surface, hotspots can appear, where the light is concentr... more When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect 1,2 , has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes 3,4 , caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1-2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200-300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy 5 , electron energy-loss spectroscopy 6 , cathode luminescence imaging 7 and two-photon photoemission imaging 8 have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect 9-11 , how the local field is distributed remains unknown.
Nature, 2009
Laser science has been successful in producing increasingly highpowered, faster and smaller coher... more Laser science has been successful in producing increasingly highpowered, faster and smaller coherent light sources . Examples of recent advances are microscopic lasers that can reach the diffraction limit, based on photonic crystals 3 , metal-clad cavities 4 and nanowires . However, such lasers are restricted, both in optical mode size and physical device dimension, to being larger than half the wavelength of the optical field, and it remains a key fundamental challenge to realize ultracompact lasers that can directly generate coherent optical fields at the nanometre scale, far beyond the diffraction limit 10,11 . A way of addressing this issue is to make use of surface plasmons 12,13 , which are capable of tightly localizing light, but so far ohmic losses at optical frequencies have inhibited the realization of truly nanometre-scale lasers based on such approaches 14,15 . A recent theoretical work predicted that such losses could be significantly reduced while maintaining ultrasmall modes in a hybrid plasmonic waveguide 16 . Here we report the experimental demonstration of nanometre-scale plasmonic lasers, generating optical modes a hundred times smaller than the diffraction limit. We realize such lasers using a hybrid plasmonic waveguide consisting of a high-gain cadmium sulphide semiconductor nanowire, separated from a silver surface by a 5-nmthick insulating gap. Direct measurements of the emission lifetime reveal a broad-band enhancement of the nanowire's exciton spontaneous emission rate by up to six times owing to the strong mode confinement 17 and the signature of apparently threshold-less lasing. Because plasmonic modes have no cutoff, we are able to demonstrate downscaling of the lateral dimensions of both the device and the optical mode. Plasmonic lasers thus offer the possibility of exploring extreme interactions between light and matter, opening up new avenues in the fields of active photonic circuits 18 , bio-sensing 19 and quantum information technology 20 .
Nano Letters, 2011
We report an invisibility carpet cloak device, which is capable of making an object undetectable ... more We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.
Biophysical Journal, 2012
ABSTRACT
Applied Physics Letters, 2012
Imaging and Applied Optics Congress, 2010
Laser science has tackled physical limitations to achieve higher power, faster and smaller light ... more Laser science has tackled physical limitations to achieve higher power, faster and smaller light sources [1-8]. The quest for ultra-compact laser that can directly generate coherent optical fields at the nano-scale, far beyond the diffraction limit of light, remains a key fundamental challenge [9, 10]. Microscopic lasers based on photonic crystals [3], micro-disks [4], metal clad cavities [5] and nanowires [6-8] can now reach the diffraction limit, which restricts both the optical mode size and physical device dimension to be larger than half a wavelength. While surface plasmons [11-13] are capable of tightly localizing light, ohmic loss at optical frequencies has inhibited the realization of truly nano-scale lasers [14, 15]. Recent theory has proposed a way to significantly reduce plasmonic loss while maintaining ultra-small modes by using a hybrid plasmonic waveguide [16]. Using this approach, we report an experimental demonstration of nano-scale plasmonic lasers producing optical modes 100 times smaller than the diffraction limit, utilizing a high gain Cadmium Sulphide semiconductor nanowire atop a Silver surface separated by a 5 nm thick insulating gap. Direct measurements of emission lifetime reveal a broadband enhancement of the nanowire's spontaneous emission rate by up to 6 times due to the strong mode confinement [17] and the signature of apparently threshold-less lasing. Since plasmonic modes have no cut-off, we show down-scaling of the lateral dimensions of both device and optical mode. As these optical coherent sources approaches molecular and electronics length scales, plasmonic lasers offer the