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Vrinda Thareja; Majid Esfandyarpour; Pieter G. Kik; Mark L. Brongersma
Anisotropic Metasurfaces as Tunable SERS Substrates for 2D Materials Journal Article
In: ACS Photonics, vol. 6, no. 8, pp. 1996–2004, 2019.
@article{thareja2019anisotropic,
title = {Anisotropic Metasurfaces as Tunable SERS Substrates for 2D Materials},
author = {Vrinda Thareja and Majid Esfandyarpour and Pieter G. Kik and Mark L. Brongersma},
doi = {10.1021/acsphotonics.9b00416},
year = {2019},
date = {2019-06-20},
journal = {ACS Photonics},
volume = {6},
number = {8},
pages = {1996\textendash2004},
abstract = {The reflection of light from metallic mirrors results in a near-zero electric field at their surface. This precludes strong light-matter interaction between such mirrors and two-dimensional (2D) materials placed in direct contact with them. Patterning of the metal surfaces with sub-wavelength grooves can produce anisotropic metasurfaces that offer robust enhancements in the magnitude and control over the direction of the surface fields. Here, we use this control to analyze the Raman tensor for vibrational modes of atomically-thin graphene. The anisotropic nature of the grooves leads to different Raman signal enhancement for the G (25 times) and 2D (50 times) Raman peaks of graphene for optimized groove dimensions. A notable suppression of these peaks by 40% for specific groove dimensions is also achieved. These findings suggest the use of metasurfaces as tunable surface enhanced Raman scattering substrates to study the vibrational modes of 2D materials with reduced background signals.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The reflection of light from metallic mirrors results in a near-zero electric field at their surface. This precludes strong light-matter interaction between such mirrors and two-dimensional (2D) materials placed in direct contact with them. Patterning of the metal surfaces with sub-wavelength grooves can produce anisotropic metasurfaces that offer robust enhancements in the magnitude and control over the direction of the surface fields. Here, we use this control to analyze the Raman tensor for vibrational modes of atomically-thin graphene. The anisotropic nature of the grooves leads to different Raman signal enhancement for the G (25 times) and 2D (50 times) Raman peaks of graphene for optimized groove dimensions. A notable suppression of these peaks by 40% for specific groove dimensions is also achieved. These findings suggest the use of metasurfaces as tunable surface enhanced Raman scattering substrates to study the vibrational modes of 2D materials with reduced background signals.
Vrinda Thareja
Enhancing the light-matter interaction in graphene PhD Thesis
Stanford University, 2017.
@phdthesis{VrindaTharejathesis,
title = {Enhancing the light-matter interaction in graphene},
author = {Vrinda Thareja},
url = {http://purl.stanford.edu/qj576jr4263},
year = {2017},
date = {2017-03-01},
school = {Stanford University},
abstract = {Graphene, a two-dimensional sheet of hexagonally arranged carbon atoms, can absorb 2.3 % of the incident light over a broad range of wavelengths. Whereas the single-layer-absorption is impressive, it is not strong enough for many optoelectronic applications as virtually all the light is transmitted. Moreover, for active devices such as electro-optical modulators and actively controlled thermal emitters, dynamic control over absorption is required as well. We will discuss an electrically-tunable Salisbury screen device configuration that involves placing graphene about a quarter wavelength (λ/4) distance away from a metal back reflector. This design is capable of achieving both increased absorption and active absorption modulation in a single graphene sheet. However, there is an increasing desire to build ultra-compact devices such that the costs of these devices are minimized without compromising on their performance. To satisfy this additional requirement, we combine graphene with metamaterial mirrors that enhance the electric field and hence light-matter interaction within graphene at the mirror surface. We employ Raman spectroscopy to explore the interesting dependence of the electric fields within graphene on the underlying metamaterial mirror dimensions and project the graphene-coated metamaterial mirrors as uniform and tunable SERS (Surface Enhanced Raman Spectroscopy) substrate. As a final step, we perform reflectance measurements on these metamaterial mirrors and establish a direct correlation of these measurements with Raman measurements. We conclude, therefore, that it is possible to predict the strength of our SERS substrate using reflectance measurements solely.},
keywords = {},
pubstate = {published},
tppubtype = {phdthesis}
}
Graphene, a two-dimensional sheet of hexagonally arranged carbon atoms, can absorb 2.3 % of the incident light over a broad range of wavelengths. Whereas the single-layer-absorption is impressive, it is not strong enough for many optoelectronic applications as virtually all the light is transmitted. Moreover, for active devices such as electro-optical modulators and actively controlled thermal emitters, dynamic control over absorption is required as well. We will discuss an electrically-tunable Salisbury screen device configuration that involves placing graphene about a quarter wavelength (λ/4) distance away from a metal back reflector. This design is capable of achieving both increased absorption and active absorption modulation in a single graphene sheet. However, there is an increasing desire to build ultra-compact devices such that the costs of these devices are minimized without compromising on their performance. To satisfy this additional requirement, we combine graphene with metamaterial mirrors that enhance the electric field and hence light-matter interaction within graphene at the mirror surface. We employ Raman spectroscopy to explore the interesting dependence of the electric fields within graphene on the underlying metamaterial mirror dimensions and project the graphene-coated metamaterial mirrors as uniform and tunable SERS (Surface Enhanced Raman Spectroscopy) substrate. As a final step, we perform reflectance measurements on these metamaterial mirrors and establish a direct correlation of these measurements with Raman measurements. We conclude, therefore, that it is possible to predict the strength of our SERS substrate using reflectance measurements solely.
Vrinda Thareja; Ju-Hyung Kang; Hongtao Yuan; Kaveh M Milaninia; Harold Y Hwang; Yi Cui; Pieter G Kik; Mark L Brongersma
Electrically tunable coherent optical absorption in graphene with ion gel Journal Article
In: Nano Lett., vol. 15, no. 3, pp. 1570-1576, 2015.
@article{Thareja:2015,
title = {Electrically tunable coherent optical absorption in graphene with ion gel},
author = { Vrinda Thareja and Ju-Hyung Kang and Hongtao Yuan and Kaveh M Milaninia and Harold Y Hwang and Yi Cui and Pieter G Kik and Mark L Brongersma},
doi = {10.1021/nl503431d},
year = {2015},
date = {2015-02-11},
journal = {Nano Lett.},
volume = {15},
number = {3},
pages = {1570-1576},
publisher = {ACS Publications},
abstract = {We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ∼3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We demonstrate electrical control over coherent optical absorption in a graphene-based Salisbury screen consisting of a single layer of graphene placed in close proximity to a gold back reflector. The screen was designed to enhance light absorption at a target wavelength of 3.2 μm by using a 600 nm-thick, nonabsorbing silica spacer layer. An ionic gel layer placed on top of the screen was used to electrically gate the charge density in the graphene layer. Spectroscopic reflectance measurements were performed in situ as a function of gate bias. The changes in the reflectance spectra were analyzed using a Fresnel based transfer matrix model in which graphene was treated as an infinitesimally thin sheet with a conductivity given by the Kubo formula. The analysis reveals that a careful choice of the ionic gel layer thickness can lead to optical absorption enhancements of up to 5.5 times for the Salisbury screen compared to a suspended sheet of graphene. In addition to these absorption enhancements, we demonstrate very large electrically induced changes in the optical absorption of graphene of ∼3.3% per volt, the highest attained so far in a device that features an atomically thick active layer. This is attributable in part to the more effective gating achieved with the ion gel over the conventional dielectric back gates and partially by achieving a desirable coherent absorption effect linked to the presence of the thin ion gel that boosts the absorption by 40%.
