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Resolving the electromagnetic mechanism of surface-enhanced light scattering at single hot spots.

Alonso-González P, Albella P, Schnell M, Chen J, Huth F, García-Etxarri A, Casanova F, Golmar F, Arzubiaga L, Hueso LE, Aizpurua J, Hillenbrand R - Nat Commun (2012)

Bottom Line: Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally.Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process.We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering.

View Article: PubMed Central - PubMed

Affiliation: CIC nanoGUNE Consolider, 20018 Donostia-San Sebastián, Spain.

ABSTRACT
Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering.

No MeSH data available.


Related in: MedlinePlus

Set-up for verifying surface-enhanced elastic light scattering.The set-up comprises a scattering-type scanning near-field optical microscope (s-SNOM, from Neaspec GmbH), which is based on an atomic force microscope (AFM). The apex of a conventional Si tip acts as the scattering object (O). The antenna and tip are illuminated at an angle of about 50° relative to the surface normal with the focused beam of a CO2 laser operating at 11.1 μm. The polarization of the illuminating beam Einc is parallel to the long axis of the metal rod (s-polarized light). The field backscattered by the antenna (A) and the tip (O) is collected and detected with a pseudo-heterodyne interferometer (M denotes the reference mirror and BS the beamsplitter). The polarizer P ensures that s-polarized light is detected. To obtain a modulation of the distance between tip and antenna, the tip is oscillating vertically at a frequency Ω=300 KHz and with an amplitude of 70 nm. Demodulation of the detector signal at a higher harmonic nΩ yields the amplitude and phase signals /En/ and Δϕn.
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f2: Set-up for verifying surface-enhanced elastic light scattering.The set-up comprises a scattering-type scanning near-field optical microscope (s-SNOM, from Neaspec GmbH), which is based on an atomic force microscope (AFM). The apex of a conventional Si tip acts as the scattering object (O). The antenna and tip are illuminated at an angle of about 50° relative to the surface normal with the focused beam of a CO2 laser operating at 11.1 μm. The polarization of the illuminating beam Einc is parallel to the long axis of the metal rod (s-polarized light). The field backscattered by the antenna (A) and the tip (O) is collected and detected with a pseudo-heterodyne interferometer (M denotes the reference mirror and BS the beamsplitter). The polarizer P ensures that s-polarized light is detected. To obtain a modulation of the distance between tip and antenna, the tip is oscillating vertically at a frequency Ω=300 KHz and with an amplitude of 70 nm. Demodulation of the detector signal at a higher harmonic nΩ yields the amplitude and phase signals /En/ and Δϕn.

Mentions: We experimentally realize the scheme proposed in Figure 1b with the use of a scattering-type scanning near-field optical microscope (s-SNOM, Fig. 2)3132. As schematically illustrated in Figure 3a, the apex of the vertically oscillating Si tip mimics the object (O), while the metal rod acts as an antenna (A) that converts the incident infrared radiation of a wavelength λ=11.1 μm (with polarization parallel to the rod) into strongly concentrated fields at the rod extremities (hot spots)31. The horizontally polarized light backscattered from the tip-antenna configuration is detected with a pseudo-heterodyne Michelson interferometer33 (Fig. 2). Demodulation of the detector signal at nΩ yields the amplitude /En/ and phase shift Δϕn. To identify the hot spots, we scan the tip across the antenna and record the sample height, as well as amplitude /En/ and phase Δϕn as a function of the tip position (Fig. 3b). In the amplitude map we observe the strongest signals /En/ at the rod extremities, which experimentally proves the existence of two hot spots on the metal rod. The phase map shows that the signal Δϕn is nearly constant across the antenna.


Resolving the electromagnetic mechanism of surface-enhanced light scattering at single hot spots.

Alonso-González P, Albella P, Schnell M, Chen J, Huth F, García-Etxarri A, Casanova F, Golmar F, Arzubiaga L, Hueso LE, Aizpurua J, Hillenbrand R - Nat Commun (2012)

Set-up for verifying surface-enhanced elastic light scattering.The set-up comprises a scattering-type scanning near-field optical microscope (s-SNOM, from Neaspec GmbH), which is based on an atomic force microscope (AFM). The apex of a conventional Si tip acts as the scattering object (O). The antenna and tip are illuminated at an angle of about 50° relative to the surface normal with the focused beam of a CO2 laser operating at 11.1 μm. The polarization of the illuminating beam Einc is parallel to the long axis of the metal rod (s-polarized light). The field backscattered by the antenna (A) and the tip (O) is collected and detected with a pseudo-heterodyne interferometer (M denotes the reference mirror and BS the beamsplitter). The polarizer P ensures that s-polarized light is detected. To obtain a modulation of the distance between tip and antenna, the tip is oscillating vertically at a frequency Ω=300 KHz and with an amplitude of 70 nm. Demodulation of the detector signal at a higher harmonic nΩ yields the amplitude and phase signals /En/ and Δϕn.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3293409&req=5

f2: Set-up for verifying surface-enhanced elastic light scattering.The set-up comprises a scattering-type scanning near-field optical microscope (s-SNOM, from Neaspec GmbH), which is based on an atomic force microscope (AFM). The apex of a conventional Si tip acts as the scattering object (O). The antenna and tip are illuminated at an angle of about 50° relative to the surface normal with the focused beam of a CO2 laser operating at 11.1 μm. The polarization of the illuminating beam Einc is parallel to the long axis of the metal rod (s-polarized light). The field backscattered by the antenna (A) and the tip (O) is collected and detected with a pseudo-heterodyne interferometer (M denotes the reference mirror and BS the beamsplitter). The polarizer P ensures that s-polarized light is detected. To obtain a modulation of the distance between tip and antenna, the tip is oscillating vertically at a frequency Ω=300 KHz and with an amplitude of 70 nm. Demodulation of the detector signal at a higher harmonic nΩ yields the amplitude and phase signals /En/ and Δϕn.
Mentions: We experimentally realize the scheme proposed in Figure 1b with the use of a scattering-type scanning near-field optical microscope (s-SNOM, Fig. 2)3132. As schematically illustrated in Figure 3a, the apex of the vertically oscillating Si tip mimics the object (O), while the metal rod acts as an antenna (A) that converts the incident infrared radiation of a wavelength λ=11.1 μm (with polarization parallel to the rod) into strongly concentrated fields at the rod extremities (hot spots)31. The horizontally polarized light backscattered from the tip-antenna configuration is detected with a pseudo-heterodyne Michelson interferometer33 (Fig. 2). Demodulation of the detector signal at nΩ yields the amplitude /En/ and phase shift Δϕn. To identify the hot spots, we scan the tip across the antenna and record the sample height, as well as amplitude /En/ and phase Δϕn as a function of the tip position (Fig. 3b). In the amplitude map we observe the strongest signals /En/ at the rod extremities, which experimentally proves the existence of two hot spots on the metal rod. The phase map shows that the signal Δϕn is nearly constant across the antenna.

Bottom Line: Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally.Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process.We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering.

View Article: PubMed Central - PubMed

Affiliation: CIC nanoGUNE Consolider, 20018 Donostia-San Sebastián, Spain.

ABSTRACT
Light scattering at nanoparticles and molecules can be dramatically enhanced in the 'hot spots' of optical antennas, where the incident light is highly concentrated. Although this effect is widely applied in surface-enhanced optical sensing, spectroscopy and microscopy, the underlying electromagnetic mechanism of the signal enhancement is challenging to trace experimentally. Here we study elastically scattered light from an individual object located in the well-defined hot spot of single antennas, as a new approach to resolve the role of the antenna in the scattering process. We provide experimental evidence that the intensity elastically scattered off the object scales with the fourth power of the local field enhancement provided by the antenna, and that the underlying electromagnetic mechanism is identical to the one commonly accepted in surface-enhanced Raman scattering. We also measure the phase shift of the scattered light, which provides a novel and unambiguous fingerprint of surface-enhanced light scattering.

No MeSH data available.


Related in: MedlinePlus