Compact nanoscale plasmonic field sampler

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Device overview. a Schematic of the nanodevice. b Depiction of the optical-field sampling process. Attosecond electron bursts are driven from an electrically-connected gold nanoantenna (see a) by a strong optical waveform driver (ED(t), red), collected by an adjacent gold wire, then measured using an external current detector. The weak signal waveform (ES(t), blue), with a peak intensity of 1x10-4 that of the driver pulses, modulates the average photocurrent generated by the driver pulse, I(𝞃), as a function of delay, 𝞃 (grey). The amplitude of the signal waveform is artificially magnified in a and b for visibility. c Photograph of the nanocircuit embedded on printed circuit board. d Scanning electron micrograph of the device. e Simulated electric field enhancement around a nanoantenna. The maximum field enhancement is ~35. FE: field enhancement factor. E: polarization direction of the incident electric field.
Figure from: Mina R. Bionta, Felix Ritzkowsky, Marco Turchetti, et al., “On-chip sampling of optical fields with attosecond resolution,” Nat. Photonics 15, 456 (2021).
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Measurement of an electric field through optical-field emission: left Time domain signal of the electric fields: Time-domain results for devices with a height of 240 nm comparing measured (blue) and simulated near-fields (red) to the calculated incident laser field (yellow). Negative delays indicate that the driver pulse arrives before the signal pulse. Note the plasmonic resonance of the antenna results in the dephasing in the time-domain between incident field (yellow) and the measured near field (blue) observed around 12 fs. right Frequency-domain comparison of measured (solid) and simulated (dashed) near-field spectra for devices with a 240 nm height to the measured incident laser spectrum (dotted). This on-resonant 240 nm device shows two peaks present in the cross-correlation data, one corresponding to the output laser spectrum (at 218 THz) and the other to the plasmonic enhancement of the antenna (at 257 THz).
Adapted from: Mina R. Bionta, Felix Ritzkowsky, Marco Turchetti, et al., “On-chip sampling of optical fields with attosecond resolution,” Nat. Photonics 15, 456 (2021).
 

Time-domain field-sampling technology has made tremendous leaps forward in the last hundred years, evolving from waveform streaking using oscilloscopes to optical attosecond streaking using sub-cycle extreme ultraviolet light pulses (G. Sansone et al., Science 314, 443 (2006)). Commercial optical-field-resolved time-domain spectroscopy systems operating in the THz regime (M. Tonouchi Nat. Photonics 1, 97 (2007)) have been available for many years. Such systems are commonly used for industrial applications such as chemical and material analysis, as well as many fundamental scientific applications, and are often found to be superior to conventional frequency-domain techniques. Extending time-domain, electric-field sampling to the optical regime, i.e. the visible and the near-infrared (near-IR), would likewise provide great benefit to both science and industry. For instance, recent extensions of optical-field-resolved sampling techniques to the mid-infrared (MIR) have been performed (I. Pupeza, et al., Nature 577, 52 (2020)) and have demonstrated the tremendous advantages of time-domain sampling compared to traditional frequency-domain spectroscopy with regard to sensitivity and limits of detection. Despite these compelling results, scaling such techniques to the visible and near-IR spectral regimes, which are highly relevant to applications in biology and medicine, has remained challenging. Sub-cycle optical-field sampling in these regions seemingly demands high-energy optical sources and complicated optical apparatuses.

We have developed a chip-scale device for directly tracing weak electric-field waveforms at optical frequencies exceeding 300 THz with attosecond-scale resolution. This enables time-domain, optical-field-resolved spectroscopy of low-energy optical waveforms spanning visible to mid-infrared wavelengths. Our compact optical-field-sampling devices use arrays of electrically-connected resonant nanoantennas that enhance incident optical fields to achieve strong-field photoelectron emission using only picojoules of pulse energy, a physical phenomenon once confined to the regime of high-intensity lasers and gas-phase systems. The sub-cycle, optical-field photoemission from the nanoantennas enables petahertz-level sampling bandwidths. The devices are easy to use, requiring no vacuum environment for operation and minimal laser energy – a picojoule-level driving pulse samples a femtojoule-level signal waveform, which, to the best of our knowledge is six orders of magnitude lower than has previously been measured. They also have the potential to operate from the visible to the terahertz spectral regime.

To our knowledge, the waveforms we sampled have a pulse energy six orders of magnitude lower than those previously reported in the near-infrared. Our devices also reveal dynamical properties of the interaction of the driving optical-field waveform with plasmonic nanoantennas in situ. The compact form-factor of the devices and measurement techniques developed will have a wide-ranging impact.

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