Light phase detection with on-chip petahertz electronic networks

Abstract

Ultrafast, high-intensity light-matter interactions lead to optical-field-driven photocurrents with an attosecond-level temporal response. These photocurrents can be used to detect the carrier-envelope-phase (CEP) of short optical pulses, and enable optical-frequency, petahertz (PHz) electronics for high-speed information processing. Despite recent reports on optical-field-driven photocurrents in various nanoscale solid-state materials, little has been done in examining the large-scale electronic integration of these devices to improve their functionality and compactness. In this work, we demonstrate enhanced, on-chip CEP detection via optical-field-driven photocurrents in a monolithic array of electrically-connected plasmonic bow-tie nanoantennas that are contained within an area of hundreds of square microns. The technique is scalable and could potentially be used for shot-to-shot CEP tagging applications requiring orders-of-magnitude less pulse energy compared to alternative ionization-based techniques. Our results open avenues for compact time-domain, on-chip CEP detection, and inform the development of integrated circuits for PHz electronics as well as integrated platforms for attosecond and strong-field science.

Fig. 1. Electrically connected plasmonic bow-tie nanoantenna arrays.

a Schematic diagram of the gold bow-tie nanoantenna devices on an insulating substrate (V_bias, bias voltage; A, ampere meter; I, net photocurrent). Left nanotriangles are electrically connected to one contact pad, while right nanotriangles are electrically connected to the other. An incident ultrafast optical pulse induces photoelectron emission across the nano-gaps between the nanotriangles. The carrier-envelope-phase (CEP) φ_ce of the pulse affects the photocurrent measured in the external circuit. For φ_ce = π/2 (blue trace), the pulse has two symmetric optical half cycles and the photocurrent in the two opposite directions cancel each other, leading to a zero net photocurrent. For φ_ce = π (orange trace), the pulse has only one strong optical half cycle, and a net photocurrent can be measured. In the experiment, φ_ce is modulated at a carrier-envelope-offset (CEO) frequency f_ceo. This CEO frequency can be measured from the photocurrent spectrum. b SEM image of a plasmonic nanoantenna array consisting of 288 bow-tie nanoantennas. The laser beam spot size in the experiment is also schematically illustrated. Inset: SEM image of a plasmonic bow-tie nanoantenna with a nano-gap of 28 nm. The superimposed color plot shows the simulated optical near-field profile (showing a spatial map of the electric field magnitude normalized by the incident electric field magnitude) of a nanoantenna with similar dimensions. c Simulated extinction spectrum (blue) and field-enhancement spectrum (orange) of the electrically connected bow-tie nanoantenna array. The two extinction peaks are labeled as the bow-tie mode and the wire mode. The gray shaded region illustrates the approximate laser bandwidth used in our experiments (see below for the measured laser spectrum). d Simulated optical near-field profiles (showing spatial maps of the electric field magnitude normalized by the incident electric field magnitude) of the bow-tie mode and the wire mode. The color scale is saturated for better visualization. e Simulated field-enhancement spectra of the plasmonic bow-tie nanoantenna arrays with different connecting wire positions (X_wire labeled in (b) representing the x-distance between the inner edge of the wire and the center of the bow-tie structure). For comparison, the field-enhancement spectrum for a bow-tie nanoantenna array without the connecting wires is also shown. f Simulated time-domain response of the plasmonic bow-tie nanoantenna arrays with different connecting wire positions. The waveforms show the optical field at the nanotriangle tip.

Fig. 2. Electromigration of electrically connected nanoantenna arrays.

a Applied voltage (blue) and current (orange) across a plasmonic nanoantenna array during the electromigration process. Electromigration transformed a short-circuit array into an open-circuit array. b SEM image of a connected plasmonic nanoantenna array after electromigration. The wire position X_wire ≈ 114 nm. The electrical connecting wires were broken and disconnected during electromigration. Inset: zoomed-in image of the connected bow-tie nanoantenna and the broken connecting wire. c If all the bow-tie nanoantennas along a connecting wire were disconnected to begin with, the wire was not affected and remained intact. d If there were shorted bow-tie nanoantennas (red dashed circles) along a connecting wire, the wire was broken via electromigration, eliminating the corresponding nanoantenna column from the functional device. In c&d, only part of the bow-tie nanoantennas along a connecting wire are shown in the image (but all the nanoantennas were inspected with the SEM).

Fig. 3. Carrier-envelope-phase-sensitive current from electrically-connected bow-tie arrays.

a Phase of I_cep as a function of time. A barium fluoride (BaF₂) wedge is inserted into the beam every twenty seconds leading to a measured average shift in φ_ce = 54.4° ± 11°. The orange dashed trace shows a fit to the measured data with a staircase function. The inset shows the average phase value while scanning over the entire array area. b Corresponding value of |I_cep| over the same scan shown in (a). The optical power absorption in the wedge is negligible. The inset shows the amplitude of I_cep while scanning the beam over the entire array. For the nanoantenna array being tested, the wire position X_wire ≈ 147 nm.

Fig. 4. Nanoantenna device degradation during photoemission measurement.

a SEM images of a bow-tie nanoantenna array before and after photoemission measurement. The wire position X_wire ≈ 113 nm. The average gap size of the bow-tie nanoantennas increased from 50.5 ± 3.2 nm to 62.2 ± 11.9 nm after illumination. The contrast variation is caused by charging issues of the insulating substrate during imaging. b Measured and simulated extinction spectra of the nanoantenna array shown in (a) before and after photoemission measurement. The gray shaded area shows the measured spectrum of the supercontinuum source. c Simulated CEP-sensitive photocurrent magnitude |I_cep| vs. the optical near-field for the nanoantenna array before and after photoemission measurement. Inset: Simulated time-domain response of the nanoantenna array before and after photoemission measurement. The waveforms show the plasmonically enhanced optical fields at the nanotriangle tip for the nanoantenna arrays before and after illumination. The shaded areas show the waveforms of the photoemission current calculated from the Fowler-Nordheim theory.

Fig. 5. Results demonstrating SNR, balanced configuration, and noise characterization.

a Spectrum of the photocurrent near f_ceo = 100 Hz, showing the CEP-sensitive current spike and surrounding noise with an SNR of ~254 at 0.5 Hz resolution bandwidth. The nanoantenna array being tested has a wire position X_wire ≈ 155 nm. b Plot of I_0,detected demonstrating the use of V_bias to null the average total detected current signal. The signal is nulled when V_bias ≈ −0.45 V. c Plot of the noise signal I_noise vs. the equivalent shot noise current I_eq for two arrays while setting V_bias = 3 V. Blue dots represent results from Array 1 at 150 Hz, the green dots represent Array 2 at 100 Hz, and the purple dots represent Array 2 at 340 Hz. The orange reference curve represents $\sqrt{2q\mathrm{\Delta }f{I}_{\mathrm{eq}}}$. d Power spectral density (psd) as a function of frequency for Array 2 of c. The CEP was unlocked. Reference curves demarcate 1/f^3/2 scaling and the shot-noise floor respectively. The visible notes are due to background power-line noise (at 120 Hz, 180 Hz, and 300 Hz, respectively).