Towards femtosecond on-chip electronics based on plasmonic hot electron nano-emitters

Abstract

To combine the advantages of ultrafast femtosecond nano-optics with an on-chip communication scheme, optical signals with a frequency of several hundreds of THz need to be down-converted to coherent electronic signals propagating on-chip. So far, this has not been achieved because of the overall slow response time of nanoscale electronic circuits. Here, we demonstrate that 14 fs optical pulses in the near-infrared can drive electronic on-chip circuits with a prospective bandwidth up to 10 THz. The corresponding electronic pulses propagate in macroscopic striplines on a millimeter scale. We exploit femtosecond photoswitches based on asymmetric, nanoscale metal junctions to drive the pulses. The non-linear ultrafast response is based on a plasmonically enhanced, multiphoton absorption resulting in a field emission of ballistic hot electrons propagating across the nanoscale junctions. Our results pave the way towards femtosecond electronics integrated in wafer-scale THz circuits.

Fig. 1

Femtosecond photoemission in nanoscale junctions and THz on-chip circuits. a Scanning electron microscope (SEM) image of Ti/Au contacts and asymmetric nanojunctions with the emitter (collector) denoted as ‘E’ (‘C’). Scale bar, 2 µm. Inset: Close-up showing the asymmetry between the emitter and collector. Scale bar, 200 nm. b Lateral map of the unipolar photoemission current I_emission at zero bias V_bias across asymmetric nanojunctions (overlaid plot), which are contacted by two Ti/Au striplines (outer SEM image). Scale bar, 5 µm. c Schematic of the on-chip THz time-domain circuit with optical femtosecond pump and probe pulses triggering the electronic read-out. I_emission describes the time-integrated current, while I_transient captures the time-resolved electromagnetic transients in the striplines at a time delay Δt. In all shown experiments, V_bias = 0 V. d Non-linear I_emission vs laser pulse energy E_pulse with a power law fit (red line). e Lower graph shows I_emission vs laser compressor position at a fixed E_pulse = 150 pJ. The three upper insets show the second-harmonic generation frequency-resolved optical gating (SHG-FROG) intensity Î_shg-frog for three given compressor positions denoted by a circle, triangle, and square. All measurements are performed at 77 K and in vacuum

Fig. 2

Asymmetric nanoscale junctions for plasmonically enhanced photoemission. a Schematic energy diagram of the gold-vacuum interface at the emitter with present electric field F_pump = 0.5 V nm⁻¹ (black line). The Fermi energy E_Fermi is ~5.1 eV below the vacuum level (dotted line). The barrier can be overcome by a multiphoton absorption (vertical dashed dotted line) or a tunneling process (horizontal dashed line). The colored lines consider the Schottky effect and a field-enhancement of 1 (blue), 2 (turquoise), 5 (green), and 10 (red). b SEM image of asymmetric nanojunctions with emitter (‘E’) and collector (‘C’) electrodes. Inset: numerically computed field enhancement g within such an asymmetric nanojunction for E_pump = 1.3 eV. c Polarization dependence of I_emission at E_pulse = 150 pJ and a FWHM of 14 fs of the pump pulse. Red line is a cosine fit, the gray area in the center indicates the noise level obtained without illumination

Fig. 3

Non-linear THz pulses in macroscopic on-chip circuits. a Time-resolved I_transient vs Δt (black squares) and fit function (red line) after exciting a nanojunction integrated in the stripline circuits with a 14 fs laser pulse at E_pulse = 124 pJ. The THz signal is detected after a propagation length of 300 µm. b Non-linear I_transient vs E_pulse with a power law fit (E_pulse)^β (red line) showing a similar power law coefficient β as found for I_emission (cf. Fig. 1d). c Microscope image of the utilized THz-circuitries on a sapphire chip. d Dispersion relation of the effective diffraction index n_eff of co-planar gold striplines on a sapphire substrate (inset): black (red) crosses depict numerical simulations with dimensions h = 300 nm, w = 5 µm (1 µm), and s = 10 µm (1 µm). The black (red) line shows an analytical solution for symmetrically spaced striplines with dimensions: h = 300 nm, w = s = 10 µm (1 µm)

Fig. 4

Femtosecond near-field coupling of NIR pulses to THz stripline modes. Simulated electric field distribution of the co-planar striplines (black) with a the odd mode and b the even mode. Color code describes the absolute electric field. The arrows denote the direction of the electric field vector in the image plane. c Spatial map of I_transient at fixed Δt for a sample without nanojunctions (white lines indicate the striplines). The odd mode is excited in the center with a minimum distance of 1 µm between the striplines (open triangles). The even mode is excited at edges where the striplines have a distance of 10 µm (filled triangles). d I_transient vs E_pulse for striplines without nanojunctions showing an almost linear dependence (red line). e Spatial map of I_transient for asymmetric nanojunctions integrated in the striplines with an overlaid SEM image