Sub-Cycle Effects in Carrier-Envelope-Phase-Sensitive Photoemission from Plasmonic Nanoparticles

Abstract

Abstract:We study carrier-envelope-phase-sensitive (CEP-sensitive) photoemission from plasmonic nanoparticles illuminated with few-cycle laser pulses of varying intensity. The CEPsensitive photocurrent exhibits antiresonant-like behavior due to competing emission from different optical half-cycles. In electronic devices like field-effect transistors, applied electric fields, ranging up to hundreds of gigahertz in frequency, control electron motion. Recently, researchers have demonstrated that electric fields of intense, waveform-controlled optical pulses, i.e. electric fields in the hundreds of terahertz regime, can control electron motion around solid-state nanostructures. Most notably, carrier-envelope-phase-sensitive (CEP-sensitive) effects have been observed in photoemission currents from metallic nano-tips [1,2] and, more recently, from metallic nanoparticles [3,4]. Here, we report measurements of CEP-sensitive photoemission from plasmonic nanoparticles driven by few-cycle laser pulses of variable intensity. As the driving intensity shifts, the CEP-sensitive photocurrent shows antiresonant-like behavior that we explain via a simple, quasistatic tunneling model.Our experimental approach follows earlier work [4] and is illustrated in Fig. 1a. Arrays of gold nanotriangles rest on a transparent and conducting indium tin oxide (ITO) film on a sapphire substrate. An ITO collector electrode sits across a gap (approximately 5 µm) from the nanotriangle emitters; micrographs are provided in Fig. 1b. We illuminate the nanotriangles with tightly-focused (≈ 5 µm beam diameter) few-cycle laser pulses (≈ 10 fs duration at a center wavelength of 1177 nm). The nanotriangles exhibit a localized surface plasmon resonance (LSPR), and when excited by few-cycle laser pulses, the nanotriangles' LSPR and sharp features result in enhanced local optical fields (prior work has shown a field enhancement of ≈ 30 under identical illumination conditions [4]). With our incident pulse energies of 50-160 pJ, we expect enhanced peak fields of 18-32 V/nm (i.e. Keldysh parameters of 0.67-0.38). These strong-fields lead to large photoemission currents. (As illustrated in Fig. 1a, these currents jump from the nanotriangle emitters to the collector.) Additionally, with such strong fields, the photoemission is in the strong-field regime and resembles tunneling. In Fig. 1c we plot the quasistatic, Fowler-Nordheim tunneling current from a gold surface driven by cos 2 -shaped model pulses with durations of 10 fs and center wavelengths of 1177 nm.