Interaction of ultrashort laser pulses with metal nanotips: a model system for strong-field phenomena

Abstract

We discuss the interaction of ultrashort near-infrared laser pulses with sharp metal tips at moderate nominal intensities (I 0 ∼ 10 11 W cm −2 ). As external electric fields are strongly enhanced at such tips (enhancement factor ∼10) our system turns out to be an ideal miniature laboratory to investigate strong-field effects at solid surfaces. We analyse the electron-energy spectra as a function of the strength of the laser field and the static extraction field and present an intuitive model for their interpretation. The size of the effective field acting on the metal electrons can be determined from the electron spectra. The latter are also reproduced by time-dependent density functional theory (TDDFT) simulations.The emerging field of attosecond science is based on the ability to control electronic wavepackets with the help of well-controlled few-cycle laser pulses [1]. The key effect is the recollision of an electronic wavepacket with the parent matter within a fraction of the laser cycle after its generation through ionization [2,3]. Prominent dynamical processes based on recollision are high-harmonic generation (HHG) and high-order above-threshold ionization (ATI), observed with atomic and molecular gases. The exquisite control over the recolliding wavepacket allows one to take, for example, diffraction images of the parent molecule [4,5] and to achieve attosecond time resolution for electronic processes [6][7][8]. Recently, laser-electricfield-driven recollision processes were also observed with solid-state systems, namely at dielectric nanoparticles [9] and at sharp metal tips [10,11].For versatile imaging techniques applied to atomic, molecular and solid-state systems it would be of great interest to vary the parameters of the returning electron over a large energy range. The same holds true in the process of the generation of high-harmonic photons, which is currently being optimized to match certain flux and energy requirements (see, e.g., [12,13]). Improving and extending the ability to control the motion of the electronic wavepacket requires additional control knobs for the strong fields driving the wavepacket. One obvious choice would be the addition of a static (dc) electric field. Theory suggests [14][15][16] that the application of relatively small static fields, one to four orders of magnitude weaker than the maximum laser field, already leads to a modification of the path of the recolliding wavepacket and, hence, to changes of the resulting HHG spectrum. For example, the maximum HHG energy could be tripled if a static field with a field strength 10% that of the maximum laser field was applied [16]. Although considerably smaller than the amplitude of the moderately strong laser itself, application of such a (macroscopic) static field with a strength of a fraction of a GV m −1 is unfeasible, rendering this control tool exceedingly difficult to employ in laser-atom or laser-molecule interactions.When nanostructures are irradiated with laser light, optical field enhancement takes place. The laser ele...

Спектр Электронов При Холодной Нелинейной Эмиссии Из Металла Под Действием Лазерного Импульса

“…На рис. 5 представлено сравнение результатов расчетов спектра электронов, выполненных в рамках предложен-ной модели, с экспериментальными данными [16]. При численном моделировании использовались следующие параметры: длина волны лазерного излучения 800 nm, длительность лазерного импульса на половине мак-симума интенсивности ∼ 6.5 fs (τ ∼ 13 fs), максимум напряженности лазерного поля F 0 = 9.26 V/nm.…”

“…Эта модуляция спектра не воспроизводится при теоретическом описании в рамках представлений о резкой однородной границе металла. Численное моделирование рассматриваемого процесса, проделанное в [16] на основе теории зависящего от времени функционала плотности с учетом воздействия внешнего поля на электронный газ в металле, также дает некоторое расхождение с экспериментальными ре-зультатами. С хорошей точностью прописывается само положение пиков в интерференционной картине, но не их относительные величины.…”

“…Далее вычисляются матричные эле-менты для амплитуды перехода электрона в конечные состояния, и на их основе получается энергетический спектр. Вторая схема развивается в рамках теории зависящего от времени функционала плотности с ис-пользованием формализма Кона−Шема [16]. При этом электроны в металле считаются независимыми, а их движение происходит в некотором эффективном потен-циале, создаваемом ионами кристаллической решетки при наличии обменного взаимодействия и электронной корреляции.…”

Attosecond physics at a nanoscale metal tip

“…In order to unmask the effects that take place at a metal surface, we have invoked a model applicable to metals: we employ a 1-dimensional time-dependent density functional theory (TDDFT) quantum simulation. Details of the model are given in [6,7], results are shown in Fig. 2.…”

Split ring resonator based THz-driven electron streak camera featuring femtosecond resolution

“…The success of these experiments has depended heavily on advances in ultrafast electron source technology. In the 10-100 kV electron energy regime, pulsed electron guns based on planar photocathodes are capable of producing electron bunches with pulse durations in the few hundred femtosecond regime 4,9-12 while nanotip sources [13][14][15][16][17] and bunch re-compression based systems 8,[18][19][20] are, at least in principle, capable of reaching the 10-100 fs time domain. Pulsed electron sources in the MeV are expected to have a similar performance in terms of electron pulse duration 21,22 .…”