Showing papers on "High harmonic generation published in 2011"
TL;DR: In this article, high-order harmonic generation is observed in a bulk crystalline solid with important implications for attosecond science, where the host medium for this interaction is typically a gas.
Abstract: High-order harmonic generation is a nonlinear optical process that enables the creation of light pulses at frequencies much higher than that from a seed laser. The host medium for this interaction is typically a gas. Now, the process has been observed in a bulk crystalline solid with important implications for attosecond science.
1,264 citations
TL;DR: In this article, the use of hollow-core photonic crystal fibers (PCFs) in the field of ultrafast gas-based nonlinear optics, including recent experiments, numerical modeling, and a discussion of future prospects, is discussed.
Abstract: We review the use of hollow-core photonic crystal fibers (PCFs) in the field of ultrafast gas-based nonlinear optics, including recent experiments, numerical modeling, and a discussion of future prospects. Concentrating on broadband guiding kagome-style hollow-core PCF, we describe its potential for moving conventional nonlinear fiber optics both into extreme regimes—such as few-cycle pulse compression and efficient deep ultraviolet wavelength generation—and into regimes hitherto inaccessible, such as single-mode guidance in a photoionized plasma and high-harmonic generation in fiber.
338 citations
TL;DR: Second- and third-harmonic generation in a centrosymmetric CMOS-compatible material using ring resonators and integrated optical waveguides is demonstrated and phase matching of the harmonic processes occurs due to the near coincidence of indices of refraction of the fundamental mode at the pump frequency.
Abstract: We demonstrate second- and third-harmonic generation in a centrosymmetric CMOS-compatible material using ring resonators and integrated optical waveguides. The χ(2) response is induced by using the nanoscale structure of the waveguide to break the bulk symmetry of silicon nitride (Si3N4) with the silicon dioxide (SiO2) cladding. Using a high-Q ring resonator cavity to enhance the efficiency of the process, we detect the second-harmonic output in the visible wavelength range with milliwatt input powers at telecom wavelengths. We also observe third-harmonic generation from the intrinsic χ(3) susceptibility of the silicon nitride. Phase matching of the harmonic processes occurs due to the near coincidence of indices of refraction of the fundamental mode at the pump frequency and the corresponding higher-order modes of the harmonic fields.
314 citations
TL;DR: Elect electrically tunable harmonic generation of light from a plasmonic nanocavity filled with a nonlinear medium is demonstrated.
Abstract: Plasmonics provides a route to develop ultracompact optical devices on a chip by using extreme light concentration and the ability to perform simultaneous electrical and optical functions. These properties also make plasmonics an ideal candidate for dynamically controlling nonlinear optical interactions at the nanoscale. We demonstrate electrically tunable harmonic generation of light from a plasmonic nanocavity filled with a nonlinear medium. The metals that define the cavity also serve as electrodes that can generate high direct current electric fields across the nonlinear material. A fundamental wave at 1.56 micrometers was frequency doubled and modulated in intensity by applying a moderate external voltage to the electrodes, yielding a voltage-dependent nonlinear generation with a normalized magnitude of ~7% per volt.
249 citations
TL;DR: The spectral broadening of ~1 μJ 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber results in efficient emission of dispersive waves in the deep-UV region.
Abstract: We report on the spectral broadening of ~1 μJ 30 fs pulses propagating in an Ar-filled hollow-core photonic crystal fiber. In contrast with supercontinuum generation in a solid-core photonic crystal fiber, the absence of Raman and unique pressure-controlled dispersion results in efficient emission of dispersive waves in the deep-UV region. The UV light emerges in the single-lobed fundamental mode and is tunable from 200 to 320 nm by varying the pulse energy and gas pressure. The setup is extremely simple, involving <1 m of a gas-filled photonic crystal fiber, and the UV signal is stable and bright, with experimental IR to deep-UV conversion efficiencies as high as 8%. The source is of immediate interest in applications demanding high spatial coherence, such as laser lithography or confocal microscopy.
198 citations
TL;DR: In this paper, an analytical theory of the nonlinear electromagnetic response of a two-dimensional (2D) electron system in the second order in the electric field amplitude is developed, and the second-order polarizability and the intensity of the second harmonic signal are calculated within the selfconsistent field approach both for semiconductor 2D electron systems and for graphene.
Abstract: An analytical theory of the nonlinear electromagnetic response of a two-dimensional (2D) electron system in the second order in the electric field amplitude is developed. The second-order polarizability and the intensity of the second harmonic signal are calculated within the self-consistent-field approach both for semiconductor 2D electron systems and for graphene. The second harmonic generation in graphene is shown to be about 2 orders of magnitude stronger than in GaAs quantum wells at typical experimental parameters. Under the conditions of 2D plasmon resonance the second harmonic radiation intensity is further increased by several orders of magnitude.
187 citations
TL;DR: Extreme-ultraviolet high-order-harmonic pulses with 1.6·10(7) photons/pulse at 32.5 eV have been separated from multiple harmonic orders by a time-preserving monochromator using a single grating in the off-plane mount to give minimum temporal broadening and high efficiency.
Abstract: Extreme-ultraviolet high-order-harmonic pulses with 1.6·10(7) photons/pulse at 32.5 eV have been separated from multiple harmonic orders by a time-preserving monochromator using a single grating in the off-plane mount. This grating geometry gives minimum temporal broadening and high efficiency. The pulse duration of the monochromatized harmonic pulses has been measured to be in the range 20 to 30 fs when the harmonic process is driven by an intense 30 fs near-infrared pulse. The harmonic photon energy is tunable between 12 and 120 eV. The instrument is used in the monochromatized branch of the Artemis beamline at the Central Laser Facility (UK) for applications in ultrafast electron spectroscopy.
145 citations
TL;DR: In this article, the most important aspects of the molecular self-probing paradigm are presented, which views the process of high harmonic generation as a molecule being probed by one of its own electrons.
Abstract: This tutorial presents the most important aspects of the molecular self-probing paradigm, which views the process of high harmonic generation as 'a molecule being probed by one of its own electrons'. Since the properties of the electron wavepacket acting as a probe allow a combination of attosecond and Angstrom resolutions in measurements, this idea bears great potential for the observation, and possibly control, of ultrafast quantum dynamics in molecules at the electronic level. Theoretical as well as experimental methods and concepts at the basis of self-probing measurements are introduced. Many of these are discussed as the example of molecular orbital tomography.
122 citations
TL;DR: In this paper, a semiclassical model for plasmon-enhanced high-order harmonic generation (HHG) in the vicinity of metal nanostructures is presented, and the authors show that both the inhomogeneity of the enhanced local fields and electron absorption by the metal surface play an important role in the HHG process and lead to the generation of even harmonics and significantly increased cutoff.
Abstract: We present a semiclassical model for plasmon-enhanced high-order harmonic generation (HHG) in the vicinity of metal nanostructures. We show that, besides the field enhancement, both the inhomogeneity of the enhanced local fields and electron absorption by the metal surface play an important role in the HHG process and lead to the generation of even harmonics and a significantly increased cutoff. For the examples of silver-coated nanocones and bowtie antennas, we predict that the required intensity reduces by up to three orders of magnitude due to plasmonic field enhancement. The study of the enhanced high-order harmonic generation is connected with a finite-element simulation of the electric field enhancement due to the excitation of the plasmonic modes.
119 citations
TL;DR: In this article, a singularity-driven enhancement of the electric field can be achieved even in extremely thin layers of material and the role of nonlinear surface sources in a realistic scenario where a 20-nm layer of CaF2 is excited at 21 μm.
Abstract: We show an alternative path to efficient second- and third-harmonic generation in proximity of the zero crossing points of the dielectric permittivity in conjunction with low absorption. Under these circumstances, any material, either natural or artificial, will show similar degrees of field enhancement followed by strong harmonic generation, without resorting to any resonant mechanism. The results presented in this paper provide a general demonstration of the potential that the zero-crossing-point condition holds for nonlinear optical phenomena. We investigate a generic Lorentz medium and demonstrate that a singularity-driven enhancement of the electric field may be achieved even in extremely thin layers of material. We also discuss the role of nonlinear surface sources in a realistic scenario where a 20-nm layer of CaF2 is excited at 21 μm, where e ∼ 0. Finally, we show similar behavior in an artificial composite material that includes absorbing dyes in the visible range, provide a general tool for the improvement of harmonic generation using the e ∼ 0 condition, and illustrate that this singularity-driven enhancement of the field lowers the thresholds for a plethora of nonlinear optical phenomena.
94 citations
TL;DR: In this paper, the authors characterized the efficiency of Lamb wave mode pairs to generate the cumulative second harmonic in an undamaged aluminum plate and experimentally confirmed the theoretically predicted ratios of the rate of accumulation of the second harmonic amplitude versus propagation distance.
Abstract: This research experimentally characterizes the efficiency of Lamb wave mode pairs to generate the cumulative second harmonic in an undamaged aluminum plate. Previous research developed the theoretical framework for the characteristics of second harmonic generation of Lamb waves in nonlinear elastic plates, and identified five mode types where the amplitude of the measured second harmonic should increase linearly with ultrasonic wave propagation distance. The current research considers one of these five mode types, Lamb wave mode pairs at the longitudinal velocity, and experimentally confirms the theoretically predicted ratios of the rate of accumulation of the second harmonic amplitude versus propagation distance for two different Lamb wave mode pairs. By comparing these rates of accumulation, these experimental results are used to characterize the measurement efficiency of the mode pairs under consideration.
TL;DR: In this article, high-order harmonic generation (HHG) by intense infrared laser in atomic and molecular targets taking into account the macroscopic propagation of both fundamental and harmonic fields is investigated.
Abstract: We report theoretical calculations of high-order harmonic generation (HHG) by intense infrared lasers in atomic and molecular targets taking into account the macroscopic propagation of both fundamental and harmonic fields. On the examples of Ar and ${\mathrm{N}}_{2}$, we demonstrate that these ab initio calculations are capable of accurately reproducing available experimental results with isotropic and aligned target media. We further present detailed analysis of HHG intensity and phase under various experimental conditions, in particular, as the wavelength of the driving laser changes. Most importantly, our results strongly support the factorization of HHG at the macroscopic level into a product of a returning electron wave packet and the photorecombination transition dipole under typical experimental conditions. This implies that the single-atom or single-molecule structure information can be retrieved from experimentally measured HHG spectra.
TL;DR: This work demonstrates control of short and long quantum trajectories in high harmonic emission through the use of an orthogonally polarized two-color field using a drive field from a femtosecond laser to control the relative phase ϕ between the two fields.
Abstract: We demonstrate control of short and long quantum trajectories in high harmonic emission through the use of an orthogonally polarized two-color field. By controlling the relative phase ϕ between the two fields we show via classical and quantum calculations that we can steer the two-dimensional trajectories to return, or not, to the core and so control the relative strength of the short or long quantum trajectory contribution. In experiments, we demonstrate that this leads to robust control over the trajectory contributions using a drive field from a femtosecond laser composed of the fundamental ω at 800 nm (intensity ∼1.2×10(14) W cm(-2)) and its weaker orthogonally polarized second harmonic 2ω (intensity ∼0.3×10(14) W cm(-2)) with the relative phase between the ω and 2ω fields varied simply by tilting a fused silica plate. This is the first demonstration of short and long quantum trajectory control at the single-atom level.
TL;DR: An experimental and theoretical study of the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse are presented and their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency combs are discussed.
Abstract: Intrinsic to the process of high-order harmonic generation is the creation of plasma and the resulting spatiotemporal distortions of the driving laser pulse. Inside a high-finesse cavity where the driver pulse and gas medium are reused, this can lead to optical bistability of the cavity-plasma system, accumulated self-phase modulation of the intracavity pulse, and coupling to higher-order cavity modes. We present an experimental and theoretical study of these effects and discuss their implications for power scaling of intracavity high-order harmonic generation and extreme ultraviolet frequency combs.
TL;DR: In this article, the Cooper minimum in high-order-harmonic generation from argon atoms by using long wavelength laser pulses was studied and shown to be systematically shifted with respect to total photoionization cross section measurements.
Abstract: We study the Cooper minimum in high-order-harmonic generation from argon atoms by using long wavelength laser pulses. We find that the minimum in high-order-harmonic spectra is systematically shifted with respect to total photoionization cross section measurements. We use a semiclassical theoretical approach based on classical trajectory Monte Carlo and quantum electron scattering methods to model the experiment. Our study reveals that the shift between photoionization and high-order-harmonic emission is due to several effects: the directivity of the recombining electrons and emitted polarization, and the shape of the recolliding electron wave packet.
TL;DR: A wideband frequency multiplier that effectively generates and combines the even harmonics from multiple transistors is proposed, taking advantage of standing-wave formation and loss cancellation in a distributed structure to generate high amplitude signals resulting in high harmonic power.
Abstract: A wideband frequency multiplier that effectively generates and combines the even harmonics from multiple transistors is proposed. It takes advantage of standing-wave formation and loss cancellation in a distributed structure to generate high amplitude signals resulting in high harmonic power. Wide bandwidth operation and odd harmonic cancellation around the center frequency are the inherent properties of this frequency multiplier. Using this methodology, we implemented a frequency doubler that operates from 220 GHz to 275 GHz in a standard 65 nm CMOS process. Output power of 6.6 dBm (0.22 mW) and conversion loss of 11.4 dB are measured at 244 GHz.
TL;DR: In this paper, a refined model of the second harmonic generation ripples spacing theory was proposed, taking into account the modified femtosecond laser excited silicon refractive index n∗ related to the Drude model.
Abstract: Periodic high spatial frequency ripples structures have been generated in silicon under femtosecond laser pulses irradiation. The period of the ripples is wavelength dependent. It increases from 110 up to 160 nm when the wavelength varies from 700 to 950 nm, respectively. We propose a refined model of the second harmonic generation ripples spacing theory Λ=λ/2nλ∗ taking into account the modified femtosecond laser excited silicon refractive index n∗ related to the Drude model. Good agreement is found between experimental results and the presented revisited model.
TL;DR: The first measurement of the attosecond emission generated from underdense plasma produced on a solid target is presented, and high-order harmonics of a femtosecond Ti:sapphire laser focused in a weakly ionized under dense chromium plasma are generated.
Abstract: We present the first measurement of the attosecond emission generated from underdense plasma produced on a solid target. We generate high-order harmonics of a femtosecond Ti:sapphire laser focused in a weakly ionized underdense chromium plasma. Using the “Reconstruction of Attosecond Beating by Interference of Two-photon Transitions” (RABITT) technique, we show that the 11th to the 19th harmonic orders form in the time domain an attosecond pulse train with each pulse having 300 as duration, which is only 1.05 times the theoretical Fourier transform limit. Measurements reveal a very low positive group delay dispersion of 4200 as2. Beside its fundamental interest, high-order harmonic generation in plasma plumes could thus provide an intense source of attosecond pulses for applications.
TL;DR: First HHG results are demonstrated with Xe showing significant cutoff extension to >85 eV with an efficiency of ~10-10 per harmonic, limited by the maximum gas pressure and flow into the chamber, demonstrating the potential of this 2.1-μm source for scaling of photon energy and flux in the water-window range when applied to Ne and He at kHz repetition rate.
Abstract: We report on a kHz, mJ-level, carrier-envelope phase (CEP)-stable ultrabroadband optical parametric chirped-pulse amplifier (OPCPA) at 2.1-μm wavelength, pumped by a high-energy, 14 ps, cryogenic Yb:YAG pump laser, and its application to high-order harmonic generation (HHG) with Xe. The pre-amplifier chain is pumped by a 12-ps Nd:YLF pump laser and both pump lasers are optically synchronized to the signal pulse of the OPCPA. An amplified pulse energy of 0.85 mJ was obtained at the final OPCPA stage with good beam profile. The pulse is compressed to 4.5 optical cycles ( 85 eV with an efficiency of ~10-10 per harmonic, limited by the maximum gas pressure and flow into the chamber. This demonstrates the potential of this 2.1-μm source for scaling of photon energy and flux in the water-window range when applied to Ne and He at kHz repetition rate.
TL;DR: A dramatically improved XUV frequency comb is reported on, producing record power levels to date in the 50-150 nm spectral region based on intracavity high harmonic generation, and dual-comb spectroscopy in this physically rich spectral region is proposed and analyzed for the first time.
Abstract: Previous work has shown that use of a passive enhancement cavity designed for ultrashort pulses can enable the up-conversion of the fs frequency comb into the extreme ultraviolet (XUV) spectral region utilizing the highly nonlinear process of high harmonic generation This promising approach for an efficient source of highly coherent light in this difficult to reach spectral region promises to be a unique tool for precision spectroscopy and temporally resolved measurements Yet to date, this approach has not been extensively utilized due in part to the low powers so far achieved and in part due to the challenges in directly probing electronic transitions with the frequency comb itself We report on a dramatically improved XUV frequency comb producing record power levels to date in the 50-150 nm spectral region based on intracavity high harmonic generation We measure up to 77 μW at the 11th harmonic of the fundamental (72 nm) with μW levels down to the 15th harmonic (53nm) Phase-matching and related design considerations unique to intracavity high harmonic generation are discussed, guided by numerical simulations which provide insight into the role played by intracavity ionization dynamics We further propose and analyze dual-comb spectroscopy in the XUV and show that the power levels reported here permit this approach for the first time Dual-comb spectroscopy in this physically rich spectral region promises to enable the study of a significantly broader range of atomic and molecular spectra with unprecedented precision and accuracy
TL;DR: A novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources (>2) within the Rayleigh range.
Abstract: High harmonic generation (HHG) is a central driver of the rapidly growing field of ultrafast science. We present a novel quasiphase-matching (QPM) concept with a dual-gas multijet target leading, for the first time, to remarkable phase control between multiple HHG sources ($g2$) within the Rayleigh range. The alternating jet structure with driving and matching zones shows perfect coherent buildup for up to six QPM periods. Although not in the focus of the proof-of-principle studies presented here, we achieved competitive conversion efficiencies already in this early stage of development.
TL;DR: In this article, the authors consider high-order harmonic generation from a single atom or molecule and show that the generated harmonic field is proportional to the dipole velocity, and derive this result by solving the wave equation for an infinitely thin gas.
Abstract: We consider high-order harmonic generation from a single atom or molecule and show that the generated harmonic field is proportional to the dipole velocity. We derive this result by solving Maxwell's wave equation for an infinitely thin gas. Hence, the dipole velocity form is the one that relates directly to the harmonic field, and it should be used as a reference when performing calculations with the dipole or acceleration forms.
TL;DR: The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence, and a cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths.
Abstract: The injection of a seed in a free-electron laser (FEL) amplifier reduces the saturation length and improves the longitudinal coherence. A cascaded FEL, operating in the high-gain harmonic-generation regime, allows us to extend the beneficial effects of the seed to shorter wavelengths. We report on the first operation of a high-gain harmonic-generation free-electron laser, seeded with harmonics generated in gas. The third harmonics of a Ti:sapphire laser, generated in a gas cell, has been amplified and up-converted to its second harmonic (?rad=133nm) in a FEL cascaded configuration based on a variable number of modulators and radiators. We studied the transition between coherent harmonic generation and superradiant regime, optimizing the laser performances with respect to the number of modulators and radiators. © 2011 American Physical Society.
TL;DR: In this paper, the authors theoretically investigate the time-frequency characteristics of high-order harmonic generation from Bloch-oscillating electrons in the band structure of a conventional bulk semiconductor driven by a single-optical-cycle two-color IR waveform.
Abstract: We theoretically investigate the time-frequency characteristics of high-order harmonic generation (HHG) from Bloch-oscillating electrons in the band structure of a conventional bulk semiconductor driven by a single-optical-cycle two-color IR waveform. Spectrally filtering out the Bloch-HHG cutoff radiation allows the generation of an isolated Bloch-HHG pulse of $\ensuremath{\sim}$1.6-fs duration.
TL;DR: In this paper, the authors investigate high-order harmonic generation from laser-irradiated systems that support a shape resonance and calculate the harmonic spectra and the time-frequency analysis of the harmonic intensity and phase.
Abstract: We investigate high-order harmonic generation from laser-irradiated systems that support a shape resonance. From the numerical solution of the time-dependent Schr\"odinger equation, we calculate the harmonic spectra and the time-frequency analysis of the harmonic intensity and phase. The analysis reveals the separate contributions of the short and long trajectories as well as the resonance. A range of harmonics is strongly enhanced by the presence of the resonance irrespective of the pulse length. The signature of the resonance remains significant after coherent summation over intensities as a simple method to simulate macroscopic effects. The time-frequency analysis supports the recently proposed four-step mechanism of the enhanced harmonic generation process.
TL;DR: It is expected that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.
Abstract: High harmonic spectra show that laser-induced strong field ionization of water has a significant contribution from an inner-valence orbital. Our experiment uses the ratio of ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ high harmonic yields to isolate the characteristic nuclear motion of the molecular ionic states. The nuclear motion initiated via ionization of the highest occupied molecular orbital (HOMO) is small and is expected to lead to similar harmonic yields for the two isotopes. In contrast, ionization of the second least bound orbital (HOMO-1) exhibits itself via a strong bending motion which creates a significant isotope effect. We elaborate on this interpretation by solving the time-dependent Schr\"odinger equation to simulate strong field ionization and high harmonic generation from the water isotopes. We expect that this isotope marking scheme for probing excited ionic states in strong field processes can be generalized to other molecules.
TL;DR: The results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.
Abstract: We report on single-pass high-harmonic generation (HHG) with amplified driving laser pulses at a repetition rate of 20.8 MHz. An Yb:YAG Innoslab amplifier system provides 35 fs pulses with 20 W average power at 1030 nm after external pulse compression. Following tight focusing into a xenon gas jet, we observe the generation of high-harmonic radiation of up to the seventeenth order. Our results show that state-of-the-art amplifier systems have become a promising alternative to cavity-assisted HHG for applications that require high repetition rates, such as frequency comb spectroscopy in the extreme UV.
TL;DR: In this paper, the influence of mismatching of the group velocities on the generation of the second harmonic by propagation of a primary Lamb wave tone burst with a finite duration was analyzed.
Abstract: Within the second-order perturbation approximation, the physical process of cumulative second-harmonic generation by the primary Lamb wave propagation has been investigated in the time domain. Based on the preconditions that the transfer of energy from the primary Lamb wave to the double frequency Lamb wave is not zero and that the phase velocity matching condition is satisfied, we focus on analyzing the influence of mismatching of the group velocities on the generation of the second harmonic by propagation of a primary Lamb wave tone burst with a finite duration. Our analysis indicates that the time-domain envelope of the second harmonic generated is dependent on the propagation distance when both the duration of the primary Lamb wave tone burst and the group velocity mismatch are given. Furthermore, it can be concluded that the integrated amplitude of the time-domain second harmonic, which quantifies the efficiency of the second-harmonic generation, grows with the propagation distance even when the grou...
TL;DR: In this paper, the authors showed that the combination of a strong terahertz (THz) field with up to 12 fs laser pulses can be an effective gating technique to generate single attosecond pulses.
Abstract: High-order harmonic generation by few-cycle 800 nm laser pulses in neon gas in the presence of a strong terahertz (THz) field is investigated numerically with propagation effects taken into account. Our calculations show that the combination of THz fields with up to 12 fs laser pulses can be an effective gating technique to generate single attosecond pulses. We show that in the presence of the strong THz field only a single attosecond burst can be phase matched, whereas radiation emitted during other half cycles disappears during propagation. The cutoff is extended and a wide supercontinuum appears in the near-field spectra, extending the available spectral width for isolated attosecond pulse generation from 23 to 93 eV. We demonstrate that phase-matching effects are responsible for the generation of isolated attosecond pulses, even in conditions when single-atom response yields an attosecond pulse train.
TL;DR: The possibilities to utilize field enhancement by specifically designed metal nanostructures for the generation of single attosecond pulses using the polarization gating technique are investigated and the possibility to generate previously inaccessible high-harmonics with circular polarization is indicated.
Abstract: We investigate possibilities to utilize field enhancement by specifically designed metal nanostructures for the generation of single attosecond pulses using the polarization gating technique. We predict the generation of isolated 59-attosecond-long pulses using 15-fs pump pulses with only a 0.6 TW/cm2 intensity. Our simulations also indicate the possibility to generate previously inaccessible high-harmonics with circular polarization by using an ensemble of vertically and horizontally orientated bow-tie structures. In the numerical simulation we used an extended Lewenstein model, which includes the spatial inhomogeneity in the hot spots and collisions of electrons with the metal surface.