In-Yong Park

High-harmonic generation by resonant plasmon field enhancement

High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light. This nonlinear conversion process requires high pulse intensities, greater than 1013 W cm-2, which are not directly attainable using only the output power of a femtosecond oscillator. Chirped-pulse amplification enables the pulse intensity to exceed this threshold by incorporating several regenerative and/or multi-pass amplifier cavities in tandem. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity. Here we demonstrate a method of high-harmonic generation that requires no extra cavities. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate. In our experiment, the output beam emitted from a modest femtosecond oscillator (100-kW peak power, 1.3-nJ pulse energy and 10-fs pulse duration) is directly focused onto the nanostructure with a pulse intensity of only 1011 W cm-2. The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to 47 nm by injection with an argon gas jet. The method could form the basis for constructing laptop-sized EUV light sources for advanced lithography and high-resolution imaging applications.

Kim et al. reply

Replying to M. Sivis, M. Duwe, B. Abel & C. Ropers 485, 10.1038/nature10978 (2012)Sivis et al. showed spectral data of extreme ultraviolet (EUV) emission from gas-exposed bow-ties, claiming high predominance of atomic line emission (ALE) of neutral and ionized gas atoms in contradiction to our data of high harmonic generation (HHG). This is not the first time the signature of ALE has been identified in conventional HHG spectral data. The two distinct phenomena, ALE and HHG, are not mutually exclusive but coexistent when gaseous atoms are illuminated by strong-field laser pulses.

High-harmonic generation by field enhanced femtosecond pulses in metal-sapphire nanostructure

Plasmonic high-harmonic generation (HHG) drew attention as a means of producing coherent extreme ultraviolet (EUV) radiation by taking advantage of field enhancement occurring in metallic nanostructures. Here a metal-sapphire nanostructure is devised to provide a solid tip as the HHG emitter, replacing commonly used gaseous atoms. The fabricated solid tip is made of monocrystalline sapphire surrounded by a gold thin-film layer, and intended to produce EUV harmonics by the inter- and intra-band oscillations of electrons driven by the incident laser. The metal-sapphire nanostructure enhances the incident laser field by means of surface plasmon polaritons, triggering HHG directly from moderate femtosecond pulses of ∼0.1 TW cm−2 intensities. The measured EUV spectra exhibit odd-order harmonics up to ∼60 nm wavelengths without the plasma atomic lines typically seen when using gaseous atoms as the HHG emitter. This experimental outcome confirms that the plasmonic HHG approach is a promising way to realize coherent EUV sources for nano-scale near-field applications in spectroscopy, microscopy, lithography and atto-second physics.

Generation of isolated attosecond pulses using a plasmonic funnel-waveguide

We theoretically investigated the possibility of generating attosecond pulses by means of plasmonic field enhancement induced in a nano-structured metallic funnel-waveguide. This study was motivated by our recent experimental demonstration of ultrashort extreme-ultraviolet (EUV) pulses using the same type of three-dimensional waveguides. Here, with emphasis on generation of isolated attosecond pulses, the finite-domain time-difference method was used to analyze the funnel-waveguide with respect to the geometry-dependent plasmonic features such as the field enhancement factor, enhanced plasmonic field profile and hot-spot location. Then an extended semi-classical model of high-order harmonic generation was adopted to predict the EUV spectra generated from the funnel-waveguide in consideration of the spatial inhomogeneity of the plasmonic field within the hot-spot volume. Our simulation finally proved that isolated attosecond pulses can be produced at fast repetition rates directly from a few-cycle femtosecond laser or by synthesizing a two-color laser consisting of two multi-cycle pulses of cross-polarized configuration.

High harmonic generation by guided surface plasmon polaritons

We discuss how the intriguing phenomenon of surface plasmon resonance (SPR) can be exploited in enhancing the intensity field of the incident femtosecond laser for the purpose of high harmonic generation (HHG). We first summarize our previous attempt made with a 2-D planar nanostructure comprised of metallic bow-tie nano-antennas, which enabled us to generate up to 21st harmonics from Xenon gas using 1-nJ pulse energy with an intensity enhancement factor of ~20 dB. Then we describe another attempt currently being made by devising a 3-D nano-waveguide with the aim of improving the HHG conversion efficiency by expanding the localized volume of field enhancement by means of propagating surface plasmon polaritons (SPPs). Our finite-difference time-domain (FDTD) calculation shows that the enhanced volume can be increased significantly by optimal selection of the waveguides geometrical parameters as verified in our preliminary experimental results.

Plasmonic field enhancement for generating ultrashort extreme-ultraviolet light pulses

High-harmonic generation to produce ultrashort EUV pulses by frequency-upconversion of near-infrared (NIR) pulses requires strong laser intensities. Here we describe a 3-dimensional metallic waveguide that enables plasmonic generation of ultrashort EUV pulses through field enhancement by means of surface-plasmon polaritons. Details on the design and fabrication of the plasmonic waveguide on the tip of a cantilever nanostructure are explained along with discussions on experimental data.

High Harmonic Generation by Plasmonic Enhancement of Femtosecond Pulse Laser

Coherent EUV light is generated directly from a modest femtosecond laser whose pulse intensity reaches only 1011 W cm− 2. This value is in fact two orders of magnitude lower than the threshold pulse intensity required for high harmonic generation using noble gases. The key to this achievement is the plasmonic field enhancement by means of metallic nanostructures fabricated in bow-tie shape. This method requires no extra cavities for amplification of pulse intensity and thereby could form the basis to construct laptop-sized coherent EUV sources for many advanced applications lithographic and high-resolution imaging.

Serial number coding and decoding by laser interference direct patterning on the original product surface for anti-counterfeiting

Here, we investigate a method to distinguish the counterfeits by patterning multiple reflective type grating directly on the surface of the original product and analyze the serial number from its rotation angles of diffracted fringes. The micro-sized gratings were fabricated on the surface of the material at high speeds by illuminating the interference fringe generated by passing a high-energy pulse laser through the Fresnel biprism. In addition, analysis of the gratings diffraction fringes was performed using a continuous wave laser.

Observation of strongly enhanced ultrashort pulses in 3-D metallic funnel-waveguide

For strong field enhancement of ultrashort light pulses, a 3-D metallic funnel-waveguide is analyzed using the finite-difference time-domain (FDTD) method. Then the maximum intensity enhancement actually developed by the funnel-waveguide upon the injection of femtosecond laser pulses is observed using two-photon luminescence (TPL) microscopy. In addition, the ultrafast dephasing profile of the localized field at the hot spot of the funnel-waveguide is verified through the interferometric autocorrelation of the TPL signal. Finally it is concluded the funnel-waveguide is an effective 3-D nanostructure that is capable of boosting the peak pulse intensity of stronger than 80 TWcm(-2) by an enhancement factor of 20 dB without significant degradation of the ultrafast spatiotemporal characteristics of the original pulses.

Plasmon-driven extreme-UV light

Extreme-UV (EUV) light of short optical wavelengths (below 120nm) is key to advances in many fields of science and technology. In microscopy and lithography, for example, this type of radiation improves the ultimate resolution achievable in imaging and patterning. Coherent EUV radiation is emitted by free electrons orbiting a synchrotron, but its use is largely limited to scientific research. For industrial applications, laser-produced plasma sources are being developed to generate high-energy photons from ionized (charged) atoms. However, EUV-generating plasmas require laser intensities higher than 1011Wcm 2. These are not easily achieved with semiconductor or solid-state lasers. Moreover, plasma-based EUV sources are not coherent. In other words, they emit in all directions at many independent wavelengths. We are investigating a ‘plasmonic’ method of generating EUV radiation that exploits the strong field enhancement typical of metallic nanostructures under illumination of femtosecond laser pulses.1, 2 Surface plasmon polaritons (SPPs) are electromagnetic waves propagating along a metal-dielectric (insulating) interface. They result from coupling between incident photons and surface plasmons. In nanostructured tapered metallic waveguides with a hollow core, SPPs can be made to follow the geometric shape of the waveguide adiabatically (i.e., no loss by scattering or absorption). Inside the tapered hole, the forward-moving and backward-reflected SPPs are superimposed constructively to create an intense plasmonic standing wave near the tip where the local cross-sectional dimension becomes much smaller than the fundamental wavelength. Consequently, SPPs can be focused beyond the diffraction limit on a subwavelength spot with dramatically enhanced intensity. This intriguing phenomenon of SPP adiabatic nanofocusing can be used to generate EUV pulses directly from near-IR (NIR) pulses Figure 1. Plasmonic funnel waveguide using adiabatic nanofocusing of propagating surface plasmon polaritons (SPPs) inside a tapered nanoscale hole to produce extreme-UV (EUV) radiation through a 30nm exit aperture at a high repetition rate. NIR: Near-IR. KAIST: Korea Advanced Institute of Science and Technology.