Frequency comb has shown remarkable potential in time/frequency metrology, atomic/molecular spectroscopy and precision LIDARs. It will create novel possibilities in nano-photonics and plasmonics; however, its interrelation with surface plasmons is unexplored despite the important role that plasmonics play in nonlinear spectroscopy and quantum optics through the manipulation of light in a subwavelength scale. We demonstrate that frequency comb can be transferred by plasmonic nanostructures without noticeable degradation of less than 6.51×10<sup>-19</sup> in absolute position and 1 Hz in linewidth, which implies frequency comb’s potential applications in nanoplasmonic spectroscopy, quantum metrology and subwavelength photonic circuits.
Recently, ultrafast strong field induced optical current in SiO<sub>2 </sub>dielectric medium has demonstrated. By foaming laser
intensity more than 10<sup>13</sup> W•cm<sup>-2</sup> in the dielectric material, the optical current was generated in a dielectric gap without
any DC bias. This phenomenon is affected by the strength electric field of incident laser field and the generated electrons
follow the speed of optical frequency enabling lightfast electronics in the future. In this study, we especially adopted
nanoplasmonic field to trigger and control current flow in a nanometer spatial resolution. Nanoplasmonic field enables to
manipulate light field in nanoscale domain. By using nanoplasmonic field, optically induced current flow can be
selectively controlled by characteristic of nanoplasmonic nanostructure.
For the first demonstration, saw tooth like 2-D nano Au pattern was numerically and experimentally investigated to boost
up the laser intensity of incident 4.5 fs laser pulse with minimum field distortion and broadening. The intensity
enhancement factor of plasmonic field at the saw tooth tip was ~40, enabling Wannier–Stark effect with incidence
intensity level of only 10<sup>11</sup>W•cm<sup>-2</sup> in the TiO<sub>2</sub> substrate. The carrier envelope phase of laser pulse is controlled to
measure ultrafast optical current generation in dielectric medium by plasmonically induced strong near-field. This will be
the basis for developing practical lightfast optical electronics in the future.
Ultrashort extreme-ultraviolet (EUV) light pulses are an important tool for time-resolved pump-probe spectroscopy to
investigate the ultrafast dynamics of electrons in atoms and molecules. Among several methods available to generate
ultrashort EUV light pulses, the nonlinear frequency upconversion process of high-harmonic generation (HHG) draws
attention as it is capable of producing coherent EUV pulses with precise control of burst timing with respect to the
driving near-infrared (NIR) femtosecond laser. In this report, we present and discuss our recent experimental data
obtained by the plasmon-driven HHG method that generate EUV radiation by means of plasmonic nano-focusing of NIR
femtosecond pulses. For experiment, metallic waveguides having a tapered hole of funnel shape inside were fabricated
by adopting the focused-ion-beam process on a micro-cantilever substrate. The plasmonic field formed within the funnelwaveguides
being coupled with the incident femtosecond pulse permitted intensity enhancement by a factor of ~350,
which creates a hot spot of sub-wavelength size with intensities strong enough for HHG. Experimental results showed
that with injection of noble gases into the funnel-waveguides, EUV radiation is generated up to wavelengths of 32 nm
and 29.6 nm from Ar and Ne gas atoms, respectively. Further, it was observed that lower-order EUV harmonics are cut
off in the HHG spectra by the tiny exit aperture of the funnel-waveguide.
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
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 waveguide's geometrical parameters as
verified in our preliminary experimental results.
High harmonic generation is a well-established optical method to produce coherent short-wavelength light in the
ultraviolet and soft-X ray range. This nonlinear conversion process requires ultrashort pulse lasers of strong intensity
exceeding the threshold of 10<sup>13</sup> Wcm<sup>-2</sup> to ionize noble gas atoms. Chirped pulse amplification (CPA) is popularly used to
increase the intensity power of a femtosecond laser produced from an oscillator. However, CPA requires long cavities
for multi-staged power amplification, restricting its practical uses due to hardware bulkiness and fragility. Recently, we
successfully exploited the phenomenon of localized surface plasmon resonance for high harmonic generation, which
enables replacing CPA with a compact metallic nanostructure. Surface plasmon resonance induced in a well-designed
nanostructure allows for intensity enhancement of the incident laser field more than 20 dB. For experimental validation,
a 2D array of gold bowtie nanostructure was fabricated on a sapphire substrate by the focused-ion-beam process. By
injection of argon and xenon gas atoms onto the bowtie nanostructure, high harmonics up to 21<sup>st</sup> order were produced
while the incident laser intensity remains at only 10<sup>11</sup> Wcm<sup>-2</sup>. In conclusion, the approach of exploiting surface plasmons
resonance offers an important advantage of hardware compactness in high harmonic generation.
When a metallic nanostructure is illuminated by ultrashort light pulses, the excitation of surface plasmons is observed
along with subsequent strong enhancement of the electric field in the vicinity of the nanostructure. This localized surface
plasmonic resonance is exploited to generate coherent extreme ultraviolet light and soft-X ray by interacting noble gas
atoms with femtosecond laser pulses. The resulting field enhancement is much affected by the 3-D shape of the used
nanostructure, so various nanostructure shapes are examined through finite-difference time-domain analysis to predict
their performance in high harmonic generation.
We report a preliminary result of scanning probe nanofabrication using an AFM (atomic force microscopy) tip with assistance of femtosecond laser pulses to enhance fabrication capability. Illumination of the AFM tip with ultra-short light pulses induces a strong electric field between the tip and the metal surface, which allows removing metal atoms from the surface by means of field evaporation. Computer simulation reveals that the intended field evaporation is triggered even in air when the induced electric field reaches the level of a few volts per nanometer, which is low enough to avoid unwanted thermal damages on most metal surfaces. For experimental validation, a Ti:sapphire femtosecond pulse laser with 10 fs pulse duration at 800 nm center wavelength was used with a tip coated with gold to fabricate nano-size holes on a thin film gold surface. Experimental results demonstrate that fine holes with a diameter of less than ~10 nm can be successfully made with precise control of the intensity of femtosecond laser pulses.