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.
Optical tweezers is a promising manipulation tool for objects in the range of micrometers to nanometers. Although there are many reported works on manipulating objects made of different materials and objects of irregular shapes, it is more suitable for non-opaque materials and objects that are symmetrical. Furthermore, there are potential damages on the objects arising from immense heat that is produced by the laser beam. These problems can be alleviated by trapping objects (micro-handles) and using them collectively as a gripper to indirectly hold and manipulate a target object. Holding denotes equilibrium of forces exerted by the tools on a target object. However, there still is a problem with this approach. When the trapping volume is larger than the size of a tool, target objects get pulled towards the center of the trapping volume. This breaks the force equilibrium and gripping thus fails. In this paper, we report a new design of tools that can overcome this problem. The tool is a slender object with one end acting as a probe while the other end is spherical so that trapping is easy. The length of the tool is designed to be larger than the radius of the trapping volume. Thus the target object is never pulled towards the trapping center. A group of multiple identical tools will surround and push a target object at the probe tips resulting in a stable grasp.
A design of a microscopy system tailored for optical tweezers with a capability of an automatic focusing is presented. In this design, we utilize lenses, motorized mechanical stage, lamp and a digital camera to magnify and see a micrometer sized spheres floating in a thin film of water. The system can automatically translate the stage that holds the specimen to obtain the best focused image. The best focused image is "sharp." Mathematically, the best focused image shows the maximum amount of high frequency terms from the images obtained by translating the stage. The metric that calculates how one image is focused is called the Focus Measure (FM). Unfortunately, low frequency components also increase this FM. And an optical imaging system is a low pass filtering system. Thus the primary concern is to lower the low frequency components in an image. The electric signals from each pixel of a CCD include noises that are inherent in each pixel. The result of this is an FM profile that has multiple local maxima. This is the most critical reason why an Automatic Focusing System (AFS) yields incorrect focusing results. Available techniques have been tested and from this experience, the most appropriate Focus Measure Filter (FMF) that has the sharpest FM despite the low frequency terms and noises has been selected. Furthermore, a maximum search algorithm that is immune to local maxima in an FM profile is discussed. Using this FMF and search algorithm, an Automatic focusing system (AFS) tailored for optical tweezers is presented. The system is implemented on personal computers equipped with Pentium 4 processors.
There are several new tools for manipulating microscopic objects. Among them, optical tweezers (OT) has two distinguishing advantages. Firstly, OT can easily release an object without the need of a complicated detaching scheme. Secondly, it is anticipated to manipulate an object with six degrees of freedom. OT is realized by tightly focusing a laser beam on microscopic objects. Grabbing and releasing is easily done by turning a laser beam on and off. For doing a dexterous manipulation on an object, a complicated potential trap must be calculated and applied. We foresee that such calculation method will be developed in the near future. One of the candidates for implementing the calculated trap is scanning optical tweezers (SOT). SOT can be built by using actuators with a scanning frequency in the order of a hundred Hertz. We need fast scanners to stably trap an object. In this study, we present our design of such SOT. The SOT uses piezo-actuated tilt mirror and objective positioner to scan full three-dimensional workspace.
There are increased needs for manipulating microscopic objects. One of enabling technologies is an instrument called optical tweezers (OT) that uses a focused laser beam to trap and move microscopic objects. OT has been shown effective for directly manipulating spherical, cylindrical or axis-symmetrical shapes. For other forms of shapes that do not show any symmetry, there have been works on using micrometer sized balls as a handle to indirectly manipulate the objects. Direct manipulation is difficult because complex trapping potential needs to be calculated to stably trap non-symmetrical shapes. User interfaces for these "indirect" systems use a computer mouse to design a layout of balls for surrounding (holding) an object and a trajectory that describes how these balls as a whole moves. The contained object pushed by these surrounding balls then moves accordingly. In this study, we introduce an intuitive user interface system for manipulating these balls. Using virtual reality gloves, each finger tip position of an operator is used to position control these balls. This user interface system enables the operator to intuitively grasp, move and release irregular formed shapes.