Yunseok Kim

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.

Distance measurements by combined method based on a femtosecond pulse laser.

We describe a combined interferometric scheme that enables absolute distance measurements using a femtosecond pulse laser. This method is combined with synthetic wavelength interferometry (SWI), time of flight (TOF) and spectrally-resolved interferometry (SRI) using the optical comb of femtosecond laser. Each technique provides distinct measuring resolutions and ambiguity ranges which are complementary to each other. These separate measurement principles are incorporated and implemented simultaneously and the unified output can enhance the dynamic range of the measuring system. Our experimental results demonstrate an example of absolute distance measurement with the proposed technique and we discuss the possibility of the combined method to measure long distances and the important factors for the implementation.

Absolute length calibration of gauge blocks using optical comb of a femtosecond pulse laser

We report an exploitation of the optical comb of a femtosecond pulse laser as the wavelength ruler for the task of absolute length calibration of gauge blocks. To that end, the optical comb was stabilized to the Rb clock of frequency standard and an optical frequency synthesizer was constructed by tuning an external single-frequency laser to the optical comb. The absolute height of gauge blocks was measured by means of multiwavelength interferometry using multiple beams of different wavelengths consecutively provided by the optical frequency synthesizer. The wavelength uncertainty was measured 1.9×10-10 that leads to an overall calibration uncertainty of 15 nm (k=1) in determining the absolute length of gauge blocks.

Origin of surface potential change during ferroelectric switching in epitaxial PbTiO3 thin films studied by scanning force microscopy

We investigated the surface potential of the ferroelectric domains of the epitaxial PbTiO3 (PTO) films using both Kelvin probe and piezoresponse force microscopy. The surface potential changes as a function of applied biases suggested that the amount and sign of surface potentials depend on the correlation between polarization and screen charges. It also suggested that the trapped negative charges exist on the as-deposited PTO surfaces. Injected charges and their resultant surface potentials are investigated by grounded tip scans. The results unveiled the origin of surface potential changes during ferroelectric switching in the epitaxial PTO films.

Magnetoelectric Coupling in Ordered Arrays of Multilayered Heteroepitaxial BaTiO3/CoFe2O4 Nanodots

Fully epitaxial BaTiO(3)/CoFe(2)O(4) ferroelectric/ferromagnetic multilayered nanodot arrays, a new type of magnetoelectric (ME) nanocomposite with both horizontal and vertical orderings, were fabricated via a stencil-derived direct epitaxy technique. By reducing the clamping effect, ferroelectric domain modification and distinct magnetization change proportional to different interfacial area around the BaTiO(3) phase transition temperatures were found, which may pave the way to quantitative introducing of ME coupling at nanoscale and build high density multistate memory devices.

Absolute length measurement with the frequency comb of a femtosecond laser

We report exploiting the frequency comb of a femtosecond laser as a means of realizing the time-based SI definition of the meter for length metrology. Specifically, an external-cavity diode laser is continuously tuned to a stabilized frequency comb, and its output frequency is modulated over an extensive range to measure the absolute value of a given length by means of multi-wavelength optical interferometry. This approach could find applications in establishing practical length standards with a small amount of uncertainty directly traceable to time standards.

Preparation and Elastic Properties of Helical Nanotubes Obtained by Atomic Layer Deposition with Carbon Nanocoils as Templates

Helical nanofibers and nanotubes of inorganic materials have application potential in various fields, such as catalysis, sensors, and functional and smart systems. Currently available helical nanomaterials include coiled carbon nanotubes, carbon nanocoils, silica nanosprings, silicon carbide nanosprings, and helical transition-metal (Ti, Ta, V) oxide nanotubes. The synthesismethods for helical nanomaterials rely mainly on catalytic chemical vapor deposition (CVD) and template approaches. The template method is usually applied to the synthesis of helical oxide nanotubes because it is relatively difficult to fabricate such tubular structures directly by CVD or physical vapor deposition (PVD). Carbon nanocoils are the most successfully synthesized by CVD with good reproducibility and high yields. Previously, helical metal and oxide nanoandmicrostructures have been synthesized by electrodeposition, electroless deposition, and sol–gel methods with carbon nanocoils or microcoils as templates. However, these coating methods do not provide convenient and simple control over the thickness and uniformity of the coatings because the templates have low chemical reactivity, small diameters, and particularly large surface curvature. Moreover, certain pretreatments and suitable surfactants are required to improve the wettability and chemical reactivity of the templates. Atomic layer deposition (ALD) is a powerful growth technique for high-quality films. It utilizes the sequential exposure of reactants to substrates to achieve layer-by-layer film growth, which allows atomic-scale thickness control. Compared to traditional PVD or CVD methods, it shows outstanding advantages including precise thickness control,

Injection charge assisted polarization reversal in ferroelectric thin films

The authors have investigated the polarization reversal on ferroelectric thin films caused by a grounded tip on 50-nm-thick Pb(Zr,Ti)O3 films. Backswitching occurred when the grounded tip recontacted a “freshly” switched area. It is believed that the upper part of the film switches back due to the field between the grounded tip and previously injected charges. During dynamic operation, partial backswitching was observed during pulsed writing using pulse widths of 1ms. The results show that polarization reversal is an issue, which has to be addressed in the writing scheme of future probe-based storage devices.

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.

Atomically Smooth p-Doped Silicon Nanowires Catalyzed by Aluminum at Low Temperature

Silicon nanowires (SiNWs) are powerful nanotechnological building blocks. To date, a variety of metals have been used to synthesize high-density epitaxial SiNWs through metal-catalyzed vapor phase epitaxy. Understanding the impact of the catalyst on the intrinsic properties of SiNWs is critical for precise manipulation of the emerging SiNW-based devices. Here we demonstrate that SiNWs synthesized at low-temperature by ultrahigh vacuum chemical vapor deposition using Al as a catalyst present distinct morphological properties. In particular, these nanowires are atomically smooth in contrast to rough {112}-type sidewalls characteristic of the intensively investigated Au-catalyzed SiNWs. We show that the stabilizing effect of Al plays the key role in the observed nanowire surface morphology. In fact, unlike Au which induces (111) and (113) facets on the nanowire sidewall surface, Al revokes the reconstruction along the [112] direction leading to equivalent adjacent step edges and flat surfaces. Our finding sets the lower limit of the Al surface density on the nanowire sidewalls at ∼2 atom/nm(2). Additionally, despite using temperatures of ca. 110-170 K below the eutectic point, we found that the incorporation of Al into the growing nanowires is sufficient to induce an effective p-type doping of SiNWs. These results demonstrate that the catalyst plays a crucial role is shaping the structural and electrical properties of SiNWs.