Coherent EUV Generation

(1) Coherence EUV Generation and ultra-fast devices

Surface plasmons are coherent electrons oscillation that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface. These oscillations make the collective oscillation of electron in a nanostructures stimulated by incident light. This is called localized surface plasmon resonance when the resonance condition is established in nanostructures. The localized surface plasmon resonance have ability to enhance the electromagnetic field intensity in the nanometer-sized volume with strong optical confinement relative to the light wavelength. This enhanced field also maintain temporal behavior of incident light. So, well-deigned nanostructures for field enhancement can act as powerful optical devices and be nanometer-sized electromagnetic amplifiers which have many possibilities for various applications.

Our first research interests are based on coherence extreme-ultraviolet (EUV) generation through using surface plasmon phenomena. High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent EUV light. This nonlinear conversion process requires high pulse intensities, greater than 10¹³W/cm², 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. Intra-cavity pulse amplification 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 localized surface plasmon resonance 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 10¹¹W/cm². The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to several tens of nm by injection with an noble gas jet. The method could form the basis of constructing laptop-sized EUV light sources for advanced lighography and high-resolution imaging applications.

Our second research interests are based on ultra-fast device through using surface plasmon phenomena. Surface plasmons which are collective modes of the electrons in metal are excited when light strikes the surface of the nano-structure have behaviors of high intensity, high spatial confinement and similar temporal properties of incident light. The highest field enhancements occur in gaps of bow-tie nanostructure and electrons in bow-tie shaped metallic nanostructure can cross the gold bowtie structure. by nonlinear electron tunneling phenomena during very short time related to incident pulse duration. The time limitations of conventional electric switch devices are about few nanosecond regime. So, well-designed nanostructure-shaped switch can reduce time response down to few femtosecond regime.