1. Billionth Uncertainty Precision Engineering Group, KAIST, Daedeok Science Town, Daejeon 305-701, South Korea
2. These authors contributed equally to this work.

Correspondence to: Seung-Woo Kim¹ Correspondence and requests for materials should be addressed to S.-W.K. (Email: swk@kaist.ac.kr).

Abstract

High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light^{1, 2, 3}. 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^{4, 5}. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity^{6, 7}. 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.

1. Billionth Uncertainty Precision Engineering Group, KAIST, Daedeok Science Town, Daejeon 305-701, South Korea
2. These authors contributed equally to this work.

Correspondence to: Seung-Woo Kim¹ Correspondence and requests for materials should be addressed to S.-W.K. (Email: swk@kaist.ac.kr).

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