Laser Technology Laboratory

Outline

Nonlinear optical process in the XUV region is of paramount importance not only in the field of quantum electronics but also in ultrafast optics. From the viewpoint of quantum electronics, new features of the interaction between intense XUV photons and matters are expected to be revealed through observation of those nonlinear phenomena. On the other hand, those nonlinear processes in the XUV region are indispensable for the progress of attosecond science, including attosecond atomic/molecular physics and chemistry, because they are very useful for investigating ultrafast phenomena directly in the attosecond time scale. Using high harmonic generation by intense femtosecond laser technology, we are pursuing extreme optical science including XUV nonlinear optics and attosecond physics/chemistry.

Recent Research Topic

Success in emitting a new X-ray with a wavelength in the region of the “water window”

Fig. 1 Absorption spectra of water and protein in the soft X-ray region

Fig. 2 Measured high harmonic spectrum from Ne by a 1.55 μm laser pulse

Fig. 3 Measured high harmonic spectrum from He by a 1.55 μm laser pulse

The realization of a powerful coherent X-ray source has been the dream of researchers of laser physics and photonics. In particular, as shown in Fig. 1, the spectral range between the K-absorption edges of carbon (280 eV) and oxygen (540 eV), which is called the water window, is attractive for high-contrast biological imaging. This spectral range has been one of the most important grails in the research and development of coherent X-ray sources. An intense ultrafast water-window X-ray pulse would allow us to capture images of live cells by instantaneously halting their motion, preserving structural information that is lost in the sample's preparation process for electron microscopy.

Here we demonstrate the generation of a coherent water window X-ray by extending the plateau region of high-order harmonics under a neutral-medium condition. The maximum harmonic photon energy attained are 300 eV and 450 eV in Ne and He, respectively. Our proposed generation scheme, combining a 1.6 μm laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window X-ray.

Fig. 2 shows the measured high harmonic spectrum from Ne gas driven by a 1.55 μm laser pulse with a pump energy of 2.2 mJ. The harmonic yield was optimized by adjusting the focusing point and by varying the backing pressure of the gas jet. The red line shows the square of a reciprocal factor of self-absorption (1/f2)2 as a function of photon energy. As expected, the high harmonic intensity increases with increasing (1/f2)2 up to 250 eV. Above 250 eV, the harmonic intensity decreases because the cut-off energy of 270 eV set by the interaction laser intensity prevents further extension of the plateau region. Thus, the high harmonic intensity reaches a maximum at 250 eV.

We further explored the generation of high harmonics under a neutral-medium condition by changing the nonlinear medium from Ne to He with the aim of obtaining a higher photon energy. Fig. 3 shows the measured high harmonic spectrum from He gas driven by a 1.55 μm laser pulse with an increased pump energy of 4.5 mJ. We can clearly see the carbon K-edge in the spectrum by inserting a Mylar filter. The maximum high harmonic photon energy attained is 450 eV, which is the highest photon energy ever reported under a neutral-medium condition.