Spatially resolved Fourier transform spectroscopy in the extreme ultraviolet

Abstract

Coherent extreme ultraviolet (XUV) radiation produced by table-top high-harmonic generation (HHG) sources provides a wealth of possibilities in research areas ranging from attosecond physics to high resolution coherent diffractive imaging. However, it remains challenging to fully exploit the coherence of such sources for interferometry and Fourier transform spectroscopy (FTS). This is due to the need for a measurement system that is stable at the level of a wavelength fraction, yet allowing a controlled scanning of time delays. Here we demonstrate XUV interferometry and FTS in the 17-55 nm wavelength range using an ultrastable common-path interferometer suitable for high-intensity laser pulses that drive the HHG process. This approach enables the generation of fully coherent XUV pulse pairs with sub-attosecond timing variation, tunable time delay and a clean Gaussian spatial mode profile. We demonstrate the capabilities of our XUV interferometer by performing spatially resolved FTS on a thin film composed of titanium and silicon nitride.A well-known feature of high-harmonic generation (HHG) is broadband spectra in the XUV and soft X-ray regions [1][2][3]. This radiation is typically emitted in a train of attosecond pulses with excellent spatial and temporal coherence, as shown in various interferometric and spectroscopic measurements [4][5][6][7][8][9][10][11][12]. As a result, interferometry with high harmonics found important applications in e.g. Molecular Orbital Tomography [13], in wavefront reconstruction [14] and electric field characterization [15] of high harmonics. Recently, interferometry with high harmonics provided added value to coherent diffractive imaging (CDI) [16,17] using the full high harmonics bandwidth and photon flux. However, in the extreme ultraviolet (XUV) spectral range, interferometry and Fourier transform spectroscopy (FTS) are challenging due to the high stability requirements of the interferometer itself. Two main types of HHG interferometers have been devised. In one scheme, the near-infrared fundamental driving pulse is split into two phase-locked pulses with an adjustable time delay, and this pulse pair is subsequently used for HHG [5,[7][8][9]. Although this method has been successfully used it is typically limited by the stability of the optical interferometer. The other scheme is based on wavefront division, whereby one HHG beam is divided into two phase-locked sources by a piezo-mounted split mirror. This configuration allows more stable interferometry [10,[18][19][20][21], but results in two beams with different spatial profiles and strong diffraction effects due to the hard edge of the split mirror. Wavefront division interferometry is also less flexible when one would like to change the intensity ratio between the two beams.In this letter we present XUV interferometry using a novel ultrastable common-path interferome-1 arXiv:1607.02386v2 [physics.optics]

Extreme ultraviolet lensless imaging without object support through rotational diversity in diffractive shearing interferometry

“…1. An object is illuminated by a pair of broadband noncollinear HHG beams, which are produced using phase-locked pairs of intense driving laser pulses [18]. The noncollinear geometry gives rise to a coherent, spatially sheared pair of diffraction patterns at a camera placed in the far field.…”

“…A coherent pair of HHG pulses is produced by two phase-stable driving laser pulses, which each focus in a separate location inside the gas jet but interfere at the object location. Further details of this method are given in previous work [8,18]. A typical HHG spectrum as used in this experiment is shown in Fig.…”

“…Our approach is based on the diffractive shearing interferometry (DSI) approach which allows wavelength-resolved CDI. For our present application we use XUV radiation generated by HHG [18]. In DSI, the interference between two sheared diffraction patterns is measured through spatially resolved Fourier-transform spectroscopy (FTS) [8].…”

Broadband extreme ultraviolet dispersion measurements using a high-harmonic source

“…To this end, we use two independently generated, but phase-locked, sources of HHG that function as a probe and reference. We ensure subattosecond accurate timing (therefore, phase) control through the use of a birefringence-based common-path interferometer [15]. By scanning the relative phase between the sources, we are able to measure the wavelength dependence of the sampleinduced phase over the full HHG spectrum in a single measurement.…”

“…This system provides 8 mJ pulses with a 25 fs duration, has a spectrum centered on 840 nm wavelength, and has a repetition rate of 300 Hz. These pulses are split into pairs of identical pulses with a controllable and intrinsically stable delay, using an interferometer based on birefringent wedges [15]. After the interferometer, the pulse pairs are focused in a gas jet for HHG.…”