Tunable high-efficiency light modulator for soft X-rays

The ability to manipulate visible light is something that we take for granted. With the wide range of optical elements available, starting from the simplest mirrors, filters or lenses to diffraction gratings or even tunable elements, such as spatial light modulators and acousto-optical deflectors, it is possible to realize almost any optical layout one can imagine. However, for manipulating light at shorter wavelengths, such as in the extreme ultraviolet (EUV) or soft X-ray spectral range, the choice of optics is much more limited. The lack of high-efficiency tunable elements significantly hampers the exploitation of such potentially useful photon energy ranges in research and technology.

In this work, we have demonstrated an ultrafast and high-efficiency light modulator, working in the EUV and soft X-ray range. The experiment was performed at the TIMER beamline of the FERMI free electron laser utilizing EUV transient grating in reflection geometry. The proposed device is based on photoinduced phase transition between the ground state and metastable hidden orders in an electronic crystal 1T-TaS₂.

To imprint a grating structure for deflecting the light “on demand”, we used two crossed coherent EUV beams, which created an interference pattern on the surface of the 1T-TaS₂ sample. The beam mintensity was chosen such that the fluence in bright fringes is high enough to stimulate the phase transition to the hidden state. On the other hand, in the areas where the light interfered destructively (dark fringes) the sample remains unswitched since the switching threshold is not reached. Consequently, a periodic pattern of alternating stripes of hidden and ground electronic states was formed on the surface of the sample.

Our experiment revealed that the imprinted periodic structure diffracts non-resonant EUV light with unprecedented efficiency. While such gratings typically relax quickly following thermal dynamics, in the present case, the structure was extremely stable at 100 K, showing almost no relaxation for several hundreds of seconds after the photoexcitation (Fig. 1 top panel). However, this structure could be “erased”, restoring the original state of the sample by a single EUV laser pulse (Fig. 1 bottom panel). The erasing pulse homogenously heats up the sample, increasing the temperature of the entire surface and inducing the relaxation of the hidden order.

Figure 1: (a) Intensity of the diffracted probe beam recorded on the detector as a function of time before (first frame) and after (subsequent frames) the photoexcitation by the two crossed pump beams. (b) Same as (a) but applying a single “erasing” pulse in between the second and thrird frames.

The two key ingredients for the device operation are the non-thermal nature of the phase transition to the hidden metastable state and the significant changes in lattice parameters, which accompanies this phase transition. Metastability ensures the stability of the imprinted structure, while the large discontinuity in lattice parameters provides high efficiency of the light modulation: after switching, the crystal lattice contracts by 0.5 % in the out-of-plane direction, producing real-space relief on the sample surface.

Our modelling shows that at grazing incidence geometry, the modulation efficiency of such a device can be further enhanced by orders of magnitude in the whole EUV and soft X-ray spectral ranges (Fig. 2a). By slight increasing the amplitude of surface modulation, which might be achievable, e.g., using different excitation wavelengths, diffraction efficiencies as large as a few percent are expected for soft X-rays (Fig. 2b). This value approaches the efficiency of the state of the art etched diffraction gratings, with the additional key benefit of being tunable with an extremely fast response time.

Figure 2: (a) modelled efficiency of the imprinted diffraction grating at different wavelengths and for different incidence angles of the light; (b) expected efficiency at a constant wavelength of 0.5 nm as a function of the amplitudes of the surface modulation.

In the present experiment, the grating has a sub-100 nm pitch, but we expect that even smaller features can be generated, up to approaching the length scale of a single domain in the hidden state of 1T-TaS₂. The presented device can offer great improvements in various fields of technology and science, which rely on using EUV and soft X-ray light, including EUV lithography and the production of high-density integrated circuits.

This research was conducted by the following research team:

Igor Vaskivskyi¹, Anze Mraz^1,2, Rok Venturini^1,3, Gregor Jecl^1,3, Yevhenii Vaskivskyi^1,3, Riccardo Mincigrucci⁴, Laura Foglia⁴, Dario De Angelis⁴, Jacopo-Stefano Pelli-Cresi⁴, Ettore Paltanin⁴, Danny Fainozzi⁴, Filippo Bencivenga⁴, Claudio Masciovecchio⁴ and Dragan Mihailovic^1,5
1 Jozef Stefan Institute, Dept. of Complex Matter, Ljubljana, Slovenia.
² Faculty for Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia.
³ Faculty for Mathematics and Physics, University of Ljubljana, Ljubljana, Slovenia.
⁴ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy.
⁵ CENN Nanocenter, Ljubljana, Slovenia.

Contact persons email:

Local contact person emails: ,