technical advances of the timer project

Nuclear Instruments and Methods in Physics Research A 635 (2011) S69–S74

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

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E-m

journal homepage: www.elsevier.com/locate/nima

Technical advances of the TIMER project

R. Cucini a,�, F. Bencivenga a, M. Zangrando b, C. Masciovecchio a

a Sincrotrone Trieste S.C.p.A., Strada Statale 14 - km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italyb IOM-CNR Laboratorio TASC, Strada Statale 14 - km 163,5 in AREA Science Park, 34149 Basovizza, Trieste, Italy

a r t i c l e i n f o

Available online 29 October 2010

Keywords:

Transient grating

Free electron laser

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.10.099

esponding author. Tel.: +39 040 375 8663; fa

ail address: riccardo.cucini@elettra.trieste.it (R

a b s t r a c t

We report on recent developments concerning the TIMER project that is based on the idea to extend the

Transient Grating technique in the Extreme Ultraviolet region. We designed a setup only based on

reflective optics, which permits to overcome the strong photon absorption in this spectral range. We also

carried out some preliminary theoretical calculations in order to study the shape of the transient grating

associated with the proposed setup.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The FERMI@Elettra Free Electron Laser (FEL) facility will makeavailable Extreme Ultraviolet (EUV) photon pulses with uniquecharacteristics [1]. In particular, the seeded scheme adopted forFERMI@Elettra guarantees important features, i.e.:

�

probe ultrafast phenomena;

sian temporal structure.

We intend to exploit these peculiarities in order to develop atime resolved instrument (TIMER [2]) based on the TransientGrating (TG) scheme [3,4]. TIMER would be able to probe thecollective atomic dynamics in the mesoscopic region, whichcorresponds to a momentum (q) and energy transfer (E) range ofabout (0.1�1 nm�1) and (0:121000 meV), respectively. Such aq-range, corresponding to the characteristic length scale(� 102100 nm) of topological disorder [5], is of special interestfor the study of disordered systems but, to date, cannot be accessedby any time or energy resolved instrument. The dynamicalbehavior of amorphous solids at these length scales still presentsunsolved and strongly debated aspects such as, e.g., the origin of theanomalous acoustic attenuation [6–9]. The unique capabilities ofTIMER would also provide a sensitive probing of interfaces and thinfilms, as well as heat transport and electron correlations innanostructured materials. In this paper, we describe the actual

ll rights reserved.

x: +39 040 375 8831.

. Cucini).

state of the art of the TIMER setup, after some feasibility testsrecently carried out [10].

2. Transient grating experiment

TG is a time resolved technique, based on non linear opticaleffects, and represents a particular case of the so called four-wave-mixing technique [11,12], which is extensively described inliterature [4]. In a TG experiment two photon pulses (pump),obtained dividing a single photon beam, interfere inside the sampleto produce a spatially periodic variation of its optical properties(e.g. refractive index and absorption coefficient) by exciting somematerial modes. This modulation is probed by a third photon pulse(probe), typically of different wavelength from that of the pump,which impinges on the induced grating at the Bragg angle and isconsequently diffracted. The dependence of diffracted intensity onpump-probe delay provides information on the relaxing TG and soon the dynamics of the excited modes.

This technique allows measuring several different physicalproperties of the sample, such as acoustic and structural para-meters or electronic relaxations, directly in the time domain. Thedynamics of the excited sample can be experimentally detected bytime delaying the probe with respect to the pump. For instance, our1.5-m-long delay line allows probing a time scale range up to 10 ns.Therefore, if sub-ps pulses are employed, the dynamics associatedto both fast (� ps) and slow (� ns) phenomena can be simulta-neously measured with the excellent signal-to-noise ratio char-acteristic of TG setups [13].

The q-value associated with a TG experiment is determined bythe wavelength of the incident radiation (lex) and the anglebetween the pump pulses (yex), i.e.

q¼4psinðyex=2Þ

lex: ð1Þ

R. Cucini et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) S69–S74S70

So far the only available sources to perform TG experiments arehigh power table-top lasers, which are characterized by wave-lengths longer than 0:2 mm. The corresponding q-region is thuslimited to values lower than � 0:06 nm�1, which do not allow toprobe the mesoscopic region. FERMI@Elettra source can provide alex range of 60–5 nm, which would enable to cover the entiremesoscopic region.

3. Experimental setup

The scheme for the experimental layout is sketched in Fig. 1(a).Peculiarity of the present setup is the use of only reflective opticalelements. This constrain is dictated by the necessity to circumventthe strong photon absorption of optical materials at EUV wave-lengths. Moreover, this configuration also permits to keep the pulsechirping to only a few fs, practically negligible considering a pulsetime duration of the source of about 50–300 fs. We recentlyperformed preliminary test of TG setup without transmissionoptics in the visible range with excellent results [10]. The FELpulse generation permits to obtain simultaneously the first andthird harmonics, which are both present inside the same pulse(ratio of 3rd to 1st harmonic intensity � 1% [14]). At first, the pulseis sent on the edge of a metallic plane mirror: half of the beam isreflected while the other half propagates freely, creating two pulseswith the shape of an half circle. In order to obtain a reflectivity of95–80% in the wavelength range of interest, an angle of incidence(AoI; angle between the radiation k-vector and the normal to themirror surface) of about 881 and a gold coating have been chosen.The operation of this mirror is shown in the inset of Fig. 1(a). Thetransmitted part of the beam is sent on a second plane mirror,which cuts again the half beam in the vertical direction creating

Fig. 1. (a) Sketch of the optical setup for TG experiment with FEL radiation. (b) Spatial profi

pumps and probe pulses at the sample position.

Table 1

Structural parameters for pump toroidal mirrors; yw: angle of incidence on the mirror; R:

using the mirror length (L) and height (H).

yex (deg) Coating yw (deg) R [60–2 nm] (%)

9.2 C 82.9 85

13.8 C 80.6 82–78

39.5 Au 67.8 50–35

52.7 Au 61.2 40–25

two pulses with a quarter-circle shape. The mirror works at an AoIof 851. Also in this case a gold coating is employed, which yields areflectivity � 85% for lex ¼ 60210 nm and � 40% for lex ¼ 5 nm.This implies that the intensity of the reflected beam is sensiblylower than the one of transmitted beam. The beams shape beforeand after the two beam splitters is reported in Fig. 1(b) and (c),respectively.

These last two pulses are used as pumps and are sent on thesample after reflecting onto a pair of focusing toroidal mirrors,which determine yex. Four pairs of mirrors can be inserted along theoptical path of the two pumps in different positions, varying in thisway the yex angle. The chosen configurations correspond toyex ¼ 9:23,13:83,39:53,52:73. One of the two pump beams makes afurther reflection, so that the footprint of the two beams at thesample position is geometrically similar. This guarantees a moreeffective overlapping and maximizes the visibility of the inducedtransient grating.

Considering the high fluency of the FEL source, the toroidalmirrors should be placed far from the source in order to avoiddamages. Moreover, in view of the large magnification of thesystem (about 15–80), it is not convenient to focus the beamsdirectly at the sample position because the high fluency mayinduce an irreversible damage of most samples. In order to over-come this problem, the sample will be positioned between themirror and the focal point. This is achieved by properly choosingthe sagittal (r) and tangential (r) radii of the toroidal mirrors. Thisconfiguration was preferred to the one with focal point between themirror and the sample because this requires a smaller tangentialradius that, in some cases, is barely within the capabilities of mirrormanufactures. All the properties of toroidal mirrors and theexpected dimensions of pump beams at the sample positionare reported in Tables 1 and 2, respectively. The longitudinal

le of the FEL pulses before the two plane mirrors/beam splitters. (c) Superposition of

reflectivity; r: tangential radius; r: sagittal radius; the optical area can be calculated

r (cm) r (cm) Optical area (L�H) (mm2)

5851.5 88.5 150�30

3379 90 150�30

680 97.6 100�30

440 103.4 70�30

Table 4Spot dimensions of probe at the sample position after focalization for different

incident angles (ypr) and wavelengths (lpr).

yprðdegÞ=lpr 20 nm 13.3 nm 6.7 nm

3.05 166�122 195�104 124�57

4.6 163�122 121�108 89�56

12.2 165�67 113�39 57�37

15.4 167�48 113�30 58�27

Focal spot: probe (FWHM, H�V, mm2).

Table 5

Number of photons per pulse for the pump pulse at different wavelengths (lex) and

angles (yex).

yexðdegÞ=lex 60 nm 40 nm 20 nm

9.2 6 5.4 7.1

13.8 5.4 4.6 5.6

39.5 4.5 3.8 3

52.7 3.4 3.1 1.9

Photon/pulse (pump, �1012).

R. Cucini et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) S69–S74 S71

dimensions of the mirrors (L) were chosen in order to catch � 95%of the beam footprint and Lo300 mm, as required. The corre-sponding photon flux at the mirror surface is low enough to avoidthe use of a cooling system. The coating is chosen in order tomaximize the reflectivity of the mirrors in the lex�range and AoI’s(reported as yw in the table) of interest. All the mirrors arecharacterized by the same errors in terms of slope errors(o5 mrad rms, tangential and sagittal), shape (r10 nm) androughness (r0:3 nm). Every mirror is connected with a manipula-tion system with six degrees of freedom: x, y, z translations, pitch,roll and yaw.

The portion of the beam reflected by the first beam splitter issent on a delay line, which is realized by using four multilayermirrors operating at 451 AoI and optimized to reflect the FEL 3rd

harmonic. Thanks to the optimization of these multilayers, it ispossible to obtain a suppression for the first harmonic of a factorabout 106. It guarantees a practically monochromatic pulse, whichis used as probe. Though the final project is aimed to reachwavelengths as low as 5 nm for the pump pulses, in the first phasewe will exploit three lex values: i.e. 60, 40 and 20 nm. Therefore, themultilayer mirrors will be optimized for the corresponding 3rd

harmonics: i.e. 20, 13.3 and 6.7 nm, respectively. In particular, forthe first case Si/Mo mirrors (reflectivity � 40%) will be used, whileSi/Mo with B4C barrier and MoB4C mirrors will be used for the lasttwo cases (reflectivity � 68% and R¼40%, respectively). The shapeof pump and probe pulses at the sample position is shown inFig. 1(c).

After the delay line, the probe impinges on a focusing mirror,which sends the beam on the sample at the correct Bragg angle(ypr). The parameters of the toroidal mirror are chosen in order toproduce an efficient superposition with the pumps in correspon-dence of the sample position. Considering the different probewavelength with respect to the pump (lpr ¼ lex=3), different coat-ings are required for the probe mirrors. The main parameters ofthese mirrors are reported is Table 3; slope errors, shape androughness requirements are the same as the pump mirrors. Thedimensions of the probe beam at the sample position for the chosenconfigurations are reported in Table 4.

Finally, considering all the previously listed parameters wecalculate the number of photons per pulse expected for the pumpand probe beams. Results are listed in Tables 5 and 6. Despite thestrong reduction of photons flux in the probe beam, mainly due tothe four consecutive reflections in the delay line, the expectednumber of photons per pulse is enough in order to obtain an

Table 2Spot dimensions of pump at the sample position after focalization for different

incident angles (yex) and wavelengths (lex).

yexðdegÞ=lex 60 nm 40 nm 20 nm

9.2 200�168 244�130 151�75

13.8 152�168 172�124 102�73

39.5 170�90 127�93 90�67

52.7 167�61 130�54 71�60

Focal spot: pump (FWHM, H�V, mm2).

Table 3

Structural parameters for probe toroidal mirrors; yw: angle of incidence on the mirror; R:

using the mirror length (L) and height (H).

ypr (deg) Coating yw (deg) R [20–6.7 nm] (%)

3.05 C 87.8 95

4.6 C 87 93

12.2 C 83.2 85–73

15.4 TiO2 81.6 80–52

appreciable diffracted signal considering typical diffraction effi-ciency of common materials (� 10�7).

In order to determine the required pump beam dimensions atthe sample position listed in Table 2, we calculated the shape (S(y))of the time-averaged interference pattern at the sample position(conventionally set at x¼0) as a function of beam dimensions,where x and y are orthogonal directions coplanar with the k-vectorsof pump beams (x being orthogonal to the sample surface andcoaxial with the bisector of pump beams). For vertically polarizedfield, S(y) can be approximated by the following equation [15]:

SðyÞ ¼

Z þ2st

�2st

I1ðx,y,z,t,dtÞþ I2ðx,y,z,tÞ

����þ2ðI1ðx,y,z,t,dtÞI2ðx,y,z,tÞÞ1=2cosðqyÞ dt

���x ¼ 0,z ¼ 0,dt ¼ 0

ð2Þ

where, in our particular case

I1ðx,y,z,t,dtÞpexpf�½ðxsinðyex=2Þþycosðyex=2ÞÞ=sH�2

�½ðxcosðyex=2Þ�ysinðyex=2Þ�cðtþdtÞÞ=ðcstÞ�2

�½z=sV �2gYð�xsinðyex=2Þ�ycosðyex=2ÞÞ ð3Þ

reflectivity; r: tangential radius; r: sagittal radius; the optical area can be calculated

r (cm) r (cm) Optical area (L�H) (mm2)

10390 15.45 300�20

5650 15.8 200�20

1100 16.1 200�20

725 16.2 100�20

Table 6

Number of photons per pulse for the probe pulse at different wavelengths (lpr) and

angles (ypr).

yprðdegÞ=lpr 20 nm 13.3 nm 6.7 nm

3.05 4.8 20 0.5

4.6 4.6 19 0.45

12.2 4.6 17 0.37

15.4 4.1 17 0.27

Photon/pulse (probe, �109).

R. Cucini et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) S69–S74S72

and

I2ðx,y,z,tÞpaexpf�½ð�xsinðyex=2Þþycosðyex=2ÞÞ=sH�2

�½ðxcosðyex=2Þþysinðyex=2Þ�ctÞ=ðcstÞ�2

�½z=sV �2gYðxsinðyex=2Þ�ycosðyex=2ÞÞ: ð4Þ

In the previous equations st is the pulse time duration (RMS), dt

and a are the time delay and intensity ratio between the two pumppulses, respectively, c is the speed of light, Y is the step functionwhile sH and sV are the vertical and horizontal dimensions (RMS)of pump beams, respectively. In the calculation the parametersst , dt, a and the variable z were set to 90 fs, 0 fs, 0.8 and 0 mm,

Fig. 2. y-dependence of interference pattern (S(y)) generated by the two crossed

pump pulses for lex ¼ 60 nm and yex ¼ 9:23 . Middle curve (full line) corresponds to

the required sH value reported in Table 2, upper/lower curves (dash/dotted lines)

refer to twice/half this value.

Fig. 3. 2D intensity plot the (x,y)-dependence of the integrand function of Eq. (2) at t¼0 and

dt ¼s (panel (d)), dt ¼ 2s (panel (e)) and dt ¼ 3s (panel (f)); all panels correspond to lex ¼

respectively. For sake of clarity in Figs. 2 and 3 we increased they-periodicity of the interference grating by replacing in Eq. (2) theparameter q with qu¼ q=200. We also note how the shape of S(y)does not depend on the coordinate z, since the latter introduces any-independent intensity decrease of S(y). Therefore, only thesH�dependence of S(y) was studied; the main requirement con-cerning the vertical dimensions of pump and probe beams is thatthey should be roughly the same for a given (lex, yex)-value.

The obtained results are reported in Fig. 2 for lex ¼ 60 nm andyex ¼ 9:23. The middle curve in Fig. 2 refers to a sH value (s)corresponding to the horizontal beam dimension reported inTable 2. The ‘‘bump’’ appreciable in the left hand side can beassociated to the portions of the beam (identifiable with the tails ofthe Gaussian profiles) which do not simultaneously overlap inspace during the beam propagation. This region does not contributeto the TG signal because of the lack of periodicity. Indeed, the finitetime duration and relative angle of the two pulses limit the spatialextent of the overlapping region at about 2cst=cosðyexÞ. Since therequired horizontal beam dimension is of the same order as theoverlapping region, it has to be determined at all yex values ofinterest. This is of great relevance in order to optimize thethroughput of the experimental setup. For instance, the upperand lower curves of Fig. 2 refer to sH values twice and half as largethan s, respectively. In the former case the relative amount ofradiation not contributing to the TG signal (i.e. the ‘‘bump’’)increases; conversely, in the latter case the ‘‘bump’’ vanishes atthe price of reducing the y-extension of the interference region,with a consequent loss of TG signal for a given fluency at the samplesurface (we also recall that in these experiments the fluency shouldbe kept below the damage threshold).

Another relevant parameter to be used for optimizing the profileof interference pattern is dt. In Fig. 3 we report in a 2D intensity plotthe (x,y)-dependence of the integrand function of Eq. (2) calculatedat t¼0 and z¼0 for different dt�values and for (lex, yexÞ ¼ ð60 nm,9.21). As can be readily appreciated from the pictures, negativevalues of dt (panels (a) and (b)) do not help in obtaining amore regular interference pattern, and neither the condition

z¼0 for different dt�values: dt¼ -2s (panel (a)), dt¼�s (panel (b)), dt ¼ 0 (panel (c)),

60 nm and yex ¼ 9:23 .

R. Cucini et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) S69–S74 S73

dt¼ 0 (panel (c)) seems to be the ‘‘best case’’. On qualitativegrounds, the optimal pattern is obtained for dt¼ s and/or dt¼ 2s(panels (d) and (e), respectively), while for larger values of dt

(e.g., panel (f)) a clear worsening is observed. Therefore, it may bepossible to partially compensate the distortions induced by theasymmetric pump line shape and intensity by controlling dt. As apreliminary result, we also found that the dt�dependence of S(y) isquite different for different (lex, yex)-values. A systematic studyaimed to fully characterize the behavior of the interference patternas a function of these parameters and the related effects on the final

Fig. 4. Ray-tracing calculation for pump and probe pulses in different configurations:

lpr ¼ 6:7 nm, yex ¼ 15:43 (d) probe: lpr ¼ 20 nm, yex ¼ 3:053 . The inset of panel (b) shows th

beam that propagates after the cut of the beam splitter mirror.

TG signal from typical samples in both reflection and transmissiongeometry is presently in progress [16].

A ray-tracing calculation using the SHADOW program [17] hasbeen made in order to estimate the deviations of the pulse profilefrom the ideal half and quarter-circle sketched in Fig. 1(c). Wereport here two representative pump/probe configurations: thefirst one for long wavelengths (lex ¼ 60 nm, lpr ¼ 20 nm) and smallangles (yex ¼ 9:23, ypr ¼ 3:053), the second one for short wave-lengths (lex ¼ 20 nm, lpr ¼ 6:7 nm) and large angles (yex ¼ 52:73,ypr ¼ 15:43). The results are shown in Fig. 4.

(a) pump: lex ¼ 20 nm, yex ¼ 52:73; (b) pump: lex ¼ 60 nm, yex ¼ 9:23; (c) probe:

e spherical aberration effect of the toroidal focusing mirror on the shape of a circular

R. Cucini et al. / Nuclear Instruments and Methods in Physics Research A 635 (2011) S69–S74S74

The panels (b), (d) and (a), (c) represent the first and secondcondition for pump and probe, respectively. All the panels report inside histograms the integrations along the vertical and horizontaldirections. The deviations from the ideal half and quarter-circleprofiles are due to the spherical aberration of the toroidal mirrors.This is pointed out in the inset of panel (b). Here we report thecalculation of a circular beam that impinges on a beam splittingmirror, which cuts a portion of the beam with a quarter-circleshape. Here it is clear how the vertical cut of the mirror appears as aparabolic shape instead of a linear cut. Moreover, the aberrationcreates a sort of tail at the side of the almost circular spot. These ray-tracing results would be used as input for a more realisticcalculation of the interference pattern and expected TG signal,which takes into account such aberrations.

4. Conclusion

We report on the recent development concerning the design of aTransient Grating setup optimized for EUV FEL radiation, whichpermits to extend the applicability of this technique in themesoscopic region, presently uncovered by any time or energyresolved instrument. The proposed setup is expected to be able tostudy a wide variety of materials, at it would permit to extend theactual knowledge of the dynamics at nanometric scale. Thecalculations suggest different solutions in order to optimize thevisibility of the interference pattern and allow defining the mainparameters of focusing devices. The estimation of the expectedphotons flux at the sample position supports the feasibility of theproposed experiment.

Acknowledgements

Authors acknowledge C. Svetina and D. Cocco for the data of theFEL source and M.G. Izzo for reading the paper.

This work is partially supported by the European ResearchCouncil-Contract ERC no. 202804.

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