1538 J. Opt. Soc. Am. B/Vol. 26, No. 8 /August 2009 Möhring et al.
Generation of phase-controlledultraviolet pulses and characterization
by a simple autocorrelator setup
Jens Möhring,1 Tiago Buckup,1 C. Stefan Lehmann,1,2 and Marcus Motzkus1,*1Physikalische Chemie, Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse,
D-35043 Marburg, Germany2Present address: Department of Chemistry and Laser Centre, Vrije University, de Boelealaan 1083,
1081 HV Amsterdam, The Netherlands*Corresponding author: [email protected]
Received April 24, 2009; revised June 2, 2009; accepted June 2, 2009;posted June 5, 2009 (Doc. ID 110580); published July 8, 2009
. INTRODUCTIONeneration of phase-modulated femtosecond ultraviolet
UV) pulses represents a key technology to study a greatumber of highly relevant molecular systems and photo-hemical reactions with the methods of coherent controlnd advanced nonlinear spectroscopy techniques [1,2].or coherent control experiments in the UV, a wavelengthunable assembly for ultrashort UV pulse generation andn associated characterization must be combined with theppropriate phase modulation technology. These methodsust solve the constraints of conventional shaping tech-
ologies in the UV and the challenge of low energy char-cterization in this spectral range. Furthermore, the com-lete configuration has to be simple and robust enough torovide sufficient measuring time for the actual spectro-copic or control experiment. Therefore, the present paperllustrates a full setup of an UV source, a modulator, and
characterization, capable of satisfying these require-ents.To gain access to modulated femtosecond pulses the
utput of an ultrashort UV pulse generator, relying onarametric processes, is fed into a direct UV-shaping spa-ial light modulator (SLM) setup. Owing to the opacity ofost phase modulators in the UV spectral range, a spe-
ial phase modulation technology has to be used here.ne possibility to overcome this issue is the application of
ndirect shaping schemes, where shaping takes place inhe visible (VIS) spectral region [3–7]. However, this con-ept is rather complex and limited by the optical convolu-ion taking place during the nonlinear process, leading tooupling between amplitude and phase or to reduction inpectral resolution of the applied shaping. For direct UV
hase modulation, acousto-optic modulators or reflectiveevices are required, which show enough transmission oreflectivity in the UV spectral region, respectively [8–12].he first direct UV phase modulation was shown with aicroelectromechanical system (MEMS) array, which
ombines phase wrapping and a comparable high densityf actuators enabling complex phase synthesis [8]. Fur-hermore, fast frame rates beyond the timing of typicalCD devices are available with such micromirror arrays.ere we show an improved setup based on this MEMShase modulator to generate shaped tunable ultrashortV pulses and discuss its advantages and limitations.The precise application of ultrafast UV pulses, espe-
ially of shaped pulse forms, requires a reliable character-zation technique to verify the experimental conditions on
daily basis and to support setup operations. Especially,he adjustment of the shaper setup or the validation ofhe generated pulse forms requires a versatile pulse char-cterization setup. There are various UV pulse character-zation devices available relying on external referenceulses [13,14]. Particularly in the context of monitoring auning process, which induces optical path length varia-ions, such as the adjustment of typical pulse compres-ors, characterization with external references is oftenumbersome. Autoreferencing capable UV characteriza-ion methods, such as frequency resolved optical gatingFROG), are often relying on third-order nonlinearities ashase-matching materials for second-order processes areardly available in the UV region. Therefore, they de-and frequently high intensities or the complex imple-entation of three-beam setups like in the case of tran-
Möhring et al. Vol. 26, No. 8 /August 2009 /J. Opt. Soc. Am. B 1539
epresent an easy adjustable characterization setup par-icularly if a two-photon absorption (TPA) detector issed, since hereby no phase-matching condition has to beet. As TPA based detectors are frequently used for ACeasurements in the VIS and the near-infrared (NIR)
pectral regions [19–21], it is desirable to extend their us-ge to the UV spectral range. To enable TPA based non-inear detectors in the UV, high bandgap materials suchs diamond, fused silica (FS), or CsTe have to be used asctive media [22,23]. As this group of materials is oftenpplied for the construction of solar blind photomultiplierube cathodes, the two-photon photoemission in these de-ices can be exploited as autocorrelation detector. A prooff principle experiment has been recently conducted byhara et al., although pulse energies above the damagehreshold of the photomultiplier (PMT) were applied tobtain autocorrelation traces [24]. In the present papere show an autocorrelator setup of a split mirror autocor-
elation device based on a solar blind PMT as nonlinearetector. The reduced degrees of freedom compared withther UV characterization devices renders this AC setupell suited to obtain everyday pulse validation and to
implify setup tuning in a coherent control application.esides the construction of an UV phase modulation andcharacterization setup, its features, discussed by shap-
ng examples, are shown in Sections 2 and 3.
. REALIZATION AND DISCUSSION OF THEXPERIMENTAL SETUPigure 1 shows a complete overview of the presented UVetup. Subsections 2.A–2.C will address the individualomponents of the setup with special focus on the phaseodulator and the pulse characterization technologies
pplied.
ig. 1. (Color online) Experimental setup overview. A nc-OPA isumped by an amplified femtosecond laser (CPA). Tunable VISadiation is generated and SFM with the temporal stretched (�”ndicates a glass block) NIR fundamental without prior pulseompression. Pulse compression is conducted in a UV prism com-ressor (Comp.). The compressed pulse is fed into our purely re-ective 4-f MEMS shaper setup (SLM). A chopper (CH) with a 1:5uty cycle is used to enable synchronization of the MEMS deflec-ion. The shaped UV pulse is fed to a split mirror TPA (TPA inMT) autocorrelator.
. UV Pulse Generationhe initial laser source of the present experiment is ahirped pulse amplifier (Clark MXR 1000), which delivers30 fs NIR pulses �795 nm/650 �J�. To enable sufficientunability for spectroscopic and coherent control applica-ions, sum frequency mixing of the output from a noncol-inear parametric amplifier was chosen. The UV genera-ion setup consists of a single stage noncollinear opticalarametric amplifier (nc-OPA), a sum frequency mixerSFM), and a UV compressor (Fig. 1). Upconversion byum frequency mixing uses uncompressed nc-OPA pulsesnd all pulse compression takes place in the UV [25].The parametric amplifier generates pulses from below
00 nm to above 600 nm. When mixed with 795 nm NIRundamental from the CPA system, this corresponds cur-ently to a potential UV tuning range from below 300 nmo above 350 nm. To avoid spectral narrowing effects ando exploit the energy present in the VIS pulse, the NIReam is chirped to about 0.4 ps before the SFM stage [25].ased on the SFM in the applied ß-barium borate (BBO)rystal �100 �m, �=37°�, pulse durations below 23 fs cane achieved, purely by UV pulse compression, without thepplication of the SLM setup.As the subsequent UV prism compressor needs hori-
ontal input polarization to exploit Brewster conditions inhe FS prisms, the polarizations of the VIS and the NIReams are rotated by 90° with achromatic � /2 phase re-arders in front of the SFM stage. UV energies of up to00 nJ are accessible directly after the SFM stage, yield-ng energies of up to 250 nJ behind the compressor.
. MEMS Phase Modulatorfter the compressor, the pulse is fed into a phase modu-
ator based on a MEMS chip. This phase modulatorFraunhofer/IPMS, 2D MEMS phase former kit) allowsor direct UV pulse shaping down to about 200 nm, lim-ted only by the reflectivity of the aluminum mirror sur-ace [8]. In contrast to nonpixelated devices such as de-ormable mirrors, the 2D-MEMS microstructured mirrorllows phase wrapping, which is necessary to synthesizeore complex phase functions in coherent control appli-
ations.Defined by the MEMS modulator operation principle,
he shaper setup is built in reflective 4-f geometry, with aylindrical mirror (f=254 mm, illustrated in Fig. 1 by aens symbol) and a grating with a groove density of 1200/mm. For the illustration of the space–time coupling ef-ects, a similar shaper geometry based on a 600 g/mmrating and a f=500 mm mirror has been used.
Owing to hardware limitations of the MEMS array theodulator permits only a temporal duty cycle of 5%. As
he minimum deflection time of the shaper is 0.2 ms, it isossible to synchronize the SLM to the laser to gain anffective pulse usage ratio of 20% on a 1 kHz laser system.ll unmodulated pulses are removed by a chopper bladeith an appropriate duty cycle. In the current setup, theD shaping is not exploited and all pixels in a column areet to equal values.
The deflection calibration was done by a Michelson in-erferometer setup, relying on a HeNe laser. A minor un-ertainty (10%) origins presumably from the fact thatnly slightly more than one interference fringe could be
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1540 J. Opt. Soc. Am. B/Vol. 26, No. 8 /August 2009 Möhring et al.
raced by the maximum deflection of the micromirrors.or future optimizations, a more precise calibrationhould be possible by multiple reflections to gain furtherodulation cycles or by white light interferometry. The
atter should also yield an exact representation of theomplete MEMS surface, and therefore a general offsetrame to actively flatten the structure above its passivelanarity.
. Autocorrelatorhe main design target of the applied characterization de-ice was the ability for fast inspection and tuning of keyomponents such as the compressor or the shaper assem-ly. As these procedures require optical path length varia-ions, a reference free characterization technique is theethod of choice. Therefore to facilitate easy adjustments
f the setup, a UV autocorrelator assembly has been de-eloped. Its construction relies on TPA in a solar blindhotomultiplier cathode made for VUV radiation detec-ion. To further simplify the autocorrelator, the device isased on a split mirror design, which has already provenersatile for ultrashort pulse characterization in the lit-rature over the last years [18,26–29]. In this setup theeam splitting is realized by spatial cutting the beam pro-le in two halves by a split mirror. To collect autocorrela-ion traces, one half mirror is moved by a piezostage. Thetage is operated in an open loop configuration, and theosition is measured by a calibrated internal positioneter. In this configuration autocorrelation traces can be
cquired as fast as four times per second, limited by thevailable laser repetition rate of 1 kHz. The split beam isocused on the cathode of a solar blind photomultiplierElectron Tubes 9423 B) by an off-axis aluminum mirrorf=101.6 mm�. The output of the PMT is amplified andetected by Boxcar integration. The presented autocorr-lator shows an excellent everyday usability owing to theeduced quantity of degrees of freedom to adjust. Thisoncept is further assisted by the application of a TPAonlinearity, as no phase-matching adjustments are re-uired for this process.The built-in low-noise amplification of photoelectronsakes a photomultiplier an ideal two-photon detector for
ow-energy pulse characterization. In case of our setup,nergies below 15 pJ in front of the PMT enable the ac-uisition of an autocorrelation trace. The verification ofwo-photon behavior of the PMT is shown in Fig. 2(a) at aenter wavelength of about 324 nm. A good TPA responseith a slope of 1.93 in the double logarithmic plot of PMTgainst a linear reference diode is observed in the rangef typical measurements. A general limit for all two-hoton-sensitive detectors is the emerging linear sensitiv-ty toward lower wavelengths. This linear signal contribu-ion of the PMT obscures its TPA response already atavelengths shifted far from the PMT linear absorptiondge. In contrast to nonlinear crystal based autocorrela-ors, no spectral filtering is possible to discriminate be-ween the fundamental and the two-photon responses ofhe detector. It is therefore important that the PMT has toe as insensitive as possible to the fundamental wave-ength. In case of our setup, AC measurements below 300m show significant parasitic linear signal contribution.
his is noteworthy, as the red edge of the CsI photocath-de absorption is around 230 nm [30].
Prior approaches in the literature used high energies3–20 �J�, partially beyond the damage threshold of theMT, to acquire an autocorrelation trace [24]. A possiblexplanation of these high-energy requirements comparedith our setup (minimum energy: 15 pJ) may be theuantum efficiency curve of the applied PMT. Itsatasheet names only a quantum yield ratio of slightlybove 700 between the fundamental and the second har-onic wavelengths for the CsTe photocathode.In the design of a two-photon photoemission based au-
ocorrelator, the possible contribution of resonance en-anced TPA with finite temporal response should alwayse considered [31]. As the obtained time bandwidth prod-cts agree with the cross correlation data of a similarulse generation setup [25], the acquired autocorrelationshould contain negligible pulse broadening effects owingo noninstantaneous detector response. Nevertheless, inpplications for sub-15-fs pulses or for different detectors,he measurement technique should always be reviewedor pulse broadening effects.
An exemplary autocorrelation trace acquired with theplit mirror UV autocorrelator discussed in this section ishown in Fig. 2(b). The background contribution resultingrom TPA of the two individual pulses is subtracted inubsequent AC figures. A minor baseline drift, as can beeen in Fig. 2(b), can presumably be attributed to weak
ig. 2. (Color online) Energy dependence of the PMT response.a) Linear fit on double logarithmic plot of the data shown in (a)eturns a gradient of 1.93 indicating good two photon response ofhe PMT. (b) A typical AC traces in an energy range covered bya).
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Möhring et al. Vol. 26, No. 8 /August 2009 /J. Opt. Soc. Am. B 1541
hading effects from the moving AC mirror and is alsoubtracted consecutively. Although the AC setup is ca-able for real-time monitoring of a tuning process, theeasurements shown here are averaged around 1500 la-
er shots per data point to increase the signal-to-noise ra-io. All AC measurements presented were taken in thepectral region between 315 and 330 nm.
. PHASE-MODULATED UV PULSEStypical phase-modulator application is the compression
f ultrashort pulses. Here we demonstrate the recompres-ion of a femtosecond pulse sent through differentmounts of FS glass substrate. Figure 3 shows a scan ofhe second-order spectral phase tailor term. A 10 or 20m FS block is inserted in the beam path of the UV
ulse, respectively. By scanning the second-order spectralhase, the pulses are recompressed nearly to the valuebserved for the unmodified pulse also shown in the fig-re. The data points shown in Fig. 3 are the measuredalues, whereas the curve representation is based on fit-ing a modeled autocorrelation to the data. The fitting re-ies on the measured spectrum and illustrates that theenerated pulses are near the spectral transform limitTL).
The compensation of a chirp imprinted on a pulse, e.g.,y glass substrate, can also be achieved by the applicationf an adaptive compression scheme based on an evolution-ry learning algorithm [32]. This compression experimenterves as a testbed application for closed-loop coherentontrol runs and illustrates the capability of the pre-ented phase modulator to handle the complex phaseunctions generated during an evolutionary algorithmun. Figure 4 shows the autocorrelation trace of the UVulse after an adaptive compression run to compensate 10m of FS substrate. The resulting pulse (pulse duration31 fs) is in excellent agreement compared with the ini-
ial pulse (pulse duration=33 fs). To enable fast acquisi-ion, the feedback value for the algorithm is computedrom the maximum autocorrelation signal and its base-ine. Only these two points of the AC trace are measuredo generate the feedback of the adaptive compression.
ig. 3. (Color online) Compensation of dispersion, caused bylass substrates, by second order spectral phase scanning. Theurves indicate a fitting based on simulation of an appropriateC and the retrieved amount of FS is given in comparison to the
nserted one.
The last example demonstrates the generation of mul-ipulses by the application of a sinus modulation of thepectral phase. As multipulses enable the vibrationalode filtering and the control of wave packet motion, they
epresent an important application for phase modulationechnology [33,34]. Figure 5 contains the autocorrelationata of three different multipulse separation values. Aulse is generated from the measured fundamental UVpectrum under the assumption of a flat spectral phase.he comparison between the given and the retrieved val-es of the multipulse separation shows again a very goodgreement. On the average the analysis of the phase
ig. 4. (Color online) Adaptive recompression after insertion of0 mm FS substrate into the UV beam path. The figure showshe autocorrelation traces of the optimal pulses with and withouthe additional substrate. The inset shows the convergence curvef the applied evolutionary algorithm.
ig. 5. (Color online) Multipulses generated by sinus phaseodulation. The parameters used for multipulse generation cor-
espond to ����=a sin��b+c�. (All: a=1.23, c=0). The points rep-esent the measured AC whereas the curves indicate a fittingased on a simulated AC of the measured pulse spectrum.
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1542 J. Opt. Soc. Am. B/Vol. 26, No. 8 /August 2009 Möhring et al.
odulations in Figs. 4 and 5 exhibits a too low modula-ion in the range of 5%–10%, indicating a minor calibra-ion error of the phase stroke.
The preceding paragraphs showed the generation ofypical phase modulations required in the application ofLM. Furthermore the characterization of the generatedV pulses was realized by the presented AC setup and il-
ustrates the good quality of the obtained measurements.owever, in case of shaped pulse forms, the impact of the
patiotemporal characteristics of these beams on splitirror autocorrelation devices has to be carefully ana-
yzed.In the last part, a more in-depth examination of the in-
erplay between a split mirror autocorrelator and a SLMs given. A typical artifact of pulse phase control is the de-endence of spatial effects on temporal modulations35–39]. As every 4-f based phase modulator shows thisspace–time coupling,” the effects on the applied autocor-elation setup should be carefully analyzed. According to35] the amount of space–time coupling CST, described inpatial walk-off per time delay, is given by
CST =cd cos��i�
�,
here c is the speed of light, d is the groove spacing of thehaper grating, �i is the input angle on this grating, and �s the center wavelength of the pulse. The formula stateshat space–time coupling is more pronounced on low dis-ersive grating setups. Calculation of the space–time cou-ling on the presented MEMS gives values between 1.5nd 0.7 mm ps−1 for 600 and 1200 g mm−1 gratings, re-pectively, for a central wavelength of 330 nm at the Lit-row condition (Littrow angles: 11.4° for 1200 g mm−1,.7° for 600 g mm−1). With respect to a beam diameter ofround 2–3 mm, especially in the case of multipulse gen-ration, the space–time coupling has to be carefully putnto consideration.
As previously described, the UV autocorrelator appliespatial beam splitting by a divided mirror. In our stan-ard setup, the split mirror cuts the beam profile perpen-icular to the horizontal layer, in which space–time cou-ling, generated by the phase modulator, takes place. Ashe phase pattern on the modulator influences the spa-iotemporal intensity profile of the beam, each mirror halfeflects a different temporal cross section of the beam. Inase of a linear phase, generating a simple temporal shiftf the pulse, a contrast reduction in the AC trace can bebserved. This is correlated with the linear spatial shift ofhe beam caused by the space–time coupling. This influ-nces the intensity distribution of the two AC beams, re-ulting in a reduced peak intensity of the AC trace. Aore subtle effect can be observed in case of multipulse
eneration. The initial multipulse measurements appearsymmetric [Fig. 6(a)]. The space–time coupling induces apatial shift of each temporal subpulse according to its de-ay [see the illustration in Fig. 6(a)]. Considering the split
irror AC setup used here, a spatial displacement of theubpulses results in an amplitude filtering based on theirverlap with the mirror surface. As illustrated in Fig. 6(c),he space–time coupling is hereby roughly approximatedy a Gaussian shaped temporal intensity filter simulating
he edge of each D-shaped mirror. This filter reduces thentensity of subpulses displaced into the gap between thewo mirrors. The AC process can therefore be simulatedy a cross correlation of the accordingly intensity-odulated pulse trains. Figure 6(a) shows the fitting ofeasured data by the above-described model. A good
greement between simulation and data is found. The in-uence of the space–time coupling can be easily checked
n the current setup: as the two-photon photoemissionnd all other optics in the setup are independent on po-arization, a periscope can be used to rotate the beam pro-le by 90° in front of the AC input. In this case the planef space–time coupling effects does not influence the in-ensity distribution between the mirror halves. Figure(b) illustrates that there is nearly no asymmetry left in
ig. 6. (Color online) Space–time coupling effect on split mirrorutocorrelators: (a) asymmetric autocorrelation trace of multi-ulse; (b) asymmetry vanishes when beam is rotated by peri-cope. This effect can be attributed to space–time coupling as il-ustrated in the insets. The fitting to the asymmetric AC trace isomputed according to the illustration in (c). The split mirrorcts as intensity filter for the pulse train, yielding an intensityltered multipulse for each mirror half. A simulated cross corre-
ation between these agrees well with the AC shown in (a).
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Möhring et al. Vol. 26, No. 8 /August 2009 /J. Opt. Soc. Am. B 1543
he AC trace, illustrating the vanishing of space–timeoupling effects.
. CONCLUSIONn the present paper we demonstrated a complete setupor the generation of phase-modulated ultrashort pulsesnd their easy characterization. Starting from a noncol-inear parametric amplifier, followed by a sum frequency
ixing, a tunable and manageable ultrashort pulseource has been implemented to supply the pulse shapingetup with very short ��30 fs� UV pulses. To generatehase-modulated UV pulses, a MEMS based SLM capablef direct UV phase modulation is applied in reflective 4-feometry. The SLM offers the advantage of a pixelatedodulator combined with the potential reflectivity range
f a deformable mirror. We illustrated the shaping capa-ility of this device by various phase examples typicallyound in coherent control applications. Especially, the suc-essful adaptive compression emphasizes the ability ofhis shaper to generate even complex phase functions.
To test the performance of the constructed system onn everyday basis, an easy applicable autocorrelator forV pulse characterization has been presented. This de-ice offers a significant improvement in the usability forltrashort UV pulses in spectroscopic experiments. Par-icularly the avoidance of external reference pulses makeshe technique a valuable tool for ultrashort temporalulse characterizations at every desired position in an ul-rafast UV setup. Additionally, the analysis of split mirrorC trace’s symmetry yields an interesting indicator forpace–time coupling effects.
ür Quantenoptik, Garching, Germany) for valuable dis-ussions at the beginning of this project, which was in-pired by his previous work on the MEMS setup. A Ph.D.cholarship of the Philipps-Universität Marburg to J.öhring is gratefully appreciated. C. S. Lehmann ac-
nowledges a travel grant from the European Scienceoundation (ESF-DYNA grant 1796).
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