Shaper-assisted ultraviolet cross correlatorJens Möhring,1 Tiago Buckup,1,2 and Marcus Motzkus1,2,*

1Physikalische Chemie, Philipps Universität Marburg, Hans-Meerwein-Strasse, D-35043 Marburg, Germany2Physikalisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 229, D-69120 Heidelberg, Germany

*Corresponding author: marcus.motzkus@pci.uni‑heidelberg.de

Received December 9, 2009; revised March 26, 2010; accepted March 29, 2010;posted April 30, 2010 (Doc. ID 121265); published May 24, 2010

We successfully demonstrate the characterization of phase- and amplitude-modulated broadband UV pulses witha shaper-assisted cross-correlation setup. A two-dimensional pulse modulator, operated in the diffractive shapingmode, is used to generate an inherently temporally overlapped reference beam. To greatly improve the usabilityof this method, we combined this setup with a split-mirror UV autocorrelator based on a solar-blind photo-multiplier tube as sensitive nonlinearity. This allows sensitive characterization of the Fourier-limited pulsesdown to a few picojoules, as well as complex-shaped ultrashort UV pulses, typically occurring in coherentcontrol experiments. © 2010 Optical Society of America

OCIS codes: 320.0320, 320.5540, 320.7160, 040.7190.

Progress in ultrafast experiments in the UV range, espe-cially the application of shaped pulse forms, requiresflexible and user-friendly pulse characterization techni-ques. Ideally, characterization methods should work withlow pulse energies, be independent of external refer-ences, and be capable of giving the most direct temporalrepresentation of the pulse. To account for these criteriain the UV spectral range, two challenges have yet tobe met.Day-to-day usability demands a highly manageable

characterization setup; hereby, especially, the lack oftemporal overlap with an external reference greatly sim-plifies the experimental effort. This requirement is, inprinciple, satisfied by an autocorrelator (AC); however,neither the sign of a chirp nor the temporal pulse shapecan be measured directly by an AC. This issue can besolved by introduction of an external reference, e.g., inspectral interferometry techniques [1], although at thecost of a much more laborious setup. The applicationof a two-dimensional (2D) spatial light modulator (SLM)can address this challenge in a very efficient way. SLMscan be used as dispersionless interferometers and as out-standing tools for full management and characterizationof the spectral phase [2–4]. Moreover, in this context, 2DSLMs offer even more potential for flexible characteriza-tion schemes unpaired by other SLMs [5].The second important challenge is a convenient nonli-

nearity to enable high-sensitivity UV pulse characteriza-tion in the nanojoule energy regime. Presently, ACs, e.g.,with theKerr effect asnonlinearity, are effective tools onlyfor high UV pulse energy characterization [6]. Because ofthe lack of second-order phase-matching materials in theUV, only the additional effort of either a three-beam ACsetup [7] or the introduction of an external reference [8]can increase the sensitivity of characterization. Photo-detectors showing only a two-photon response, widelyused in the visible and near-IR (NIR) regions for auto-correlation measurements, avoid the problems of phasematching, handling of vacuum UV radiation, and externalreferences. In this regard, two-photonphotoelectronemis-sion from the cathode of a solar-blind photomultiplier(PMT) is an ideal candidate for UV pulse characterization.Proof-of-principle experiments already illustrated the fea-sibility of this concept, although the UV pulses were withenergies in the microjoule region [9].

In order to address these two issues in the characteriza-tion of UV pulses, we combine the sensitivity of a two-photon absorption (TPA) PMT with the beam steeringand phase modulation capability of a 2D SLM. This way,we are able to generate an inherently temporally over-lapped and spatially separated reference beam using a dif-fractive shaping scheme [10]: spatial splitting of such anunmodified reference and a modulated working pulseyields two separated beams and allows us to perform ex-act cross-correlation (XAC) measurements [Fig. 1]. Thecross correlation is hereby executed in a split-mirror AC,offering the benefits of both techniques within the samesetup.

To enable the required direct 2D UV phase modulation,a special modulator technology based on a microelectro-mechanical system (MEMS) mirror arrays (MEMS PhaseFormer Kit, Fraunhofer IPMS) is applied [11–13]. TheSLM setup offers now an optimized duty cycle (333 Hz,software upgrade enables 30 μs minimum on time) andbroadband pulse handling capability (sub-35 fs). Thisreflective modulator controls spectral phases by a pixe-lated structure (200 × 240 pixels) of individually electro-static deflectable aluminum micromirrors. The pulseshaper is operated in a reflective 4f setup, where the ima-ging component is a cylindrical mirror (f ¼ 254 mm) anda 1200 g mm−1 grating acts as the dispersive element.The microstructure of the mirror generates a relativelystrong 2D diffraction pattern. However, the beams gen-erated by diffractive shaping can be easily separatedfrom these artifacts by introduction of a spatial aperture.Altogether, the energy throughput of the complete 4fsetup is around 5%–10%, mainly caused by mirror andgrating losses.

For full control over the electric field by a phase-onlymodulator and to introduce the capability of spatial split-ting of the two beams, a diffractive shaping scheme isused [10]. Hereby, along the vertical axis of the SLM,where no spectral dispersion takes place, a grating iswritten directing the shaped beam into its first diffractiveorder [see Fig. 1(a) for two exemplary phase patterns].For spectral phase modulation of the pulse, the phaseof this grating is shifted. To obtain amplitude modulation,the groove depth of the grating is reduced. As a blazedgrating structure is generated by a sawtooth function,energy can be directed into first or minus first order

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by mirroring the structure. The spatial period (dgrating ¼0:3 mm) of the diffractive pattern is chosen to guideshaped pulse and reference onto each half of the splitmirror in the correlator setup, respectively [as illustratedin Fig. 1(b)].In principle, the required delay between the shaped

pulse and reference can be generated in two differentways. The first method is to introduce the delay by the2D shaper using a linear phase; however, this comes withsome drawbacks: first, if the XAC delay is also generatedby the shaper, the phase modulation induced by the SLMitself is recursively characterized. A long-delay timerange can also introduce shaping artifacts into the refer-ence beam [14,15].These potential issues are solved by implementation of

a mechanical cross-correlation delay. The introduction ofsuch a delay line is straightforward if a split-mirror ACsetup is derived for this purpose. By using a split-mirrorcorrelation scheme, whereupon one mirror half ismounted on a piezo delay line equipped with capacitiveinternal position metrology (PI P-625.1CD), a precise, in-dependent calibration of the XAC time axis is guaran-teed. Further, an open-loop-driven mirror can acquire

XAC traces at a higher speed, enabling real-time charac-terization. In the present case the mirror is driven by atriangular waveform with a frequency of 0:5–2 Hz.

For nonlinearity, a solar-blind PMT (Electron Tubes,9423B) with a cesium iodide photocathode is used [13].In TPA measurements, this PMT was used down to 15 pJat the photocathode, enabling low power, femtosecondUV cross- and autocorrelation. The TPA detector was ap-plied in the spectral range between 300 to 350 nm andshould be usable up to 440 nm, limited only by the quan-tum efficiency spectrum of the PMT. The high-frequencylimit of the AC is set by the increasing one-photon ab-sorption of the CsI photocathode, which may becomea serious issue between 250–290 nm. This effect has tobe especially considered when a PMT is chosen forTPA applications [13]. To extend the TPA detectionscheme to lower wavelengths, large bandgap materialsmust be used as the photocathode, and they must bescreened for parasitic pulse broadening effects.

To illustrate typical applications where a cross-correlation measurement is required, the characteriza-tion of a double pulse and the results of a third-orderphase modulation are shown in Figs. 2 and 3, respec-tively. Double-pulse generation is achieved by periodicphase (square wave) and amplitude (j sin j waveform)modulation based on the diffractive shaping scheme. Aperfect characterization of the resulting double pulseis shown in Fig. 2(a)]. The cross correlation clearlyshows that no zero-point artifacts are present in the

Fig. 1. (Color online) Principle of UV cross correlation andthe applied shaping scheme. (a) Comparison between diffrac-tive and conventional shaping patterns written on a 2Dphase-only modulator (SLM). The left part shows a zero phase,whereas on the right side, a quadratic phase is shown. In a dif-fractive shaping scheme, a grating, perpendicular to the planeof dispersion, is written on the SLM. This diffractive shaping isalso used to obtain the beam splitting shown in (b). (b) Princi-pal illustration of the cross-correlation setup consisting of asplit-mirror AC and a two-dimensional direct UV phase modu-lator operated in the diffractive shaping mode. The elements ofthe 4f setup for the SLM are omitted for clarity.

Fig. 2. (Color online) (a) Double pulse generated by diffrac-tive shaping and characterized by shaper-assisted XAC. TheXAC shows clean double-pulse generation without visible arti-facts due to the shaping. (b) Cross correlations of doublepulses; symmetric around time zero (left) and with one pulseat time zero (right). Here XAC shows the capability of measur-ing absolute time, which is not possible in AC measurements.

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double-pulse structure, a diagnosis not possible by an ACmeasurement. Furthermore, the XAC measurementenables us to differentiate between a double pulse sym-metric around time zero and a temporal structure inwhich one replica is situated at time zero [see Fig. 2(b)].Figure 3 shows the temporal structure of two third-orderphase functions with opposite signs generated by theSLM.As the applied correlator is based on a standard split-

mirror correlator device, one can switch between anautocorrelation measurement and a cross correlationin the same setup with minimum amount of adjustment.The presented measurement scheme can also be ap-

plied on 2D visible or NIR phase modulators, such as li-quid crystal on silicon shapers. Further, the presentedsimple integration of a mechanical delay in a 2D sha-per-assisted spectroscopy technique [16] could relievethe mask from the linear phase load, increasing the op-erating range of these setups.In conclusion, we have successfully shown a UV

characterization scheme based on a shaper-assisted

cross-correlation measurement with high sensitivity.Altogether, its versatility and straightforward implemen-tation make the presented setup a particularly useful toolfor the characterization of shaped broadband UV pulses,especially for complex pulse forms encountered in emer-ging coherent control applications in the UV.

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Fig. 3. (Color online) Third-order phase functions character-ized by shaper-assisted cross correlation. A cross correlation iscapable of differentiating between the signs of a third-orderphase, such as is shown here.

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