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Extreme-ultraviolet high-order-harmonic pulses with 1.6·107 photons/pulse at 32.5eV have been separated from multiple harmonic orders by a time-preserving monochromator using a single grating in the off-plane mount. This grating geometry gives minimum temporal broadening and high efficiency. The pulse duration of the monochromatized harmonic pulses has been measured to be in the range 20 to 30 fs when the harmonic process is driven by an intense 30 fs near-infrared pulse. The harmonic photon energy is tunable between 12 and 120 eV. The instrument is used in the monochromatized branch of the Artemis beamline at the Central Laser Facility (UK) for applications in ultrafast electron spectroscopy.
©2011 Optical Society of America
1. Introduction
High-order laser harmonics (HHs) that are generated by the interaction between a very intense ultrashort pulsed laser and a gas jet represent a source of high brightness extreme-ultraviolet (XUV) and soft X-ray radiation of table-top size, high peak intensity, very short time duration and high degree of coherence [1]. The HH spectrum is generally described as a sequence of peaks corresponding to the odd harmonics of the fundamental laser wavelength with an intensity distribution characterized by a plateau whose extension is related to the pulse intensity. The combination of the use of advanced phase matching mechanisms and interaction geometries as well as intense ultrafast lasers has made it possible to obtain HH radiation up to the water window region (2.3-4.4 nm) [2]. Moreover, the radiation generated with HHs using few-optical-cycles laser pulses has recently become the tool for the investigation of matter with sub-femtosecond resolution [3,4].
Short pulses of HH radiation are potentially a powerful tool for studies of electronic structure and dynamics. At XUV wavelengths, single-photon ionisation of core and inner valence electrons occurs, even at low XUV intensities. However, since the HH spectrum extends over a broad wavelength range, there are some difficulties in the application of HHs to electron spectroscopy, since the electron energy spectrum produced by the required harmonic will overlap with that produced by other harmonic orders. Therefore, the use of HH emission as a probe of electronic structure and dynamics requires the spectral selection of a single harmonic with a suitable low-resolution monochromator that has to keep the temporal duration of the XUV pulse to as short as that at the HH source. A modest time broadening can be tolerated if the pulse duration at the output of the monochromator is still shorter than the temporal resolution required by the experiment.
The monochromator can be modelled as a filter with a complex frequency response including the spectral transmission and the distortion in the spectral phase [5]. Since the XUV pulse at the source may be close to its transform limit, any modification of its complex spectrum results in a time broadening as described by its Fourier transform. For a Gaussian profile with no modulation centred at wavelength λ with half-width spectral band ΔλP, the half-width duration is Δτp = (0.44/c)·(λ2 /ΔλP), where c is the speed of light in vacuum. In order to preserve the duration of the pulse at the output of the monochromator, the selected bandwidth has to be larger than the spectral width of one harmonic and the transfer function has to be almost constant within the bandwidth. The former condition is easily verified since harmonic peaks are clearly separated. The latter condition is verified if the monochromator is built with reflecting optics since the reflectivity is almost constant within the bandwidth of a single harmonic.
The simplest way to obtain HH spectral selection is the use of a multilayer (ML) mirror at normal-incidence, which does not alter the pulse time duration up to fractions of a femtosecond, gives almost negligible aberrations in compact focusing schemes and is moreover very efficient [6,7]. ML focusing systems are normally designed to work at a fixed wavelength, nevertheless tunable monochromators using two ML plane mirrors have been designed [8]. The main drawback of ML-based monochromators is the requirement to have different sets of mirrors to cover a spectral region as wide as the whole HH spectrum. Furthermore, the reflectance contrast given by ML mirrors can be poor for some types of measurements, such as the detection of very weak photoelectron signals created by two-photon absorption of HHs [9].
Broadband XUV monochromators with high spectral purity are usually based on reflection gratings at grazing incidence. Unfortunately, the grating inevitably gives a distortion of the temporal profile of the pulse because of diffraction. Each ray diffracted at first order by two adjacent grooves is delayed by one wavelength. The total difference in the optical paths of the diffracted beam is given by ΔOP = λΝ, where N is the number of illuminated groves. For example, the total delay of radiation at 30 nm diffracted by a 100 gr/mm grating illuminated over a surface of 20 mm is 60 μm, i.e. 200 fs. This effect is clearly dramatic in the case of femtosecond pulses, since it reduces both the time resolution capability and the peak intensity at the exit of the monochromator.
Nevertheless, it is possible to design grating monochromators that do not alter the temporal duration of an ultrafast pulse by using at least two gratings in subtractive configuration to compensate for the dispersion: the second grating compensates both for the time and the spectral spread introduced by the first one. Such a configuration is called a time-delay compensated monochromator (TDCM). The most compact solution for building a TDCM requires the use of two toroidal gratings in the classical diffraction mount (CDM) [10], since the dispersion and the focusing are done by the same optical element. In this configuration, the first grating disperses the radiation in its focal plane, where an intermediate slit carries out spectral selection of a single harmonic. The second grating finally compensates for the temporal stretching and focuses the radiation on to the same point for each of the wavelengths that are transmitted through the slit. It has been theoretically demonstrated using ray-tracing programs that such a configuration may be able to compensate for the grating temporal stretching to an accuracy of one femtosecond. The configuration with concave gratings at grazing incidence has been already accomplished in the XUV, both with a pair of a spherical and a toroidal grating [11] and with two equal toroidal gratings [12]. The shorter XUV temporal response at the output of the monochromator has been measured to be respectively about 180 fs and 50 fs. In both cases, the measured time response is definitely longer than the intrinsic duration of the XUV HHs, that is expected to be in the plateau half the duration of the generating laser pulse [13], therefore about 30 fs for the 55-fs Ti:Sapphire of Ref. 11,14 fs for the 30-fs system of Ref. 12. Furthermore, the efficiency of two-grating monochromators is very poor, since the efficiency of a single grating in the CDM may be estimated in the 0.1-0.2 range at best. As an example, the global efficiency of the monochromator presented in Ref. 12 has been measured to be 0.026.
An alternative configuration for TDCMs with high efficiency uses the gratings in the off-plane mount (OPM), where the incident light direction belongs to a plane parallel to the direction of the grooves [15]. It is known that the efficiency in the OPM may be close to the reflectivity of the coating, so much higher efficiency than the CDM is expected [14]. In particular, a monochromator operating in the 20-60 nm wavelength region with two plane gratings in the CDM has been designed [16] and built [17]. Spectral selection of HHs while maintaining few-femtosecond pulse-length has been experimentally demonstrated [18]: the response measured at the output was 13 fs at H19 (42 nm) and 8 fs at H23 (35 nm), that is almost the intrinsic duration of the HHs. This is the shortest time response measured up to now from a TDCM. Furthermore, the global efficiency of the monochromator has been measured to be in the range 0.10-0.18 in the whole region of operation, definitely higher than the values achieved in the CDM. Finally, the configuration has been demonstrated to be quite robust to misalignments [19], although it has a relatively complex optical design with six optical elements, namely four toroidal mirrors and two plane gratings.
The use of two gratings that is necessary for a completely optimised TDCM, reduces the transmission and increases the complexity and the cost of the design. Therefore it is worthwhile analysing the single-grating configuration to identify the condition that gives the minimum temporal broadening from just one grating. It can be demonstrated that, once the spectral resolution R = λ/Δλ is defined, the minimum number of illuminated grooves for such a resolution is Nmin = R when the grating is operated at first diffracted order [20]. The corresponding variation of the optical paths at the grating output is ΔOP = λNmin = λ2/Δλ and the half-width duration is ΔτG,min ≈(0.5/c)·(λ2 /Δλ), that is close to the Fourier limit ΔτP for a Gaussian profile with no modulation. Here Δλ is defined as the half-width bandwidth at the output of the monochromator. Therefore, a single grating can be used for the spectral selection of ultrashort pulses without altering the pulse duration significantly, provided that the number of illuminated grooves is close to the actual resolution. A monochromator with a single grating operated close to such a condition will be called a time-preserving monochromator (TPM). The use of a single grating simplifies the optical and mechanical design of the instrument, maximizes the efficiency and reduces the costs.
In case of an almost transform-limited pulse with bandwidth ΔλP and duration ΔτP, a TPM used to spectrally select the whole bandwidth of the pulse in the first diffracted order should have the grating illuminated over N = c ΔτP / λ grooves. If the actual number of illuminated grooves is lower than N, the bandwidth on the exit slit is broader than the pulse bandwidth, i.e. Δλ > ΔλP, resulting in a grating time response which is shorter than the pulse duration, i.e. ΔτG < ΔτP. Vice versa, if the actual number of illuminated grooves is higher than N, the grating time response is longer than the pulse duration, i.e. ΔτG > ΔτP, even if the output resolution is kept lower than the intrinsic resolution of the pulse by opening the exit slit.
In this paper, we will discuss the design of a high-efficiency TPM for XUV ultrashort pulses which introduces a minimum temporal broadening at the output of a few tens of femtoseconds. This monochromator was built for the Artemis beamline at the Central Laser Facility (UK). Artemis is a facility based on high repetition rate and ultra-short tunable laser sources, which are used to produce ultra-fast XUV pulses through HH generation [21]. A schematic is shown in Fig. 1 . The facility is based on a 14-mJ, 30-fs, 1-kHz Ti:Sapphire system operated at 780 nm with carrier-envelope phase stabilization (Red Dragon, KMLabs Inc, USA). Part of the output energy is split and used to drive a widely tunable OPA system which provides 30-100 fs pulses from 235 nm to 20 microns. Another part of the output can be spectrally broadened in a gas-filled hollow fiber and recompressed using chirped mirrors to give sub-10 fs pulses. Any of the laser beamlines can be used to generate HHs, or to provide pump or probe pulses. Two vacuum beamlines, for monochromatized or broadband XUV pulses, transport the HHs to end-stations for materials science and atomic and molecular physics and chemistry. The facility aims to combine femtosecond optical science and synchrotron technologies to enable experiments in the field of ultrafast x-rays and to exploit HH generation for applications [22]. A grating design has been adopted suitable to realize TPMs with ultrashort time response and high efficiency. In this paper, we discuss the design of the monochromator and present its spectral and temporal characterization.
2. Time-preserving monochromators in the off-plane mount
The TPM monochromator here presented uses gratings in the OPM. The latter differs from the CDM in that the incident and diffracted wave vectors are almost parallel to the grooves, as shown in Fig. 2(a) [23]. The direction of the incoming rays is described by two parameters: the altitude γ, that is defined as the angle between the direction of the incoming rays and the direction of the grooves, and the azimuth. The azimuth α of the incoming rays is defined to be zero if they lie in the plane perpendicular to the grating surface and parallel to the rulings, so −α is the azimuth of the light reflected at zeroth order. Let β define the azimuth of the first diffracted order at wavelength λ. The grating equation is written as sinγ (sinα + sinβ) = λσ, where σ is the groove density. When the grating is used as a monochromator, it is operated in the condition α = β [24] and the grating equation becomes 2 sinγ sinα = λσ. In the following discussion, the grating will be supposed to be operated at first diffracted order.
Fig. 2 a) Geometry of the off-plane mount. Incident light arrives along a cone of half-angle γ at an azimuthal angle α. The first diffraction order leaves on a cone of the same half-angle at an azimuthal angle β. b) Blaze condition of maximum efficiency where δ is the blaze angle.
The blaze condition for which the diffraction efficiency is maximized is fulfilled when the diffracted light is reflected specularly from the groove surface and when the shadowing effect from adjacent grooves is minimized, i.e. α = β = δ, where δ is the blaze angle of the grating. In blaze condition, each groove of the grating is seen by the incident ray as a portion of a plane mirror, as shown in Fig. 2(b). It has been theoretically demonstrated and experimentally measured [25–29] that the diffraction efficiency at the blaze wavelength in the OPM is close to the reflectivity of the coating at the altitude angle. This makes the OPM very attractive for constructing XUV monochromators with high efficiency [30].
Usually, the gratings in the OPM are plane and illuminated by collimated light. This implies that a monochromator needs at least two additional concave mirrors at grazing incidence with toroidal or paraboloidal shape, the first in front of the grating to collimate the light coming from the entrance slit, the second after the grating to focus the diffracted light on the exit slit.
The schematic of the configuration is shown in Fig. 3 and the three optical elements are indicated as TM1, G and TM2. The first mirror (TM1) acts as the collimator, the second mirror (TM2) as the condenser. The two mirrors are operated at equal grazing angle and unity magnification to minimize the aberrations: the input arm of the collimator, p, i.e. the distance between the entrance source and the centre of TM1 is equal to the output arm of the condenser, q, i.e. the distance between the centre of TM2 and exit slit. For toroidal mirrors, the best performances are obtained with a pair of identical mirrors, i.e. p = q, since this minimizes the coma aberration. A spectrally dispersed image of the source is created on the exit plane, where the slit carries out the spectral selection. Wavelength scanning is performed by rotating the grating around an axis that is tangent to the surface, passes through the grating centre and is parallel to the grooves. The azimuth changes with rotation while the altitude is kept constant, so that the maximum efficiency condition is exactly fulfilled only at the blaze wavelength λΒ = 2 sinγ sinδ/σ, which obviously depends on the blaze angle δ. At different wavelengths, the efficiency decreases because the grating is operated off-blaze. Nevertheless, it has been shown that such a mounting is very efficient in a broad spectral region for the spectral selection of HHs [31].
Fig. 3 Schematic of a monochromator with plane gratings in the off-plane mount and three optical elements: TM1 indicates the first collimating mirror, G the grating, TM2 the second focusing mirror, p is the entrance arm, i.e. the distance between the source and the centre of TM1, q the exit arm, i.e. the distance between the centre of TM2 and the slit, ΔS the width of the slit.
The monochromator can be operated either with or without an entrance slit. The latter case is the normal case for analysis and utilization of HHs, since the size of the XUV source is in the range of few tens of micrometers. Let us suppose that the width ΔS of the exit slit is equal to or wider than the size of the XUV source at the input of the monochromator. The half-width output bandwidth is Δλ = ΔS (σq)−1. The grating temporal response depends on the number of illuminated grooves, that is N = 2 D p σ, where D is the half-width beam divergence at the input of the monochromator and p is the length of the entrance arm. Here and in the following, the beam divergence is assumed so small that the approximation tan(D) ~ D is valid, as verified in the experiments of HH generation (D < 5 mrad). The pulse temporal half-width after diffraction is ΔτG = ½ N λ / c = λ D p σ / c. The condition to preserve the temporal duration of the pulse with half-width duration ΔτP is finally λ D p σ ≤ cΔτP.
Since the number of illuminated grooves on a grating in the CDM is equal to Ncd = 2 D p σcd / cos(αcd), where σcd is the groove density and αcd the incidence angle, a monochromator in the CDM gives the same temporal response as the OPM if its groove density is σcd = σ cos(αcd). Since the incidence angle to be used for XUV radiation is in the range αcd≈80°-85°, the groove density required to maintain the same temporal response is 5-10 times lower in the CDM than the OPM. For time responses in the range of few tens of femtoseconds, the required groove density and the corresponding blaze angle to achieve high efficiency in the CDM is definitely beyond the present capabilities of the manufacturers, so laminar gratings with very low groove density and poor efficiency have to be adopted as discussed in detail in Ref. 20. On the contrary, the OPM offers the advantage of requiring both higher groove density and higher blaze angle than the CDM, and so it can then be adopted for ultrashort time responses while maintaining high efficiency.
As a general rule, the CDM should be chosen for single-grating XUV monochromators with relatively long responses, i.e. few hundreds of femtoseconds, while the OPM has to be selected for ultrashort responses in the range of few tens of femtoseconds, as in the case of the instrument presented here.
3. Monochromator design and characterization
The parameters of the monochromator are summarised in Table 1 . The instrument is operated without the entrance slit, the source being directly the HH generation point. The ideal mirrors to collimate and focus the beam with no aberrations are paraboloidal, but in this case toroidal mirrors have been adopted (provided by Standa, Lithuania) since the divergence of the XUV beam is expected to be less than 10 mrad. It has been verified by ray-tracing simulations that the residual aberrations due to the toroidal shape are negligible for such small angular acceptance. The input arm p and the output arm q are equal: p = q = 300 mm. The central section has four gratings (manufactured by Richardson Grating, USA) which are mounted on a motorized linear stage and can be selected by the user for different resolutions and blazed wavelengths. All the optical elements are gold coated. A schematic of the monochromator connected to the vacuum chamber for HH generation is shown in Fig. 4(a) and a photograph of the instrument is shown for comparison in Fig. 4(b). The half-width output bandwidth in the OPM is almost constant with wavelength, since it depends only on the geometric parameters and on the groove density: Δλ = ΔS (σq)−1. The values for a 50-μm-wide slit are also reported in Table 1.
Table 1. Parameters of the ARTEMIS monochromator.
Fig. 4 Artemis monochromator as (a) schematic and (b) as a photograph with optical elements annotated.
The calculated time response of the monochromator is shown in Fig. 5 for a beam aperture of 6 mrad × 6 mrad, which is close to what has been previously measured for HHs generated with a focusing set-up similar to that used at Artemis [32]. The time response has been calculated as the half-width spread of the optical paths at the output slit: ΔτG = λ D p σ / c. The response is in the range 5-18 fs for the two low-resolution gratings and 30-80 fs for the medium-resolution ones.
Fig. 5 Time response of the monochromator for a beam aperture of 6 mrad × 6 mrad, calculated as the half-width spread of the optical paths at the output slit.
Clearly, the time response depends on the actual divergence of the beam, since it defines the area illuminated on the grating in the direction perpendicular to the grooves and correspondingly the number of grooves that are involved in the diffraction process. In particular, the response increases linearly with the beam divergence in the vertical direction, that is the direction perpendicular to the grooves in the OPM. A diaphragm can be placed just in front of the first mirror to limit the vertical aperture: this reduces the temporal broadening but obviously decreases also the photon flux for HH beams with large angular apertures. If the aperture is reduced to 3 mrad in the vertical direction, the corresponding time broadening is reduced by a factor of two with respect to that shown in Fig. 5, i.e. it assumes values smaller than 10 fs and 40 fs respectively for the low-resolution and the medium-resolution gratings. The expected time response of the single-grating configuration is comparable or even shorter to the time response of the existing TDCMs using concave gratings in the CDM. This confirms the advantage of the OPM for ultrafast responses in a simpler optical set-up.
The calibration of the spectral response has been performed using a hollow-cathode lamp in the 25-90 nm interval and a microfocus X-ray source in the 10-25 nm interval. A channel-electron-multiplier detector working in the photon counting regime has been used to detect the output radiation. The spectral scans have been performed by rotating the grating. Calibration spectra obtained with grating number 4 (G4) are shown in Fig. 6(a) . It has been verified that the width of the spectral lines is limited by the slit width, confirming that the spectral aberrations of the optical system are negligible.
Fig. 6 a) Calibration spectra with the 500 gr/mm grating (G4) and the hollow-cathode lamp with He and Ne gases. b) Monochromator efficiency, defined as the ratio between the radiation diffracted at first order and measured at the output and the radiation at the same wavelength measured at the input.
The measurement of the efficiency has been performed using the facilities available at CNR-IFN Padova (Italy) and described elsewhere [33]. The efficiency is here intended as the global monochromator efficiency, that is the ratio between the radiation diffracted at first order and measured at the output and the radiation at the same wavelength measured at the input. The measurements are shown in Fig. 6(b). The peak efficiencies at the blaze wavelength of each grating are in the 0.2-0.3 range. These values are rather high for a grazing-incidence monochromator and confirm the advantages of the OPM in terms of efficiency.
4. Spectral characterization of high-order harmonic generation
XUV radiation in the wavelength range 10–100 nm (12–120 eV) is produced by HH generation in a pulsed gas target, using 30-fs Ti:Sa laser pulses at 1 kHz repetition rate. The amount of laser energy that can be used to generate harmonics is limited by the distance between the laser focus and the first toroidal mirror, which has to be operated in safe conditions when illuminated by the IR beam that co-propagates with the XUV. The longer the distance, the higher the IR laser energy that can be used to generate HHs. On the contrary, once the accepted angular divergence of the monochromator has been fixed, the longer the distance, the longer the grating temporal response, since the number of illuminated grooves is linearly increasing with the distance. In the Artemis design, the input arm has been chosen to be 300 mm. This limits the amount of laser energy that can be used to generate HHs to less than ~3 mJ.
The IR beam is focused by a 25 cm lens onto a gas jet controlled by a piezoelectric valve operated at 1 kHz (provided by Attotech, Sweden). The IR co-propagates with the HHs generated in gas up to the grating. When the grating is rotated to perform the XUV wavelength scanning, the IR beam is totally diffracted on the zero-order, hits the toroidal mirror out of plane, is reflected in a direction far from the slit aperture and is finally blocked along the pipe connecting the monochromator chamber to the slit. No IR radiation can be measured at the output of the monochromator when the grating is rotated out from the zero order.
The monochromatized XUV flux is measured after the exit slit with an absolutely calibrated XUV channel-electron-multiplier which can be inserted into and removed from the optical path. By using 1-mJ laser pulses to generate HHs in argon, we have measured a flux as high as 1.6·1010 photons/s for the 21st harmonic (32.5eV) at the output of the monochromator, corresponding to an XUV energy of 83 pJ/pulse, with a pulse repetition rate of 1 kHz at the full harmonic bandwidth of 900 meV. Figure 7 shows a typical scanned HH spectrum obtained with Ar gas and exit slit closed to 100µm, corresponding to a 700 meV energy resolution. For symmetry reasons only the odd harmonic of the fundamental wavelength (785 nm) are generated. The small lines observed at low energy correspond to the second order of the grating and are well separated from the main harmonics.
Fig. 7 HH spectrum in argon, 1-mJ 30-fs IR laser pulses, grating G3. The peaks identified with dots at low energy are second diffraction orders.
When compared to other schemes that are used to obtain a monochromatized XUV beam, such as the combination of multilayer mirrors and thin metal foils, the use of the time-preserving grating monochromator gives some clear advantages: the spectral purity of the radiation is higher, since adjacent harmonics are completely stopped at the slit plane; thin metal foils are not needed to stop the fundamental IR laser beam, therefore the throughput of the system is improved; and the XUV photon energy is tunable in a broad region, a clear advantage over using multilayers.
5. Temporal response
The duration of the XUV pulses at the output of the monochromator has been measured using a method already demonstrated in [34]. The experimental set-up is shown in Fig. 8 . The 30 fs, 780 nm Ti:Sa laser beam is split into two by a beam-splitter. One beam is focused into the argon gas jet in the generation chamber and generates HHs. The XUV beam is monochromatized and is then focused by a toroidal mirror into a krypton gas jet. The second laser beam passes through a variable delay stage, propagates parallel to the XUV beamline, and is finally focused by a lens and recombined with the XUV monochromatized beam at an annular mirror. An ion time-of-flight spectrometer (Ion TOF), which has previously been used to study strong-field ionization effects in argon [35], is mounted on the common IR-XUV focal point and detects the ions emitted in the interaction region. The temporal overlap between the two beams is found by scanning the zero-order (XUV + IR) pump coming out from the monochromator and the IR probe from the delay stage and observing the interference fringes in the common focus. The complete optical set-up forms an interferometer with ~6 m arms.
Fig. 8 Experimental set-up for measuring the duration of monochromatized XUV pulses. BS is the beamsplitter, J1 the argon gas jet to generate HHs, TM1-G-TM2 the optics inside the monochromator (see Fig. 3), TM3 the refocusing toroidal mirror, DL the delay line, HM the recombination mirror, J2 the krypton gas jet, Ion ToF the ion time-of-flight spectrometer.
The ionization yield of krypton, requiring an IR intensity of ~1013 Wcm−2 or a photon energy of more than ~14 eV depending on the ionization regime, has been measured by the ion spectrometer as a function of the temporal overlap between the IR beam from the monochromator set at the zero-order and the IR external beam. As the delay line is scanned, temporal fringes are clearly apparent in the measured Kr+ yield, as visible in Fig. 9 . The fine structure with ½ laser-cycle period (1.3 fs) is well resolved and shows that the interferometer is stable to better than ¼ cycle (0.6 fs) over the duration of the measurement and delay range. The fringes were still visible after 12 hours of operation, illustrating that the two foci remained spatially and temporally overlapped over this time, proving the reliability of the beamline for multi-pulse pump-probe experiments with femtosecond resolution.
Fig. 9 Measured Kr+ yield as a function of the temporal overlap between zero-order (IR + XUV) pump and IR probe, showing interference fringes between the two IR pulses. The asymmetry in the Kr+ yield is a consequence of the XUV also transmitted in the zero-order.
By tuning the monochromator to transmit a single XUV harmonic, i.e. H23 at 35.7 eV, we observe single XUV photon absorption followed by IR strong field ionization. The monochromator slit was 160-um wide and the corresponding resolution λ/Δλ was about 40. The electronic structure of krypton is such that when an XUV pump photon is absorbed, 4s or 4p photoionization results. Direct or conjugated shake-up or the configuration interaction causes an additional 4p electron to be promoted to a highly excited satellite state of Kr+. The satellite state is readily strong-field ionized to Kr2+ by the IR pulse, as shown in Fig. 10 . An XUV photon generates the Kr2+ population only on the condition that the strong field pulse is also present, hence the shape of the temporal step is a measure of the temporal cross-correlation. If the pump and probe are reversed in time, no energetically favorable route to Kr2+ exists. The convolution of a 28-fs Gaussian IR pulse with the cumulative distribution function of a Gaussian of a variable duration is overlaid on the experimental data of Fig. 10, indicating that the XUV pulse duration is in the range 10 to 50 fs. Analysis and modeling of the temporal behavior of the Kr2+ yield for a range of harmonics has enabled us to map out partial cross-sections of satellite states for the first time, demonstrating the advantages of the tunability and high throughput of this monochromator [36]. This more detailed analysis indicates the duration of the XUV is 24 ± 2 to 28 ± 2 fs over the H21 to H27 spectral range from 32.8 to 42.1 eV.
Fig. 10 Yield of Kr2+ with H23 (35.7 eV) pump and 800 nm probe as a function of the delay between the two pulses.
To our knowledge, this is the first measurement showing that a single-grating monochromator specially designed for ultrashort pulses gives a temporal response as short as few tens of femtoseconds, providing also high efficiency. This result has been achieved thanks to the grating geometry adopted to design the monochromator.
6. Conclusions
We have presented the design and realization of a configuration for a single-grating time-preserving monochromator for femtosecond ultrashort pulses. One of these instruments has been installed on the monochromatized branch of the Artemis beamline at the Central Laser Facility (UK). The core of the design is the use of a grating in the off-plane mount, which has been shown to give minimum temporal broadening after diffraction and very high grating efficiency. We have measured XUV pulses with 1.6·107 ph/pulse at 32.5 eV at 1 kHz repetition rate. The pulse duration of the monochromatized pulses has been temporally characterized to be less than 30 fs.
The monochromator provides the Artemis beamline with an intense XUV beam with high spectral purity, tunability in a broad energy region and ultrashort duration. These properties make it very attractive for ultrafast time-resolved electron spectroscopy.
Acknowledgments
We thank CLF engineering staff, particularly Dan Wolff, for their support in the design of the monochromatized beamline. This work was financially supported by STFC Facility Development scheme, STFC facility access grants and Laserlab Europe.
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