- Email: [email protected]
Atomic streak camera
I September 1996 OPTICS COMMUNICATIONS Optics Communications 129 (1996) 361-368
ELSEVIER
A t o m i c streak camera G.M. Lankhuijzen,
L.D. Noordam
FOM lnstitute Jbr Atomic and Molecular Physics. Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
Received 29 December 1995; accepted 27 February 1996
Abstract An atomic streak camera is developed to study the ultrafast ionization dynamics of Rydberg atoms excited by short laser pulses in an electric field. The electron flux released in the ionization process is imaged on a position sensitive detector. The atomic streak camera converts the time of ionization into a displacement on the detector. A sub-picosecond temporal resolution is obtained. The properties of the atomic streak camera are discussed in detail. PACS: 32.60.+i; 42.65.Re; 32.80.Dz
1. Introduction Highly excited electrons in atoms have been studied extensively over the last years. The dynamics of these so-called Rydberg electrons [ 1 ] occur on a picosecond time scale, and have been studied with ultra-short laser pulses. Such a short laser pulse creates an electronic wave packet that mimics the classical electron motion in the atomic potential. Several probe techniques have been developed to study the dynamics of these electronic wave packets. Up till now optical probe techniques have been used because of the high temporal resolution that is required to follow the electron motion. Using optical pump-probe techniques the motion of the wave packet in the atomic potential is revealed: the pump laser created a wave packet near the core, the optical probe monitors the return of the electron to the atomic core. In one technique, photoionization by the probe pulse is measured versus delay. In this way the amount of wavefunction near the core is probed since the ionization probability is largest when the electron is near the core [ 2 - 4 ] . In another optical technique two identical laser pulses are applied, creating two
identical wave packets. When the firstly created wave packet has returned to the core, interferences between this wave packet and the wave packet created by the second pulse occur, giving rise to an enhancement or reduction of the Rydberg population. By measuring this modulation in the Rydberg population as a function of the delay between the two laser pulses, the return of the wave packet to the core is measured [5,6]. If the Rydberg atom is placed in a static electric field the potential in the direction of the anode is lowered and the wave packet can escape over the saddle point in the potential (see Fig. 1). Using the double wave packet pump-probe technique the dynamics of the Rydberg wave packet in the tilted potential has been studied [5,7,8]. However, in these experiments the return of the wave packet to its starting conditions is measured, and not the time of escape of the electron. In this paper we present a new device, the atomic streak camera, which is able to measure the ionization dynamics, i.e., the escape of the electron from the atomic potential, with picosecond resolution. The design of the atomic streak camera (see Fig. 2) is based on a conventional streak camera [9,10]. In a conven-
0030-4018/96/$12.00 Copyright (~) 1996 Elsevier Science B.V. All rights reserved. PII S0030-401 8 (96)00166-6
OPTICS 3658
G.M. Lankhuijzen, L.D. Noordam/ Optics Communications 129 (1996) 361-368
362
4 kV/cm
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Fig. 1. Potential energy curve of an electron in a combined Coulomb and electric field (V = - 1 / r - Fz). The classical ionization energy is lowered by the external electric field. An electron with energy higher than the saddle point energy can escape from the atomic potential.
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~ d e l a y e d switchiDulse
Fig. 2. Principle o f the atomic streak camera. Atoms in an electric field are ionized by a short laser pulse. For clarity the ionization is depicted in two separate bursts. The electrons are accelerated and
pass through three slits where they are focused in the x-direction. After focusing they enter the deflection region. The voltage of the upper deflection plate is ramped from - 1 . 3 kV to +1.2 kV within 500 ps. The deflection of the electrons depends on the time of arrival between the deflection plates. The temporal profile of the electron pulse is thus transformed into a deflection angle along the x-direction which is detected further downstream by a position sensitive detector, consisting of a three stages channelplate electron multiplier, a phosphor screen and CCD camera.
tional streak camera the optical pulse is transformed into an electron pulse by means of a photocathode. In the atomic streak camera, instead of a photocathode, a low pressure atomic gas is used to create the electrons. Atoms in a static electric field are excited by a short laser pulse. Let us consider the case where the
wave packet dynamics is such that the electrons leave the atoms in two bursts. The double electron pulse is accelerated by the electric field and passes the slit in the anode. After the slit the electrons enter the deflection region. The voltage applied to one of the two deflection plates is ramped. Therefore, the electron pulse arriving first between the deflection plates will be less deflected than the second one, resulting in a different position on the detector. By measuring the position of the electrons on the detector, the time resolved ionization spectrum of the atoms is retrieved. For the generation of the voltage ramp an optical switch is used [ 10]. The laser pulse used to ionize the atoms is also used to trigger the photoswitch. In this way there is no noticeable time jitter between these two events, making it possible to average over many shots. In order to calibrate the conversion from position on the detector to time of ionization two identical pulses, which are delayed with respect to each other, are sent into the atomic streak camera. The excitation is chosen far above the saddle point, leading to prompt ionization of the atoms. Both optical pulses will ionize a small fraction of the atoms and as a result two electron bursts will be launched. The time separation between the electron pulses is identical to the delay between the optical pulses: 18 ps. In Fig. 3 a single shot image of the detector using this double pulse is shown. Each line corresponds to electrons created by one of the optical pulses. The measured horizontal spacing between the lines is now transferred to a time scale. The tilt of both lines can be explained as follows: the ionizing laser beam travels along the direction of the slit (y-direction). The atoms are ionized at different times due to the transit time of the laser pulse along the direction of the slit. The total slit length of 1.0 cm corresponds to 33 ps travel time for the ionizing laser beam. Therefore, electrons created at the start of the slit will reach the deflection plates earlier than electrons created further downstream. This intrinsic property of the atomic streak camera is another way of in situ calibration of the position to time scaling of the detector and is in excellent agreement with the calibration obtained with the double pulse experiment.
G.M. Lankhuijzen. L.D. Noordam/Optics Communications 129 (1996) 361-368
363
Fig. 3. Image of a single shot readout of the CCD camera. Two identical pulses, delayed by 18 ps, are used to ionize rubidium atoms in an electric field of 4 kV/cm. The central wavelength of the laser pulses (595.0 nm) is such that the created wave packet ionizes immediately. Hence two optical pulses will give rise to two electron pulses. The horizontal x-axis shows the time, the vertical y-axis the position along the slit. The angle of the tilted lines is determinedby the speed of light.
2. Design of the atomic streak camera In order to optimize the performance of the atomic streak camera electron trajectory calculations are performed. These calculations give insight in the potential temporal resolution of the atomic streak camera as
a function of all experimental parameters. The calculations show that for a sweep rate of 5 V / p s applied to the deflection plates (length 15 mm, spacing 5 m m ) the deflection as a function of time (after 20 cm) is 4 6 0 / z m / p s . The electrons are accelerated to 3.2 keV. Both the width of the electron pulse hitting the de-
364
G.M. Lankhuijzen, L.D. Noordam/Optics Communications 129 (1996) 361-368
tector without any deflection and the resolution of the detector itself has to be smaller than 460/zm in order to obtain sub-picosecond resolution. The spatial resolution of the detector used here is 200/xm (FWHM), and the spotsize of the electron pulse is focused down to 200/zm (FWHM). In order to focus the electrons two additional electric field regions are used through which the electrons pass before entering the deflection region (see Fig. 2). Due to the electric field change before and after the slits, the electrons are focused. Three other effects will limit the temporal resolution of the atomic streak camera: (i) The energy spread of the ejected electrons causes a broadening of the electron peak while traveling towards the deflection plates. (ii) Due to the width of the laser beam electrons will be created at different positions along the z-axis in the interaction region, giving different times of arrival between the deflection plates. (iii) The shape of the voltage pulse applied to the deflection plates varies from shot to shot due to intensity fluctuations of the optical switch pulse. All these effects are studied by performing the trajectory calculations and averaging over (i) the energy of the ejected electrons, (ii) the spatial profile of the laser beam, and (iii) the shape of the voltage pulse. We will now discuss the differences between the atomic streak camera and a conventional streak camera. The aim of a conventional streak camera is to measure optical pulse durations whereas the atomic streak camera probes the electron flux as emitted by a gas of autoionizing atoms. Also the principle of operation of the two types is somewhat different. The main difference lies in the process of creation of the electrons. In a conventional streak camera the electrons are created using a photocathode. The created electrons leaving the photocathode have a relatively large energy spread ( ~ 0.2 eV). Due to this energy spread the transit time of the individual electrons to the sweeping device is spread out over time, giving rise to a decrease in temporal resolution. To minimize this spreading of the electrons, the electrons are accelerated as quickly as possible to keV energies. To obtain picosecond resolution electric fields > I0 k V / c m are required [9]. In the atomic streak camera however the initial electron energy is extremely low, typically less than 0.01 eV, reducing the restrictions on the electric fields in which the atoms are ionized. Typical values are a width of 0.5 ps (FWHM) for an energy spread of 0.01 eV in
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Fig. 4. Due to the finite size of the laser beam, electronsare created at different positions in the interaction region. Electrons created at a higher potential (position I, filled circle) will take longer to reach the anode, but their final velocity will be higher. They will therefore catch up with the slower electrons (open circle) at the time focus.
an electric field of 4 kV/cm. The creation of the electrons in a conventional streak camera is in the plane of the photocathode, whereas the electrons are created over a volume in the atomic streak camera. The creation of electrons at different positions in the z-direction (see Fig. 4) gives rise to a different time of arrival and a different energy at the exit of the acceleration region. Electrons that are created at a higher potential, position I in Fig. 4, will arrive later at the anode, but their energy is higher than electrons created at position II. After the slit these electrons will catch up with the electrons created at point II at a given point along the z-axis. This point is known from ion time of flight apparatuses as the time focus and is calculated easily [ 11 ]. When we position the deflection plates at this time focus the effect of the laser beam diameter is strongly reduced. Using a post-acceleration region the position of the time focus can be adjusted. The post acceleration region is also used to focus the electrons in the x-direction. Trajectory calculations show that for an electric field of 4 k V / c m and a post acceleration of 2 kV/cm, a beam diameter of 100 # m (FWHM) gives rise to a temporal broadening of less than 0.6 ps over a range of 100 ps. In Fig. 5 the effect of the time resolution as a function of the laser beam diameter is shown. The time focus is set deliberately at a different position from the deflection plates. By moving the lens that is focusing the laser beam, the laser beam diameter can be increased and hence the width of the electron peak. The line in Fig. 5 is given by a trajectory calculation involving Gaussian beam focusing and the electron trajectories discussed above.
G.M. Lankhuijzen, L.D. Noordam/Optics Communications 129 (1996) 361-368 beam diameter (l~m)
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Fig. 6. Schematic representation of the electronic circuit for the generation of the fast voltage ramp. The 150 pF capacitor is charged up to +1.6 kV. At t = 0 the 200 fs laser pulse is applied to the photoswitch, lowering the resistance to < 10 II. As a result the deflection plate is charged up to + 1.6 kV within 500 ps. During the rising edge of the voltage ramp the electrons pass the deflection plates. The life time of the free carriers in the GaAs phot0switch is 1.5 ns: after a few nanoseconds the resistance of the photoswitch is high again ( > 100 MI~). In a few microseconds the condenser plates are discharged over the 1 MI~ resistor to the - 0 . 9 kV bias voltage.
3. Production of voltage ramp The heart of the streak camera is formed by the two deflection plates to which the fast voltage ramp is applied. The electronic circuit used for the generation of such a fast electrical pulse is shown in Fig. 6. A ceramic chip capacitor with low inductance and effective series resistance is charged up to 1.6 kV typically. A piece of semi-insulating GaAs (15 x 5 x 0.35 mm) doped with Fe (concentration > 1016 cm -3) connects
365
the capacitor with the upper deflection plate; the lower one is at ground potential. The resistance of the GaAs photoswitch is > 100 MIL The upper deflection plate is biased to - 0 . 9 kV by means of a 1 MO resistor. When the GaAs is illuminated with a short laser pulse, valence electrons are promoted to the conduction band lowering the resistance over 7 orders of magnitude (down to ,,~ 10 f~). A current starts to flow from the capacitor to the upper deflection plate, charging it up to +1.6 kV. The lifetime of the charge carriers in this particular piece of GaAs is ~ 1.5 ns [ 12], which is longer than the time required to charge the capacitance of the deflection plates. By varying the delay between the optical ionizing pulse and the optical switch pulse the most linear part of the electrical pulse is selected. Since the detector is centered at the z-axis, the bias voltage is used to let the voltage pulse go through zero volt at the linear part of the response. When the resistance of the GaAs switch is high again, the capacitor is discharged by the 1 Mf~ resistor to the - 0 . 9 kV in a few microseconds. The switch is operated in saturation, i.e., a laser flux > 4 0 / z J / c m 2 for a laser pulse with central wavelength z ~ '~20 nm [ 10], to minimize the effects of laser intensity fluctuations. Great care is taken to minimize the Amplified Spontaneous Emission (ASE) background of the laser pulses. The ASE background gives rise to a non-stable operation of the photoswitch. The creation of the valence electrons in the photoswitch is a single-photon process. Therefore, the ASE background, which has a width of 5 ns, already reduces the resistance of the photoswitch before the femtosecond pulse arrives. An extra 360 resistor is put in series with the GaAs photoswitch to minimize effects of fluctuations in the resistance of the GaAs due to fluctuations in laser intensity. The inductance in the circuit is determined by the geometry of the design. To minimize this inductance, strips are used ( 1.5 cm wide) to connect the electronic components. The overall size of the circuit is minimized to a few centimetres. The pulse characteristics are fully determined by this electronic scheme and are calculated easily. In Fig. 7 the measured deflection as a function of the delay between the optical ionizing and optical switch pulse is plotted. The electrons are created in an electric field of 4 kV/cm. The sweep voltage is set to a low value to reduce the deflection angle, so that the entire voltage pulse can be observed within the range
366
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By measuring the position of the electrons after they have passed the deflection region the temporal profile of the electron pulse is retrieved. The detector consists of a three stages channelplate electron multiplier, a phosphor screen which transforms the electrons into photons, and a CCD camera (256 x 256 pixels) to read out the phosphor screen. The channelplates have an overall amplification of ~ 107. The electron to photon conversion at the phosphor screen has a gain of ,~ 10. The phosphor screen releases ~, 108 photons (spotsize 200 /zm FWHM) per incoming electron. Using a f = 50 mm f/D = 1.2 lens the phosphor screen is imaged on the CCD pixels. The number of photons per incoming electron is well above the noise level of the CCD camera. The total image size of the detector is 23.4 x 23.4 ram. The overall resolution of the detector, mainly determined by the size of the light pulse from the phosphor screen is 200/xm (FWHM).
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Time (ps) Fig. 7. (a) Measured deflection (circles) of the electrons as a function of delay between the optical ionizing laser pulse and the optical switch pulse. The dash-dotted curve represents the calculated response of the circuit in Fig. 6 convolutedwith the transit time (560 ps) of the electronsthroughthe deflectionplates. (b) Deviation of the deflection from a linear response for a time window of 200 ps. of the detector. The energy of the electrons entering the deflection region is 2 keV which gives them a transit time through the deflection plates (1.5 cm long) of 560 ps. The measured deflection is therefore a convolution of the voltage pulse with a block of 560 ps wide. The dash-dotted curve in Fig. 7 shows the calculation of the response of the electrical circuit convoluted with the 560 ps block pulse. The inductance L and capacitance C are used as fit parameters. The electrical pulse as deduced from Fig. 7 has a rise time of 430 ps. In the lower part of Fig. 7 the deviation from a linear response is plotted. Over a region of 200 ps the linearity is better than 0.5%.
The images recorded by the CCD camera at 10 Hz are transferred to a PC via a frame grabber. Several data analyses are performed on a shot to shot basis before the data is stored. To obtain the position to time transformation the angle of the lines as depicted in Fig. 3 is determined in situ. Once the angle is known the data of the image is integrated along this line, resulting in a single array, the so-called ionization spectrum, as shown in the top part of Fig. 3. Due to intensity fluctuations of the laser pulses that trigger the photoswitch, the voltage ramp has a small time jitter (typically < 15 ps) with respect to the ionizing laser pulse. Therefore, the individual streak images are displaced with respect to each other on the detector. If these images were added without any correction the resolution will be as bad as this time jitter. Therefore a Centre Of Mass (COM) correction is used in order to shift the individual images on top of each other. To determine the centre of mass with sufficient accuracy, many electrons need to hit the detector due to the statistical nature of this ionization process.
G.M. Lankhuijzen. L.D. Noordarn/ Optics Communications 129 (1996) 361-368
6. Experimental results
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To test the specification of the atomic streak camera, short laser pulses are needed both for triggering the photoswitch as for the creation of sub-picosecond electron pulses. For the trigger of the GaAs photoswitch an amplified CPM laser (10 Hz) is used [13]. The CPM laser produces pulses of 100 fs at 80 MHz repetition rate at a central wavelength of 620 nm. These pulses are amplified using four single pass Bethunetype dye cells which are pumped by the second harmonic of a nanosecond Nd:YAG (where YAG denotes yttrium aluminum garnet) laser operating at 10 Hz. The photoswitch is illuminated with 200 fs pulses with an energy of > 100/zJ per pulse. This is well above the saturation fluence of the photoswitch minimizing the effects of laser intensity fluctuations on the voltage pulse characteristics. In order to minimize the ASE background a saturable absorber is installed after the third Bethune cell reducing the ASE to < 0.2% of the total pulse energy. Part of the amplified CPM light ( 113 /zJ) is used for continuum generation in a water cell. By means of an interference bandpass filter the desired frequency range (590-605 nm) is selected and further amplified. A shaper [ 14] is used to select the central wavelength and bandwidth, typically 0.1-1.5 nm, of the pulses. A number of experiments are performed to obtain the specifications of the atomic streak camera. In all experiments the atomic vapour consisted of rubidium atoms coming out of an oven. The binding energy of the 5s groundstate electron of rubidium is 33688.3 c m - l which corresponds to a two photon excitation at 593.6 nm. When a rubidium atom is placed in an electric field the ionization threshold is given by Ec = -6.12v/if,
(1)
where F is the electric field strength ( V / c m ) and Ec is the ionization threshold (cm -1 ). If the excitation energy of the electron is far above this threshold the ionization will be prompt, i.e., the electron will leave the atom immediately. The electron pulse will therefore be as short as the optical pulse (200 fs). In Fig. 8a a single shot image is shown. The rubidium atoms are ionized in a two-photon process at 597.0 nm (at 0.25Ec) in a static electric field of 4.0 kV/cm. The electrons are post accelerated to 5.0 keV before entering the deflection plates. The upper de-
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flection plate is biased at - 1 . 2 kV and switched to +1.3 kV within 500 ps. The sweep rate of the electrons over the detector is 3.8 • 108 m/s. The gain of the detector is deliberately set to a low value (gain channel plates ,-~ 1 0 6 ) . In this way single electrons hitting the detector are not detected and the measured peak consists of > 100 electrons. In Fig. 8b a 100 shot average is plotted. The fluctuations in the laser intensity which give rise to a small change in the field ramp account for the broadening of the peak to 2.5 ps (FWHM). Under normal experimental conditions the shot to shot fluctuation gives rise to a larger broadening ( > 10 ps). Therefore a Centre Of Mass (COM) correction is used to shift the individual shots. A resolution of 2.5 ps (FWHM) is then easily obtained. In Fig. 9 the decay of an autoionizing electronic wave packet is shown. The electric field is set at 2.0 kV/cm. The excitation is just above the classical field ionization limit (0.87Ec) using the frequency dou-
G.M. Lankhuijzen, L.D. Noordam/ Optics Communications 129 (1996) 361-368
368
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Time (ps) Fig. 9. Time resolved ionization spectrum of an electronic wave packet excited above the classical field ionization limit at 0.87Ec using a single photon excitation at 298.78 nm and a bandwidth of 0.2 nm (FWHM). The laser polarization is chosen perpendicular to the electric field of 2.0 kV/cm.
bled light at 298.78 nm. The laser polarization is chosen perpendicular to the electric field of 2.0 kV/cm, launching the electron perpendicular to the z-axis. Therefore the electron does not escape immediately from the atomic potential giving rise to the observed structure in the ionization spectrum. From Fig. 9 we learn that the ionization occurs in several bursts, revealing the wave packet motion in the potential. The time spacing between the bursts is understood in terms of the motion of an angular wavepacket. The physics involved in the ionization dynamics will be discussed in detail elsewhere [ 15].
7. Conclusion A new device, the atomic streak camera, that can be used to study the electron flux of autoionizing atoms in a static electric field is presented. A sub-picosecond resolution is obtained making it possible to resolve the ionization dynamics of electronic wave packets in rubidium, created above the classical field ionization limit.
It is a pleasure to thank A.N. Buyserd for technical assistance, E Karouta and T. Uitterdijk for supplying photoconductive switches, W.H. Knox and A. Vijftigschild for suggestions on the switching device, E van Deenen and T. Uitterdijk for carefully reading the manuscript. The work in this paper is part of the research program of the "Stichting voor Fundamenteel Onderzoek van de Materie" (Foundation for Fundamental Research on Matter) and was made possible by financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" (Netherlands Organization for the Advancement of Research).
References [1] T.E Gallagher, Rydberg atoms, Ist Ed. (Cambridge University Press, Cambridge, 1994). [2] A. Giusti-Suzor and E Zoller, Phys. Rev. A 36 (1987) 5178. [3] A. ten Wolde, L.D. Noordam, A. Lagendijk and H.B. van Linden van den Heuvell, Phys. Rev. Lett. 61 (1988) 2099. [4] J.A. Yeazell and C.R. Stroud Jr., Phys. Rev. A 43 (1991) 5153. [5] B. Broers, J.E Christian, J.H. Hoogenruad, W.J. van der Zande, H.B. van Linden van den Heuvell and L.D. Noordam, Phys. Rev. Lett. 71 (1993) 344. [6] L.D. Noordam, D.I. Duncan and T.E Gallagher, Phys. Rev. A 45 (1992) 4734. [7] B. Broers, J.E Christian and H.B. van Linden van den Heuvell, Phys. Rev. A 49 (1994) 2498. [8] J.E Christian, B. Broers, J.H. Hoogenraad, W.J. van der Zande and L.D. Noordam, Optics Comm. 103 (1993) 79. [9] Y. Tsuchiya, IEEE J. Quantum Electron. QE-20 (1984) 1516. [10] W. Knox and G. Mourou, Optics Comm. 37 (1981) 203. [11] W.C. Wiley and I.H. McLaren, Rev. Sci. Instr. 26 (1955) 1150. [ 12l E Karouta, private communication. [ 13] L.D. Noordam, W. Joosen, B. Broers, A. ten Wolde, A. Lagendijk, H.B. van Linden van den Heuvell and H.G. Muller, Optics Comm. 85 (1991) 331. [ 14] A.M. Weiner, J.E Heritage and E.M. Kirschner, J. Opt. Soc. Am. B 5 (1988) 1563. [15] G.M. Lankhuijzen and L.D. Noordam, Phys. Rev. Lett. 76 (1996) 1784.
ELSEVIER
A t o m i c streak camera G.M. Lankhuijzen,
L.D. Noordam
FOM lnstitute Jbr Atomic and Molecular Physics. Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
Received 29 December 1995; accepted 27 February 1996
Abstract An atomic streak camera is developed to study the ultrafast ionization dynamics of Rydberg atoms excited by short laser pulses in an electric field. The electron flux released in the ionization process is imaged on a position sensitive detector. The atomic streak camera converts the time of ionization into a displacement on the detector. A sub-picosecond temporal resolution is obtained. The properties of the atomic streak camera are discussed in detail. PACS: 32.60.+i; 42.65.Re; 32.80.Dz
1. Introduction Highly excited electrons in atoms have been studied extensively over the last years. The dynamics of these so-called Rydberg electrons [ 1 ] occur on a picosecond time scale, and have been studied with ultra-short laser pulses. Such a short laser pulse creates an electronic wave packet that mimics the classical electron motion in the atomic potential. Several probe techniques have been developed to study the dynamics of these electronic wave packets. Up till now optical probe techniques have been used because of the high temporal resolution that is required to follow the electron motion. Using optical pump-probe techniques the motion of the wave packet in the atomic potential is revealed: the pump laser created a wave packet near the core, the optical probe monitors the return of the electron to the atomic core. In one technique, photoionization by the probe pulse is measured versus delay. In this way the amount of wavefunction near the core is probed since the ionization probability is largest when the electron is near the core [ 2 - 4 ] . In another optical technique two identical laser pulses are applied, creating two
identical wave packets. When the firstly created wave packet has returned to the core, interferences between this wave packet and the wave packet created by the second pulse occur, giving rise to an enhancement or reduction of the Rydberg population. By measuring this modulation in the Rydberg population as a function of the delay between the two laser pulses, the return of the wave packet to the core is measured [5,6]. If the Rydberg atom is placed in a static electric field the potential in the direction of the anode is lowered and the wave packet can escape over the saddle point in the potential (see Fig. 1). Using the double wave packet pump-probe technique the dynamics of the Rydberg wave packet in the tilted potential has been studied [5,7,8]. However, in these experiments the return of the wave packet to its starting conditions is measured, and not the time of escape of the electron. In this paper we present a new device, the atomic streak camera, which is able to measure the ionization dynamics, i.e., the escape of the electron from the atomic potential, with picosecond resolution. The design of the atomic streak camera (see Fig. 2) is based on a conventional streak camera [9,10]. In a conven-
0030-4018/96/$12.00 Copyright (~) 1996 Elsevier Science B.V. All rights reserved. PII S0030-401 8 (96)00166-6
OPTICS 3658
G.M. Lankhuijzen, L.D. Noordam/ Optics Communications 129 (1996) 361-368
362
4 kV/cm
A
Rydberg serie ~
ionization
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Fig. 1. Potential energy curve of an electron in a combined Coulomb and electric field (V = - 1 / r - Fz). The classical ionization energy is lowered by the external electric field. An electron with energy higher than the saddle point energy can escape from the atomic potential.
P P
~ d e l a y e d switchiDulse
Fig. 2. Principle o f the atomic streak camera. Atoms in an electric field are ionized by a short laser pulse. For clarity the ionization is depicted in two separate bursts. The electrons are accelerated and
pass through three slits where they are focused in the x-direction. After focusing they enter the deflection region. The voltage of the upper deflection plate is ramped from - 1 . 3 kV to +1.2 kV within 500 ps. The deflection of the electrons depends on the time of arrival between the deflection plates. The temporal profile of the electron pulse is thus transformed into a deflection angle along the x-direction which is detected further downstream by a position sensitive detector, consisting of a three stages channelplate electron multiplier, a phosphor screen and CCD camera.
tional streak camera the optical pulse is transformed into an electron pulse by means of a photocathode. In the atomic streak camera, instead of a photocathode, a low pressure atomic gas is used to create the electrons. Atoms in a static electric field are excited by a short laser pulse. Let us consider the case where the
wave packet dynamics is such that the electrons leave the atoms in two bursts. The double electron pulse is accelerated by the electric field and passes the slit in the anode. After the slit the electrons enter the deflection region. The voltage applied to one of the two deflection plates is ramped. Therefore, the electron pulse arriving first between the deflection plates will be less deflected than the second one, resulting in a different position on the detector. By measuring the position of the electrons on the detector, the time resolved ionization spectrum of the atoms is retrieved. For the generation of the voltage ramp an optical switch is used [ 10]. The laser pulse used to ionize the atoms is also used to trigger the photoswitch. In this way there is no noticeable time jitter between these two events, making it possible to average over many shots. In order to calibrate the conversion from position on the detector to time of ionization two identical pulses, which are delayed with respect to each other, are sent into the atomic streak camera. The excitation is chosen far above the saddle point, leading to prompt ionization of the atoms. Both optical pulses will ionize a small fraction of the atoms and as a result two electron bursts will be launched. The time separation between the electron pulses is identical to the delay between the optical pulses: 18 ps. In Fig. 3 a single shot image of the detector using this double pulse is shown. Each line corresponds to electrons created by one of the optical pulses. The measured horizontal spacing between the lines is now transferred to a time scale. The tilt of both lines can be explained as follows: the ionizing laser beam travels along the direction of the slit (y-direction). The atoms are ionized at different times due to the transit time of the laser pulse along the direction of the slit. The total slit length of 1.0 cm corresponds to 33 ps travel time for the ionizing laser beam. Therefore, electrons created at the start of the slit will reach the deflection plates earlier than electrons created further downstream. This intrinsic property of the atomic streak camera is another way of in situ calibration of the position to time scaling of the detector and is in excellent agreement with the calibration obtained with the double pulse experiment.
G.M. Lankhuijzen. L.D. Noordam/Optics Communications 129 (1996) 361-368
363
Fig. 3. Image of a single shot readout of the CCD camera. Two identical pulses, delayed by 18 ps, are used to ionize rubidium atoms in an electric field of 4 kV/cm. The central wavelength of the laser pulses (595.0 nm) is such that the created wave packet ionizes immediately. Hence two optical pulses will give rise to two electron pulses. The horizontal x-axis shows the time, the vertical y-axis the position along the slit. The angle of the tilted lines is determinedby the speed of light.
2. Design of the atomic streak camera In order to optimize the performance of the atomic streak camera electron trajectory calculations are performed. These calculations give insight in the potential temporal resolution of the atomic streak camera as
a function of all experimental parameters. The calculations show that for a sweep rate of 5 V / p s applied to the deflection plates (length 15 mm, spacing 5 m m ) the deflection as a function of time (after 20 cm) is 4 6 0 / z m / p s . The electrons are accelerated to 3.2 keV. Both the width of the electron pulse hitting the de-
364
G.M. Lankhuijzen, L.D. Noordam/Optics Communications 129 (1996) 361-368
tector without any deflection and the resolution of the detector itself has to be smaller than 460/zm in order to obtain sub-picosecond resolution. The spatial resolution of the detector used here is 200/xm (FWHM), and the spotsize of the electron pulse is focused down to 200/zm (FWHM). In order to focus the electrons two additional electric field regions are used through which the electrons pass before entering the deflection region (see Fig. 2). Due to the electric field change before and after the slits, the electrons are focused. Three other effects will limit the temporal resolution of the atomic streak camera: (i) The energy spread of the ejected electrons causes a broadening of the electron peak while traveling towards the deflection plates. (ii) Due to the width of the laser beam electrons will be created at different positions along the z-axis in the interaction region, giving different times of arrival between the deflection plates. (iii) The shape of the voltage pulse applied to the deflection plates varies from shot to shot due to intensity fluctuations of the optical switch pulse. All these effects are studied by performing the trajectory calculations and averaging over (i) the energy of the ejected electrons, (ii) the spatial profile of the laser beam, and (iii) the shape of the voltage pulse. We will now discuss the differences between the atomic streak camera and a conventional streak camera. The aim of a conventional streak camera is to measure optical pulse durations whereas the atomic streak camera probes the electron flux as emitted by a gas of autoionizing atoms. Also the principle of operation of the two types is somewhat different. The main difference lies in the process of creation of the electrons. In a conventional streak camera the electrons are created using a photocathode. The created electrons leaving the photocathode have a relatively large energy spread ( ~ 0.2 eV). Due to this energy spread the transit time of the individual electrons to the sweeping device is spread out over time, giving rise to a decrease in temporal resolution. To minimize this spreading of the electrons, the electrons are accelerated as quickly as possible to keV energies. To obtain picosecond resolution electric fields > I0 k V / c m are required [9]. In the atomic streak camera however the initial electron energy is extremely low, typically less than 0.01 eV, reducing the restrictions on the electric fields in which the atoms are ionized. Typical values are a width of 0.5 ps (FWHM) for an energy spread of 0.01 eV in
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an electric field of 4 kV/cm. The creation of the electrons in a conventional streak camera is in the plane of the photocathode, whereas the electrons are created over a volume in the atomic streak camera. The creation of electrons at different positions in the z-direction (see Fig. 4) gives rise to a different time of arrival and a different energy at the exit of the acceleration region. Electrons that are created at a higher potential, position I in Fig. 4, will arrive later at the anode, but their energy is higher than electrons created at position II. After the slit these electrons will catch up with the electrons created at point II at a given point along the z-axis. This point is known from ion time of flight apparatuses as the time focus and is calculated easily [ 11 ]. When we position the deflection plates at this time focus the effect of the laser beam diameter is strongly reduced. Using a post-acceleration region the position of the time focus can be adjusted. The post acceleration region is also used to focus the electrons in the x-direction. Trajectory calculations show that for an electric field of 4 k V / c m and a post acceleration of 2 kV/cm, a beam diameter of 100 # m (FWHM) gives rise to a temporal broadening of less than 0.6 ps over a range of 100 ps. In Fig. 5 the effect of the time resolution as a function of the laser beam diameter is shown. The time focus is set deliberately at a different position from the deflection plates. By moving the lens that is focusing the laser beam, the laser beam diameter can be increased and hence the width of the electron peak. The line in Fig. 5 is given by a trajectory calculation involving Gaussian beam focusing and the electron trajectories discussed above.
G.M. Lankhuijzen, L.D. Noordam/Optics Communications 129 (1996) 361-368 beam diameter (l~m)
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Fig. 6. Schematic representation of the electronic circuit for the generation of the fast voltage ramp. The 150 pF capacitor is charged up to +1.6 kV. At t = 0 the 200 fs laser pulse is applied to the photoswitch, lowering the resistance to < 10 II. As a result the deflection plate is charged up to + 1.6 kV within 500 ps. During the rising edge of the voltage ramp the electrons pass the deflection plates. The life time of the free carriers in the GaAs phot0switch is 1.5 ns: after a few nanoseconds the resistance of the photoswitch is high again ( > 100 MI~). In a few microseconds the condenser plates are discharged over the 1 MI~ resistor to the - 0 . 9 kV bias voltage.
3. Production of voltage ramp The heart of the streak camera is formed by the two deflection plates to which the fast voltage ramp is applied. The electronic circuit used for the generation of such a fast electrical pulse is shown in Fig. 6. A ceramic chip capacitor with low inductance and effective series resistance is charged up to 1.6 kV typically. A piece of semi-insulating GaAs (15 x 5 x 0.35 mm) doped with Fe (concentration > 1016 cm -3) connects
365
the capacitor with the upper deflection plate; the lower one is at ground potential. The resistance of the GaAs photoswitch is > 100 MIL The upper deflection plate is biased to - 0 . 9 kV by means of a 1 MO resistor. When the GaAs is illuminated with a short laser pulse, valence electrons are promoted to the conduction band lowering the resistance over 7 orders of magnitude (down to ,,~ 10 f~). A current starts to flow from the capacitor to the upper deflection plate, charging it up to +1.6 kV. The lifetime of the charge carriers in this particular piece of GaAs is ~ 1.5 ns [ 12], which is longer than the time required to charge the capacitance of the deflection plates. By varying the delay between the optical ionizing pulse and the optical switch pulse the most linear part of the electrical pulse is selected. Since the detector is centered at the z-axis, the bias voltage is used to let the voltage pulse go through zero volt at the linear part of the response. When the resistance of the GaAs switch is high again, the capacitor is discharged by the 1 Mf~ resistor to the - 0 . 9 kV in a few microseconds. The switch is operated in saturation, i.e., a laser flux > 4 0 / z J / c m 2 for a laser pulse with central wavelength z ~ '~20 nm [ 10], to minimize the effects of laser intensity fluctuations. Great care is taken to minimize the Amplified Spontaneous Emission (ASE) background of the laser pulses. The ASE background gives rise to a non-stable operation of the photoswitch. The creation of the valence electrons in the photoswitch is a single-photon process. Therefore, the ASE background, which has a width of 5 ns, already reduces the resistance of the photoswitch before the femtosecond pulse arrives. An extra 360 resistor is put in series with the GaAs photoswitch to minimize effects of fluctuations in the resistance of the GaAs due to fluctuations in laser intensity. The inductance in the circuit is determined by the geometry of the design. To minimize this inductance, strips are used ( 1.5 cm wide) to connect the electronic components. The overall size of the circuit is minimized to a few centimetres. The pulse characteristics are fully determined by this electronic scheme and are calculated easily. In Fig. 7 the measured deflection as a function of the delay between the optical ionizing and optical switch pulse is plotted. The electrons are created in an electric field of 4 kV/cm. The sweep voltage is set to a low value to reduce the deflection angle, so that the entire voltage pulse can be observed within the range
366
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By measuring the position of the electrons after they have passed the deflection region the temporal profile of the electron pulse is retrieved. The detector consists of a three stages channelplate electron multiplier, a phosphor screen which transforms the electrons into photons, and a CCD camera (256 x 256 pixels) to read out the phosphor screen. The channelplates have an overall amplification of ~ 107. The electron to photon conversion at the phosphor screen has a gain of ,~ 10. The phosphor screen releases ~, 108 photons (spotsize 200 /zm FWHM) per incoming electron. Using a f = 50 mm f/D = 1.2 lens the phosphor screen is imaged on the CCD pixels. The number of photons per incoming electron is well above the noise level of the CCD camera. The total image size of the detector is 23.4 x 23.4 ram. The overall resolution of the detector, mainly determined by the size of the light pulse from the phosphor screen is 200/xm (FWHM).
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Time (ps) Fig. 7. (a) Measured deflection (circles) of the electrons as a function of delay between the optical ionizing laser pulse and the optical switch pulse. The dash-dotted curve represents the calculated response of the circuit in Fig. 6 convolutedwith the transit time (560 ps) of the electronsthroughthe deflectionplates. (b) Deviation of the deflection from a linear response for a time window of 200 ps. of the detector. The energy of the electrons entering the deflection region is 2 keV which gives them a transit time through the deflection plates (1.5 cm long) of 560 ps. The measured deflection is therefore a convolution of the voltage pulse with a block of 560 ps wide. The dash-dotted curve in Fig. 7 shows the calculation of the response of the electrical circuit convoluted with the 560 ps block pulse. The inductance L and capacitance C are used as fit parameters. The electrical pulse as deduced from Fig. 7 has a rise time of 430 ps. In the lower part of Fig. 7 the deviation from a linear response is plotted. Over a region of 200 ps the linearity is better than 0.5%.
The images recorded by the CCD camera at 10 Hz are transferred to a PC via a frame grabber. Several data analyses are performed on a shot to shot basis before the data is stored. To obtain the position to time transformation the angle of the lines as depicted in Fig. 3 is determined in situ. Once the angle is known the data of the image is integrated along this line, resulting in a single array, the so-called ionization spectrum, as shown in the top part of Fig. 3. Due to intensity fluctuations of the laser pulses that trigger the photoswitch, the voltage ramp has a small time jitter (typically < 15 ps) with respect to the ionizing laser pulse. Therefore, the individual streak images are displaced with respect to each other on the detector. If these images were added without any correction the resolution will be as bad as this time jitter. Therefore a Centre Of Mass (COM) correction is used in order to shift the individual images on top of each other. To determine the centre of mass with sufficient accuracy, many electrons need to hit the detector due to the statistical nature of this ionization process.
G.M. Lankhuijzen. L.D. Noordarn/ Optics Communications 129 (1996) 361-368
6. Experimental results
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To test the specification of the atomic streak camera, short laser pulses are needed both for triggering the photoswitch as for the creation of sub-picosecond electron pulses. For the trigger of the GaAs photoswitch an amplified CPM laser (10 Hz) is used [13]. The CPM laser produces pulses of 100 fs at 80 MHz repetition rate at a central wavelength of 620 nm. These pulses are amplified using four single pass Bethunetype dye cells which are pumped by the second harmonic of a nanosecond Nd:YAG (where YAG denotes yttrium aluminum garnet) laser operating at 10 Hz. The photoswitch is illuminated with 200 fs pulses with an energy of > 100/zJ per pulse. This is well above the saturation fluence of the photoswitch minimizing the effects of laser intensity fluctuations on the voltage pulse characteristics. In order to minimize the ASE background a saturable absorber is installed after the third Bethune cell reducing the ASE to < 0.2% of the total pulse energy. Part of the amplified CPM light ( 113 /zJ) is used for continuum generation in a water cell. By means of an interference bandpass filter the desired frequency range (590-605 nm) is selected and further amplified. A shaper [ 14] is used to select the central wavelength and bandwidth, typically 0.1-1.5 nm, of the pulses. A number of experiments are performed to obtain the specifications of the atomic streak camera. In all experiments the atomic vapour consisted of rubidium atoms coming out of an oven. The binding energy of the 5s groundstate electron of rubidium is 33688.3 c m - l which corresponds to a two photon excitation at 593.6 nm. When a rubidium atom is placed in an electric field the ionization threshold is given by Ec = -6.12v/if,
(1)
where F is the electric field strength ( V / c m ) and Ec is the ionization threshold (cm -1 ). If the excitation energy of the electron is far above this threshold the ionization will be prompt, i.e., the electron will leave the atom immediately. The electron pulse will therefore be as short as the optical pulse (200 fs). In Fig. 8a a single shot image is shown. The rubidium atoms are ionized in a two-photon process at 597.0 nm (at 0.25Ec) in a static electric field of 4.0 kV/cm. The electrons are post accelerated to 5.0 keV before entering the deflection plates. The upper de-
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flection plate is biased at - 1 . 2 kV and switched to +1.3 kV within 500 ps. The sweep rate of the electrons over the detector is 3.8 • 108 m/s. The gain of the detector is deliberately set to a low value (gain channel plates ,-~ 1 0 6 ) . In this way single electrons hitting the detector are not detected and the measured peak consists of > 100 electrons. In Fig. 8b a 100 shot average is plotted. The fluctuations in the laser intensity which give rise to a small change in the field ramp account for the broadening of the peak to 2.5 ps (FWHM). Under normal experimental conditions the shot to shot fluctuation gives rise to a larger broadening ( > 10 ps). Therefore a Centre Of Mass (COM) correction is used to shift the individual shots. A resolution of 2.5 ps (FWHM) is then easily obtained. In Fig. 9 the decay of an autoionizing electronic wave packet is shown. The electric field is set at 2.0 kV/cm. The excitation is just above the classical field ionization limit (0.87Ec) using the frequency dou-
G.M. Lankhuijzen, L.D. Noordam/ Optics Communications 129 (1996) 361-368
368
Acknowledgements 1.0
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bled light at 298.78 nm. The laser polarization is chosen perpendicular to the electric field of 2.0 kV/cm, launching the electron perpendicular to the z-axis. Therefore the electron does not escape immediately from the atomic potential giving rise to the observed structure in the ionization spectrum. From Fig. 9 we learn that the ionization occurs in several bursts, revealing the wave packet motion in the potential. The time spacing between the bursts is understood in terms of the motion of an angular wavepacket. The physics involved in the ionization dynamics will be discussed in detail elsewhere [ 15].
7. Conclusion A new device, the atomic streak camera, that can be used to study the electron flux of autoionizing atoms in a static electric field is presented. A sub-picosecond resolution is obtained making it possible to resolve the ionization dynamics of electronic wave packets in rubidium, created above the classical field ionization limit.
It is a pleasure to thank A.N. Buyserd for technical assistance, E Karouta and T. Uitterdijk for supplying photoconductive switches, W.H. Knox and A. Vijftigschild for suggestions on the switching device, E van Deenen and T. Uitterdijk for carefully reading the manuscript. The work in this paper is part of the research program of the "Stichting voor Fundamenteel Onderzoek van de Materie" (Foundation for Fundamental Research on Matter) and was made possible by financial support from the "Nederlandse Organisatie voor Wetenschappelijk Onderzoek" (Netherlands Organization for the Advancement of Research).
References [1] T.E Gallagher, Rydberg atoms, Ist Ed. (Cambridge University Press, Cambridge, 1994). [2] A. Giusti-Suzor and E Zoller, Phys. Rev. A 36 (1987) 5178. [3] A. ten Wolde, L.D. Noordam, A. Lagendijk and H.B. van Linden van den Heuvell, Phys. Rev. Lett. 61 (1988) 2099. [4] J.A. Yeazell and C.R. Stroud Jr., Phys. Rev. A 43 (1991) 5153. [5] B. Broers, J.E Christian, J.H. Hoogenruad, W.J. van der Zande, H.B. van Linden van den Heuvell and L.D. Noordam, Phys. Rev. Lett. 71 (1993) 344. [6] L.D. Noordam, D.I. Duncan and T.E Gallagher, Phys. Rev. A 45 (1992) 4734. [7] B. Broers, J.E Christian and H.B. van Linden van den Heuvell, Phys. Rev. A 49 (1994) 2498. [8] J.E Christian, B. Broers, J.H. Hoogenraad, W.J. van der Zande and L.D. Noordam, Optics Comm. 103 (1993) 79. [9] Y. Tsuchiya, IEEE J. Quantum Electron. QE-20 (1984) 1516. [10] W. Knox and G. Mourou, Optics Comm. 37 (1981) 203. [11] W.C. Wiley and I.H. McLaren, Rev. Sci. Instr. 26 (1955) 1150. [ 12l E Karouta, private communication. [ 13] L.D. Noordam, W. Joosen, B. Broers, A. ten Wolde, A. Lagendijk, H.B. van Linden van den Heuvell and H.G. Muller, Optics Comm. 85 (1991) 331. [ 14] A.M. Weiner, J.E Heritage and E.M. Kirschner, J. Opt. Soc. Am. B 5 (1988) 1563. [15] G.M. Lankhuijzen and L.D. Noordam, Phys. Rev. Lett. 76 (1996) 1784.