A tunable low-energy photon source for high-resolution angle-resolved photoemission spectroscopy
John W. Harter, Philip D. C. King, Eric J. Monkman, Daniel E. Shai, Yuefeng Nie, Masaki Uchida, Bulat Burganov, Shouvik Chatterjee, Kyle M. Shen
AA tunable low-energy photon source for high-resolution angle-resolved photoemissionspectroscopy
John W. Harter, Philip D. C. King,
1, 2
Eric J. Monkman, Daniel E. Shai, YuefengNie, Masaki Uchida, Bulat Burganov, Shouvik Chatterjee, and Kyle M. Shen
1, 2, a)1)
Laboratory of Atomic and Solid State Physics, Department of Physics,Cornell University, Ithaca, New York 14853, USA Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853,USA (Dated: 12 April 2017)
We describe a tunable low-energy photon source consisting of a laser-driven xenonplasma lamp coupled to a Czerny-Turner monochromator. The combined tunability,brightness, and narrow spectral bandwidth make this light source useful in laboratory-based high-resolution photoemission spectroscopy experiments. The source suppliesphotons with energies up to ∼ > ph/swithin a 10 meV spectral bandwidth, which is comparable to helium plasma lampsand many synchrotron beamlines. We first describe the lamp and monochromatorsystem and then characterize its output, with attention to those parameters whichare of interest for photoemission experiments. Finally, we present angle-resolvedphotoemission spectroscopy data using the light source and compare its performanceto a conventional helium plasma lamp. a) Author to whom correspondence should be addressed: [email protected] a r X i v : . [ phy s i c s . i n s - d e t ] A p r . INTRODUCTION Angle-resolved photoemission spectroscopy (ARPES) plays a valuable role in the fieldof condensed matter physics by offering a direct momentum-space probe of the underlyingelectronic structure of solids. Motivated by the demand for high-quality data, both the in-strumental energy and momentum resolution and the detection efficiency of ARPES systemshave improved substantially over the last decades . ARPES is based on the photoelectriceffect, in which an emitted electron’s kinetic energy, E kin , is given by E kin = hν − φ − E B , (1)where hν is the incident photon energy, φ is the work function of the system, and E B isthe binding energy of the electron within the solid . A key component of photoemission,therefore, is the photon source, which is often a synchrotron because the light must besimultaneously bright and possess a narrow spectral bandwidth in order to maintain a highenergy resolution. A common alternative to synchrotrons is the laboratory-based noble gasplasma lamp. A major drawback of plasma lamps is the fact that the photon energy is fixedat a discrete set of atomic lines. For a helium lamp, this encompasses the He-I α line at 21.2eV and the He-II α line at 40.8 eV.The spectral lines from plasma lamps, as well as typical photon energies used at syn-chrotrons, sit close to the minimum of the so-called “universal curve” describing the inelas-tic mean free path of photoexcited electrons within a metal as a function of their kineticenergy . Some benefits of using low-energy light sources therefore include an enhancedsensitivity to more bulk-like properties of the system under study, an increased toleranceto unwanted adsorbates on the sample surface, and a higher momentum resolution. As aresult, the development and use of low-energy sources have recently become common inthe ARPES community. These are typically laser sources: one such source supplies 7 eVphotons by using the nonlinear optical crystal KBe BO F to generate the second harmonicof a frequency-tripled Nd:YVO laser , and a similar 6 eV light source based on the crystal β -BaB O generates the fourth harmonic of a titanium-sapphire laser . Another low-energysource in use employs a low-pressure xenon discharge lamp, which has a number of discreteatomic lines in the energy range 8 - 11 eV .One major disadvantage of the laser sources described above is a fixed photon energy,which is a particularly important issue at low energies because of the sensitivity to photon2avelength caused by final-state matrix element effects. The ability to tune the photonenergy, however, can mitigate this issue. Therefore, it is important to develop a tunablelow-energy photon source that can allow for the selection of final states. In this paperwe describe a novel laboratory-based low-energy photon source consisting of a laser-drivenxenon plasma lamp in conjunction with a Czerny-Turner monochromator. The total cost ofthe device is less than $45,000, which is significantly cheaper than noble gas plasma lampsand laser-based systems. Under typical operation, the device delivers > ph/s at a 10meV spectral bandwidth, and its continuous output avoids space-charge complications thatare present in pulsed sources. The brightness, energy tunability, and adjustable spectralbandwidth of the light source make it ideally suited for laboratory-based high-resolutionARPES experiments. II. THE INSTRUMENT
There are three main components of the photon source: a bright laser-driven xenonplasma lamp, a high-resolution Czerny-Turner monochromator, and a pair of lenses thatfocus the light onto the sample. As shown in Fig. 1, the output of the plasma lamp isdirected onto the entrance slit of the monochromator. At the exit slit, the light is guidedthrough an ultra-high vacuum (UHV) viewport into the ARPES vacuum chamber and ontothe sample. At the operating energy range of the lamp for photoemission (up to ∼ -coated) aluminum; aluminum has a high reflectivity at ultraviolet wavelengths and a MgF overlayer prevents oxidation of the metallic surface. Both CaF and MgF are acceptablematerials for lenses and windows, provided they are of “vacuum grade” quality, because theyhave a sufficiently large band gap to allow transmission of photons.The entire optical beam path of the lamp must be placed within a sealed oxygen-freeatmosphere such as dry nitrogen. The ultraviolet light generated by the xenon plasmalamp is capable of generating ozone. This process absorbs a significant fraction of photons,diminishing the brightness of the source. In addition, ozone is harmful to the componentsof the photon source and will degrade the optical elements within it.3 ampleViewportLens LampMonochromator
Grating Entrance slitExit slit Xe bulb LaserOAPAssemblyLens Mesh Within UHV
FIG. 1. Schematic diagram of the xenon plasma lamp and Czerny-Turner monochromator, asdescribed in the text. Within the lamp housing, an internal laser excites a plasma containedinside the xenon bulb. The light emitted from the plasma is focused onto the entrance slit of themonochromator with a pair of off-axis parabolic mirrors. At the exit slit of the monochromator, apair of lenses focus the light through an ultra-high vacuum viewport and onto the sample.
A. The xenon plasma lamp
The plasma lamp (EQ-1500, Energetiq Technology, Inc.) is a patented commercial instru-ment featuring an internal continuous wave 980 nm diode laser running at 60 W and drivinga plasma within the xenon bulb. The broadband output of the lamp has a divergence of 60 ◦ and extends from ultraviolet to infrared wavelengths (170 nm to 2100 nm). The electrode-less design of the lamp offers greater stability, a longer lifetime, and higher brightness thanxenon or deuterium arc lamps, with a spectral radiance exceeding 10 mW/(mm · nm · sr) overthe entire operational wavelength range. The diverging output of the lamp is coupled toan off-axis parabolic (OAP) mirror assembly which focuses the light onto the monochro-mator entrance slit. The OAP assembly reduces the divergence of the beam to 20 ◦ , with acorresponding magnification of the plasma image by 3 × .4 . The Czerny-Turner monochromator The monochromator (SP-2355, Princeton Instruments) has a Czerny-Turner configu-ration with a 300 mm focal length and a UV-optimized holographic plane grating (3600grooves/mm). The acceptance angle of the instrument is 15 ◦ , receiving roughly half of thetotal output of the xenon plasma lamp. At the exit slit of the monochromator, the measuredlinear dispersion of photons is better than 0.33 nm/mm over the entire operating range ofthe lamp. Based on these parameters, one can compute the required monochromator slitwidth for a desired spectral bandwidth using the formula s = (cid:32) hcσ (cid:33) ∆ E ≈ (cid:32) . · eV meV (cid:33) ∆ E , (2)where s is the slit width (the entrance and exit slits should be matched), σ is the lineardispersion of the monochromator, ∆ is the desired spectral bandwidth of the lamp, and E = hc/λ is the energy of the photons. For example, at an operating energy of 6 eV, aspectral bandwidth of 5 meV can be achieved with monochromator slits set to 0.5 mm. C. The focusing lenses
At the exit slit of the monochromator, a pair of lenses focus the emitted light through aUHV viewport and onto the sample. The focal lengths of these lenses depend on their diam-eter and on the minimum lens-to-sample distance, which will vary between photoemissionsystems. As an example, we consider standard lens diameters of 25 mm and a lens-to-sampledistance of 130 mm. In order to capture all of the light, the external lens must be within 95mm of the monochromator exit slit. Choosing focal lengths of 65 mm (external lens) and130 mm (internal lens), we form an image of the exit slit at the sample position with a 2 × magnification. In the diagram of Fig. 1, the focusing lens is shown within the UHV chamber.This is not strictly necessary, but is done in order to minimize the spot size on the sample,as the magnification of the lens system is proportional to the lens-to-sample distance.An important complication to consider is that of chromatic aberration. The index ofrefraction of CaF has an average dispersion of 9 × − nm − in the operating energy rangeof the photon source . The focal lengths of the lenses will therefore vary by as much as ±
5% as the photon energy is adjusted. In order to correct for this effect, one must adjustthe position of the external lens in order to keep the focus at the sample position. Another5omplication involves the lens close to the sample, which may charge up from photoemittedelectrons. Electrostatic shielding in the form of a grounded mesh between the lens and thesample can prevent the charge field from interfering with the photoemission measurements .Alternatively, the focusing lens may be moved outside of the vacuum chamber, at the expenseof an increased spot size. III. CHARACTERIZATIONA. Intensity
The most important characteristic of a photon source for photoemission is photon flux (fora given spectral bandwidth). The instrument described here provides a very bright source,rivaling that of conventional laboratory-based plasma lamps. Figure 2 shows the absoluteintensity of the source as a function of photon energy for a series of monochromator slitwidths, measured at the output of the monochromator with a calibrated silicon photodiode(AXUV-20, International Radiation Detectors, Inc.). Also shown is the expected intensityas a function of spectral bandwidth (assuming the slits are adjusted at each energy in orderto keep the bandwidth constant). Superimposed on Fig. 2(c) is data for a helium plasmalamp (VUV5000, VG Scienta, Inc.). Although the spectral bandwidth of the helium lampis narrow ( ∼ α and He-II α ). B. Spectral bandwidth
One feature of the photon source described here that is absent in conventional laboratory-based plasma lamps is the ability to adjust the spectral bandwidth of the source based onthe requirements of the user: a significant intensity gain may be achieved by a moderateincrease of the bandwidth of the light. This is especially useful, for example, when mea-suring momentum-space ARPES maps, as a large number of measurements must be taken,but each of which can be acquired with a somewhat lower energy resolution. Figure 3shows the measured spectral bandwidth and intensity of the photon source as a function ofmonochromator slit width. The bandwidth is determined by measuring the angle-integratedFermi step of freshly evaporated polycrystalline gold and deconvolving the broadening dueto instrumental resolution (4.9 meV) and finite temperature (12 K). The measured spectral6 Spectral Bandwidth (meV)Photon Energy (eV)Photon Energy (eV) I n t en s i t y ( ph / s ) I n t en s i t y ( ph / s ) I n t en s i t y ( ph / s ) He-IIαHe-Iα
Photon EnergySpectral BandwidthMonochromator Slit Width (a)(b)(c)
FIG. 2. Absolute intensity of the photon source. (a) Experimentally measured intensity as afunction of photon energy for a series of fixed monochromator slit widths. (b) Calculated intensityas a function of photon energy for a series of desired spectral bandwidths. (c) Calculated intensityas a function of spectral bandwidth for a series of photon energies and for a conventional heliumplasma lamp. bandwidth agrees well with Equation 2, and the photoemission intensity follows the expectedquadratic dependence on slit width due to the larger entrance slit and the wider bandwidth.7 lit Width (mm)Kinetic Energy (eV) S pe c t r a l B and w i d t h ( m e V ) I n t en s i t y ( a r b . un i t s ) I n t en s i t y ( a r b . un i t s ) E F T = 12 KE = 6.2 eVSlits = 0.10 mm (a)(b) (Slit width) FIG. 3. Spectral bandwidth and intensity of the photon source as a function of slit width. (a) Anangle-integrated energy distribution curve of polycrystalline gold taken at E = 6 . T = 12K and with monochromator slits set to 0.10 mm. Also shown is a Fermi-Dirac distribution fit tothe data, from which the spectral bandwidth can be extracted after deconvolving the broadeningdue to instrumental resolution (4.9 meV). (b) Photoemission intensity and spectral bandwidthextracted using the procedure outlined in panel (a) as a function of monochromator slit width for E = 6 . C. Spot size
Another important characteristic of a light source is a narrow spot size. This is requiredbecause samples studied by ARPES are usually small ( ∼ ). If the spot size of thelamp is too large, most of the photons will contribute to a background signal, possiblyoverwhelming the signal from the sample itself. In addition, smaller spot sizes allow forhigher momentum resolution in ARPES experiments. Figure 4 shows the size of the xenonplasma image formed at the entrance slit of the monochromator by the OAP assembly.A full width at half maximum (FWHM) of 0.5 mm (horizontal direction) and 0.9 mm8vertical direction) are measured by the photodiode. Because the OAP assembly has a 3 × magnification, we infer the xenon plasma to be roughly 0.17 mm by 0.3 mm within thebulb. For ARPES experiments, the slits of the monochromator usually stay within therange 0.1 – 0.5 mm, which is on the order of the xenon lamp spot size at the entrance slit.As discussed above, a pair of lenses form an image of the exit slit at the sample positionwith a magnification of 2 × . The resulting spot size on the sample will thus be twice the slitwidth (0.2 – 1.0 mm) in the horizontal direction and 1.8 mm in the vertical direction. Afixed vertical aperture, however, may be placed at the exit slit in order to reduce the verticaldimension. I n t en s i t y ( a r b . un i t s ) Horizontal ScanVertical Scan FWHM = 0.5 mmFWHM = 0.9 mm
Displacement (mm) -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
FIG. 4. Size of the xenon plasma image formed at the monochromator entrance slit by the OAPassembly (3 × magnification of the actual plasma). Solid lines are fits to a Gaussian function, fromwhich a horizontal spot size of 0.5 mm (FWHM) and a vertical spot size of 0.9 mm (FWHM) areextracted. IV. PHOTOEMISSION
To demonstrate the usefulness of the xenon photon source, we present ARPES data ofa two-dimensional electron gas on the surface of CdO(001), which manifests itself as a pairof small concentric electron pockets centered at k x = k y = 0 in momentum space . TheCdO sample was grown by metalorganic vapor phase epitaxy on a sapphire substrate andmeasures ∼ × . In order to produce a clean surface for photoemission, the samplewas annealed in vacuum at 600 ◦ C for one hour before measurement. Figure 5 comparesenergy–momentum cuts and Fermi surface maps of the surface electronic structure of CdOusing a conventional helium plasma lamp (He-I α ) and using the xenon light source described9n this paper. The Fermi surface maps were generated by integrating spectral intensitywithin ±
30 meV of the Fermi level; an energy resolution much smaller than this energyscale is therefore unnecessary, and the ability to increase the photon flux by widening thespectral bandwidth is desirable. This feature enabled the Fermi surface map to be acquiredover a span of only 60 minutes with the xenon source, while 160 minutes was required toobtain a similar signal-to-noise ratio using the helium lamp. Both the helium and xenonplasmas are nominally unpolarized. In each case, however, reflection off the monochromatorgrating and optical elements partially polarizes the photons. This can account for the slightdifference between the symmetry of the Fermi surface map intensity of Fig. 5(c) and Fig.5(d). Photoemission matrix elements, which depend sensitively on photon energy, also playa role in the polarization dependence of the measured intensity.
V. CONCLUDING REMARKS
Future improvements to the photon source discussed in this paper include the possibilityof controlling the polarization of the emitted light A MgF double Rochon prism or atriple-reflector Brewster polarizer placed after the lens at the monochromator exit slit canlinearly polarize the light before it enters the vacuum chamber. By rotating the prism orreflectors, one may control the direction of polarization of light incident upon the sample. Aquarter waveplate operating in the ultraviolet wavelength range (for example, birefringentCaF ) can transform the linear polarization into circular polarization.In conclusion, we have presented a novel photon source that is comprised of a laser-drivenxenon plasma lamp coupled to a Czerny-Turner monochromator. We believe that this devicewill be a valuable addition to laboratory-based light sources for angle-resolved photoemissionspectroscopy. ACKNOWLEDGMENTS
We acknowledge helpful discussions with H. Padmore and F. Baumberger. Photoemissionwas performed on a CdO sample provided by C. McConville and grown by V. Mu˜noz-Sanjos´e, and we thank D. G. Schlom for allowing us to utilize his vacuum chamber forsample preparation. This work was supported by the National Science Foundation through10 omentum, k (Å ) -1 B i nd i ng E ne r g y ( e V ) Xenon LampHelium Lamp (a) (b)(c) (d) x M o m en t u m , k ( Å ) - y FIG. 5. Comparative ARPES data showing the surface electronic structure of CdO using a conven-tional helium lamp (He-I α , 21.2 eV) and the xenon photon source ( E = 6 . T ≈
100 K. The inner electron pocket is not visible in the helium map due to matrixelement effects, but can be detected with the xenon light source. (a),(b) Energy–momentum cutsthrough the center of the electron pocket ( k y = 0). For each photon source, data was accumulatedfor 60 minutes. (c),(d) Fermi surface maps generated by integrating spectral weight within ± α map in panel (c) was acquired over a span of 160 minuteswhereas the map in panel (d) taken with the xenon source required only 60 minutes. DMR-0847385 and the MRSEC program under DMR-1120296 (Cornell Center for MaterialsResearch). E.J.M. acknowledges support from an NSERC PGS.
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