Using our techniques, we gain valuable information about the fundamental physics and chemistry of complex many-body systems, and about the properties of technologically important materials in states relevant to device-operating conditions.  Our particular expertise is in the study of ultrafast dynamics, on timescales from tens of femtoseconds (fs) to picoseconds (ps).  Our users study new materials that have promise for novel applications in computation [1,2,3,4], optoelectronics (e.g., [5,6]), photocatalysis [7], and solar energy harvesting [7,8], and they pursue curiosity-driven research that expands our understanding of quantum mechanics and of deep questions underlying physics and chemistry [9,10,11,12].

Please read below for an overview of our techniques.  If you are interested in submitting an application for facility access, you can find detailed technical information about our capabilities here, along with guidance for planning your experiment and preparing your application.

Photoemission spectroscopy is a mature and widely used technique for both fundamental and applied research within the fields of physics, chemistry, and material science.  In photoemission from the solid state, photons interact with a solid sample, and electrons are photoemitted into free space.  The electrons travel through free space to a photoelectron analyzer, which detects both emission angle and kinetic energy.  Because energy is conserved in the photoemission process, we can use the measured kinetic energy of the electrons to calculate the binding energy of the initial electronic states in the sample [13].

Electronic states in crystalline solids can be loosely grouped into three categories:  core level states, valence band states, and conduction band states.  From measurements of all three of these, fundamental physical and chemical properties of materials can be extracted.  Core level states are tightly bound to atomic cores, and electron wave vectors (k_x, k_y, k_z) are not defined for these states.  By contrast, electronic states in the valence and conduction bands arise from the bonding properties of solids, and their binding energy depends on their wave vector (i.e., they disperse).  In the photoemission process, in-plane wave vector is conserved, and therefore the initial-state wave vector (kx, ky) can be calculated from the emission angle that is measured by the analyzer, so that the relationship inside the crystal between binding energy and electronic wave vector (i.e., the dispersion relation) for conduction and valence band states can be experimentally determined.  This type of measurement constitutes angle-resolved photoemission spectroscopy (ARPES) [13,14]. 

Photoemission spectroscopy can only detect occupied electronic states (since unoccupied states have no electrons that can be photoemitted).  In systems at equilibrium, all electronic states up to the thermally-broadened Fermi level are filled, and no states are filled at higher energies.  Thus, core level and valence band states constitute common topics of study via photoemission.  However, one can gain tremendous insight into fundamental many-body interactions and into states of matter relevant to device applications by studying the evolution and decay of quasiparticle excitations to the conduction band [15], and into complex correlated ground states (e.g., charge density waves, superconducting states, etc.)  via observations of how they melt and recover after a perturbation.  These investigation are the domain of pump-probe photoemission.  We use a “pump” laser pulse to generate excited electronic states, and then measure the excited-state system with a second pulse (the probe).  The timing of the probe is precisely calibrated relative to the pump so that it lags the pump pulse by a known period of time (the “delay,” Δt).  The probe thus generates a snapshot of the system after it has evolved by an amount of time Δt from its excitation.  By making multiple measurements across a range of delay times, one observes the process of the excitation’s decay from the initial excitation to the reestablishment of equilibrium after a long time has elapsed [16,17,18,19].

The realization of stable pump and probe beams with sufficiently short pulses and useful energy ranges is a technologically demanding challenge.  However, laser technology has developed rapidly in recent years.  As laser physics has opened new research frontiers, pump-probe spectroscopy has opened the door to whole new fields of research that move beyond the study of equilibrium states to the study of complex dynamical processes.

Click here ​to read about our materials science capabilities and to get advice on preparing beamtime proposals.

[1] Cacho, et al., “Momentum-Resolved Spin Dynamics of Bulk and Surface Excited States in the Topological Insulator Bi₂Se₃,” Phys. Rev. Lett., 114 (2015) 097401. https://doi.org/10.1103/PhysRevLett.114.097401​

[2] Bertoni, et al., “Generation and Evolution of Spin-, Valley-, and Layer-Polarized Excited Carriers in Inversion-Symmetric WSe₂,” Phys. Rev. Lett. 117 (2016) 277201. https://doi.org/10.1103/PhysRevLett.117.277201

[3] Ulstrup, et al., “Spin and valley control of free carriers in single-layer WS₂,” Phys. Rev. B 95 (2017) 041405(R). https://doi.org/10.1103/PhysRevB.95.041405​

[4] Liu, et al., “Symmetry-breaking and spin-blockage effects on carrier dynamics in single-layer tungsten diselenide,” Phys. Rev. B. 100 (2019) 214309.​ https://doi.org/10.1103/PhysRevB.100.214309

[5] Gierz, et al., “Snapshots of non-equilibrium Dirac carrier distributions in graphene,” Nat. Mat. 12 (2013) 1119. https://doi.org/10.1038/nmat3757​

[6] Gierz, et al., “Population inversion in monolayer and bilayer graphene,” J. Phys.:  Cond. Matt. 27 (2015) 164204. https://doi.org/10.1088/0953-8984/27/16/164204

[7] Zhang, et al., “State-Selective Dynamics of TiO₂ Charge-Carrier Trapping and Recombination,” J. Phys. Chem. Lett. 10 (2019) 5265. https://doi.org/10.1021/acs.jpclett.9b02153​

[8] Johannsen, et al., “Tunable Carrier Multiplication and Cooling in Graphene,” Nano Lett. 15 (2015) 326. https://doi.org/10.1021/nl503614v​

[9] Crepaldi, et al., “Enhanced ultrafast relaxation rate in the Weyl semimetal phase of MoTe₂ measured by time- and angle-resolved photoelectron spectroscopy,” Phys. Rev. B 96 (2017) 241408(R). https://doi.org/10.1103/PhysRevB.96.241408​​

[10] Petersen, et al., “Clocking the melting transition of charge and lattice order in 1T-TaS₂ with ultrafast extreme-ultraviolet angle-resolved photoemission spectroscopy,” Phys. Rev. Lett. 107 (2011) 177402. https://doi.org/10.1103/PhysRevLett.107.177402

[11] Monney, et al., “Revealing the role of electrons and phonons in the ultrafast recovery of charge density wave correlations in 1T-TiSe₂,” Phys. Rev. B 94 (2016) 165165. https://doi.org/10.1103/PhysRevB.94.165165​

[12] Cilento, et al., “Dynamics of correlation-frozen antinodal quasiparticles in superconducting cuprates,” Sci. Adv. 4 (2018) eear1998. https://doi.org/10.1126/sciadv.aar​1998

[13] Hüfner, Photoelectron Spectroscopy:  Principles and Applications, Springer (2003).

[14]  Damascelli, “Probing the Electronic Structure of Complex Systems by ARPES,” Phys. Script. T109 (2004) 61.  https://doi.org/10.1238/Physica.Topical.109a00061​

[15] Bovensiepen, et al., eds., Dynamics at Solid State Surfaces and Interfaces, Vols. 1 and 2, Wiley-VCH (2010, 2012). https://doi.org/10.1002/9783527633418

[16] Haight, “Electron dynamics at surfaces,” Surf. Sci. Rep. 21 (1995) 275. https://doi.org/10.1016/0167-5729(95)00002-X

[17] Bovensiepen and Kirchmann, “Elementary relaxation processes investigated by femtosecond photoelectron spectroscopy of two-dimensional materials,” Laser Photon. Rev. 6 (2012) 589. https://doi.org/10.1002/lpor.201000035

[18] Lisowski, et al., “Ultra-fast dynamics of electron thermalization, cooling, and transport effects in Ru(001),” Appl. Phys. A 78 (2004) 165. https://doi.org/10.1007/s00339-003-2301-7

[19] Smallwood, et al., “Ultrafast angle-resolved photoemission spectroscopy of quantum materials,” Eur. Phys. Lett. 115 (2016) 27001.​ https://doi.org/10.1209/0295-5075/115/27001​