首页|Resonant four-photon photoemission from SnSe2(001)
Resonant four-photon photoemission from SnSe2(001)
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High-order nonlinear multiphoton absorption is usually inefficient,but can be enhanced by designing resonant excitations between occupied and unoccupied energy levels.We conducted angle-resolved multi-photon photoemission(mPPE)studies on the SnSe2(001)surfaces excited by ultra-short laser pulses.By tuning photon energy and light polarization,we demonstrate the presence of a resonant four-photon photoemission(4PPE)process involving the occupied valence band(VB),the unoccupied second conduction band(CB2)and the unoccupied image-potential state(IPs)of SnSe2.In this 4PPE process,VB electrons of SnSe2 are resonantly excited into CB2 by adsorbing two photons,followed by the adsorption of another photon to populate the n=1 IPs before being emitted out to the vacuum by adsorbing one more photon.This results in a double-resonant 4PPE process,which exhibits approximately a 40 times enhancement in photoe-mission yields compared to cases where one of the resonant pathways,CB2 →IPs,is inhibited by involving a virtual state instead of the IPs in the 4PPE.The double-resonant 4PPE process efficiently excite the bulk VB electrons outside the vacuum,like taking advantage of resonant"ladders"through two real empty electronic states of SnSe2.Our results highlight the important applications of mPPE in probing the band-structure,partic-ularly the unoccupied states,of recently emerging main group dichalco-genide semiconductors.Furthermore,the discovered resonant mPPE process contributes to the exploration of their promising optoelectronic applications.
School of Physics and Technology and Key Laboratory of Artificial Micro-and Nano-Structures of Ministry of Education,Wuhan University,Wuhan 430072,China
Department of Physics and Astronomy,California State University Northridge,Northridge,CA 91330-8268,USA
Institute for Advanced Study,Wuhan University,Wuhan 430072,China
Strategic Priority Research Program of the Chinese Academy of SciencesNational Key R&D Program of ChinaNational Key R&D Program of ChinaNational Natural Science Foundation of China
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