title: Biexciton formation and exciton coherent coupling in layered GaSe.
abstract: Nonlinear two-dimensional Fourier transform (2DFT) and linear absorption spectroscopy are used to study the electronic structure and optical properties of excitons in the layered semiconductor GaSe. At the 1s exciton resonance, two peaks are identified in the absorption spectra, which are assigned to splitting of the exciton ground state into the triplet and singlet states. 2DFT spectra acquired for co-linear polarization of the excitation pulses feature an additional peak originating from coherent energy transfer between the singlet and triplet. At cross-linear polarization of the excitation pulses, the 2DFT spectra expose a new peak likely originating from bound biexcitons. The polarization dependent 2DFT spectra are well reproduced by simulations using the optical Bloch equations for a four level system, where many-body effects are included phenomenologically. Although biexciton effects are thought to be strong in this material, only moderate contributions from bound biexciton creation can be observed. The biexciton binding energy of ∼2 meV was estimated from the separation of the peaks in the 2DFT spectra. Temperature dependent absorption and 2DFT measurements, combined with "ab initio" theoretical calculations of the phonon spectra, indicate strong interaction with the A1 (') phonon mode. Excitation density dependent 2DFT measurements reveal excitation induced dephasing and provide a lower limit for the homogeneous linewidth of the excitons in the present GaSe crystal.
authors.affiliation: Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620, USA.
authors.forename: P
authors.initials: P
authors.lastname: Dey
authors.affiliation: Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620, USA.
authors.forename: J
authors.initials: J
authors.lastname: Paul
authors.affiliation: National Institute of Standards and Technology, 325 Broadway, Boulder, Colarado 80305, USA.
authors.forename: G
authors.initials: G
authors.lastname: Moody
authors.affiliation: Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620, USA.
authors.forename: C E
authors.initials: CE
authors.lastname: Stevens
authors.affiliation: Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620, USA.
authors.forename: N
authors.initials: N
authors.lastname: Glikin
authors.affiliation: Chernivtsi Department, Frantsevich Institute of Material Sciences Problems, The National Academy of Sciences of Ukraine, 5, Iryna Vilde St., 58001 Chernivtsi, Ukraine.
authors.forename: Z D
authors.initials: ZD
authors.lastname: Kovalyuk
authors.affiliation: Chernivtsi Department, Frantsevich Institute of Material Sciences Problems, The National Academy of Sciences of Ukraine, 5, Iryna Vilde St., 58001 Chernivtsi, Ukraine.
authors.forename: Z R
authors.initials: ZR
authors.lastname: Kudrynskyi
authors.affiliation: Physics Department, West Virginia University, Morgantown, West Virginia 26506-6315, USA.
authors.forename: A H
authors.initials: AH
authors.lastname: Romero
authors.affiliation: Materials Science Institute, University of Valencia, P.O. Box 2205, 46071 Valencia, Spain.
authors.forename: A
authors.initials: A
authors.lastname: Cantarero
authors.affiliation: Department of Physics, University of Alabama at Birmingham, Birmingham, Alabama 35294, USA.
authors.forename: D J
authors.initials: DJ
authors.lastname: Hilton
authors.affiliation: Department of Physics, University of South Florida, 4202 East Fowler Ave., Tampa, Florida 33620, USA.
authors.forename: D
authors.initials: D
authors.lastname: Karaiskaj