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Slowdown of light due to exciton-polariton propagation in ZnO
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.ORCID iD: 0000-0002-6405-9509
Linköping University, Department of Physics, Chemistry and Biology, Functional Electronic Materials. Linköping University, The Institute of Technology.ORCID iD: 0000-0001-7155-7103
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 24, 245212- p.Article in journal (Refereed) Published
Abstract [en]

By employing time-of-flight spectroscopy, the group velocity of light propagating through bulk ZnO is demonstrated to dramatically decrease down to 2044 km/s when photon energy approaches the absorption edge of the material. The magnitude of this decrease is found to depend on light polarization. It is concluded that even though the slowdown is observed in the vicinity of donor bound exciton (BX) resonances, the effect is chiefly governed by dispersion of free exciton (FX) polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion is accurately determined.

Place, publisher, year, edition, pages
American Physical Society , 2011. Vol. 83, no 24, 245212- p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-69847DOI: 10.1103/PhysRevB.83.245212ISI: 000292254000003OAI: oai:DiVA.org:liu-69847DiVA: diva2:433444
Note

Original Publication: Shula Chen, Weimin Chen and Irina Boyanova, Slowdown of light due to exciton-polariton propagation in ZnO, 2011, Physical Review B. Condensed Matter and Materials Physics, (83), 24, 245212. http://dx.doi.org/10.1103/PhysRevB.83.245212 Copyright: American Physical Society http://www.aps.org/

Available from: 2011-08-10 Created: 2011-08-08 Last updated: 2017-12-08
In thesis
1. Excitonic effects in ZnO
Open this publication in new window or tab >>Excitonic effects in ZnO
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Zinc Oxide (ZnO) is an extensively researched II-VI wide bandgap semiconductor material. As a promising material for future optoelectronic and spintronic applications, it continues to attract enormous amount of interest. Though over the past decades extensive experimental and theoretical work has been done to deepen the understanding of its fundamental material properties, there are still controversial and unexplored areas. The research work summarized in this thesis was aimed at clarifying and exploring some of these areas, as will be introduced below.

One of attractive properties of ZnO is a very large binding energy of free excitons (FX), which makes excitonic effects of particular importance in this material. The excitons couple with other elementary excitations inside the material such as longitudinal optical (LO) phonons or photons. The former leads to the intense LO phonon-assisted radiative transitions, while the latter causes formation of the exciton-polariton.

The exciton-phonon coupling was suggested to be enhanced in ZnO-based nano- and microstructures. This conclusion was based on the prevalence in these structures at room temperature of LO phonon-assisted FX transitions, which is in contrast with bulk ZnO photoluminescence (PL) where the no-phonon (NP) FX emission dominates. The exact mechanism for this effect, however, was not clear. In paper 1, we have clarified these issues by employing PL and cathodoluminescence (CL) measurements performed for bulk ZnO material and ZnO tetrapods. From spatially resolved CL studies, we have shown that the suppression of the NP FX emission strongly depends on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. The effect is interpreted using a model based on re-absorption due to multiple internal reflections in the vicinity of the FX resonance.

As to the exciton-photon coupling, it usually leads to formation of mixed or coupled states of excitons and photons known as exciton-polaritons. The exciton-polariton formation has been demonstrated to lead to slow-down of light in several semiconductor materials such as CdZnTe, GaN, etc. Due to the strong exciton-photon coupling in ZnO, the polariton formation may also affect light velocity in this medium. To explore this effect, we have performed timeof-flight measurement using pulsed laser light. Our studies that are summarized in paper 2 have shown that the group velocity of light in bulk ZnO could be decreased down to 2044km/s and the magnitude of this decrease depends on light polarization. The main physical mechanism responsible for this effect was singled out as being due to the formation of free exciton-polaritons that propagate coherently via ballistic transport. Based on the experimentally determined spectral dependence of the polariton group velocity, the polariton dispersion was also determined.

Excitonic effects in ZnO could also be utilized to investigate fundamental properties of ZnO. For example, previous magneto-optical studies of donor bound excitons allowed to establish ordering of valence band (VB) states and also provided consistent information on the sign and g-factor of holes from the upper A-valence subband. On the other hand, properties of the higher lying B-VB subband were not fully understood. To clarify this issue, we have performed time-resolved and magneto-PL studies for the so-called I6 B and I7 B excitonic transitions which involved a hole from the B-VB subband as summarized in paper 3. From the magneto-PL measurements, values of effective g-factors for conduction band electrons and B valence band holes were determined as ge =1.91, gh =1.79 and gh =0, respectively.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. 27 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1534
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-77170 (URN)LIU-TEK-LIC-2012:19 (Local ID)978-91-7519-874-3 (ISBN)LIU-TEK-LIC-2012:19 (Archive number)LIU-TEK-LIC-2012:19 (OAI)
Presentation
2012-05-31, Schrödinger E324 , Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2012-05-07 Created: 2012-05-07 Last updated: 2017-03-27Bibliographically approved
2. Excitonic Effects and Energy Upconversion in Bulk and Nanostructured ZnO
Open this publication in new window or tab >>Excitonic Effects and Energy Upconversion in Bulk and Nanostructured ZnO
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Zinc Oxide (ZnO), a II-VI wurtzite semiconductor, has been drawing enormous research interest for decades as an electronic material for numerous applications. It has a wide and direct band gap of 3.37eV and a large exciton binding energy of 60 meV that leads to intense free exciton (FX) emission at room temperature. As a result, ZnO is currently considered among the key materials for UV light emitting devices with tailored dimensionality and solid state white lighting. Full exploration of ZnO in various applications requires detailed knowledge of its fundamental and materialrelated properties, which remains incomplete. The research work summarized in this thesis addresses a selection of open issues on optical properties of ZnO based on (but not limited to) detailed time-resolved photoluminescence (PL) and magneto-optical studies of various excitonic transitions as specified below.

Papers 1 and 2 analyze recombination dynamics of FX and donor bound excitons (DX) in bulk and tetrapod ZnO with the aim to evaluate contributions of radiative and nonradiative carrier recombination processes in the total carrier lifetime. We show that changes in relative contributions of these processes in “bulk” and near-surface areas are responsible for bi-exponential exciton decays typically observed in these materials. The radiative FX lifetime is found to be relatively long, i.e. >1 ns at 77 K and >14 ns at room temperature. In the case of DX, the radiative lifetime depends on exciton localization. Radiative recombination is concluded to dominate the exciton dynamics in “bulk regions” of high-quality materials. It leads to appearance of a slow component in the decays of no-phonon (NP) FX and DX lines, which also determines the dynamics of the longitudinal optical (LO) phonon-assisted and two-electronsatellite DX transitions. On the other hand, the fast component of the exciton decays is argued to be a result of surface recombination.

Paper 3 evaluates exciton-phonon coupling in bulk and tetrapod ZnO. It is found that, in contrast to bulk ZnO, the NP FX emission in ZnO tetrapods is weak as compared with the LO phonon assisted transitions. We show that the observed high intensity of the FX-1LO emission does not reflect enhanced exciton-phonon coupling in nanostructured ZnO. Instead, it is a result of stronger suppression of the NP FX emission in faceted regions of the tetrapods as revealed from spatially resolved cathodoluminescence (CL) studies. This is attributed to enhanced re-absorption due to multiple internal reflections, which become especially pronounced in the vicinity of the FX resonance.

Effects of exciton-photon coupling on light propagation through the ZnO media are studied in Papers 4 and 5. By employing the time-of-flight spectroscopy, in Paper 4 we demonstrate that the group velocity of laser pulses propagating through bulk ZnO can be slowed down to as low as 2044 km/s when photon energies approach the optical absorption edge of the material. The magnitude of this decrease can be manipulated by changing light polarization. In Paper 5 we show that the observed slow-down is caused by the formation of free exciton-polaritons and is determined by their dispersion. On the other hand, contributions of DX polaritons become important only in the proximity to their corresponding resonances.

Excitonic effects can also be utilized to investigate fundamental properties and defect formation in ZnO. In Paper 6, we employ DX to study magneto-optical properties of the B valence band (B-VB) states as well as dynamics of inter-VB energy relaxation. We show that PL decays of the emissions involving the B-VB holes are faster than that of their counterparts involving the A-VB holes, which is interpreted as being due to energy relaxation of the holes assisted by acoustic phonons. Values of effective Landé g factors for the B-VB holes are also accurately determined. In paper 7, we uncover the origin of a new class of bound exciton lines detected within the nearband-edge region. Based on their magnetic behavior we show that these lines do not stem from DXs bound to either ionized or neutral donors but instead arise from an exciton bound to an isoelectronic center with a hole-attractive local potential.

In Paper 8, DX emissions are used to monitor energy upconversion in bulk and nanorod ZnO. Based on excitation power dependent PL measurements performed with different energies of excitation photons, the physical processes responsible for the upconversion are assigned to two-photon-absorption (TPA) via virtual states and twostep TPA (TS-TPA) via real states. In the former case the observed threshold energy for the TPA process is larger than half of that for one-photon absorption across the bandgap, which can be explained by the different selection rules between the involved optical transitions. It is also concluded that the TS-TPA process occurs via a defect/impurity with an energy level lying within 1.14-1.56 eV from one of the band edges, likely a zinc vacancy.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 57 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1560
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-102594 (URN)10.3384/diss.diva-102594 (DOI)978-91-7519-464-6 (ISBN)
Public defence
2014-01-31, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-12-16 Created: 2013-12-16 Last updated: 2017-03-27Bibliographically approved

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