Browsing by Author "Elmali, A."

Now showing 1 - 7 of 7

Open Access
Attractive versus repulsive excitonic interactions of colloidal quantum dots control blue-to red-shifting (and non-shifting) amplified spontaneous emission
(American Chemical Society, 2013-11-21) Cihan, A. F.; Kelestemur, Y.; Guzelturk, B.; Yerli, O.; Kurum, U.; Yaglioglu, H. G.; Elmali, A.; Demir, Hilmi Volkan
Tunable, high-performance, two-photon absorption (TPA)-based amplified spontaneous emission (ASE) from near-unity quantum efficiency colloidal quantum dots (CQDs) is reported. Besides the absolute spectral tuning of ASE, the relative spectral tuning of ASE peak with respect to spontaneous emission was shown through engineering excitonic interactions in quasi-type-II CdSe/CdS core/shell CQDs. With core shell size adjustments, it was revealed that Coulombic exciton-exciton interactions can be tuned to be attractive (type-I-like) or repulsive (type-II-like) leading to red- or blue-shifted ASE peak, respectively, and that nonshifting ASE can be achieved with the right core shell combinations. The possibility of obtaining ASE at a specific wavelength from both type-I-like and type-II-like CQDs was also demonstrated. The experimental observations were supported by parametric quantum-mechanical modeling, shedding light on the type-tunability. These excitonically engineered CQD-solids exhibited TPA-based ASE threshold as low as 6.5 mJ/cm(2) under 800 nm excitation, displaying one of the highest values of TPA cross-section of 44 660 GM.
Open Access
Blue-and red-shifting amplified spontaneous emission of CdSe/CdS core/shell colloidal quantum dots
(IEEE, 2013) Kelestemur, Yusuf; Cihan, Ahmet Fatih; Güzeltürk, Burak; Yerli, Ozan; Kurum, U.; Yaglioglu H.G.; Elmali, A.; Demir, Hilmi Volkan
We report blue- and red-shifting amplified spontaneous emission of CdSe/CdS quantum dots, controlled by varying core/shell dimensions and modifying exciton-exciton interactions, with low optical gain threshold of two-photon absorption pumping. © 2013 The Optical Society.
Open Access
Crystallization of Ge in SiO2 matrix by femtosecond laser processing
(American Vacuum Society, 2012-01-19) Salihoglu, O.; Kürüm, U.; Yaglioglu, H. G.; Elmali, A.; Aydınlı, Atilla
Germanium nanocrystals embedded in a siliconoxide matrix has been fabricated by single femtosecond laser pulse irradiation of germanium doped SiO2 thin films deposited with plasma enhanced chemical vapor deposition. SEM and AFM are used to analyze surface modification induced by laser irradiation. Crystallization of Ge in the oxide matrix is monitored with the optic phonon at 300 cm(-1) as a function of laser fluence. Both the position the linewidth of the phonon provides clear signature for crystallization of Ge. In PL experiments, strong luminescence around 600 nm has been observed.
Open Access
Electrochemically tunable ultrafast optical response of graphene oxide
(A I P Publishing LLC, 2011) Kürüm, U.; Ekiz, O. Ö.; Yaglioglu, H. G.; Elmali, A.; Ürel, M.; Güner, H.; Mızrak, A. K.; Ortaç, B.; Dâna, A.
We demonstrate reversible and irreversible changes in the ultrafast optical response of multilayer graphene oxide thin films upon electrical and optical stimulus. The reversible effects are due to electrochemical modification of graphene oxide, which allows tuning of the optical response by externally applied bias. Increasing the degree of reduction in graphene oxide causes excited state absorption to gradually switch to saturable absorption for shorter probe wavelengths. Spectral and temporal properties as well as the sign of the ultrafast response can be tuned either by changing the applied bias or exposing to high intensity femtosecond pulses. © 2011 American Institute of Physics.
Open Access
Encapsulation of a zinc phthalocyanine derivative in self-assembled peptide nanofibers
(The Royal Society of Chemistry, 2012) Garifullin, R.; Erkal, T. S.; Tekin, S.; Ortaç, B.; Gürek, A. G.; Ahsen, V.; Yaglioglu, H. G.; Elmali, A.; Güler, Mustafa O.
In this article, we demonstrate encapsulation of octakis(hexylthio) zinc phthalocyanine molecules by non-covalent supramolecular organization within self-assembled peptide nanofibers. Peptide nanofibers containing octakis(hexylthio) zinc phthalocyanine molecules were obtained via a straight-forward one-step self-assembly process under aqueous conditions. Nanofiber formation results in the encapsulation and organization of the phthalocyanine molecules, promoting ultrafast intermolecular energy transfer. The morphological, mechanical, spectroscopic and non-linear optical properties of phthalocyanine containing peptide nanofibers were characterized by TEM, SEM, oscillatory rheology, UV-Vis, fluorescence, ultrafast pump-probe and circular dichroism spectroscopy techniques. The ultrafast pump-probe experiments of octakis(hexylthio) zinc phthalocyanine molecules indicated pH controlled non-linear optical characteristics of the encapsulated molecules within self-assembled peptide nanofibers. This method can provide a versatile approach for bottom-up fabrication of supramolecular organic electronic devices. © 2012 The Royal Society of Chemistry.
Open Access
Femtosecond laser crystallization of amorphous Ge
(American Institute of Physics, 2011) Salihoglu, O.; Kürüm, U.; Yaglıoglu, G. H.; Elmali, A.; Aydınlı, Atilla
Ultrafast crystallization of amorphous germanium (a-Ge) in ambient has been studied. Plasma enhanced chemical vapor deposition grown a-Ge was irradiated with single femtosecond laser pulses of various durations with a range of fluences from below melting to above ablation threshold. Extensive use of Raman scattering has been employed to determine post solidification features aided by scanning electron microscopy and atomic force microscopy measurements. Linewidth of the Ge optic phonon at 300 cm -1 as a function of laser fluence provides a signature for the crystallization of a-Ge. Various crystallization regimes including nanostructures in the form of nanospheres have been identified.
Open Access
Probing ultrafast energy transfer between excitons and plasmons in the ultrastrong coupling regime
(American Institute of Physics Inc., 2014) Balci, S.; Kocabas, C.; Küçüköz, B.; Karatay, A.; Akhüseyin, E.; Yaglioglu, H. G.; Elmali, A.
We investigate ultrafast energy transfer between excitons and plasmons in ensembles of core-shell type nanoparticles consisting of metal core covered with a concentric thin J-aggregate (JA) shell. The high electric field localization by the Ag nanoprisms and the high oscillator strength of the JAs allow us to probe this interaction in the ultrastrong plasmon-exciton coupling regime. Linear and nonlinear optical properties of the coupled system have been measured using transient absorption spectroscopy revealing that the hybrid system shows half-plasmonic and half-excitonic properties. The tunability of the nanoprism plasmon resonance provides a flexible platform to study the dynamics of the hybrid state in a broad range of wavelengths. © 2014 AIP Publishing LLC.