Browsing by Author "Zhang, Z."

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    All-fiber-integrated soliton-similariton laser with in-line fiber filter
    (Optical Society of America, 2012-08-16) Zhang, Z.; Öktem, B.; Ilday, F. Ö.
    We demonstrate an all-fiber-integrated Er-doped fiber laser operating in the soliton-similariton mode-locking regime. In the similariton part of the cavity, a self-similarly evolving parabolic pulse with highly linear chirp propagates in the presence of normal dispersion. Following an in-line fiber-based birefringent filter, the pulse evolves into a soliton in the part of the cavity with anomalous dispersion. The similariton and the soliton pulses are dechirped to 75.5 and 167.2 fs, respectively, outside of the cavity. Mode-locked operation is very robust, owing to the influence of the two similariton and soliton attractors that predominate each half of the laser cavity. The experimental results are supported with numerical simulations, which provide good agreement.
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    Effect of Mg doping in the barriers on the electrical performance of InGaN/GaN-based light-emitting diodes
    (Elsevier B. V., 2018-04) Zhu, B.; Zhang, Z.; Tan, S. T.; Lu, S.; Zhang, Y.; Kang, X.; Wang, N.; Hasanov, N.; Demir, Hilmi Volkan
    In this work, we report how the Mg doping in the barriers affects the electrical performance of InGaN/GaN-based light-emitting diodes. When compared with the reference device that does not have Mg doped quantum barriers, the turn-on voltage for the proposed device is reduced and the electrical thermal stability is improved. The superior electrical performance is analyzed through the temperature dependent current-voltage and capacitance-voltage characteristics. Meanwhile a reduced depletion length and increased acceptor concentration are achieved in the control devices which is consistent with the simulated results.
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    From aluminum foil to two-dimensional nanocrystals using ultrasonic exfoliation
    (American Chemical Society, 2021-04-15) Lu, W.; Birmingham, B.; Voronine, D. V.; Stolpman, D.; Ambardar, S.; Erdoğan, Deniz Altunöz; Özensoy, Emrah; Zhang, Z.; Solouki, T.
    Al nanostructures have unique optical properties such as widely tunable surface plasmon resonances from deep UV to NIR that can be used for label-free fluorescence enhancement and surface-enhanced Raman scattering. Various Al nanostructures have been fabricated using sophisticated “top-down” lithographic and “bottom-up” colloidal methods. Here, we developed a simple and efficient method of synthesizing two-dimensional (2D) aluminum (Al) nanocrystals from commercially available Al foil using ultrasonic exfoliation under ambient environment. 2D Al nanocrystals with sizes from a few hundred nanometers to several micrometers and thickness in the tens of nanometers were isolated through centrifugation separation. The exfoliated 2D Al nanocrystals are covered with a passivated Al2O3 nanolayer. The determined exfoliation mechanism is a combination of the preferred cleavage along the (111) surface planes and layer-by-layer Al2O3 exfoliation from the surface of the 2D Al nanocrystals. We demonstrate that the 2D Al nanocrystals can be assembled at water/air interface and transferred to different substrates to form 2D Al nanocrystal films. These 2D Al nanocrystal films exhibit surface plasmon resonance in the visible spectral range and show enhanced Raman signals of adenine using a 532 nm excitation. These 2D Al nanocrystal films could be further developed for new optical and sensing applications.
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    Narrow electroluminescence in bromide ligand-capped cadmium chalcogenide nanoplatelets
    (AIP Publishing LLC, 2022-06-15) Thung, Y.T.; Zhang, Z.; Yan, F.; Demir, Hilmi Volkan; Sun, H.
    Colloidal zinc blende II–VI semiconductor nanoplatelets (NPLs) demonstrate as a promising class of materials for optoelectronic devices due to their unique excitonic characteristics, narrow emission linewidth, and quantum well-structure. Adopting heterostructures for these nanocrystals allows tuning of their optical features and enhances their photostability, photoluminescence (PL), quantum yield (QY), and color purity for further device integration. Exchanging of carboxylate capping ligands on top and bottom [001] facets of CdSe NPLs with halide ligands is an alternative to achieve the aims of spectral tunability and improve surface passivation, but to date there have been no reports on integrating the advantages of halide ligand exchanged CdSe NPLs for device fabrication. In this work, we demonstrate green electroluminescence (EL) of bromide ligand-capped CdSe NPLs as active emitters in an electrically driven light emitting diode (LED) with a low turn-on voltage of 3.0 V. We observed EL emission at 533.1 nm with a narrow linewidth of 19.4 nm, a maximum luminance of 1276 cd/m2, and the highest external quantum efficiency (EQE) of 0.803%. These results highlight the ability of halide ligand exchange in tuning the EL properties of CdSe NPL-LEDs and potential of bromide ligand-capped CdSe NPLs in contributing to the green emission region of NPL-LEDs, demonstrating its potential for future device integration and contribution to a high color rendering index of future NPL displays.
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    Seizure detection using least eeg channels by deep convolutional neural network
    (Institute of Electrical and Electronics Engineers Inc., 2019) Avcu, Mustafa Talha; Zhang, Z.; Chan, D. W. S.
    This work aims to develop an end-to-end solution for seizure onset detection. We design the SeizNet, a Convolutional Neural Network for seizure detection. To compare SeizNet with traditional machine learning approach, a baseline classifier is implemented using spectrum band power features with Support Vector Machines (BPsvm). We explore the possibility to use the least number of channels for accurate seizure detection by evaluating SeizNet and BPsvm approaches using all channels and two channels settings respectively. EEG Data is acquired from 29 pediatric patients admitted to KK Woman's and Children's Hospital who were diagnosed as typical absence seizures. We conduct leave-one-out cross validation for all subjects. Using full channel data, BPsvm yields a sensitivity of 86.6% and 0.84 false alarm (per hour) while SeizNet yields overall sensitivity of 95.8 % with 0.17 false alarm. More interestingly, two channels seizNet outperforms full channel BPsvm with a sensitivity of 93.3% and 0.58 false alarm. We further investigate interpretability of SeizNet by decoding the filters learned along convolutional layers. Seizure-like characteristics can be clearly observed in the filters from third and forth convolutional layers.
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    Sub-50 fs Yb-doped laser with anomalous-dispersion photonic crystal fiber
    (Optical Society of America, 2013) Zhang, Z.; Şenel, Ç.; Hamid, R.; Ilday, F. Ö.
    We report on the generation of 42 fs pulses at 1 μm in a completely fiber-integrated format, which are, to the best of our knowledge, the shortest from all-fiber-integrated Yb-doped fiber lasers to date. The ring fiber cavity incorporates anomalous-dispersion, solid-core photonic crystal fiber with low birefringence, which acts as a broadband, in-fiber Lyot filter to facilitate mode locking. The oscillator operates in the stretched-pulse regime under slight normal net cavity dispersion. The cavity generates 4.7 ps long pulses with a spectral bandwidth of 58.2 nm, which are dechirped to 42 fs via a grating pair compressor outside of the cavity. Relative intensity noise (RIN) of the laser is characterized, with the integrated RIN found to be 0.026% in the 3 Hz-250 kHz frequency range.