| 1. |
- Guina, Mircea, et al.
(author)
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Semiconductor saturable absorbers with recovery time controlled by lattice mismatch
- 2007
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In: Optical Components and Materials IV. - : SPIE. ; , s. 64690P-1-64690P-7
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Conference paper (peer-reviewed)abstract
- We propose and demonstrate a new method to reduce the absorption recovery time of semiconductor saturable absorber mirrors operating at the 1060-nm wavelength range. The method is based on controlling the amount of nonradiative recombination centers within the absorbing region by incorporating an InGaP epitaxial layer with a relative large lattice mismatch to GaAs (~2.2 %). The defect density within the absorbing region can be controlled by the thickness of a GaAs buffer layer grown between the InGaP lattice mismatched layer and the InGaAs/GaAs quantum-wells. For thickness of the GaAs buffer of ∼110 nm and∼570 nm the absorption recovery time was∼5 ps and ∼10 ps, respectively. It is important to note that the fast recovery time was achieved without degrading the nonlinear optical properties of the saturable absorber mirror. The practicality of the structures was proved by demonstrating a reliable self-starting operation of a mode-locked Yb-doped fiber laser.
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| 2. |
- Guina, Mircea, et al.
(author)
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Semiconductor saturable absorbers with recovery time controlled through growth conditions
- 2007
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In: Solid State Lasers XVI. - : SPIE. ; , s. 645113-1-645113-7
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Conference paper (peer-reviewed)abstract
- We discuss a new method to shape the temporal response of saturable absorption in semiconductors. In particular, we investigate the possibility to control independently the absorption recovery time of each quantum-well forming the semiconductor absorber. The recovery time is tailored by irradiation with nitrogen ions produced by an RF-plasma source. The irradiation is performed in-situ as one step of the epitaxial growth process; the quantum-wells are individually exposed to a flux of N-ions after they are grown. The amount of non-radiative recombination centers within the quantum-wells is strongly related to the time interval during which the N-ions flux is active and to the thickness of the semiconductor layer grown on top of each quantum-well before the irradiation is performed. We apply this method to fabricate fast semiconductor saturable absorbers operating in the 1-μm wavelength range. The absorption recovery time could be changed from 300 ps to 10 ps without degradation of the nonlinear optical response. The practicality of the design is finally proved by using the semiconductor saturable absorbers for mode-locking Yb-doped fiber lasers.
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| 3. |
- Hakkarainen, Teemu, et al.
(author)
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Te incorporation and activation as n-type dopant in self-catalyzed GaAs nanowires
- 2019
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In: Physical Review Materials. - 2475-9953. ; 3:8
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Journal article (peer-reviewed)abstract
- Dopant atoms can be incorporated into nanowires either via the vapor-liquid-solid mechanism through the catalyst droplet or by the vapor-solid growth on the sidewalls. Si is a typical n-type dopant for GaAs, but in nanowires it often suffers from a strongly amphoteric nature in the vapor-liquid-solid process. This issue can be avoided by using Te, which is a promising but less common alternative for n-type doping of GaAs nanowires. Here, we present a detailed investigation of Te-doped self-catalyzed GaAs nanowires. We use several complementary experimental techniques, such as atom probe tomography, off-axis electron holography, micro-Raman spectroscopy, and single-nanowire transport characterization, to assess the Te concentration, the free-electron concentration, and the built-in potential in Te-doped GaAs nanowires. By combing the experimental results with a theoretical model, we show that Te atoms are mainly incorporated by the vapor-liquid-solid process through the Ga droplet, which leads to both axial and radial dopant gradients due to Te diffusion inside the nanowires and competition between axial elongation and radial growth of nanowires. Furthermore, by comparing the free-electron concentration from Raman spectroscopy and the Te-atom concentrations from atom probe tomography, we show that the activation of Te donor atoms is 100% at a doping level of 4×1018cm-3, which is a significant result in terms of future device applications.
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| 4. |
- Hamed, Tareq Abu, et al.
(author)
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Multiscale in modelling and validation for solar photovoltaics
- 2018
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In: EPJ Photovoltaics. - : EDP Sciences. - 2105-0716. ; 9
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Journal article (peer-reviewed)abstract
- Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges.
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| 5. |
- Huang, Yuqing, et al.
(author)
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Room-temperature electron spin polarization exceeding 90% in an opto-spintronic semiconductor nanostructure via remote spin filtering
- 2021
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In: Nature Photonics. - : NATURE RESEARCH. - 1749-4885 .- 1749-4893. ; 15, s. 475-482
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Journal article (peer-reviewed)abstract
- An exclusive advantage of semiconductor spintronics is its potential for opto-spintronics, which will allow integration of spin-based information processing/storage with photon-based information transfer/communications. Unfortunately, progress has so far been severely hampered by the failure to generate nearly fully spin-polarized charge carriers in semiconductors at room temperature. Here we demonstrate successful generation of conduction electron spin polarization exceeding 90% at room temperature without a magnetic field in a non-magnetic all-semiconductor nanostructure, which remains high even up to 110 degrees C. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter. We further show that the quantum-dot electron spin can be remotely manipulated by spin control in the adjacent spin filter, paving the way for remote spin encoding and writing of quantum memory as well as for remote spin control of spin-photon interfaces. This work demonstrates the feasibility to implement opto-spintronic functionality in common semiconductor nanostructures. An electron spin polarization of 90% is achieved in a non-magnetic nanostructure at room temperature without magnetic field. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter.
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| 6. |
- Kivisto, Samuli, et al.
(author)
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Pulse dynamics of a passively mode-locked Bi-doped fiber laser
- 2010
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In: Optics Express. - 1094-4087. ; 18:2, s. 1041-1048
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Journal article (peer-reviewed)abstract
- The pulse evolution in Bi-doped soliton fiber laser with slow and fast saturable absorber has been studied both experimentally and numerically. Semiconductor saturable absorbers with balanced slow and fast absorption recovery mechanisms exhibit a bi-temporal recovery dynamics which permits both reliable start-up of passive mode-locking and short pulse generation and stabilization. The pulse dynamics within the Bi fiber laser cavity have been investigated.
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| 7. |
- Laukkanen, Pekka, et al.
(author)
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Surface Studies by Low-Energy Electron Diffraction and Reflection High-Energy-Electron Diffraction
- 2012
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In: Springer Series in Materials Science. ; 150, s. 1-21
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Book chapter (peer-reviewed)abstract
- In this chapter, we present the basic concepts of the low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) experiments. The main goal is to provide an overview of the exploitation of these instrumental methods for analyzing the surfaces of technologically important III–V compound semiconductors. In particular, the interpretation of LEED and RHEED patterns is discussed for the most representative reconstructions of GaAs(100), GaInAsN(100), and Bi-stabilized III–V(100) surfaces. Other application examples concern the use of RHEED for optimizing the growth conditions and growth rates used in molecular beam epitaxy of III–V device heterostructures.
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| 8. |
- Metaferia, Wondwosen, 1980-, et al.
(author)
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High quality InP nanopyramidal frusta on Si
- 2014
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In: CrystEngComm. - : Royal Society of Chemistry (RSC). - 1466-8033. ; 16:21, s. 4624-4632
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Journal article (peer-reviewed)abstract
- Nanosized octagonal pyramidal frusta of indium phosphide were selectively grown at circular hole openings on a silicon dioxide mask deposited on indium phosphide and indium phosphide pre-coated silicon substrates. The eight facets of the frusta were determined to be {111} and {110} truncated by a top (100) facet. The size of the top flat surface can be controlled by the diameter of the openings in the mask and the separation between them. The limited height of the frusta is attributed to kinetically controlled selective growth on the (100) top surface. Independent analyses with photoluminescence, cathodoluminescence and scanning spreading resistance measurements confirm certain doping enrichment in the frustum facets. This is understood to be due to crystallographic orientation dependent dopant incorporation. The blue shift from the respective spectra is the result of this enrichment exhibiting the Burstein-Moss effect. Very bright panchromatic cathodoluminescence images indicate that the top surfaces of the frusta are free from dislocations. The good optical and morphological quality of the nanopyramidal frusta indicates that the fabrication method is very attractive for the growth of site-, shape-, and number-controlled semiconductor quantum dot structures on silicon for nanophotonic applications.
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| 9. |
- Prytz, Nicklas Bjärnhall, et al.
(author)
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Edge-enhanced optical parametric generation in periodically poled LiNbO3
- 2020
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In: Optics Express. - : The Optical Society. - 1094-4087. ; 28:14, s. 20879-20887
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Journal article (peer-reviewed)abstract
- We demonstrate enhanced optical parametric gains occurring at the edge of periodically poled LiNbO3 (PPLN) regions. Experiments performed in MgO-doped PPLN samples, pumped at 532 nm with parametric signal outputs around 800 nm and 1550 nm, exhibit good agreement with numerical simulations of the nonlinear wave dynamics in the system, based on the assumption of an average refractive index increase Delta n = 5.3 x 10(-5) in the PPLN region. Excitation in proximity to the PPLN edge with a pump power of 8.1 mW results in a 3.6-fold output power increase with respect to parametric generation inside the PPLN area.
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| 10. |
- Puustinen, J., et al.
(author)
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1.22 µm GaInNAs saturable absorber mirrors with tailored recovery time
- 2010
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In: Emerging trends and novel materials in photonics. - : American Institute of Physics (AIP). - 9780735408432 ; , s. 200-203
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Conference paper (peer-reviewed)abstract
- The effect of in-situ N-ion irradiation on the recombination dynamics of GaInNAs/GaAs semiconductor saturable absorber mirrors has been studied. The samples were fabricated by molecular beam epitaxy using a radio frequency plasma source for nitrogen incorporation in the absorber layers as well as for the irradiation. The recombination dynamics of irradiated samples were studied by pump-probe measurements. The recombination time of the absorbers could be reduced by increasing the irradiation time. The effect of the reduced recombination time on the pulse dynamics of a mode-locked laser setup was studied with a Bi-doped fibre laser. The pulse quality was found to improve with increased irradiation time and reduced recombination time, demonstrating the potential of the in-situ irradiation method for device applications.
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