| 1. |
- Aghaeipour, Mahtab, et al.
(författare)
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Optical response of wurtzite and zinc blende GaP nanowire arrays
- 2015
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Ingår i: Optics Express. - 1094-4087. ; 23:23, s. 30177-30187
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Tidskriftsartikel (refereegranskat)abstract
- We compare the optical response of wurtzite and zinc blende GaP nanowire arrays for varying geometry of the nanowires. We measure reflectance spectra of the arrays and extract from these measurements the absorption in the nanowires. To support our experimental findings and to allow for more detailed investigations of the optical response of the nanowire arrays than possible in experiments, we perform electromagnetic modeling. This modeling highlights the validity of the extraction of the absorptance from reflectance spectra, as well as limitations of the extraction due to anti-reflection properties of the nanowires. In our combined experimental and theoretical study, we find for both zinc blende and wurtzite nanowires an absorption resonance that can be tuned into the ultraviolet by decreasing the diameter of the nanowires. This peak stops blue-shifting with decreasing nanowire diameter at a wavelength of approximately 330 nm for zinc blende GaP. In contrast, for the wurtzite GaP nanowires, the resonance continues blue-shifting at 310 nm for the smallest diameters we succeeded in fabricating. We interpret this as a difference in refractive index between wurtzite and zinc blende GaP in this wavelength region. These results open up for optical applications through resonant absorption in the visible and ultraviolet wavelength regions with both zinc blende and wurtzite GaP nanowire arrays. Notably, zinc blende and wurtzite GaP support resonant absorption deeper into the ultraviolet region than previously found for zinc blende and wurtzite InP and InAs. (C) 2015 Optical Society of America
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| 2. |
- Aghaeipour, Mahtab
(författare)
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Tailoring the Optical Response of III-V Nanowire Arrays
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Semiconductor nanowires show a great deal of promise for applications in a wide range of important fields, including photovoltaics, biomedicine, and information technology. Developing these exciting applications is strongly dependent on understanding the fundamental properties of nanowires, such as their optical resonances and absorption spectra. In this thesis we explore optical absorption spectra of arrays of vertical III-V nanowires with a special emphasis on structures optimized to enhance absorption in the solar spectrum. First, we analyze experimentally determined absorption spectra of both indium phosphide (InP) and gallium phosphide (GaP) nanowire arrays. The study provides an intuitive understanding of how the observed absorption resonances in the nanowires may be tuned as a function of their geometrical parameters and crystal structure. As a consequence, the spectral position of absorption resonances can be precisely controlled through the nanowire diameter. However, the results highlight how the blue-shift in the optical absorption resonances as the diameter of the nanowires decreases comes to a halt at low diameters. The stop point is related to the behavior of the refractive indices of the nanowires. The wavelength of the stop is different for nanowire polytypes of similar dimensions due to differences in their refractive indices. We then present a theoretical argument that it is important to consider symmetry properties when tailoring the optical modes excited in the nanowires for enhanced absorption. We show that absorption spectra may be enhanced compared to vertical nanowires at normal incidence by tilting the nanowires with normal incidence light, or by using off-normal incidence with vertical nanowires. This is because additional optical modes inside the nanowires are excited when the symmetry is broken. Looking forward to omnidirectional applications, we consider branched nanowires as a way to enhance the absorption spectra at normal incidence by taking advantage of simultaneous excitation of the spectrally different optical modes in the branches and the stems. Third, we describe in theoretical terms how integrating distributed Bragg reflectors (DBRs) with the nanowires can improve absorption spectra compared to conventional nanowires. DBRs provide a way to employ light trapping mechanisms which increases the optical path length of the excited modes and thereby improves the absorption of the excited modes. At normal incidence, DBR-nanowires improve the absorption efficiency to 78%, compared to 72% for conventional nanowires. We show that the efficiency is increased to 85% for an off-normal incident angle of 50˚. Overall, our results show that studies of optical resonances in nanowires that take the light-matter interaction into account provide opportunities to develop novel optical and optoelectronic functionalities in nanoscience and nanotechnology.
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| 3. |
- Aghaeipour, Mahtab, et al.
(författare)
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Tunable absorption resonances in the ultraviolet for InP nanowire arrays
- 2014
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Ingår i: Optics Express. - 1094-4087. ; 22:23, s. 29204-29212
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Tidskriftsartikel (refereegranskat)abstract
- The ability to tune the photon absorptance spectrum is an attracting way of tailoring the response of devices like photodetectors and solar cells. Here, we measure the reflectance spectra of InP substrates patterned with arrays of vertically standing InP nanowires. Using the reflectance spectra, we calculate and analyze the corresponding absorptance spectra of the nanowires. We show that we can tune absorption resonances for the nanowire arrays into the ultraviolet by decreasing the diameter of the nanowires. When we compare our measurements with electromagnetic modeling, we generally find good agreement. Interestingly, the remaining differences between modeled and measured spectra are attributed to a crystal-phase dependence in the refractive index of InP. Specifically, we find indication of significant differences in the refractive index between the modeled zinc-blende InP nanowires and the measured wurtzite InP nanowires in the ultraviolet. We believe that such crystal-phase dependent differences in the refractive index affect the possibility to excite optical resonances in the large wavelength range of 345 < lambda < 390 nm. To support this claim, we investigated how resonances in nanostructures can be shifted in wavelength by geometrical tuning. We find that dispersion in the refractive index can dominate over geometrical tuning and stop the possibility for such shifting. Our results open the door for using crystal-phase engineering to optimize the absorption in InP nanowire-based solar cells and photodetectors. (C) 2014 Optical Society of America
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| 4. |
- Anttu, Nicklas, et al.
(författare)
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Absorption and transmission of light in III-V nanowire arrays for tandem solar cell applications
- 2017
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 28:20
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Tidskriftsartikel (refereegranskat)abstract
- III-V semiconductor nanowires are a platform for next-generation photovoltaics. An interesting research direction is to embed a nanowire array in a transparent polymer, either to act as a stand-alone flexible solar cell, or to be stacked on top of a conventional Si bottom cell to create a tandem structure. To optimize the tandem cell performance, high energy photons should be absorbed in the nanowires whereas low energy photons should be transmitted to and absorbed in the Si cell. Here, through optical measurements on 1.95 eV bandgap GaInP nanowire arrays embedded in a polymer membrane, we identify two mechanisms that could be detrimental for the performance of the tandem cell. First, the Au particles used in the nanowire synthesis can absorb >50% of the low-energy photons, leading to a <40% transmittance, even though the Au particles cover <15% of the surface area. The removal of the Au particles can recover the transmission of low energy photons to >80%. Second, after the removal of the Au particles, a 40% reflectance peak shows up due to resonant back-scattering of light from in-plane waveguide modes. To avoid the excitation of these optical modes in the nanowire array, we propose to limit the pitch of the nanowire array.
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| 5. |
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| 6. |
- Anttu, Nicklas, et al.
(författare)
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Absorption of light in InP nanowire arrays
- 2014
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Ingår i: Nano Reseach. - : Springer Science and Business Media LLC. - 1998-0124 .- 1998-0000. ; 7:6, s. 816-823
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Tidskriftsartikel (refereegranskat)abstract
- An understanding of the absorption of light is essential for efficient photovoltaic and photodetection applications with III-V nanowire arrays. Here, we correlate experiments with modeling and verify experimentally the predicted absorption of light in InP nanowire arrays for varying nanowire diameter and length. We find that 2,000 nm long nanowires in a pitch of 400 nm can absorb 94% of the incident light with energy above the band gap and, as a consequence, light which in a simple ray-optics description would be travelling between the nanowires can be efficiently absorbed by the nanowires. Our measurements demonstrate that the absorption for long nanowires is limited by insertion reflection losses when light is coupled from the air top-region into the array. These reflection losses can be reduced by introducing a smaller diameter to the nanowire-part closest to the air top-region. For nanowire arrays with such a nanowire morphology modulation, we find that the absorptance increases monotonously with increasing diameter of the rest of the nanowire.
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| 7. |
- Anttu, Nicklas, et al.
(författare)
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Beyond ray optics absorption of light in CsPbBr3perovskite nanowire arrays studied experimentally and with wave optics modelling
- 2023
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Ingår i: Nanotechnology. - 0957-4484. ; 35:9
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Tidskriftsartikel (refereegranskat)abstract
- We study experimentally and with wave optics modelling the absorption of light in CsPbBr3perovskite nanowire arrays fabricated into periodic pores of an anodized aluminum oxide matrix, for nanowire diameters from 30 to 360 nm. First, we find that all the light that couples into the array can be absorbed by the nanowires at sufficient nanowire length. This behavior is in strong contrast to the expectation from a ray-optics description of light where, for normally incident light, only the rays that hit the cross-section of the nanowires can be absorbed. In that case, the absorption in the sample would be limited to the area fill factor of nanowires in the hexagonal array, which ranges from 13% to 58% for the samples that we study. Second, we find that the absorption saturates already at a nanowire length of 1000-2000 nm, making these perovskite nanowires promising for absorption-based applications such as solar cells and photodetectors. The absorption shows a strong diameter dependence, but for all diameters the transmission is less than 24% already at a nanowire length of 500 nm. For some diameters, the absorption exceeds that of a calculated thin film with 100% coverage. Our analysis indicates that the strong absorption in these nanowires originates from light-trapping induced by the out-of-plane disorder due to random axial position of each nanowire within its pore in the matrix.
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| 8. |
- Anttu, Nicklas
(författare)
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Connection between modeled blackbody radiation and dipole emission in large-area nanostructures
- 2016
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Ingår i: Optics Letters. - 0146-9592. ; 41:7, s. 1494-1497
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Tidskriftsartikel (refereegranskat)abstract
- When modeling the emission of light from nanostructures, we typically study either (1) blackbody radiation or (2) dipole emission. For effective analysis, it is important to know how results from these two types of modeling are related. Here, we use Kirchoff's reciprocity to study how interference affects the emissivity and number of emitted blackbody photons from a thin film for varying thickness. Next, we use Lorentz's reciprocity to study how interference modifies the emission rate of a dipole placed within the same film. Finally, to find the connection between these two emission types, we use Kirchoff's and Lorentz's reciprocity simultaneously for an arbitrary three-dimensional large-area nanostructure. We show analytically how the blackbody radiation can be represented as the integrated emission from homogeneously distributed dipoles in the nanostructure. In this case, the dipole moment density is determined by the refractive index of the nanostructure.
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| 9. |
- Anttu, Nicklas, et al.
(författare)
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Coupling of Light into Nanowire Arrays and Subsequent Absorption
- 2010
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Ingår i: Journal of Nanoscience And Nanotechnology. - : American Scientific Publishers. - 1533-4899 .- 1533-4880. ; 10:11, s. 7183-7187
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Konferensbidrag (refereegranskat)abstract
- We present a theoretical study of the absorption of light in periodic arrays of InP nanowires. The absorption in the array depends strongly on the diameter and the length of the nanowires, as well as the period of the array. Nanowires of a length of just 2 Am are able, after an appropriate choice for the other parameters, to absorb more than 90% of the incident energy of TE and TM polarized light, with photon energies almost all the way down to the band gap energy and an incidence angle up to 50 degree. This high total absorption arises from a good coupling of the incident light into the nanowire array at the top interface between air and the array and absorption inside the array before the light reaches the interface between the nanowires and the substrate. We find that for a given photon energy there exists a critical nanowire diameter above which a dramatic increase in the absorption occurs. The critical diameter decreases for increasing photon energies, and is directly related to the dispersion of waveguiding modes in single isolated nanowires. A characterization showed that the absorption characteristics of the nanowire arrays are very promising for photovoltaic applications.
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| 10. |
- Anttu, Nicklas, et al.
(författare)
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Crystal Phase-Dependent Nanophotonic Resonances in InAs Nanowire Arrays
- 2014
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Ingår i: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 14:10, s. 5650-5655
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructures have many material, electronic, and optical properties that are not found in bulk systems and that are relevant for technological applications. For example, nanowires realized from III-V semiconductors can be grown into wurtzite crystal structure. This crystal structure does not naturally exist in bulk where these materials form the zinc-blende counterpart. Being able to concomitantly grow these nanowires in the zinc-blende and/or wurtzite crystal structure prlovides an important degree of control for the design and optimization of optoelectronic applications based on these semiconductor nanostructures. However, the refractive indices of this new crystallographic phase have so far not been elucidated. This shortcoming makes it impossible to predict and utilize he full potential of these new nanostructured materials for optoelectronics applications a careful design and optimization of optical resonances by tuning the nanostrucuted geometry is needed to achieve optimal performance. Here, we report and analyze striking differeences in the optical response of nanophotonic resonances in wurtzite and zinc-blend InAs nanowire arrays. Specifically, through reflectance measurements we find that the resonance can be tuned down to lambda approximate to 380 nm in wurtzite nanowires by decreasing the nanowire diameter. In stark contrast, a similar tuning to below approximate to 500 nm is not possible in the zinc-blende nanowires. Furthermore, we find that the wurtzite nanowires can absorb twice as strongly as the zinc-blende nanowires. We attribute these strikingly large differences in resonant behavior to large differences between the refractive indices of the two crystallographic phases realized in these nanostructures. We anticipate our finding to be relevant for other III-B materials as well as for all material systems that manifest polytypism. Taken together, our results demonstrate crystal phase engineering as a potentially new design dimension for optoelectronics applications.
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