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Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostructures. [corrected].

Schmitt SW, Sarau G, Christiansen S - Sci Rep (2015)

Bottom Line: After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference.Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference.Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for the Science of Light, Photonic Nanostructures, Günther-Scharowsky-Str. 1, 91058 Erlangen/Germany Helmholtz-Zentrum Berlin für Materialien und Energie, Institute Nanoarchitectures for Energy Conversion, Hahn-Meitner-Platz 1, 14109 Berlin/Germany.

ABSTRACT
Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/n eff)(3) < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

No MeSH data available.


Related in: MedlinePlus

Mode analysis.(a) Radiative power from the modes through the top (arbitrary units) of the SiNC C4 plotted with respect to the wavelength. The upper (red) and lower (blue) graphs show modes excited by a dipole polarized along the x and z direction (), respectively. The numbers indicate the peaks corresponding to the modes shown in (d). (b) Circumferences (heights) of the SiNC C4 hosting WGMs. Blue and red symbols correspond to WGMs excited by  and , respectively. Closed and open stars show WGMs visualized in (c) and (d). (c) Characteristic xy cross sections of the energy density for the HE61, HE81, HE101, HE81a, and HE101a WGMs. (d) xz and xy cross sections of the energy density for HE101a WGMs hosted at five discrete heights of the structure C4.
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f3: Mode analysis.(a) Radiative power from the modes through the top (arbitrary units) of the SiNC C4 plotted with respect to the wavelength. The upper (red) and lower (blue) graphs show modes excited by a dipole polarized along the x and z direction (), respectively. The numbers indicate the peaks corresponding to the modes shown in (d). (b) Circumferences (heights) of the SiNC C4 hosting WGMs. Blue and red symbols correspond to WGMs excited by and , respectively. Closed and open stars show WGMs visualized in (c) and (d). (c) Characteristic xy cross sections of the energy density for the HE61, HE81, HE101, HE81a, and HE101a WGMs. (d) xz and xy cross sections of the energy density for HE101a WGMs hosted at five discrete heights of the structure C4.

Mentions: To understand the formation of strongly confined optical emission peaks in the SiNC that could not be found in the SiNW reference, a mode analysis using finite difference time domain simulations (FDTD solutions, Lumerical) was performed. For 660 nm pumping of a SiNC and a SiNW, the most laser light is absorbed at the maxima of the energy density as seen in Fig. 1c. Here, the highest spontaneous band-to-band emission from excited photo-carriers will emerge that, in turn, will give rise to NIR modes between 850 and 1250 nm that can be hosted by the SiNC and the SiNW. Therefore, for the mode analysis, broadband dipole pulses (850–1250 nm) polarized in x and z direction (, ) were excited in the maxima of the pump laser absorption in a SiNC with the geometry of C4 and a SiNW with the geometry of NW1. However, as is described in more detail in supplementary information S3, the SiNW attached to the wafer substrate is by far more optically ‘leaky’ than the SiNC, which is able to retain a higher amount of optical energy over a longer time span. This radiative energy is stored in optical modes. In the following, these modes, which are obviously unique to the SiNC geometry, will be analyzed in more detail. To find their spectral location, the radiative power emitted through the top facet of the SiNC was monitored in the numerical analysis. By plotting over the wavelength in Fig. 3a, a multitude of modes strongly radiating in the z direction can be identified, which is in good agreement with the experimental results in Fig. 2a. The peaks observed in the measured spectra show a good position coincidence with those in the simulated spectra. Moreover, numerically determined Q-factors of peaks in C4 ranging from 331 to 801 compare well with the measured ones ranging from 181 to 732 (for direct comparison see supplementary information S2 and S4). The amplitude of each resonance is not very meaningful, as it depends on the position of the sources used to excite the system and the position of the time monitors used to measure the response. However, in this case it is not necessary to calculate the absolute amplitude of the mode profile. The Q-factors and relative mode profile are the quantities of interest. Systematically smaller Q values in the measurements compared to the simulations can be attributed to surface roughness induced optical losses as e.g. already reported for Si micro disks or losses that are induced by free carrier absorption in the high injection regime (also compare next section)2122. Furthermore, it can be seen that the set of excited modes is different for the two excitation polarizations (, ). It turns out that all identified spectral radiation maxima correspond to WGMs that occur in discrete heights of the SiNC. Within the SiNCs, WGM positions are related to a circumference according to with , , and are the characteristic dimensions of the SiNC specified in supplementary information S1. Plotting this circumference with respect to the spectral position of the maxima in Fig. 4b reveals the formation of five branches, along which WGMs align depending on the excitation by different polarizations (, ). Each branch corresponds to a characteristic xy cross section of the energy density for the associated WGMs that in the following will be denoted as HE61, HE81, HE101, HE81a, and HE101a, respectively. For the indexing of the modes see supplementary information S5. Figure 4c shows the typical xy cross sectional energy density for the WGMs in the five different branches with a wavelength of 1152 nm, 1119 nm, 1175 nm (2x), and 1103 nm. Note that HE81a and HE101a WGMs coincide at 1175 nm. xz and xy cross sections of the energy density of HE101a WGMs are shown in Fig. 3d. Surprisingly, the WGMs are hosted in discrete heights of the SiNC that in principle should offer a continuum of circumferences from bottom to top. The discretization can be explained by the fact that the high WGMs found in the SiNC all show a long but much less intense ‘leaking’ branch towards the upper edge of the structure. As it can be seen in the xz cross sections of the energy density of the exemplary modes in Fig. 3d, this geometrically extended branch confines the modes to their discrete positions. The upward branch of the modes appears to be also responsible for the directional emission of the SiNC. Based on the numerical results for the 922 nm mode emission (see Fig. 3d), the NA of the SiNC C4 was estimated to be 0.22, which is about the NA of a typical optical fiber (see supplementary information S6), which implies the possibility of an efficient vertical NIR light coupling.


Observation of strongly enhanced photoluminescence from inverted cone-shaped silicon nanostructures. [corrected].

Schmitt SW, Sarau G, Christiansen S - Sci Rep (2015)

Mode analysis.(a) Radiative power from the modes through the top (arbitrary units) of the SiNC C4 plotted with respect to the wavelength. The upper (red) and lower (blue) graphs show modes excited by a dipole polarized along the x and z direction (), respectively. The numbers indicate the peaks corresponding to the modes shown in (d). (b) Circumferences (heights) of the SiNC C4 hosting WGMs. Blue and red symbols correspond to WGMs excited by  and , respectively. Closed and open stars show WGMs visualized in (c) and (d). (c) Characteristic xy cross sections of the energy density for the HE61, HE81, HE101, HE81a, and HE101a WGMs. (d) xz and xy cross sections of the energy density for HE101a WGMs hosted at five discrete heights of the structure C4.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4660596&req=5

f3: Mode analysis.(a) Radiative power from the modes through the top (arbitrary units) of the SiNC C4 plotted with respect to the wavelength. The upper (red) and lower (blue) graphs show modes excited by a dipole polarized along the x and z direction (), respectively. The numbers indicate the peaks corresponding to the modes shown in (d). (b) Circumferences (heights) of the SiNC C4 hosting WGMs. Blue and red symbols correspond to WGMs excited by and , respectively. Closed and open stars show WGMs visualized in (c) and (d). (c) Characteristic xy cross sections of the energy density for the HE61, HE81, HE101, HE81a, and HE101a WGMs. (d) xz and xy cross sections of the energy density for HE101a WGMs hosted at five discrete heights of the structure C4.
Mentions: To understand the formation of strongly confined optical emission peaks in the SiNC that could not be found in the SiNW reference, a mode analysis using finite difference time domain simulations (FDTD solutions, Lumerical) was performed. For 660 nm pumping of a SiNC and a SiNW, the most laser light is absorbed at the maxima of the energy density as seen in Fig. 1c. Here, the highest spontaneous band-to-band emission from excited photo-carriers will emerge that, in turn, will give rise to NIR modes between 850 and 1250 nm that can be hosted by the SiNC and the SiNW. Therefore, for the mode analysis, broadband dipole pulses (850–1250 nm) polarized in x and z direction (, ) were excited in the maxima of the pump laser absorption in a SiNC with the geometry of C4 and a SiNW with the geometry of NW1. However, as is described in more detail in supplementary information S3, the SiNW attached to the wafer substrate is by far more optically ‘leaky’ than the SiNC, which is able to retain a higher amount of optical energy over a longer time span. This radiative energy is stored in optical modes. In the following, these modes, which are obviously unique to the SiNC geometry, will be analyzed in more detail. To find their spectral location, the radiative power emitted through the top facet of the SiNC was monitored in the numerical analysis. By plotting over the wavelength in Fig. 3a, a multitude of modes strongly radiating in the z direction can be identified, which is in good agreement with the experimental results in Fig. 2a. The peaks observed in the measured spectra show a good position coincidence with those in the simulated spectra. Moreover, numerically determined Q-factors of peaks in C4 ranging from 331 to 801 compare well with the measured ones ranging from 181 to 732 (for direct comparison see supplementary information S2 and S4). The amplitude of each resonance is not very meaningful, as it depends on the position of the sources used to excite the system and the position of the time monitors used to measure the response. However, in this case it is not necessary to calculate the absolute amplitude of the mode profile. The Q-factors and relative mode profile are the quantities of interest. Systematically smaller Q values in the measurements compared to the simulations can be attributed to surface roughness induced optical losses as e.g. already reported for Si micro disks or losses that are induced by free carrier absorption in the high injection regime (also compare next section)2122. Furthermore, it can be seen that the set of excited modes is different for the two excitation polarizations (, ). It turns out that all identified spectral radiation maxima correspond to WGMs that occur in discrete heights of the SiNC. Within the SiNCs, WGM positions are related to a circumference according to with , , and are the characteristic dimensions of the SiNC specified in supplementary information S1. Plotting this circumference with respect to the spectral position of the maxima in Fig. 4b reveals the formation of five branches, along which WGMs align depending on the excitation by different polarizations (, ). Each branch corresponds to a characteristic xy cross section of the energy density for the associated WGMs that in the following will be denoted as HE61, HE81, HE101, HE81a, and HE101a, respectively. For the indexing of the modes see supplementary information S5. Figure 4c shows the typical xy cross sectional energy density for the WGMs in the five different branches with a wavelength of 1152 nm, 1119 nm, 1175 nm (2x), and 1103 nm. Note that HE81a and HE101a WGMs coincide at 1175 nm. xz and xy cross sections of the energy density of HE101a WGMs are shown in Fig. 3d. Surprisingly, the WGMs are hosted in discrete heights of the SiNC that in principle should offer a continuum of circumferences from bottom to top. The discretization can be explained by the fact that the high WGMs found in the SiNC all show a long but much less intense ‘leaking’ branch towards the upper edge of the structure. As it can be seen in the xz cross sections of the energy density of the exemplary modes in Fig. 3d, this geometrically extended branch confines the modes to their discrete positions. The upward branch of the modes appears to be also responsible for the directional emission of the SiNC. Based on the numerical results for the 922 nm mode emission (see Fig. 3d), the NA of the SiNC C4 was estimated to be 0.22, which is about the NA of a typical optical fiber (see supplementary information S6), which implies the possibility of an efficient vertical NIR light coupling.

Bottom Line: After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference.Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference.Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

View Article: PubMed Central - PubMed

Affiliation: Max Planck Institute for the Science of Light, Photonic Nanostructures, Günther-Scharowsky-Str. 1, 91058 Erlangen/Germany Helmholtz-Zentrum Berlin für Materialien und Energie, Institute Nanoarchitectures for Energy Conversion, Hahn-Meitner-Platz 1, 14109 Berlin/Germany.

ABSTRACT
Silicon nanowires (SiNWs) attached to a wafer substrate are converted to inversely tapered silicon nanocones (SiNCs). After excitation with visible light, individual SiNCs show a 200-fold enhanced integral band-to-band luminescence as compared to a straight SiNW reference. Furthermore, the reverse taper is responsible for multifold emission peaks in addition to the relatively broad near-infrared (NIR) luminescence spectrum. A thorough numerical mode analysis reveals that unlike a SiNW the inverted SiNC sustains a multitude of leaky whispering gallery modes. The modes are unique to this geometry and they are characterized by a relatively high quality factor (Q ~ 1300) and a low mode volume (0.2 < (λ/n eff)(3) < 4). In addition they show a vertical out coupling of the optically excited NIR luminescence with a numerical aperture as low as 0.22. Estimated Purcell factors Fp ∝ Q/Vm of these modes can explain the enhanced luminescence in individual emission peaks as compared to the SiNW reference. Investigating the relation between the SiNC geometry and the mode formation leads to simple design rules that permit to control the number and wavelength of the hosted modes and therefore the luminescent emission peaks.

No MeSH data available.


Related in: MedlinePlus