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Nanoscale optical positioning of single quantum dots for bright and pure single-photon emission.

Sapienza L, Davanço M, Badolato A, Srinivasan K - Nat Commun (2015)

Bottom Line: Self-assembled, epitaxially grown InAs/GaAs quantum dots (QDs) are promising semiconductor quantum emitters that can be integrated on a chip for a variety of photonic quantum information science applications.However, self-assembled growth results in an essentially random in-plane spatial distribution of QDs, presenting a challenge in creating devices that exploit the strong interaction of single QDs with highly confined optical modes.Here, we present a photoluminescence imaging approach for locating single QDs with respect to alignment features with an average position uncertainty <30 nm (<10 nm when using a solid-immersion lens), which represents an enabling technology for the creation of optimized single QD devices.

View Article: PubMed Central - PubMed

Affiliation: 1] Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA [2] Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA [3] School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.

ABSTRACT
Self-assembled, epitaxially grown InAs/GaAs quantum dots (QDs) are promising semiconductor quantum emitters that can be integrated on a chip for a variety of photonic quantum information science applications. However, self-assembled growth results in an essentially random in-plane spatial distribution of QDs, presenting a challenge in creating devices that exploit the strong interaction of single QDs with highly confined optical modes. Here, we present a photoluminescence imaging approach for locating single QDs with respect to alignment features with an average position uncertainty <30 nm (<10 nm when using a solid-immersion lens), which represents an enabling technology for the creation of optimized single QD devices. To that end, we create QD single-photon sources, based on a circular Bragg grating geometry, that simultaneously exhibit high collection efficiency (48%±5% into a 0.4 numerical aperture lens, close to the theoretically predicted value of 50%), low multiphoton probability (g(2)(0) <1%), and a significant Purcell enhancement factor (≈3).

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Circular dielectric gratings tailored to specific QD emitters.(a) Normalized cavity mode electric field intensity /E/2 superimposed on a scanning electron microscope image of the centre of one of the cavities. Scale bar represents 200 nm. (b) Experimental central wavelength of 50 circular grating cavities with varying period and central radius, plotted as a function of the simulated central wavelength. When only one peak is observed in the spectrum, black squares are used to denote the peak wavelength. When two peaks are observed, red circles and blue triangles are used. Such two-peak behaviour is also seen in simulations depending on the device parameters, and is due to coupling to a second cavity mode. Top inset: Atomic Force Microscope image of a circular grating cavity and a line cut (along the dashed line) showing the etch depth of the trenches. Bottom inset: examples of photoluminescence spectra of circular grating cavities, measured from a high-QD density region.
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f3: Circular dielectric gratings tailored to specific QD emitters.(a) Normalized cavity mode electric field intensity /E/2 superimposed on a scanning electron microscope image of the centre of one of the cavities. Scale bar represents 200 nm. (b) Experimental central wavelength of 50 circular grating cavities with varying period and central radius, plotted as a function of the simulated central wavelength. When only one peak is observed in the spectrum, black squares are used to denote the peak wavelength. When two peaks are observed, red circles and blue triangles are used. Such two-peak behaviour is also seen in simulations depending on the device parameters, and is due to coupling to a second cavity mode. Top inset: Atomic Force Microscope image of a circular grating cavity and a line cut (along the dashed line) showing the etch depth of the trenches. Bottom inset: examples of photoluminescence spectra of circular grating cavities, measured from a high-QD density region.

Mentions: The specific nanophotonic structure we focus on is a circular Bragg grating ‘bullseye' geometry, which has been developed as a planar structure in which QD photons are funneled into a near-Gaussian far-field pattern over a moderate spectral bandwidth (few nm) with high efficiency (theoretical efficiency of 50% into a 0.4 numerical aperture) and with the potential for Purcell enhancement of the radiative rate1415. The cavity mode of interest is tightly confined, and optimal performance requires the QD to be within a couple hundred nanometres of the centre of the bullseye structure. This is illustrated in Fig. 3a, which plots the normalized electric field intensity superimposed on a scanning electron microscope image of the centre of a fabricated device. An important parameter in the fabrication of these devices is the etch depth of the asymmetric grating, as this determines the fraction of emission in the upwards direction (towards our collection optics) compared with the downwards direction (towards the substrate). Furthermore, given the high refractive index difference between GaAs and air, a change in etch depth of 1 nm results in a shift of the optical resonances of about 1 nm. We use AFM to determine the GaAs dry etch rate within the grating grooves (Fig. 3b, top inset), and based on this calibration, we fabricate (see Methods section) 50 circular gratings whose parameters (pitch and central diameter) have been adjusted so that the cavity resonances cover the 930–1,000 nm range of wavelengths. These samples were fabricated in a region of the wafer with a high density of QDs, so that the resulting emission under high power excitation is broad enough to feed the cavity modes. Example spectra collected from different circular grating cavities are shown in the bottom inset of Fig. 3b. These measurements allow us to calibrate the experimental cavity resonances with respect to simulations, as shown in the main panel of Fig. 3b, and tailor the design to match the specific QD emission wavelength.


Nanoscale optical positioning of single quantum dots for bright and pure single-photon emission.

Sapienza L, Davanço M, Badolato A, Srinivasan K - Nat Commun (2015)

Circular dielectric gratings tailored to specific QD emitters.(a) Normalized cavity mode electric field intensity /E/2 superimposed on a scanning electron microscope image of the centre of one of the cavities. Scale bar represents 200 nm. (b) Experimental central wavelength of 50 circular grating cavities with varying period and central radius, plotted as a function of the simulated central wavelength. When only one peak is observed in the spectrum, black squares are used to denote the peak wavelength. When two peaks are observed, red circles and blue triangles are used. Such two-peak behaviour is also seen in simulations depending on the device parameters, and is due to coupling to a second cavity mode. Top inset: Atomic Force Microscope image of a circular grating cavity and a line cut (along the dashed line) showing the etch depth of the trenches. Bottom inset: examples of photoluminescence spectra of circular grating cavities, measured from a high-QD density region.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4525159&req=5

f3: Circular dielectric gratings tailored to specific QD emitters.(a) Normalized cavity mode electric field intensity /E/2 superimposed on a scanning electron microscope image of the centre of one of the cavities. Scale bar represents 200 nm. (b) Experimental central wavelength of 50 circular grating cavities with varying period and central radius, plotted as a function of the simulated central wavelength. When only one peak is observed in the spectrum, black squares are used to denote the peak wavelength. When two peaks are observed, red circles and blue triangles are used. Such two-peak behaviour is also seen in simulations depending on the device parameters, and is due to coupling to a second cavity mode. Top inset: Atomic Force Microscope image of a circular grating cavity and a line cut (along the dashed line) showing the etch depth of the trenches. Bottom inset: examples of photoluminescence spectra of circular grating cavities, measured from a high-QD density region.
Mentions: The specific nanophotonic structure we focus on is a circular Bragg grating ‘bullseye' geometry, which has been developed as a planar structure in which QD photons are funneled into a near-Gaussian far-field pattern over a moderate spectral bandwidth (few nm) with high efficiency (theoretical efficiency of 50% into a 0.4 numerical aperture) and with the potential for Purcell enhancement of the radiative rate1415. The cavity mode of interest is tightly confined, and optimal performance requires the QD to be within a couple hundred nanometres of the centre of the bullseye structure. This is illustrated in Fig. 3a, which plots the normalized electric field intensity superimposed on a scanning electron microscope image of the centre of a fabricated device. An important parameter in the fabrication of these devices is the etch depth of the asymmetric grating, as this determines the fraction of emission in the upwards direction (towards our collection optics) compared with the downwards direction (towards the substrate). Furthermore, given the high refractive index difference between GaAs and air, a change in etch depth of 1 nm results in a shift of the optical resonances of about 1 nm. We use AFM to determine the GaAs dry etch rate within the grating grooves (Fig. 3b, top inset), and based on this calibration, we fabricate (see Methods section) 50 circular gratings whose parameters (pitch and central diameter) have been adjusted so that the cavity resonances cover the 930–1,000 nm range of wavelengths. These samples were fabricated in a region of the wafer with a high density of QDs, so that the resulting emission under high power excitation is broad enough to feed the cavity modes. Example spectra collected from different circular grating cavities are shown in the bottom inset of Fig. 3b. These measurements allow us to calibrate the experimental cavity resonances with respect to simulations, as shown in the main panel of Fig. 3b, and tailor the design to match the specific QD emission wavelength.

Bottom Line: Self-assembled, epitaxially grown InAs/GaAs quantum dots (QDs) are promising semiconductor quantum emitters that can be integrated on a chip for a variety of photonic quantum information science applications.However, self-assembled growth results in an essentially random in-plane spatial distribution of QDs, presenting a challenge in creating devices that exploit the strong interaction of single QDs with highly confined optical modes.Here, we present a photoluminescence imaging approach for locating single QDs with respect to alignment features with an average position uncertainty <30 nm (<10 nm when using a solid-immersion lens), which represents an enabling technology for the creation of optimized single QD devices.

View Article: PubMed Central - PubMed

Affiliation: 1] Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA [2] Maryland NanoCenter, University of Maryland, College Park, Maryland 20742, USA [3] School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK.

ABSTRACT
Self-assembled, epitaxially grown InAs/GaAs quantum dots (QDs) are promising semiconductor quantum emitters that can be integrated on a chip for a variety of photonic quantum information science applications. However, self-assembled growth results in an essentially random in-plane spatial distribution of QDs, presenting a challenge in creating devices that exploit the strong interaction of single QDs with highly confined optical modes. Here, we present a photoluminescence imaging approach for locating single QDs with respect to alignment features with an average position uncertainty <30 nm (<10 nm when using a solid-immersion lens), which represents an enabling technology for the creation of optimized single QD devices. To that end, we create QD single-photon sources, based on a circular Bragg grating geometry, that simultaneously exhibit high collection efficiency (48%±5% into a 0.4 numerical aperture lens, close to the theoretically predicted value of 50%), low multiphoton probability (g(2)(0) <1%), and a significant Purcell enhancement factor (≈3).

No MeSH data available.


Related in: MedlinePlus