ARTICLE IN PRESS
Physica E 40 (2008) 2156–2159 www.elsevier.com/locate/physe
Optical emission of InAs/GaAs quantum rings coupled to a two-dimensional photonic crystal microcavity D. Sarkara, L.J. Martı´ nezb, I. Prieto-Gonza´lezb, H.P. van der Meulena,, J.M. Callejaa, D. Granadosb, A.G. Taboadab, J.M. Garcı´ ab, A.R. Alijab, P.A. Postigob a Departamento de Fı´sica de Materiales, Universidad Auto´noma de Madrid, E-28049 Madrid, Spain Instituto de Microelectro´nica de Madrid, Centro Nacional de Microelectro´nica, Consejo Superior de Investigaciones Cientı´ficas, Isaac Newton 8, PTM Tres Cantos, E-28760 Madrid, Spain
b
Available online 13 November 2007
Abstract Microphotoluminescence measurements on InAs/GaAs quantum rings embedded in a bi-dimensional photonic crystal cavity display enhanced emission intensity of single rings depending on the coupling strength to the cavity modes. The cavity is formed by three holes missing at the center of the photonic crystal structure (a linear 3 defect, L3). Light emission by the quantum rings show sharp lines at low excitation power. They undergo different enhancement factors by the separate effects of the photonic crystal and by coupling to the resonant modes, which show full linear polarization. Upon changing temperature, the uncoupled emission of single quantum rings and the resonant modes undergo different frequency shifts. This allows for an external control of the coupling. r 2007 Elsevier B.V. All rights reserved. PACS: 78.67.Hc; 42.70.Qs; 73.63.Kv Keywords: Photonic crystal microcavity; Quantum rings; Photoluminescence
1. Introduction Microcavities in photonic crystals (PC) have received much attention because of the possibility of observing cavity quantum electrodynamics (QED). Cavities based on PCs have a small mode volume (V), and high quality factors (Q), and they lead to large Q/V ratios. The Q/V ratio determines the increase in spontaneous emission on resonance with a cavity mode [1,2]. Quantum dots (QDs) are widely used as source for this spontaneous emission, as they can be seen as atom-like emitters with a discrete set of states. Various cavity QED phenomena have been realized with these systems [3–5], which are of great interest in the field of quantum information processing and single-photon sources. A variation on this scheme is the use of quantum rings (QRs) as emitters instead of QDs. In QRs the ground state has a well-defined angular momentum, which is Corresponding author. Tel.: +34 914973818; fax: +34 914978579.
E-mail address:
[email protected] (H.P. van der Meulen). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.095
tunable by an external magnetic field. This opens possibilities of new quantum effects and their applications. QRs are formed in the process of self-assembled growth of InAs QDs on GaAs by introducing a pause after overgrowing the dots with a few nm of GaAs [6]. Compared to dots, these QRs show a blue shift of the emission energy and a decreased number of excited states [7]. Besides, the oscillator strength for the excitonic ground state in QRs is three times higher than for QDs [8]. This makes QRs more favorable than QDs for the achievement of the strong coupling regime, since lower quality factors would be required [5]. In this work, we present microphotoluminescence experiments on two samples containing QRs as emitters and coupled to optical microcavities. The emission of the rings off and on resonance is compared, as well as the effect of temperature on both QR transitions and resonant mode energies. Variation of the nominal air hole radius via applied electron dose in their fabrication offers a way to change the mode Q factor and resonance energy.
ARTICLE IN PRESS D. Sarkar et al. / Physica E 40 (2008) 2156–2159
2. Experimental The samples consist of one layer of molecular beam epitaxy-grown self-assembled QRs (density around 8 109 cm2) embedded in a slab of 158 nm of GaAs to form the active layer. A 500 nm thick sacrificial layer of Al0.7Ga0.3As is used for wet etching in order to obtain the PC membrane. Details of the growth process can be obtained from Ref. [9]. A two-dimensional PC structure with a triangular lattice of air holes has been fabricated. The designed cavities have a lattice constant a ¼ 255 nm K
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and a hole radius r varying from 60 to 85 nm. The simulation of the cavity band structure was performed using 3D-FDTD commercial software [10]. The processing of the PC was done by electron-beam lithography of a polymethyl methacrylate (PMMA) layer on top of a SiO2 layer (200 nm thick) deposited by plasma-enhanced chemical vapor deposition. Reactive ion beam etching was used to open the holes in the SiO2 by CHF3:N2 and reactive ion etching with SiCl4:Ar gas mixture was used to transfer the holes at the semiconductor material. To make the L3 microcavities three holes are eliminated in the GK direction. The first nearest air holes at both ends of the cavity can be shifted outward to obtain a higher Q [1]. Finally, the sacrificial layer was eliminated by HF:H2O [1:5] wet etching. A drawing of a typical cavity is shown in the inset of Fig. 1. The measurements were performed exciting with a HeNe laser or a Ti-sapphire laser using a microscope setup with a spot size of 3 mm. The emitted light was detected with a double-grating spectrometer and a charge-coupled device detector. All the measurements were taken at 9 K in a continuous flow He cryostat for microscope application. 3. Results and discussion
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Wavelength (nm) Fig. 1. Microphotoluminescence spectrum of quantum rings in a microcavity, taken with an excitation power of 20 W/cm2 of a HeNe laser. At low wavelengths emission of individual quantum rings is observed, at higher values (around 1000 nm) emission of resonant modes is observed. The inset shows the design of a hexagonal photonic crystal with an L3 cavity in the center.
Intensity (arb. units)
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In Fig. 1, a representative photoluminescence spectrum of the first sample is shown, taken with an excitation power of 20 W/cm2 of a HeNe laser. The emission of individual QRs can be seen at low wavelength as single peaks and a resonant modes appear at higher wavelength. The observed mode multiplicity in this sample (also shown in Fig. 3b) is probably due to defect-induced resonances linked to radius inhomogeneities in the PC [11]. At low excitation power E (eV)
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Fig. 2. Temperature effect on the microphotoluminescence of a single quantum ring (a) and emission on resonance with a resonant mode (b).
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Position (μm)
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Fig. 3. The spatial localization of resonant modes (1 and 2) and off-resonance-quantum rings emission (3 and 4) is compared in (a). The location and width of the PC is shown by the horizontal bar. The wavelength of these four emissions is shown in (b).
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only single QR emission is seen. Upon increasing the power resonant modes appear. At high excitation they are more pronounced, whereas in the QR emission the filling of higher states smears out the spectrum. In spectra taken with an intermediate excitation of 8 W/cm2 emission peaks both on resonance and offresonance can be detected. In Fig. 2a, an example of QRluminescence at 980 nm is presented which shows a shift of 1.8 meV upon increasing the temperature from 6 to 36 K and in Fig. 2b, a resonant mode at 996 nm shows only a shift of 0.5 meV. The different wavelength shifts of the QR emission and of the resonant mode with temperature allows tuning individual QRs in and out of resonance. [12,13]. The spatial localization of the emission is studied (Fig. 3) by scanning the microscope objective along the y direction of the cavity. Two resonant modes (1 and 2 in Fig. 3b) and off-resonance-QRs (3 and 4 in Fig. 3b) are measured. The emission of the resonant modes is strongly localized (width 2 mm) at the position of the cavity center (Fig. 3a), as compared to the wider peak (width 10 mm) of the QR signal. The location and width of the PC is shown by the horizontal bar. The intensity increase at the left of Fig. 3a is due to a neighboring PC. In a second series of PC microcavities, we obtained single modes with high Q (up to 3380), and one or more low Q modes (300) at higher energies. Spectra of a series of cavities with the same lattice parameter a ¼ 255 nm and different radius are presented in Fig. 4. The observed trend is a shift to higher energies of the cavity mode with increasing radius of the hole. The high Q mode is fully
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Fig. 4. Microphotoluminescence of the second series of photonic crystal microcavities. A high Q mode is observed at low energies, at high energies one or more lower Q modes exist. The cavity modes shift to higher energies when increasing the hole radius r.
linearly polarized along the y direction (see the inset of Fig. 1). A more extensive study on these cavities will be presented elsewhere. 4. Summary In summary, we have reported resonances of QR emission in PC microcavities, as seen before for QDs in such cavities. A study of the microluminescence of QRs
ARTICLE IN PRESS D. Sarkar et al. / Physica E 40 (2008) 2156–2159
on- and off-resonance show different frequency shifts by changing the temperature and the hole radius. This allows tuning of the QR emission on the resonance. Acknowledgments L.J. Martinez thanks an I3P-CSIC fellowship, I. Prieto an FPI-MEC fellowship. The authors gratefully acknowledge financial support by the Spanish MEC and CAM through Projects NANOSELFII TEC-2005-05781-C03-01, MEC MAT2005-01388, NAN2004-09109-C04, NAN200408843-C05-04, Consolider-CSD 2006-19 and CAM S0505-/ESP-0200, the European Commission through SANDIE NMP4-CT-2004-500101 and PHOREMOST IST-2-511616-NOE Networks of Excellence.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Y. Akahane, et al., Nature 425 (2003) 944. K. Hennesy, et al., Photon. Nanostruct. 2 (2004) 65. Wen-Hao Chang, et al., Phys. Rev. Lett. 96 (2006) 117401. A. Imamoglu, et al., J. Appl. Phys. 101 (2007) 081602. K. Hennesy, et al., Nature 445 (2007) 896. J.M. Garcia, et al., Appl. Phys. Lett. 73 (1997) 2252. B. Ale´n, et al., Phys. Rev. B 72 (2005) 155331. H. Pettersson, et al., Physica E 6 (2000) 510. D. Granados, et al., Appl. Phys. Lett. 82 (2003) 2401. Lumerical Solutions, Inc., Vancouver, BC, Canada, 2004. K. Hennessy, et al., J. Vac. Sci. Technol. B 21 (2003) 2918. T.D. Happ, et al., Phys. Rev. B 66 (2002) 041303. A. Kiraz, et al., Appl. Phys. Lett. 78 (2001) 3932.
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