phys. stat. sol. (c) 3, No. 11, 3685–3688 (2006) / DOI 10.1002/pssc.200671564

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

InAs/InP quantum dot photonic crystal microcavities

Simon Frederick∗1,2, Dan Dalacu1, Philip J. Poole1, Jean Lapointe1, Geof C. Aers1,and Robin L. Williams1,2

1 Institute for Microstructural Sciences, National Research Council, Ottawa, Canada, K1A 0R62 Department of Physics, University of Ottawa, Ottawa, Canada, K1N 6N5

Received 1 May 2006, revised 14 June 2006, accepted 18 July 2006

Published online 24 November 2006

PACS 42.55.Sa, 42.70.Qs, 68.65.Hb, 78.55.Cr, 78.67.Hc

We examine the optical mode structure of two-dimensional, photonic crystal microcavities based on triplemissing hole defects in hexagonal symmetry, etched air-hole, suspended InP membranes. Polarisation de-pendent photoluminescence from InAs/InP quantum dots embedded within the cavities is used to exploremode energies and quality factors, Q, as a function of cavity design parameters. Optimised Q values are inexcess of 7,500 for the modified y-dipole mode.

1 Introduction

Experimental demonstrations of single photon sources based on self-assembled quantum dots (SQD) cou-

pled to the optical modes of a photonic microcavity have been focused primarily on GaAs-based devices

using InAs dots [1–5]. This is also the case for experiments exploring the strong coupling regime (for a

review see Ref. [6]). Although such dots can be tuned to emit photons around λ = 1.3 µm [7], a more

appropriate choice for fibre-based applications, around λ = 1.55 µm, is the InAs/InP quantum dot system.

InAs/InP SQDs grown by chemical beam epitaxy (CBE) emit strongly in the wavelength regime around

λ = 1.55 µm and the growth technique affords the capability to control the nucleation site of individual

quantum dots with nanometre precision [8–10]. InAs/InP quantum dot ensembles have been used previ-

ously to investigate the optical mode structure of both micropillar cavities [11, 12] and photonic crystal

(PC) microcavities [12–14]. In the work presented here we use embedded InAs/InP quantum dots to study

the mode structure, energies and quality factors, Q, of triple missing hole defect (H3), hexagonal symme-

try, photonic crystal microcavity membranes. In Si [15], such cavities have shown Q-factors in excess of

40,000 and in GaAs [16] values around 20,000 have been observed.

2 Sample

Suspended membrane cavities were fabricated from InP layers deposited on 1 µm thick, lattice matched

InGaAs etch stop layers deposited by CBE on semi-insulating InP substrates. Details of the fabrication

process can be found in Ref. [12]. To allow characterisation of the cavity modes in emission, a high

density (∼ 100 µm−2) InAs quantum dot ensemble was embedded midway through the InP membrane.

The design of the H3 cavities is described with reference to three main parameters, the pitch between

etched holes, a, the hole radii, r, and the membrane thickness, d. A schematic illustration of a typical H3cavity, along with scanning electron microscope (SEM) images of a particular structure, is shown in Fig.

1. To ensure that cavity Qs are not limited by in-plane losses, 12 rings of etched holes are used.

∗ Corresponding author: e-mail: simon.frederick@cnrc-nrc.gc.ca, Phone: 613 993 7458, Fax: 613

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

3686 S. Frédérick et al.: InAs/InP QD cavities

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

Fig. 1 Schematic illustration and scanning electron micrographs of a triple missing hole defect (H3), hexagonal

symmetry, InAs/InP quantum dot photonic crystal microcavity membrane.

Fig. 2 The top-left panel: typical low resolution PL for a triple missing hole cavity (a = 500 nm, d = 231 nm,r =170 nm and δ = 75 nm). Middle-left panel: PL intensity as a function of the polarizer angle in polar coordinates for

each peak. Bottom-left panel: PL intensity for different polarizer angle, curves are offset for clarity. Right panel: mode

A high resolution PL. The thick curve is a Lorentzian fit to the experimental data.

3 Experiment

Photoluminescence (PL) measurements were performed at room temperature using 7.3 µW of HeNe ex-

citation with a spot size of approximately 3 µm2. Excitation and collection were performed through the

same, large numerical aperture objective (N.A.=0.8). Luminescence was dispersed using either an FTIR

spectrometer (Bomem MB-160) or a 0.6m single grating spectrometer. Polarisation resolved measure-

ments were made by rotating a Glan prism in the collection path between the sample and spectrometer.

Figure 2 presents experimental PL data for a modified H3 cavity in which a = 500 nm, d = 231 nm and

r = 170 nm. To increase Q, the holes closest to the cavity centre, along the x-axis, have been moved

outwards by a distance δ = 75 nm. The upper-left panel of Fig. 2 shows a low resolution PL spectrum

in which five individual peaks are easily visible. The lowest energy peak observed, mode A, is shown at

higher resolution in the right hand panel, where a Lorentzian fit to the peak profile gives a Q in excess of

7,500, limited by the instrumental resolution. The lower left hand panel of Fig. 2 shows polarisation re-

solved PL as a function of the polariser angle. Each of the peaks observed is found to be linearly polarised,

either along or perpendicular to the long axis of the defect. To identify the characteristics of the modes

phys. stat. sol. (c) 3, No. 11 (2006) 3687

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Fig. 3 FDTD simulated electric field intensity profiles for the TE confined modes of the H3 cavity. Insets show the

expected linear polarisation orientation in the far field.

observed in PL, finite difference time domain (FDTD) simulations were performed using commercial soft-

ware [17]. Details of the finite difference time domain simulations can be found in Ref. [13]. Fig. 3 shows

the FDTD simulated electric field intensity profiles for the six confined TE modes supported by the H3microcavity. The inset in each panel shows the far field polarisation expected for each mode. By virtue of

the mode energies and polarisation orientation, the modes observed experimentally (labelled A to E) can

be identified with those predicted by simulation. Modes D1 and D2 identified by simulation are very nearly

degenerate and are labelled D in Fig. 2. From the FDTD simulated mode profiles, it is clear that the modes

can be viewed as appropriately symmetrised linear combinations of the x- and y-dipole modes of simple,

single missing hole cavites. With this in mind, modes D1, D2 and C can be viewed as single, double and

triple x-dipole-like, whilst modes E and B can be viewed as being derived from triple and double y-dipole

modes respectively. This leads us to speculate that mode A, the high-Q mode suitable for coupling to a

single quantum dot located at the defect cavity centre, may be a direct derivative of the single y-dipole

observed in single missing hole cavities. Experiments tracking the development of the y-dipole mode in

single missing hole cavities, as the two innermost holes along the x-axis are moved progressively outward

towards the H3 configuration, confirm these observations and details will be presented elsewhere.

To manipulate the energy and Q of mode A, further modifications to the H3 cavities can be made. In Fig.

4 we present experimental data showing the variation of Q and energy as a function of the position of two

further holes added to the H3 cavity. The bottom panel shows a schematic of the particular modification

under consideration, with the dotted circles representing the normal lattice positions of the missing holes.

The additional holes, with a radius equal to the normal lattice holes, are added a distance ∆ away from the

existing inner holes, as indicated in Fig.4. A zero value of ∆ indicates that the additional holes overlap

the existing inner holes exactly, whilst a positive (negative) value of ∆ indicates that the additional holes

are shifted away from (towards) the cavity centre. For negative values of ∆ we see an increase in the

cavity mode energy for increasing shift, as expected. For positive values of ∆, the mode energy is fairly

insensitive to the hole location, but the cavity Q rises with increasing hole shift, allowing one to optimise

Q without affecting the mode energy. With the particular parameters shown in Fig. 4, the optimised Qvalue is close to 5,600 for r = 150 nm and ∆=81 nm.

4 Conclusion

We have examined the optical mode structure of InAs/InP quantum dot photonic crystal microcavity mem-

branes based on three missing hole defects. Modes are observed in emission around λ = 1.55 µm that

correspond to linear combinations of simple x- and y-dipole modes with appropriate symmetrisation. For

the high-Q mode derived from the single y-dipole, we demonstrate a maximum Q in excess of 7,500. Such

3688 S. Frédérick et al.: InAs/InP QD cavities

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

-100 -75 -50 -25 0 25 50 75 100

825

830

835

840

845

850

855 r=155nm r=150nm r=145nm r=140nm

Ene

rgy

[meV

]

∆ [nm]

-100 -75 -50 -25 0 25 50 75 100

5001000150020002500

30003500400045005000

55006000

r=155nm r=150nm r=145nm r=140nm

Q

∆ [nm]

∆∆∆∆ 2r

Fig. 4 Top panels: Experimental mode A peak energy and Q as a function of ∆ for four different host lattice hole

radii with a = 441.3 nm and d = 295 nm. Bottom panel: Schematic illustration of modifications to the H3 cavity

structures are promising candidates for single photon sources based on site-selected InAs/InP quantum

dots.

Acknowledgements The authors wish to acknowledge valuable help from J. McKee and M. Tomlinson and finan-

cial support from the National Science and Engineering Research Council and the Canadian Institute for Photonic

Innovations.

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