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Publication Title | Physics Letters A 373 (2009) 1185–1188

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Physics Letters A 373 (2009) 1185–1188

Contents lists available at ScienceDirect Physics Letters A www.elsevier.com/locate/pla

Lasing of whispering-gallery modes in asymmetric waveguide GaInP micro-disks with InP quantum dots

Y. Chu a,∗, A.M. Mintairov b, Y. He b, J.L. Merz b, N.A. Kalyuzhnyy c, V.M. Lantratov c, S.A. Mintairov c

a Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA

b Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA c Ioffe Physical Technical Institute, St. Petersburg, Russia

article info

Article history:

Received 5 December 2008

Received in revised form 2 February 2009 Accepted 5 February 2009

Available online 13 February 2009 Communicated by R. Wu

abstract

Using wafer bonding (WB) and wet oxidation (WO) techniques, GaInP microdisks having an asym- metric waveguide (diameters D = 1–3 μm) with embedded InP quantum dots (size/density ∼100 nm/ ∼109 cm−2 ) have been fabricated on Si and GaAs substrates, respectively. The TEm,l (m = 28–12, l = 1, 2) and TMm,l (m = 25–10, l = 1–4) whispering gallery modes with quality factors Q ∼ 2–5 × 103 have been identified in photoluminescence spectra of these microdisks (MDs) in the spectral range 720–770 nm. Lasing thresholds of 6 (30) μW and mode coupling constants 0.9 (0.7) have been demonstrated for WO (WB) MDs.

Whispering-gallery-mode (WGM) semiconductor microdisk (MD) resonators with embedded quantum dots (QDs) are inter- esting for the realization of low threshold lasers [1,2] and for solid-state cavity quantum electrodynamics (CQED) experiments [3,4]. Up to now the most studied are GaAs MDs with InAs and In- GaAs QDs emitting from 900 to 1300 nm [1–3,5–9]. For these MDs quality factors (Q) as high as 105 have been achieved for fun- damental WGMs and room temperature lasing thresholds as low as 1 μW has been demonstrated [8]. High Q (∼104) MDs with GaAs/AlGaAs QDs were used to observe large (400 μeV) Rabi split- ting (strong coupling regime of CQED) in low temperature emission spectra [4]. Recently, blue-green CdSe/ZnSe MDs lasers emitting at 520 nm were reported having Q ∼ 2000 and lasing threshold ∼20 μW at room temperature [10]. Silicon based MDs with Ge QDs emitting in the range 1200–1600 nm have also been reported [11]. To date, much less work has been reported on GaInP MDs with embedded InP QDs, which offer emission wavelength 700–800 nm. Depending on the growth process, the lateral size and density of InP/GaInP QDs can have values ∼20 nm and ∼5 × 1010 cm−2 [12, 13] or ∼100 nm and ∼109 cm−2 [14,15]. Larger dots are favorable for CQED experiments [4,16] or single dot lasers [6] since the cou- pling of the cavity mode to the emission transition and gain are proportional to dot size. We are aware of only one report on the fabrication on GaInP MDs with InP QDs, [17] in which, however, no clear WGM in the QD emission spectral range was reported, and lasing was observed only for a wetting layer at 650 nm and at temperatures below 40 K.

* Corresponding author.

E-mail address: ychu@nd.edu (Y. Chu).

0375-9601/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2009.02.003

© 2009 Elsevier B.V. All rights reserved.

In the present Letter we used an InP/GaInP QD waveguide structure having lateral dot size 100 nm and density 5 × 109 cm−2 to fabricate microdisks emitting at ∼750 nm. Asymmetric waveg- uide MDs having diameters D = 1–4 μm were fabricated on Si and GaAs substrates using wafer bonding (WB) or wet oxidation (WO) techniques, respectively. Quality factors Q ∼ 2 and 5 × 103 have been achieved for TEm,1 (m = 14–28) modes in WB and WO disks, respectively. Lasing thresholds of 6 μW and 30 μW have been demonstrated and mode coupling constants 0.9 and 0.7 have been measured for TE21,1 mode at room temperature in WO and WB disks.

Experimental details. Our samples were prepared from a 150 nm thick GaInP waveguide structure with embedded self-organized InP quantum dots (QDs) grown by Metal-Organic Chemical Vapor De- position (MOCVD). The structures have been grown in a horizontal AIX200/4 reactor under pressure of 100 mbar. Initially a 700 nm thick AlAs layer was deposited on the GaAs wafer. Then a 150 nm thick Ga0.52 In0.48 P waveguide layer lattice-matched to GaAs was grown with InP QDs at the center. The QDs were grown at 725◦C by depositing 7 monolayers of InP, which results in room tempera- ture emission at 750 nm [18]. The dot density (1–5 × 109 cm−2) and their sizes (∼100 nm) were estimated on uncapped sam- ples using atomic force microscopy (AFM). The dot density of the waveguide structures was measured by near-field optical scanning microscopy [19]. The photoluminescence (PL) spectra of the InP QD ensemble shows a peak at 750 nm and half width of about 30 nm.

Microdisks having D = 1, 1.5, 2, 2.5 and 4 μm were fabricated using electron beam lithography (EBL) and inductively-coupled plasma reactive ion etching (ICP-RIE) by using either a wafer bond- ing (WB) or a wet oxidation (WO) technique. In the case of WB,

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