Coherent coupling dynamics in a quantum-dot microdisk laser

Abstract

Luminescence intensity autocorrelation (LIA) is employed to investigate coupling dynamics between (In,Ga)As QDs and a high-Q (~7000) resonator with ultrafast time resolution (150 fs), below and above the lasing threshold at T = 5 K. For QDs resonant and non-resonant with the cavity we observe both a six-fold enhancement and a 0.77 times reduction of the spontaneous emission rate, respectively. In addition, LIA spectroscopy reveals the onset of coherent coupling at the lasing threshold through qualitative changes in the dynamic behavior and a tripling of the resonant QD emission rate. PACS: 03.67.Lx; 42.55.Sa ; 78.47.+p; The ability to control spontaneous emission 1 in the solid state is expected to improve laser devices 2 as well as enable novel systems for quantum cryptography, such as single photon sources. 3 'Self-assembled' InAs quantum dots (QD)s integrated within high-Q microcavities provide an ideal testbed for studying the control of QD emission through cavity QED (Purcell effect). 4,5 Moreover, understanding the coupling dynamics between the QDs and microcavity may provide an avenue for performing quantum computation and communication with electron spins. 6 Previous measurements in microcavities (i.e.micropillars, 4 microdisks, 5 and microspheres 7 ) have explored enhanced spontaneous QD emission (relative to the unprocessed material), and have yet to investigate the coupling dynamics near the onset of lasing.Here we report time-resolved measurements in microdisks with integrated QDs below and above the lasing threshold using luminescence intensity autocorrelation (LIA) spectroscopy, 8 where the time resolution is theoretically limited by the optical pump/probe pulse widths (~ 150 fs). Detailed studies of the QD-cavity coupling dynamics have shown forQDs resonant and non-resonant with the cavity, a six-fold enhancement and a 0.77 times reduction of the spontaneous emission rate, respectively. Moreover, we use LIA spectroscopy to measure the onset of coherent coupling between the QDs and cavity at the lasing threshold, where we observe a clear signature at zero time delay between pump and probe pulses and an additional tripling of the resonant QD emission rate due to stimulated emission.The device structure is grown by molecular beam epitaxy upon a semi-insulating GaAs substrate with an AlAs/GaAs buffer. A 1 µm layer of Al x Ga 1-x As, (x ranging from 0.65-0.85) is subsequently grown, determining the microdisk post height. The disk region has a single layer of (In,Ga)As self-assembled QDs with a dot density of 10 9 cm -2 and is clad symmetrically by 100 nm GaAs, 20 nm Al 0.3 Ga 0.7 As, and 4 nm GaAs [right inset of Figure 1 where ∆E is the FWHM of the emission energy. The enhancement of the spontaneous emission rate is given by the Purcell factor, 1 F P, (which indicates the degree of coupling between emitter and cavity): F P = (3/4π 2 )(Qλ 3 /V). For our highest Q WGM, with transition energy at 1.378 eV, we estimate F P ~ 50. This calculation assumes an ideal monochromatic 3 emitter that is sp...

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