Deterministic single-photon source with on-chip 5.6 GHz acoustic clock

TL;DR

Problem: achieving deterministic on-demand SPSs with emission rates in the multi-GHz range remains challenging for solid-state emitters. Approach: embed InAs QDs in a hybrid photon-phonon microcavity and drive dynamic Purcell enhancement with on-chip GHz bulk acoustic waves ($F_\textrm{BAW}$) that modulate the QD energy. Key results: energy modulation up to $14$ GHz with $\Delta E$ up to $3$ meV, a dynamic crossing at $F_\textrm{BAW}=5.6$ GHz yielding emission enhancement consistent with a Purcell factor of $F_\textrm{P} \approx 5$ and a shortened lifetime $\tau \approx 170$ ps (rate ≈ $5.9$ GHz); single-photon emission is confirmed with $g^{(2)}(0) \approx 0.2$. Significance: demonstrates a scalable, on-chip GHz-rate SPS platform with an acoustic clock and potential for GHz-rate transduction in photonic quantum technologies.

Abstract

Scalable solid state single-photon sources (SPSs) with triggered single-photon emission rates exceeding a few GHz would aid in the wide technological adoption of photonic quantum technologies. We demonstrate triggering of a quantum dot (QD) single photon emission using dynamic Purcell effect induced at a frequency of several GHz by acoustic strain. To this end, InAs QDs are integrated in a hybrid photon-phonon patterned microcavity, where the density of optical states is tailored by the lateral confinement of photons in um-sized traps defined lithographically in the microcavity spacer. The single-photon character of the emission form a QD in a trap is confirmed by measuring single-photon statistics. We demonstrate modulation of the QD transition energy in a trap with a frequency up to 14 GHz by monochromatic longitudinal bulk acoustic phonons generated by piezoelectric transducers. For the modulation frequency of 5.6 GHz, corresponding to the acoustic mode of the microcavity, the QD energy is periodically shifted through a spectrally narrow confined photonic mode leading to an appreciable enhancement of the QD emission due to the dynamic Purcell effect. The platform thus enables the implementation of scalable III-V-based SPSs with on-chip tunable acoustic clocks with frequencies that can exceed several GHz under continuous wave optical excitation.

Figures (3)

• Figure 1: Dynamic Purcell effect under acoustic modulation.a Schematic energy diagram of the system. QD energy is $E_\mathrm{QD}$. The energy of an $i^\mathrm{th}$ confined cavity mode is $E_\mathrm{C,i}$. The spectral width of the resonances is indicated by Gaussian peaks. Purcell enhancement factor is $F_\mathrm{P}$. The initial cavity-QD detuning (i.e., without the modulation -- RF Off) is $\Delta E$. The QD energy is modulated by a BAW, RF On, which results in a periodic resonance with the cavity mode. For simplicity, we assume that the energy modulation amplitude is equal to the static QD-cavity detuning. At resonance there is a large Purcell enhancement leading to the triggered emission of single photons (wiggly lines). b A sketch of the MC platform to realize the modulation of panel $a$. An InAs QD is located within a spacer of an AlGaAs MC at the center of a lateral photonic trap, defined by the thicker part of the spacer. A bulk acoustic resonator, BAR, converts an applied radio-frequency signal into a BAW of GHz frequency ($F_\mathrm{BAW}$). The BAW propagates towards the center of the trap and modulates the QD via the deformation potential coupling. The device converts a CW pump into a stream of single photons with a rate equal to $F_\mathrm{BAW}$. c An exemplary calculated spectrum of a $4 \times 4 ~ \mu \mathrm{m}^2$ photonic trap. The dashed line designates the confining potential, defined by the variation of the spacer thickness. The solid lines are spatial profiles of the squared confined wavefunctions.
• Figure 2: Quantum dots in photonic traps.a Spatially and spectrally resolved PL of a $2 \times 2 ~ \mu \mathrm{m}^2$ trap in a high-density of QDs part of the sample under weak non-resonant excitation. The panel on the right is the spatially integrated spectrum. Confined photonic modes are designated $E_\mathrm{C,0}$ and $E_\mathrm{C,1}$. b, c PL of a $1 \times 1 ~ \mu \mathrm{m}^2$ trap in the high-density and low-density part of the sample, respectively. The excitation conditions are similar to those of panel $a$. The color bars encode the PL intensity in arbitrary units. d A second-order auto-correlation measurement as a function of delay ($g^{(2)}(\tau)$) of a single PL peak from a trap in a low-density region.
• Figure 3: GHz acoustic modulation of QDs in photonic traps.a Spectral dependence of a single narrow line resonant to a confined level of a $4 \times 4 ~ \mu m^2$ trap on BAW frequency ($F_\mathrm{BAW}$) for a fixed amplitude ($A_\mathrm{BAW} = 0.56$). The energy is referenced to 1286.7 meV. $\Delta E$ designates the energy modulation amplitude for a given value of $F_\mathrm{BAW}$. For clarity, the spectrum for each frequency was normalized to its maximum. b Dependence of the lower confined levels of the same trap on $A_\mathrm{BAW}$ for $F_\mathrm{BAW} = 5.6$ GHz. To improve the QDs energy shifts, the data was filtered using a bandpass filter. The original data and the explanation of the filtering procedure is described in SI-Sec.7. The color bars in panels $a$ and $b$ encode the PL intensity in arbitrary units. c Intensity profiles taken along the dashed lines in the panel $b$ for the resonances designated as $QD_2$ and $QD_3$.