Abstract
Multiply excited states in semiconductor quantum dots feature intriguing physics and play a crucial role in nanocrystal-based technologies. While photoluminescence provides a natural probe to investigate these states, room-temperature single-particle spectroscopy of their emission has proved elusive due to the temporal and spectral overlap with emission from the singly excited and charged states. Here, we introduce biexciton heralded spectroscopy enabled by a single-photon avalanche diode array based spectrometer. This allows us to directly observe biexciton–exciton emission cascades and measure the biexciton binding energy of single quantum dots at room temperature, even though it is well below the scale of thermal broadening and spectral diffusion. Furthermore, we uncover correlations hitherto masked in ensembles of the biexciton binding energy with both charge-carrier confinement and fluctuations of the local electrostatic potential. Heralded spectroscopy has the potential of greatly extending our understanding of charge-carrier dynamics in multielectron systems and of parallelization of quantum optics protocols.
Highlights
Over the past three decades, numerous types of semiconductor nanocrystals with varying compositions, shapes, sizes, and structures have been fabricated and studied[1−3] with some even making their way into mass produced consumer products.[4]
Considerable efforts were invested in the design of time-resolved light spectrometers with high sensitivity.[32−36] As a replacement for the standard CCD camera, different research groups adopted photomultiplier tube (PMT) arrays,[33] superconducting nanowire single photon detectors (SNSPDs),[35−37] or single-photon avalanche diode (SPAD) arrays.[32,34,38,39]
While these implementations harbor great potential for applications such as Raman spectroscopy and on-chip quantum communications, none is able to provide the combination of high overall detection efficiency, low dark counts, and parallel time and spectrum detection at single-photon level
Summary
Over the past three decades, numerous types of semiconductor nanocrystals with varying compositions, shapes, sizes, and structures have been fabricated and studied[1−3] with some even making their way into mass produced consumer products.[4]. This result suggests that the BX binding energy, much like 1X emission, is subject to the quantum confined Stark effect. At high enough fields the sign of the BX binding energy flips, indicating repulsive interaction of the excitons This observation agrees with past results on the effect of charge separation in type-II QD heterostructures on the BX binding energy.[45] it strengthens the assertion that spectral diffusion originates from fluctuations in the local electrostatic potential. Details of the spectroSPAD system and the linear SPAD array; details of the quantum dots used in this work including synthesis, sample preparation and excitation saturation estimation; fluorescence decay lifetime by intensity state analysis; 2D lifetime-spectrum analysis; BX quantum yield estimation; heralded spectroscopy analysis and correction details; published values of BX binding energies in II−VI nanocrystals (PDF). Any bias due to this effect is mitigated here by a combination of the chip design and temporal gating (see Supporting Information Section S6), bringing the crosstalk probability down to ∼10−5, and of a statistical correction as described in ref 44
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