31 research outputs found
Ultrafast emission from colloidal nanocrystals under pulsed X-ray excitation
Fast timing has emerged as a critical requirement for radiation detection in medical and high energy physics, motivating the search for scintillator materials with high light yield and fast time response. However, light emission rates from conventional scintillation mechanisms fundamentally limit the achievable time resolution, which is presently at least one order of magnitude slower than required for next-generation detectors. One solution to this challenge is to generate an intense prompt signal in response to ionizing radiation. In this paper, we present colloidal semiconductor nanocrystals (NCs) as promising prompt photon sources. We investigate two classes of NCs: two-dimensional CdSe nanoplatelets (NPLs) and spherical CdSe/CdS core/giant shell quantum dots (GS QDs). We demonstrate that the emission rates of these NCs under pulsed X-ray excitation are much faster than traditional mechanisms in bulk scintillators, i.e. 5d-4f transitions. CdSe NPLs have a sub-100 ps effective decay time of 77 ps and CdSe/CdS GS QDs exhibit a sub-ns value of 849 ps. Further, the respective CdSe NPL and CdSe/CdS GS QD X-ray excited photoluminescence have the emission characteristics of excitons (X) and multiexcitons (MX), with the MXs providing additional prospects for fast timing with substantially shorter lifetimes
Scintillation Detectors for Charged Particles and Photons
As any radiation detector, a scintillator is an absorbing material, which has the additional property to convert into light a fraction of the energy deposited by ionizing radiation. Charged and neutral particles interact with the scintillator material through the well-known mechanisms of radiation interactions in matter described by many authors [1, 2]. Charged particles continuously interact with the electrons of the scintillator medium through Coulomb interactions, resulting in atomic excitation or ionization. Neutral particles will first have to undergo a direct interaction with the nucleus producing recoil protons or spallation fragments, which will then transfer their energy to the medium in the same way as primary charged particles
Radiative Auger process in the single-photon limit
In a multi-electron atom, an excited electron can decay by emitting a photon.
Typically, the leftover electrons are in their ground state. In a radiative
Auger process, the leftover electrons are in an excited state and a redshifted
photon is created. In a semiconductor quantum dot, radiative Auger is predicted
for charged excitons. Here we report the observation of radiative Auger on
trions in single quantum dots. For a trion, a photon is created on
electron-hole recombination, leaving behind a single electron. The radiative
Auger process promotes this additional (Auger) electron to a higher shell of
the quantum dot. We show that the radiative Auger effect is a powerful probe of
this single electron: the energy separations between the resonance fluorescence
and the radiative Auger emission directly measure the single-particle
splittings of the electronic states in the quantum dot with high precision. In
semiconductors, these single-particle splittings are otherwise hard to access
by optical means as particles are excited typically in pairs, as excitons.
After the radiative Auger emission, the Auger carrier relaxes back to the
lowest shell. Going beyond the original theoretical proposals, we show how
applying quantum optics techniques to the radiative Auger photons gives access
to the single-electron dynamics, notably relaxation and tunneling. This is also
hard to access by optical means: even for quasi-resonant -shell excitation,
electron relaxation takes place in the presence of a hole, complicating the
relaxation dynamics. The radiative Auger effect can be exploited in other
semiconductor nanostructures and quantum emitters in the solid state to
determine the energy levels and the dynamics of a single carrier