Colloidally synthesized nanocrystals, also termed quantum dots (QDs), have developed over the last three decades from the subject of scientific curiosity to an integral part of display technology with further forthcoming applications. This successful journey is certainly not the end of road and the rapid development of new types of structures, materials and applications raises many new fundamental questions, especially regarding the energies and dynamics of excitons – bound electron-hole pairs.
A spectroscopic technique that attempts to shed light on the nature of excitons is the measurement of the quantum confined Stark effect. This effect consists in the modification of the dynamics and energies of excitons confined in a nanoparticle by means of an electric field. Microscopically, the field shifts the center-of-mass of the electron and hole wavefunctions in opposite directions thus reducing their radiative recombination rate and shifting their energy. The magnitude and field dependence of this effect in different types of nanostructures provides valuable information about the arrangement of excitons within these structures. Recently, a new all-optical way to measure Stark effect in QDs has been introduced. The electric field is applied through a (relatively) slowly oscillating electro-magnetic wave with a THz-regime (1012 Hz) frequency, a THz pulse. Such a pulse, generated from an intense ultra-short laser pulse, can produce electric fields in the range of 0.1 MV/cm at a 1 kHz repetition rate, when tightly focused with a parabolic reflective mirror on the sample. To probe the effect of the electric field on the sample we send a second beam consisting of ultra-short (<100 fs) pulses in the visible wavelength spectral range through the sample and measure its absorption with a spectrometer, equipped with a low-noise cooled CCD. To isolate the effect of the electric field, we subtract alternating spectra measurement with and without the THz pump from one another (at 0.5 kHz repetition rate // sample frequency each). Delaying the pump respect to the probe pulses defines the electric field amplitude at which the QDs are probed. Comparing the absorption with and without the presence of the THz pump (electric field) isolates the Stark effect - the influence of the electric field on the excitons.
This technique allows us to measure the effect of very strong electric fields on different types of nanoparticles in a new frequency range (THz) and without the challenging task of integrating them into a device, a process that often alters their properties.