Abstract
For possible applications of colloidal nanocrystals in optoelectronics and nanophotonics, it is of high interest to study their response at low excitation intensity with high repetition rates, as switching energies in the pJ/bit to sub-pJ/bit range are targeted. We develop a sensitive pump-probe method to study the carrier dynamics in colloidal PbS/CdS quantum dots deposited on a silicon nitride waveguide after excitation by laser pulses with an average energy of few pJ/pulse. We combine an amplitude modulation of the pump pulse with phase-sensitive heterodyne detection. This approach permits to use co-linearly propagating co-polarized pulses. The method allows resolving transmission changes of the order of 10−5 and phase changes of arcseconds. We find a modulation on a sub-nanosecond time scale caused by Auger processes and biexciton decay in the quantum dots. With ground state lifetimes exceeding 1 μs, these processes become important for possible realizations of opto-electronic switching and modulation based on colloidal quantum dots emitting in the telecommunication wavelength regime.
Highlights
We have investigated the response of PbS/CdS quantum dots coupled to a silicon nitride waveguide to an optical excitation at high repetition rates
In order to resolve the small modulation in population and refractive index induced by an excitation with a frequency much higher than the ground state (GS) exciton decay rate, we developed a side-band heterodyne pump-probe approach, which allows us to detect very small changes in amplitude and phase of the probe pulse by a drift-resistant selfreferencing approach
To push the limits for the application of the PbS/CdS material system in fast telecommunications, our results suggest a route for further investigations
Summary
In small semiconductor nanocrystals or quantum dots (QDs), the optical transition energy of an exciton is determined by the dimensions of the crystallite rather than the bandgap of the bulk material.[1,2,3,4] This tunability, their long coherence lifetime,[5,6] and the relatively low cost and scalability of the wet-chemical production, i.e., vacuum-free deposition methods such as spincoating, make colloidal QDs an attractive choice as active medium in photonic applications.[7,8] Based on a variety of different material systems, light emitting devices and lasers have been fabricated using colloidal nanocrystals, recently even on chip.[9,10,11,12,13]. A competing process is efficient broadband intraband absorption, which has been observed in pump-probe experiments.[25] Recently, it has been demonstrated that the effect of intra- and interband processes cancels at a specific wavelength, making ultrafast switching operations based on nonlinear interaction possible.[19] In this contribution, we investigate the processes governing the modulation response of colloidal PbS/CdS core-shell[4,26] QDs under a low power pulsed laser excitation with a high repetition rate. A second lock-in amplifier can be used to track the sideband of the heterodyne signal that originates from the pump amplitude modulation Such usage of parallel lock-in amplifiers has been demonstrated for four-wave-mixing[42] as well as pump-probe experiments.[43] Compared to Ref. 43, where mechanical choppers are used, fast modulation by an AOM yields a greatly improved separability of the relevant frequency bands which allows us to suppress strong bands by analog filtering (this signal pre-conditioning is the first and major key to small signal detection). When using shorter pump wavelengths, we adjusted the incoupling angle such as to predominantly excite the TE00 mode for the pump pulse
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