[paper] Ultrafast imaging of THz electric waveforms using quantum dots

Moritz B. Heindl, Nicholas Kirkwood, Tobias Lauster, Julia A. Lang, Markus Retsch, Paul Mulvaney and Georg Herink;

Ultrafast imaging of terahertz electric waveforms using quantum dots;

Light: Science & Applications; Vol. 11, No. 5 (2022)

DOI: 10.1038/s41377-021-00693-5

Abstract: Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz (THz) regime. However, only few imaging schemes are able to resolve sub-wavelength fields in the THz range, such as scanning-probe techniques, electro-optic sampling, and ultrafast electron microscopy. Still, intrinsic constraints on sample geometry, acquisition speed and field strength limit their applicability. Here, we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms. By capturing ultrafast movies, we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime. The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution introducing a direct route towards ultrafast field imaging of complex nanodevices in-operando.

Fig: Quantum-Probe Field Microscopy (QFIM): a.) Imaging of THz electric near-fields in a fluorescence microscope using quantum dot (QD) luminescence. The absorption of ultrashort visible sampling pulses (green) is modulated via the quantum-confined Stark effect in a layer of nanocrystals (red); b.) The THz-induced change in the QD band structure can increase the absorption and translates to enhanced luminescence emission, accessible by optical microscopy. The modulated fluorescence yield SQFIM = STHz−S0 encodes the instantaneous local electric field and snapshot images resolve the spatio-temporally evolution of the near-field waveform

Acknowledgements: We [the authors] thank J. Koehler and M. Lippitz for experimental equipment and valuable discussions. This work was funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) via project 403711541. T.L. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research program (grant agreement no. 714968). N.K. and P.M. thank the ARC for support through grant CE170100026. Open Access funding enabled and organized by Projekt DEAL.

[paper] Organic Transistor Memory Based on Black Phosphorus Quantum Dots

P. Kumari, J. Ko, V. R. Rao, S. Mhaisalkar and W. L. Leong

Non-Volatile Organic Transistor Memory Based on Black Phosphorus Quantum Dots as Charge Trapping Layer,

in IEEE Electron Device Letters, vol. 41, no. 6, pp. 852-855, June 2020

doi: 10.1109/LED.2020.2991157

Abstract: High performance organic nano-floating gate transistor memory (NFGTM) has important prerequisites of low processing temperature, solution–processable layers and charge trapping medium with high storage capacity. We demonstrate organic NFGTM using black phosphorus quantum dots (BPQDs) as a charge trapping medium by simple spin-coating and low processing temperature ( 120 °C). The BPQDs with diameter of 12.6 ± 1.5 nm and large quantum confined bandgap of ~2.9 eV possess good charge trapping ability. The organic memory device exhibits excellent memory performance with a large memory window of 61.3 V, write-read-erase-read cycling endurance of 10 3 for more than 180 cycles and reliable retention over 10,000 sec. In addition, we successfully improved the memory retention to ON/OFF current ratio 10E4 over 10,000 sec by introducing PMMA as the tunneling layer.

 

FIG: a.) Schematic of bottom gate top contact NFGTM device; b.) Band diagram explaining memory mechanism under positive gate bias 

Acknowledgement: W.L. Leong would like to acknowledge funding support from her NTU start-up grant (M4081866), Ministry of Education (MOE) under AcRF Tier 1 grant (2016-T1-002- 097), Tier 2 grant (2018-T2-1-075), ASTAR AME IAF-ICP Grant (No.I1801E0030) and A*STAR AME Young Individual Research Grant (Project No. A1784c019).