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Exceptionally Strong THz Field Can Manipulate the Optical Properties of Quantum Dots

Using organic DAST crystals pumped with mid-IR pulses, researchers have created an exceptionally strong terahertz (THz) field that can directly drive a large change in transmission of a visible probe pulse in quantum dots. This accomplishment is an important step toward wireless communication that utilizes THz wavelengths to move large amounts of data with terabit per second speeds.

Claudia Gollner from TU Wien in Austria will present the new findings at the Optica (formerly OSA) Laser Congress virtual web conference 03 – 07 October 2021. Gollner's talk is scheduled for Monday, 04 October at 17:00 EDT (UTC – 04:00).

"The modulation speed and signal contrast in conventional electro-absorption modulators is fundamentally limited by the time scale necessary to change the electric field and the optimization of the RF electrodes, respectively," said Gollner. "Naturally, a route to overcome these physical limits is to use high frequency THz wave-forms as a driving field, to implement a Tbit/s system."

Recently, scientists have made considerable progress in converting near-IR light into THz wavelengths via an optical phenomenon known as optical rectification. This conversion process, which takes place in organic crystals, is typically driven by near-IR femtosecond pulses centered at 1.5 µm. Although THz conversion efficiencies between 1-3% with pulse energies of up to 0.9 millijoules have been reported, optical rectification is limited by multiphoton absorption and the crystal's threshold for optical damage.

In the new work, the researchers overcame this limitation by pumping an organic crystal DAST (4-N, N dimethylamino-4'-N' methylstilbazolium tosylate) with mid-IR pulses shorter than 100 femtoseconds and centered at either 3.9 µm or the second harmonic of that wavelength at 1.95 µm. Because multiphoton absorption is suppressed at these wavelengths, the researchers were able to achieve a record conversion efficiency for THz generation approaching 6% and report on a crystal damage threshold which is almost an order of magnitude higher than for 1.5 µm femtosecond pulses.

The generated THz pulses, with electric field strengths exceeding 10 MW/cm, drive electro-absorption modulation in CdSe/CdS quantum dots through the quantum confined Stark effect (QCSE), resulting in a 15% change in transmission in the visible spectral range.

"The extreme change in transmission is, to the best of our knowledge, the highest value ever reported for solution processed electro-absorption materials at room temperature," said Gollner. "We manipulate the electronic structure of quantum dots deposited onto a glass substrate using a simple drop-casting method. The fact that we can access the QCSE directly without field enhancing structures, allows us to study a large variety of samples and excludes possible artefacts from the enhancement structure. This paves the way for future developments in the areas of THz opto-electronics, fast wireless communication and THz-driven non-linear optics."


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