30 September 2021

Mid-IR Pulses Create Terahertz Fields That Can Directly Manipulate the optical Properties of Quantum

Research lays groundwork for high-speed optical communication systems

WASHINGTON — 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.”

About the Optica Laser Congress

The Optica Laser Congress presents a comprehensive view of the latest advances in solid-state laser development and applications. This year’s meeting will be presented in a virtual, web-conference format 03 – 07 October 2021. The five-day event includes two topical meetings – Advanced Solid State Lasers Conference (ASSL) and Laser Applications Conference (LAC). ASSL highlights new sources, advanced technologies, components, and system design to extend the operation and application of solid-state lasers. LAC focuses on materials processing and applications for high power lasers. Technical sessions and Short Courses will be presented live from the Eastern Daylight Time Zone (EDT). Learn more here.

About Optica

Optica (formerly OSA), Advancing Optics and Photonics Worldwide, is the society dedicated to promoting the generation, application, archiving and dissemination of knowledge in the field. Founded in 1916, it is the leading organization for scientists, engineers, business professionals, students and others interested in the science of light. Optica’s renowned publications, meetings, online resources and in-person activities fuel discoveries, shape real-life applications and accelerate scientific, technical and educational achievement. Discover more at: Optica.org

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