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Our research program focuses on the dynamic visualisation of ultrafast electronic processes of matter with light. To this end, we develop and utilise intense, ultrashort, laser pulses whose field-waveform is controlled with sub-cycle precision. Under such fields, the interaction of laser and matter becomes extremely nonlinear and offers the possibility to devise new methodologies of probing the fundamental properties of electrons in a wide range of materials, ranging from atoms to complex solids. Understanding and controlling the microcosm at the level of electrons may become essential in future photonic and electronic technologies whose building blocks' size is gradually reducing from the mesoscopic to the atomic and molecular scales.

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Zekun Pi defends PhD

Rostock, Germany: Zekun Pi defended his PhD thesis - Synthesis and vectorial characterization of ultrafast light transients

The Xtreme Photonics Labs (XPL) group is proud to announce that Zekun has successfully defended his PhD thesis, marking a major milestone in the pursuit of ever-shorter, ever-more-controllable pulses of light.

His work tackles a key limitation of ytterbium-doped potassium gadolinium tungstate (Yb:KGW) lasers: while attractive for their scalability, high repetition rates, and lower cost compared to Ti:Sapphire systems, their pulses — typically ~100 fs — have been too long for strong-field and attosecond science. Using a neon-filled hollow-core fiber, Zekun achieved spectral broadening exceeding one petahertz, compressing pulses from 170 fs down to 8 fs, and ultimately to waveform-controlled single-cycle pulses of just 3.2 fs directly from a Yb:KGW amplifier.

Extending this approach, a dual-fiber module generated a supercontinuum spanning over 3.5 octaves (170–1900 nm). Coherently synthesizing these spectral channels, Zekun produced pulses as short as 1.8 fs with carrier-envelope-phase stabilization and a 600 as intensity envelope. Using two-dimensional homochromatic attosecond streaking, he then fully characterized the temporal evolution of these vectorial light fields with attosecond resolution.

This work establishes Yb:KGW-based systems as a compact, cost-effective platform for petahertz-scale spectral synthesis and attosecond field sampling.

Congratulations, Zekun on this excellent achievement!

Santanu Mandal defends PhD

Rostock, Germany: Santanu Mandal defended his PhD thesis - Single-cycle probing of the dynamics of the insulator-to-metal phase transition in VO₂.

Santanu’s thesis pioneers the use of ultrashort, high-power femtosecond laser pulses to probe two fundamental questions in condensed matter and strong-field physics. The first part resolves the mechanism of the insulator-to-metal transition (IMT) in monoclinic M1 vanadium dioxide (VO₂). With single-cycle visible light pulses lasting just 2.4 fs, the research shows that photoexcitation first drives VO₂ into an excited rutile metallic state. Subsequent electron and lattice dynamics play out on sub-100 fs timescales, revealing that the vanadium–vanadium bond rearrangement is stepwise and highly anharmonic—contradicting theories attributing IMT to purely electronic processes.

Its second part develops a high harmonic generation (HHG) platform for solids, enabling precise, broadband detection of nonlinear optical responses. Studies across crystals like diamond, MgO, and fluorides reveal that the transition to nonperturbative nonlinear optics cannot be captured by the standard Keldysh parameter but fits better with an effective potential model.

The thesis thus provides ground-breaking, phase-resolved visualization of ultrafast phase transitions and fundamentally advances experimental methods for strong-field studies in solids, laying a robust foundation for probing correlated oxides and complex light–matter interactions in future research.

Farewell to Hee-Yong Kim: From Attoseconds to Semiconductors

After 11 years in our group, Hee-Yong is off to his next adventure at KLA Semiconductors!

He joined us as a master’s student, tackled a PhD on attosecond field emission, and then stayed on for three more years as a postdoc - driving experiments forward, solving problems no one else wanted to touch, and somehow keeping both his plants and his old laptops alive in the process.

While we’ll miss having him around, we’re excited to see what he builds (or repairs!) next at KLA. If history is any guide, it’ll work beautifully, even if it’s running on a laptop from the early 2000s. XTreme Photonics Labs wishes Hee-Yong every success in his next chapter!