Date:May 1, 2025

Time: 12:45 PM to 1:45 PM

Location: PAIS, Room 2540

Speaker: 

Dr. Ursula Keller, Professor at ETH Zurich and recipient of OSA Frederic Ives Medal/Jarus W. Quinn Prize (2020) and SPIE Gold Medal (2020) recipient

Abstract:

This lecture provides an introduction to the fundamental principles of mode-locking and ultrafast laser physics, with a focus on optically pumped semiconductor lasers. We begin with the basic concepts of pulse formation in modelocked lasers, including linear pulse propagation in both time and frequency domains, supported by Fourier analysis. The Siegman-Haus theory of active modelocking is introduced, leading to the master equation that governs pulse evolution. Emphasis is placed on understanding the balance of loss modulation and gain dispersion in pulse shaping.

The second part of the lecture explores ultrafast optically pumped semiconductor lasers, particularly vertical-external-cavity surface-emitting lasers (VECSELs) and their modelocked integrated counterparts (MIXSELs). We examine their power-scaling potential, spectral flexibility from the visible to the short-wave infrared (SWIR), and the role of semiconductor saturable absorber mirrors (SESAMs) in enabling stable femtosecond pulse generation. Recent developments toward high-repetition-rate frequency combs and dual-comb spectroscopy in the 2–3 µm regime are also discussed, illustrating the pathway from fundamental theory to practical applications in precision metrology and spectroscopy.

Biography

Ursula Keller has been a tenured professor of physics at ETH Zurich since 1993 (www.ulp.ethz.ch) and also a director of the Swiss multi-institute NCCR MUST program in ultrafast science from 2010-2022 SNSF Link. She received the Ph.D. from Stanford University in 1989 and the Physics "Diplom" from ETH in 1984. She was a Member of Technical Staff (MTS) at AT&T Bell Laboratories from 1989 to 1993, a “Visiting Miller Professor” at UC Berkeley 2006 and a visiting professor at the Lund Institute of Technologies 2001. She has been a co-founder and board member for Time-Bandwidth Products (acquired by JDSU in 2014) and for a venture capital funded telecom company GigaTera (acquired by Time-Bandwidth in 2003). She is a co-founder of K2 Photonics (2023) to commercialize dual-comb lasers. She was a member of the research council of the Swiss National Science Foundation from 2014-2018. She is the founding president of the ETH Women Professors Forum (WPF).

The focus of her group (foto) in research is exploring and pushing the frontiers in ultrafast science and technology (online info). She invented the semiconductor saturable absorber mirror (SESAM) which enabled passive modelocking of diode-pumped solid-state lasers and established ultrafast solid-state lasers for science and industrial applications. She pushed the frontier of few-cycle pulse generation in the near infrared, full electric field control at close to petahertz frequencies and petahertz electronics with attosecond transient absorption spectroscopy. Pioneered frequency comb stabilization from modelocked lasers, which was also noted by the Nobel committee for Physics in 2005 Link. She enabled passively modelocked optically pumped semiconductor disk lasers (i.e. VECSELs and MIXSELs) with her SESAM technology. The SESAM technology also allowed the invention of two new methods for multiplexing a single cavity to support a pair of pulse trains with different pulse repetition rates ideally suited for dual-comb applications. In time-resolved attosecond metrology highlight achievements are the invention of the attoclock which measured the electron tunneling delay time, observed the dynamical Franz-Keldysh effect in condensed matter, and measured attosecond photoemission time delays from atoms and molecules using coincidence detection and angular resolution for the first time.