Ultra-short and Ultra-power Lasers

Dear Colleagues and Friends: I was delighted to invite you earlier to the First Lecture of the Memorial Lecture series on 29 Jan'24. Dr. G. Ravindra Kumar, a leading experimental physicist from the Tata Institute of Fundamental Research (TIFR) in Mumbai, delivered the inaugural lecture: "Intense Light - A Tale of Two Nobel Prizes." His wonderful talk was about powerful lasers and light. Below, I summarise my thoughts and takeaways from the lecture.

He illustrated light-matter interaction and the electronic response in the form of induced dipole re-radiation. In the linear regime of oscillation, when hit by a light wave, an electron makes small displacements around some equilibrium point. He told us about additional non-linear effects, which are of great importance as discovered later, when the energy of incident light intensifies. Normal light in day-to-day life is in the range of 1 watt per cm^2.?

The photo-electric effect of light was touched upon: An incident light wave whose photon energy is important, not the photon flux, needs to be at least the ionization energy (‘work function’) in order to exhibit the photoelectric effect. (The energy difference between the incident photon energy and the emitted photon can convert to electric energy.)

Laser was discovered in 1960 giving rise to enormous power in scientific discovery and multitudinous applications including health. He said “Optics is a Power Game” where one pushes the limits of power (intensity). How do we increase this power or intensity? We focus the (same amount of) light on a smaller area (thus squeezing the space), and we pulse it by squeezing the light in time. Ah, the duality of space and time!

If we focus the ordinary light, of one cm^2 cross-section, on one mm^2 area, we will get about 100W/cm^2. On the other hand, if one constrains the light to a narrow pulse, the peak power rises greatly. A mere one Joule of energy in a 1-second pulse when squeezed into a pulse of 1 nanosecond width, we get one Gigawatt of peak power! Electronic engineers may have contributed to the idea of how to increase the power of the laser. Coherent incidences of multiple beams can combine where Fields multiply and Frequencies of emission have additive/subtractive effects due to the nonlinear nature of the interaction. The anharmonic oscillations involved give rise to lasers no more monochromatic. However, he described that using the Mode-locking technique, these multiple ‘modes’ of laser can be exploited to generate ultrashort pulses due to phase locking. ?The spectrum of a train of narrow pulses consists of equally spaced frequencies, called ‘modes’.

The 2018 Nobel in Physics recognized Art Ashkin’s seminal work in using lasers and trapping techniques to manipulate the motion of small particles and cells, and Gérard Mourou and Donna Strickland for inventing an impactful method that enables high-intensity, ultra-short optical pulses. On the other hand, the 2023 Prize in Physics was conferred upon Pierre Agostini, Ferenc Krausz, and Anne L’Huillier in recognition of their pioneering work in developing experimental techniques capable of generating attosecond pulses of light. This breakthrough has significantly advanced the study of electron dynamics in matter. It may be noted that both the years included women laureates, a rare historical occurrence for Physics. (And Dr. Strickland got the prize for her doctoral work – what a feat!).

Prof. Kumar explained the “Chirped Pulse Amplification” (CPA) principles discovered by the aforementioned laureates. He shared a peek at his Ultrashort Pulse High-Intensity Laser Laboratory in Mumbai.

Extremely energetic incident laser light, when it matches the strength of nuclear Coulomb force in atoms, can intensely ionize matter to a plasma state. ?Generation and manipulation of extreme ultraviolet (XUV) and X-ray pulses are vital for imaging ultrafast processes at the atomic and molecular scale.

In conclusion, such enormously high-power, narrow-width lasers have enabled scientific discovery in multiple areas as well as engendered practical applications in health, quantum computing, and nanomaterials. The audience too had interesting observations and questions such as the fundamental principles of duality and reversibility in laser techniques and new opportunities for research at TIFR.