Photonics seminars

Florian Marquardt surveys approaches to reducing the energy and computational cost of neural networks by implementing their functions in physical devices. The central question is how experiments can efficiently determine updates to adjustable parameters, allowing the hardware itself to learn. Examples include learning through time reversal, obtaining nonlinear computation from linear optical scattering, and training analog quantum simulators as neuromorphic systems. The seminar is available in person and through the registration link on the organizer’s event page.

Anne L’Huillier explains how the interaction of an intense laser with atoms in a gas generates high-order harmonics and, in the time domain, trains of pulses lasting roughly one hundred attoseconds. These pulses provide a way to investigate electron motion in atoms and molecules through pump–probe measurements. The lecture traces major experimental steps from harmonic generation to the measurement of attosecond pulses, then discusses applications of the resulting techniques. It connects the production and characterisation of extremely short light pulses with the physical processes they make accessible, showing how attosecond science supports the study of electron dynamics.

Anne L’Huillier, Pierre Agostini and Ferenc Krausz explain how light pulses became short enough to resolve electron dynamics. The lectures follow the development of high-order harmonics generated when intense laser light interacts with atoms, the formation and characterisation of attosecond pulse trains, and the isolation of individual attosecond pulses. These experimental advances provide complementary ways to probe processes that occur too rapidly for earlier optical techniques. The programme connects the physical mechanisms of pulse generation with the measurement of subatomic motion and the research possibilities created by access to this timescale, covering the distinct contributions recognised by the 2023 physics prize.

This talk will present a low-cost protocol for fabricating an easily constructed femtosecond (fs) fiber laser system suitable for routine multiphoton microscopy (1060–1080 nm, 1 W average power, 70 fs pulse duration, 30–70 MHz repetition rate). Concepts well-known in the laser physics community essential to proper laser operation, but generally obscure to biophysicists and biomedical engineers, will be clarified. The parts list (~$13K US dollars), the equipment list (~$40K+), and the intellectual investment needed to build the laser will be described. A goal of the presentation will be to engage with the audience to discuss trade-offs associated with a custom-built fs fiber laser versus purchasing a commercial system. I will also touch on my research group’s plans to further develop this custom laser system for multiplexed cancer imaging as well as recent developments in the field that promise even higher performance fs fiber lasers for approximately the same cost and ease of construction.

Two-photon (2P) in vivo functional imaging of genetically encoded fluorescent Ca2+indicators (GECIs) for neuronal activity has become a broadly applied standard tool in modern neuroscience, because it allows simultaneous imaging of the activity of many neurons at high spatial resolution within living animals. Unfortunately, the most commonly used light-sources – tunable femtosecond pulsed ti:sapphire lasers – can be prohibitively expensive for many labs and fall short of delivering sufficient powers for some new ultra-fast 2P microscopy modalities. Inexpensive homebuilt or industrial light sources such as Ytterbium fiber lasers (YbFLs) show great promise to overcome these limitations as they are becoming widely available at costs orders of magnitude lower and power outputs of up to many times higher than conventional ti:sapphire lasers. However, these lasers are typically bound to emitting a single wavelength (i.e., not tunable) centered around 1020-1060 nm, which fails to efficiently excite state of the art green GECIs such as jGCaMP7 or 8. To this end, we designed and characterized spectral variants (yellow CaMP = YCaMP) of the ultrasensitive genetically encoded calcium indicator jGCaMP7, that allows for efficient 2P-excitation at wavelengths above 1010nm. In this talk I will give a brief overview over some of the reasons why using a fiber laser for 2P excitation might be right for you. I will talk about the development of jYCaMP and some exciting new experimental avenues that it has opened while touching on the prospect that shifting biosensors yellow could have for the 2P imaging community. Please join me for an interesting and fun discussion on whether “yellow is the new green” after the talk!