Research

Non-hermitian topology

In photonics, topology represents a new approach for guiding light in a way that is robust against structural disorder. Light is confined along the edge of a corresponding material and this confinement remains for straight or zig-zag interfaces alike. Traditionally, the considered media consist of periodic photonic-crystal structures. In contrast, I produced a topological confinement in disordered media through an external-control scheme that compensates for the effect of disorder. For instance, the left figure (upper part) displays a chain of resonators with random sizes that initially has no topological order ("Non-topological"). When an optical field is either injected from the left-end or right-end side (blue and orange arrows, respectively), its intensity exponentially decays towards the centre of the system. With a control scheme (lower part), the couplings between elements are locally tuned (green arrows) to reduce the impact of disorder and develop a topological order along the chain ("Topological"). When an optical field is either injected from the left-end or right-end side, its intensity always localises to the right-end side. This ability to confine photons "on-demand" could reveal a game changer to imprint and dynamically reconfigure topological effects in photonic structures.

Nicolas Bachelard and Stefan Rotter, “Controlling topological order in random media”, in preparation.

Control of random lasers

A laser is not necessarily a sophisticated device: pumping an amplifying medium randomly filled with scatterers makes a perfectly viable "random laser". The absence of mirrors greatly simplifies laser design, but control over the emission wavelength and directionality is lost. Thus, random-laser spectra are typically both multimode and disordered (blue curve). I developed an external-control scheme, in which the pump-excitation signal is spatially shaped with a spatial light modulator (schematic). The distribution of pump intensity is iteratively adjusted to spectrally shape the emission into a singlemode spectrum (red curve). This innovative method transforms non-reproducible light sources into versatile and tunable devices and has been reproduced in different lasing structures since my original paper.

Bachelard, Nicolas, Sylvain Gigan, Xavier Noblin, and Patrick Sebbah. ”Adaptive pumping for spectral control of random lasers.” Nature Physics 10, 426–431 (2014). → Link.
Bachelard, Nicolas, Jonathan Andreasen, Sylvain Gigan, and Patrick Sebbah. ”Taming random lasers through active spatial control of the pump.” Physical Review Letters 109 (3), 033903 (2012). → Link.

Self-organisation

Material systems that reside far from thermodynamic equilibrium have the potential to exhibit properties and behaviours resembling those of living organisms. Using the forces produced by an external drive field, I dynamically organised a set of moving particles (upper part) into a non-equilibrium material characterised by a transmission bandgap (lower part). The emergent structure is bestowed with lifelike properties, such as the ability to self-heal to perturbations and adapt to sudden changes in the drive. This work presents a framework for conceiving lifelike non-equilibrium materials and emphasises the potential for the dynamic imprinting of material properties through external degrees of freedom.

Bachelard, Nicolas, Chad Ropp, Marc Dubois, Rongkuo Zhao, Yuan Wang, and Xiang Zhang. ”Emergence of an enslaved phononic bandgap in a non-equilibrium pseudo-crystal” Nature Materials 16 (8), 808-813 (2017). → Link.
Ropp, Chad, Nicolas Bachelard, David Bart, Yuan Wang, and Xiang Zhang. ”Dissipative self-organization in optical space”, Nature Photonics 12 (12), 739-743 (2018). → Link.

SupplementaryMovieM1_40fps.mp4

Optical levitation

In optical levitation, nanometer-size objects are manipulated in vacuum through optical forces to assemble opto-mechanical structures such as resonators. Decoupled from their environment, these structures can form pristine mechanical resonances that are envisioned to perform high-sensitivity measurements or even question the limits of quantum physics. Yet, owing to the difficulty of precisely adjusting light forces, levitation has been so far limited to single-element systems and simple resonators. In contrast, I used spatially modulated light field to control the optical forces exerted onto multi-element systems. Thanks to such complex light fields, I adjusted the motion of several nano-objects in parallel and demonstrated the first collective cooling of a multi-particle system (video).

Kaczvinszki, Markus, Nicolas Bachelard, Jakob Hüpfl, Michael Horodynski, Matthias Kühmayer and Stefan Rotter "Optimal Cooling of Multiple Levitated Particles through Far-Field Wavefront-Shaping." arXiv:2103.12592 (2021). → Link.

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