Harmonic Nanoparticles for Regenerative Research

In this visualized experiment, protocol details are provided for in vitro labeling of human embryonic stem cells (hESC) with second harmonic generation nanoparticles (HNPs). The latter are a new family of probes recently introduced for labeling biological samples for multi-photon imaging. HNPs are capable of doubling the frequency of excitation light by the nonlinear optical process of second harmonic generation with no restriction on the excitation wavelength. Multi-photon based methodologies for hESC differentiation into cardiac clusters (maintained as long term air-liquid cultures) are presented in detail. In particular, evidence on how to maximize the intense second harmonic (SH) emission of isolated HNPs during 3D monitoring of beating cardiac tissue in 3D is shown. The analysis of the resulting images to retrieve 3D displacement patterns is also detailed.

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

Industrial laser precision machining is predominately based on pulsed Nd:YAG lasers operated at a wavelength of 1.06 µm in a free running pulse regime. The objective of this thesis work has been to investigate second and third harmonic generation for such "long pulse" Nd:YAG lasers. Visible and UV radiation can be better focused. The absorption coefficient at these wavelengths is higher in most materials. Processing at the doubled or tripled frequency increases machining precision and allows processing of new classes of materials such as, e.g., high reflective metals or ceramics. Frequency doubling and tripling of Q-switched, modelocked, or CW operated Nd:YAG lasers has been the subject of numerous publications. However, the particular problems associated with the free running pulse regime have not yet been addressed. The generation of harmonics requires a laser source that can deliver radiation with high brightness. A system based on a slab crystal with a beam quality factor of M2 ≈ 1.6 and a power of up to 1.5 kW has been used in the experiments. An external electro-optical switch has been implemented in order to precisely control the laser pulse shape and duration. The non-linear coefficient and the angular acceptance have been the main criteria for selecting LBO (LiB3O5), KTP (KTiOPO4), and MgO:LiNbO3 for the frequency conversion experiments. The conversion efficiency has been measured for these crystals as a function of the intensity at the fundamental wavelength. In contrast to simple model predictions, the efficiency does not increase linearly with intensity under long pulse conditions. An intensity dependant phase mismatch is at the origin of the deviations. A heuristic model for the efficiency prediction has been developed. It is based on a linear increase of phase mismatch with intensity and it allows describing the experimental findings with sufficient accuracy. The conversion efficiency for frequency doubling is limited to about 20% at pulse durations of 100 µsec. It decreases with increasing pulse duration. Catastrophic thermal damage is at the origin of the limitation in this pulse regime. Our experiments indicate that the damage is initiated by the frequency doubled light. Temporary color centers are created in the presence of the second harmonic light. The dynamic of the creation and annihilation of these centers has been investigated. It could be shown that finally the absorption of the fundamental harmonic (infrared) radiation at these centers results in irreversible thermal damages. A power of 140 W in the second harmonic (178 MW/cm2 on a waist of 5 µm) and 12 W in the third harmonic (61 MW/cm2 in a waist of 2.5 µm) was obtained with pulses of 200 µs. This allowed to demonstrate the feasibility of material processing with very high reproducibility and definition, which for instance allowed the design and the manufacturing of micro-grippers and several other prototypes in the field of robotics and micro-engineering.

EPFL2001

Nonlinear plasmonics of metallic heptamers

Olivier Martin, Simon Rivier, Krishnan Thyagarajan

Second-harmonic generation (SHG) from centrosymmetric nanostructures originating from the breaking of inversion symmetry at their surfaces is a well-known phenomenon and is extensively used as a surface probe in nonlinear optical microscopy. In recent years, SHG and its subsequent enhancement using plasmonics has been observed from nanostructures such as sharp metallic tips, nanoantennae and nanodimers. However, the process is still inefficient, its mechanism not well understood, and an improvement is required. In order to achieve a higher conversion efficiency, we investigate experimentally a way to minimize the radiative losses at the fundamental frequency. In the present investigation, we use silver heptamer nanostructures and tune the subradiant mode of the Fano resonance to the fundamental of the pump source, while tuning a higher order multipolar term to the second harmonic and in the process we obtain a significant enhancement of the second harmonic signal. A detailed explanation and analysis of this is provided by considering the contribution and effect of varying different parameters, such as gap size and radius, as well as the overall symmetry of the structure. In fact, recently gold heptamers have been studied and have indeed shown strong hybridization of their constituent resonant primitive plasmonic modes, leading to new hybridized superradiant 'bright' and subradiant 'dark' modes(1, 2). The ease of fabrication and possible tunability achievable, make these structures very versatile tools for studying surface SHG in nanostructures.

Spie-Int Soc Optical Engineering, Po Box 10, Bellingham, Wa 98227-0010 Usa2012

Gd3+-Functionalized Lithium Niobate Nanoparticles for Dual Multiphoton and Magnetic Resonance Bioimaging

Luigi Bonacina, Anne-Sophie Chauvin, Raphaël Jovita De Matos, Dario Diviani, Sandrine Gerber, Adrian Stefan Gheata, Ileana Ozana Jelescu, Fiorella Lucarini, Marinella Mazzanti, Laura Camille Louise Nicolle, Katarzyna Pierzchala, Jérémy Vuilleumier

Harmonic nanoparticles (HNPs) have emerged as appealing exogenous probes for optical bioimaging due to their distinctive features such as long-term photostability and spectral flexibility, allowing multiphoton excitation in the classical (NIR-I) and extended near-infrared spectral windows (NIR-II and -III). However, like all other optical labels, HNPs are not suitable for whole-body imaging applications. In this work, we developed a bimodal nonlinear optical/magnetic resonance imaging (MRI) contrast agent through the covalent conjugation of Gd(III) chelates to coated lithium niobate HNPs. We show that the resulting nanoconjugates exert strong contrast both in T1-weighted MRI of agarose gel-based phantoms and in cancer cells by harmonic generation upon excitation in the NIR region. Their capabilities for dual T1/T2 MRI were also emphasized by the quantitative mapping of the phantom in both modes. The functionalization protocol ensured high stability of the Gd-functionalized HNPs in a physiological environment and provided a high r1 relaxivity value per NP (5.20 × 105 mM–1 s–1) while preserving their efficient nonlinear optical response.

2022

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

EPFL2001