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Dynamics of femtosecond laser ablation of trapped dielectric and metallic nanoparticles

Periodic Reporting for period 1 - ADMEP (Dynamics of femtosecond laser ablation of trapped dielectric and metallic nanoparticles)

Reporting period: 2016-05-01 to 2018-04-30

In many applications in research, industry and healthcare, pulsed lasers are used to cut, remove or deposit material. In even more extreme examples, pulsed lasers are used to trigger nuclear fusion or more practically, to generate EUV light for a new generation of lithography machines. What all the above processes have in common is that so much energy is deposited in the target that its optical properties change during an individual laser pulse. Thus, to predict/optimize the energy absorption in this regime, one needs to understand the complex interplay between the laser and the dynamically changing material.
The ADMEP project aims to theoretically and experimentally study the dynamics of the material properties in nano- to micro-scale particles and their influence on the optical properties upon irradiation with femtosecond (1 fs = 10-15 s) laser pulses. By using fs-laser pulses, we can ensure that the shape of the particle does not change during the pulse interaction, allowing us to focus on studying mainly the carrier dynamics of the particle. To isolate the effects of the dynamics of the carrier density and temperature from the effects of their spatial inhomogeneity, we perform experiments on small spherical nanoparticles. For small enough spheres, the transient material properties can be assumed to remain homogeneous inside the particles. Hence, they are the ideal platform to investigate the transient material properties while interacting with fs-laser pulses.

The objectives of the project involve the study of nanoparticles in three different scenarios in order to address specific questions:

• Influence of the laser-induced carrier density on the scattering cross-section of small spherical nanoparticles under ablation conditions.
• Scattering and absorption by microparticles under ablation conditions.
• Plasma dynamics of trapped nanoparticles upon fs-laser irradiation.

Main outputs:

• We have designed and built a working experimental apparatus that allows us to study the interaction dynamics of fs-laser pulses with levitating nanoparticles. In this way we have studied the ultrafast optical response of single gold nanoparticles upon fs-laser irradiation and its laser-induced evaporation.

• We have developed a method that combines a theoretical model with a numerical algorithm that successfully predicts the dynamics of the energy deposition of femtosecond laser pulses in dielectrics under tight focusing conditions. We have benchmarked this method by studying the transient optical properties of a laser-induced electron plasma micro-disk in water.
Within this project, we have studied the interaction of ultrashort laser pulses with nanoparticles and microparticles, both from an experimental and a theoretical framework. Two main platforms have been used to investigate this problem and achieve the goals previously listed.

The first platform used in the project consists of a quadrupole ion trap to study the interaction of ultrashort laser pulses with trapped single gold nanoparticles. We first use the electrospray technique to introduce gold nanoparticles with a radius of 50 nm inside the trap. Then, we individually illuminate the nanoparticles using an infra-red fs-laser pulse. In this way the electrons in the conduction band gain kinetic energy through the interaction with the laser, which results in a temperature increase that modifies the optical properties of the excited particle. At a precise time after the excitation, we send a second laser pulse with a different wavelength, which is chosen to be particularly sensitive to small changes of the optical properties of the nanoparticle. Using the experimental setup, we study the laser induced evaporation of trapped nanoparticles. We observe an overall decrease of the scattering intensity as the nanoparticle size decreases due to the loss of mass.

Second, we studied the interaction of ultra-violet femtosecond laser pulses with a micron-sized electron plasma disc at a water/air interface. We used an infra-red laser pulse with a duration of 150 fs to generate the disc of excited electrons with a radius of 1.5 μm and a thickness of 75 nm. The electron density and temperature change as the excitation pulse propagates inside the water surface, thus the optical properties of the disc also vary in time. We experimentally measured the reflectivity of the excited water surface at different time delays during and after the excitation. To model the propagation and absorption of light we combined a FDTD numerical routine with the equations that govern the laser-water interaction (i.e. multiple rate equations). The FDTD algorithm stands for finite difference time domain and aids to solve the Maxwell’s equations and thus to simulate the propagation of the laser while the optical properties of water are periodically updated due to the excitation of an electron plasma. The model accurately predicts the dynamics of the system, which ultimately provides crucial information about the laser energy deposited into the water molecules.
The combination of the presented experimental platforms with our theoretical models and numerical techniques allow researchers to easily test new theories and more importantly to be able to predict the behaviour of single isolated nanoparticles upon extreme conditions. Undoubtedly, the use of this approach will find its way into a number of applications that include for instance the study of nanoparticle synthesis via ablation of immersed targets, extreme ultra-violet nanolithography and laser-based cell and neuro surgery.
Ultrafast dynamics of the interaction of fs-laser pulses with trapped nanoparticles inside a QIT.