Report
Teaser, summary, work performed and final results
Periodic Reporting for period 1 - NANO-DIELECTRICS (Nanostructured dielectric platforms for electric and magnetic field-enhanced spectroscopies and nonlinear photonics with low-loss characteristics)
Teaser
It is widely known that metals, such as gold and silver, when suitably structured at the nanometre scale, are able to focus light into very small (sub-wavelength) volumes, greatly enhancing its local intensity. For this unique capability such metallic nanostructures are often...
Summary
It is widely known that metals, such as gold and silver, when suitably structured at the nanometre scale, are able to focus light into very small (sub-wavelength) volumes, greatly enhancing its local intensity. For this unique capability such metallic nanostructures are often referred to as “nanoantennasâ€. In the last decades, metallic nanoantennas have opened up a wide range of applications in numerous fields, spanning (bio)imaging and sensing to the development of optoelectronic hybrid devices. However, since the mechanism of light confinement relies on the collective oscillation of free electrons, generation of heat via Joule dissipation is inherent to the process and detrimental effects arise for several uses, such as ultrafast and nonlinear optics.
The central objective of this project was to circumvent this issue through the use of dielectric nanoantennas, which have been proposed recently as low-loss alternatives with relatively high light confinement capability. Specifically, this research focused on investigating dielectric-based resonators for high-efficiency infrared-to-visible light conversion, as well as ultrafast optical modulation, including the study of hybrid arrangements of dielectric and metallic nanostructures to combine the best of both worlds. Dielectric ultra-thin novel perovskite materials were also analysed.
The project found that a silicon nano-disk surrounded by a gold nano-ring delivers a record nonlinear light-conversion efficiency close to 0.01%. The nano-system uses a third harmonic generation process, which involves tripling the energy of the incident photons. Slightly less efficient, but still very promising, sub-100 nm thickness lead halide perovskites were also found to produce significant third harmonic emission. In addition, this investigation discovered that these structures can produce sub-20 fs all-optical switching, enabling modulation speeds over 20 THz. The results of this work could have potential impact in solar cell technology, nano-medicine, and the development of photonic and optoelectronic nano-devices.
Work performed
The project involved measurements of third-order nonlinear optical processes of nanostructures such as third harmonic generation (THG) and four wave mixing (FWM). THG coherently combines three photons of equal frequency to create a new photon of triple the incident photon frequency, while FWM mixes two or three different incident photon energies enabling multiple output wavelengths.
Numerical simulations were initially carried out to design optical nanoantennas so that they resonate at the desired wavelengths. The designed nanoantennas were then fabricated through an electron-beam lithography procedure, and studied using a tuneable (300-2600 nm wavelength) ultrafast (150 fs pulse duration) laser as the excitation source. The samples were placed on a piezoelectric stage in an optical microscope to allow controlled positioning in the three axes, and the emission was collected in reflection geometry and sent to an optical detector for intensity measurements or a spectrometer for spectral analysis.
Results showed that a nanoantenna composed by a silicon nano-disk placed in the centre of a gold nano-ring can highly concentrate the electric field of the incoming light, giving rise to a record THG conversion efficiency on the nanometre scale of 0.007%. By suitably modifying geometrical parameters of the hybrid nanoantenna, we could tune the enhanced third harmonic emission throughout the whole optical range. A similar performance of 0.006% was obtained by studying sub-100 nm thickness chemically synthesised lead halide perovskites when tuning the THG wavelength to the absorption peak of the material. By changing the chemical composition of the perovskite, the wavelength of optimal response could be tuned throughout the optical regime. Regarding FWM experiments, we found efficient emission when studying a single germanium nanodisk supporting multiple optical modes. We analysed the effect of using different excitation beams to simultaneously excite the different resonances. We found that the FWM performance peaks when the spatial overlap of the mixed wavelengths within the disk is maximised.
Ultrafast pump-probe spectroscopy measurements of silicon/gold nanoantennas were also carried out in this project. In a typical experiment, a short femtosecond optical pump pulse transfers energy to the electrons of the material, and the dynamics of the relaxation pathways is then characterised using a probe pulse as a function of pump-probe time delay and wavelength. In this work, using 7 fs laser pulses, we found positive and negative reflectivity modulations of the probe pulse of 0.3% in magnitude at either side of the nanoantenna\'s resonance, with an associated time response (full-width-at-half-maximum) that is less than 20 fs, enabling ultrafast all-optical switching.
The outcomes of this project were disseminated mainly through four peer-reviewed publications in high impact factor journals, seven invited oral presentations at international conferences and other scientific events, and three contributed talks and three poster presentations at international conferences.
Final results
This project demonstrated unprecedented coherent infrared-to-visible light conversion efficiencies on the nanometre scale through third harmonic generation using dielectric-based nanoantennas and ultra-thin films. This could potentially improve solar cell technology by helping transform non-absorbing frequencies by the cell into absorbing ones, enhancing light-to-electricity conversion efficiencies, which would contribute to European Union goals for energy and environment policies. In addition, (bio)imaging techniques could also benefit as efficient nonlinear nano-emitters would allow high-contrast imaging. Moreover, nano-medicine applications could also emerge from this work. Intracellular drug delivery, for example, could be locally activated with visible light using infrared light from outside the human body. Finally, this project obtained record optical modulation speeds using a silicon/gold nanoantenna, which could lead to the development of ultrafast photonic nano-circuitry, helping overcome conventional electronics speed limitations.