FWO.103-2022.01

The general objective of this proposal is, by the enhancement of Vietnam-Belgium collaboration in research and education, to obtain fundamental understanding of plasmonic behavior in novel systems that combine top-down fabricated metamaterials with bottom-up grown nanoclusters, and of the dynamical processes following radiation absorption in these hybrid systems. The gained knowledge will be used to create novel metamaterials for ultrasensitive molecular detection.

The particular scientific objectives are therefore as following:
- Obtain fundamental understanding about the optical properties of metamaterial resonators made from nanoclusters.
- Understand the dissipation processes following radiation absorption and field enhancement in hybrid nanoclusters-metamaterials systems, and to quantify their corresponding time scales
- Utilize hot-spot engineering to improve the near-field enhancement for sensing applications based on the hybrid nanocluster-metamaterials systems.

- F. Han, T. L. Pham, K. Pilarczyk, N. T. Tung, D. H. Le, G. A. E. Vandenbosch, J. Van de Vondel, N. Verellen, X. Zheng, E. Janssens, Tunable Mid-Infrared Multi-Resonant Graphene-Metal Hybrid Metasurfaces. Adv. Optical Mater. 2024, 12, 2303085. IF 7.2, Q1 (selected for front cover).
- Matias Bejide, The Linh Pham, Amirmostafa Amirjani, Guillaume Libeert, Nils Deßmann, Thanh Tung Nguyen, Ewald Janssens, Picosecond dynamics of hot carriers in infrared plasmonic metasurfaces. Materials Advances 2025, 6, 5035. IF 4.7, Q1 (selected for front cover).
- The Linh Pham, Amirmostafa Amirjani, Kacper Pilarczyk, Fei Han, Nils Deßmann, Joris Van de Vondel, Thanh Tung Nguyen, Ewald Janssens. Ultrafast Dynamics in Strongly-Coupled Plasmonic Metasurfaces. Laser & Photonics Reviews (2026): e03113. IF 10.0, Q1.
- Nguyen Hai Anh, Pham The Linh, Nguyen Hoang Tung, Nguyen Nhat Linh, Nguyen Thi Giang, Nguyen Thi Mai, Amirmostafa Amirjani, Bui Xuan Khuyen, Vu Dinh Lam, Ewald Janssens, Nguyen Thanh Tung, Dual-plasmonic architectures: unraveling coupling dynamics between nanoparticle chains and nanoantennas. J. Phys. D: Appl. Phys. 2026, 59, 085103. IF 3.2, Q1.
- The Linh Pham, Nguyen Hai Anh, Nguyen Thi Mai, Vu Dinh Lam, Ewald Janssens, Nguyen Thanh Tung, Polarization-selective electromagnetically induced transparency in asymmetric mid-infrared plasmonic metamaterials. Communications in Physics. 2026, 36, 27. 

The Vietnam-Belgium collaboration in the field of MMs was established in 2010, when NTT pursued his PhD program at KU Leuven under the supervision of Peter Lievens (PL) and co-supervision by EJ and Prof. Margriet Van Bael. His PhD (completed in May 2014) focused on stability studies of bimetallic atomic clusters and the development of a Stern-Gerlach magnetic deflection setup. Besides the study of clusters in the gas phase, NTT was the driving force behind a collaboration on metamaterials (bilateral NAFOSTED-FWO 2011.35 project “Taming negative refractive metamaterials” with PL and Prof. Vu Dinh Lam as principal promotors). After returning to Vietnam, NTT strengthened IMS-KU Leuven collaboration, first via student exchanges (EJ as host) of Nguyen Thi Hien and Bui Son Tung (both obtained their PhD and became good researchers) and later via the project “Exploring the physical principles of electromagnetic energy harvesting in GHz and THz metamaterials” (FWO.2017.103.01) (with EJ, PL, and JVDV). This project resulted in eight ISI papers, four papers in domestic journals, two PhDs (Tran Van Huynh and Matias Bejide), and several student/scholar exchanges. PL and EJ have also been regular visitors to IMS and frequent invited speakers at the IWAMSN workshop series co-organized by NIMS Japan, IMS-VAST, and the Institute of Néel (CNRS, France). The KU Leuven promoters (EJ and JVDV) have a long-standing tradition of close and successful collaboration on joint projects, the use of shared research infrastructure, and the joint supervision of young researchers.

The project exploits the complementary expertise of the three experienced researchers who will be involved. The teams in Belgium and Vietnam have strong expertise in nanocluster fabrication (EJ and NTT), cluster deposition (EJ and JVDV), and nanofabrication techniques (JVDV and NTT). NTT is a leading expert in metamaterials research in Vietnam, while EJ has studied specific properties of metamaterials, in particular their dynamical relaxation processes. Sensing applications using SERS have been studied extensively by NTT and his colleagues at IMS. Given the complementary expertise of the teams in Belgium and Vietnam, intensive interactions are planned, including joint supervision of PhD students and research exchanges. EJ will coordinate the project on the Leuven side (project planning, take care of the dissemination of the results), and NTT will be in charge of the project at IMS-VAST. 

01 Postdoctoral position:
- Description: PhD in Physics, Chemistry, or Materials Science, with experience in designing and fabricating plasmonic nanoclusters and nanostructures to study their optical properties. Candidates with strong publications are preferred.
- Deadline: June 14, 2023
- Results: filled by Dr. Amirmostafa Amirjani at KU Leuven [more info]

02 PhD-student positions: [filled]
- Description: Master degree in Physics, Chemistry, or Materials Science, with expertise/publication in fabricating/measuring metamaterials and/or synthesizing/characterizing nanoparticles.
- Deadline: September 1, 2022
- Results: filled by  Mr. The Linh Pham at KU Leuven [more info], expected to be defended in June 2026, and Mr. Hai Anh Nguyen at IMS [more info], defended in December 2025. 

Contact: Prof. Ewald Janssens (ewald.janssens@kuleuven.be) and Prof. Nguyen Thanh Tung (tungnt@ims.vast.ac.vn)

Nanocluster production and deposition

The research group at KU Leuven has two home-built setups for the growth and deposition of nanoclusters. Both setups make use of physical techniques to create a beam of pure metallic clusters (no ligands or impurities) with control over their size distribution and composition, and that can be deposited on any solid material. The setup choice to make a particular sample will depend on the targeted particle size and flux, as well as on setup availability, considering other ongoing projects.

The first setup has a liquid nitrogen-cooled source that combines magnetron sputtering with condensation in a carrier gas. By variation of the source parameters, the size distribution of the clusters can be tuned from a few atoms up to nanoparticles with diameters of 5-8 nm. This sputter source produces a continuous beam of clusters containing a large fraction of charged particles. The setup is equipped with a high-resolution (M/deltaM=200) radio-frequency quadrupole mass filter, allowing precise mass-selection for clusters with masses up to 10000 u. Alternatively, charged larger particles can be size-selected with a quadrupole ion bender.  The clusters neutralize after deposition. The deposition (impact) energy of the clusters can be controlled by a bias voltage on the substrate, allowing impact energies down to 0.1 eV/atom, i.e., the soft-landing regime. Monitoring the beam current incident on the sample during deposition allows control of the deposited density.

The second setup has a dual-target laser ablation source, allowing for the controlled production of bimetallic clusters and small NPs (< 5 nm in diameter), in which size and composition are tunable by varying parameters such as relative power and timings of both lasers. After formation, the size distribution within the (charged particles in the) cluster beam can be analyzed by a time-of-flight mass spectrometer. The entire beam (including neutral clusters) can be deposited, in the soft-landing regime, on the substrate of interest. In this project, the substrates are the metamaterials fabricated as described under point 5.2.

Nanofabrication

State-of-the-art micro- and nanofabrication tools are available in the cleanrooms of the KU Leuven Nanocentre and the Institute of Materials Science (IMS), such as spin coaters, photolithography, optical microscopes, and oxygen plasma cleaning. A customized Raith electron beam lithography (EBL) tool at KU Leuven can be used to write metamaterial structures operating at near- and mid-IR frequencies with a resolution of 10 nm. Complementary, the cleanroom in IMS is equipped with a UV-laser lithography system (resolution of 1 um), which is suitable for the fabrication of mid-IR samples.

The metamaterials will be fabricated using a similar procedure as for CMOS devices [*Bej21b]. First, the substrates go through several cleaning steps (rinsing, exposing under UV laser light). Then, a Co-PMMA and PMMA photoresist polymer bilayer will be deposited on the substrate by spin coating. After that, the samples will be exposed to a (20 keV) electron beam or an UV laser, which writes a specific pattern (e.g. the regular array of CWs or split rings). Thereafter, the samples will be developed by submersion in isopropyl alcohol and dried in a nitrogen flow. The next step depends on the required morphology. For bulky metallic structures, we will use e-beam evaporation of the precursor material and molecular beam epitaxy (at KU Leuven), or magnetron sputtering (at IMS). For resonators made from nanoclusters, we will use the cluster deposition. For metallic resonators decorated with a thin layer of nanoclusters, we will first perform metallic layer deposition and thereafter cluster deposition. Finally, the remaining photoresist polymer will be removed by a lift-off process, revealing the final nanocluster-based metamaterial pattern. The material selection is made such that it is compatible with the electromagnetic properties in the infrared region, i.e., Al, Ag, or Au is used for the resonators and SiO2, Al2O3, MgF2, or GaAs as dielectric substrate. The quality of the materials will be inspected by scanning electron microscopy (SEM) and optical microscopy.

Optical characterization 

At KU Leuven, the transmission and reflection spectra of the samples are characterized by Fourier transform infrared (FTIR) spectroscopy in a Bruker Vertex 80V, covering the 2.5-27 m wavelength range. A FTIR Microscope Hyperion 2000 is integrated into this setup, allowing for local spectroscopy on areas of 1x1 mm². At IMS, a micro-FTIR (InfraLUM FT-08/µMAX) can be used for IR analysis in the 1.2-16 m range of small-area samples down to 100x100 µm2. A UV-Vis-NIR spectrometer (Cary 5000, Agilent) at IMS can be utilized as a complement for shorter near-IR and visible wavelength ranges. The combination of two FTIR spectrometers can cover all the considered wavelength ranges of this project.

Transient transmission studies

For nanocluster-based metamaterials operating at mid-IR frequencies, the time-resolved optical properties will be investigated by time-resolved pump-probe spectroscopy. These experiments require short (picosecond) laser pulses of high intensity and with a wavelength that can be tuned to the resonance of the materials. This light can be delivered by the Free Electron Lasers for Infrared eXperiments (FELIX) at the RU Nijmegen, the Netherlands [FEL_web], a user facility that is frequently visited by the KU Leuven team. The wavelength of FELIX is tunable from 2.7 to 150 µm. Pump-probe experiments will be performed in transmission mode with a train of single micropulses. Each micropulse is 8 ps long and has a fluence in the range of 2–12 J/m2. A stronger pulse (the pump) excites the metamaterial, and the weaker, time-delayed pulse (the probe) probes the subsequent dynamics. The transient response, reflecting the underlying relaxation dynamics, is monitored as a function of the delay time of the probe with respect to the pump. In practice, we will use an advanced scheme involving a third pulse (reference pulse), which allows for a drastic improvement of the signal-to-noise ratio by eliminating pulse-to-pulse energy fluctuations, thereby increasing the sensitivity of the technique. 

Throughout the excitation and relaxation processes, the concentration and momentum of charge carriers change, and thus also the refractive index of the metamaterial changes, which causes a transient change in the transmission spectrum. The carrier concentration and momentum evolution can be described by an impact ionization model. As the ground state electron population recovers, so too does the infrared transmission. In this manner, the transmission of the probe sheds light on the relaxation dynamics of the electrons and phonons.

For samples operating at near-IR frequencies, the synergic effects will be dynamically examined in a similar way as for the mid-IR, but using a time-resolved laser system (Chameleon Ultra II) at IMS. This laser can be continuously tuned over the 690-1080 nm spectral range and imposes extreme conditions in the samples because of the ultrashort pulses (10−15 s) and ultrahigh intensities (>1013 Wcm−2), focused onto micrometer-sized areas. During time-resolved laser-sample interactions, photons are mainly absorbed by electrons, and the subsequent energy transfer from electrons to ions and lattice motion can be investigated.  

SERS measurements

The fabricated traditional and nanocluster-based metamaterials with strong plasmon modes will be utilized as a SERS substrate for determining the concentration of regularly used insecticides for horticultural and agricultural pests (i.e. methyl parathion, dithiocarbamates, or carbendazim). Raman scattering spectroscopy will be performed by a home-built system composed of a Nd:YAG laser (Teem Photonics, 1064 nm, 2 mW at the sample) as the excitation source and a 55 cm focal length spectrometer (iHR550 Horiba) equipped with a 2-stage Peltier-cooled CCD (Synapse Horiba). The fabricated samples will have a strong plasmon resonance at 1064 nm to have the best FE. The analytes will be prepared at various low concentrations (0.01, 0.1, 1, 10, 30, to 50 ppm) before adding them to the surface of the metamaterials. Raman scattering spectra of metamaterial surfaces with various analyte concentrations will be measured and compared with a bare metal reference surface that has a 100 ppm analyte concentration.

Numerical and analytical studies

Licensed commercial software, in particular the CST Microwave Studio package (available at IMS) and the COMSOL Multiphysics package (available at KU Leuven), will be employed for electromagnetic simulations of the hybrid nanocluster-based metamaterials. The modelling makes use of the finite integration technique (FIT), including finite-difference time-domain (FDTD) and finite element methods (FEM). Those numerical methods solve Maxwell‘s equations for light-matter interaction problems and should provide identical solutions. Since the permittivity and permeability of the nanoclusters differ from those of the corresponding bulk material, the quantities should be calculated and used as input parameters to model the nanocluster resonators as an effective medium in the simulations. The modified classical Drude model can be used to calculate the dielectric function of larger (> 5nm) nanoclusters, while for smaller clusters (<5 nm), the quantum model proposed by Scholl et al. or the extended discrete interaction model proposed by Sorensen et al. will be utilized.