Research Topics

Hyperuniformity in Photonics: Theory and Applications

Hyperuniform disordered (HuD) photonic architectures merge strong spatial correlations with structural disorder, producing isotropic photonic band gaps and high-Q localized resonances despite the absence of periodicity. First, we established theoretically that Anderson-like localized modes in HuD patterns obey photonic-crystal-like scaling laws, linking the field distribution, Q factor, and frequency to structural parameters. Building on this, we investigated HuD dielectric membranes embedded with quantum dots, taking advantage of a hyperspectral scanning near-field optical microscope (SNOM). We directly imaged the transition between extended, band-edge, and strongly localized modes with sub-wavelength resolution. The near-field studies confirmed theoretical predictions: coexistence of multiple classes of modes, scaling consistent with photonic crystals, and robustness against fabrication imperfections. By introducing a Engineered HuD cavities further reached experimental Q factors of several thousand, while maintaining favorable Q/V ratios. This combined theoretical–experimental program establishes HuD platforms as promising candidates for scalable nanophotonics, quantum emitters, and integrated photonic devices.

Selected Publications

"Photonic Crystal–Like Scaling Behavior of localized Anderson modes in Hyperuniform Disordered Systems"

N.Granchi, G. Calusi, C. Gonzini, M. Lodde, K. Stokkereit, P. .J. van Veldhoven, A. Fiore, M. Florescu and F. Intonti

Journal of the European Optical Society, accepted (2025)

"High spatial resolution imaging of light localization in hyperuniform disordered patterns of circular air pores in a dielectric slab"

N. Granchi, R. Spalding, K. Stokkereit, M. Lodde, M. Petruzzella, F. V. Otten, R. Sapienza, A. Fiore, M. Florescu and F. Intonti

Front. Photonics 4:1199411 (2023)

"Near-field imaging of optical nanocavities in hyperuniform disordered materials"

N. Granchi, M. Lodde, K. Stokkereit, R. Spalding, P. J. van Veldhoven, R. Sapienza, A. Fiore, M. Gurioli, M. Florescu and F. Intonti

Phys. Rev. B 107, 064204 (2023)

"Near-Field Investigation of Luminescent Hyperuniform Disordered Materials"

N. Granchi, R. Spalding, M. Lodde, M. Petruzzella, F. W. Otten, A. Fiore, F. Intonti, R. Sapienza, M. Florescu and M. Gurioli

Adv. Optical Mater. 2022, 10, 210256 (2022)

Non-Hermitian Photonics &

Photonic Design Optimization

One of the major breakthroughs in nanophotonics over the past decades has been the ability to tailor the spectral line shape and spatial extent of the local density of optical states (LDOS). This capability is at the core of light–matter interaction control and lies behind key phenomena such as the Purcell effect, i.e., the modification of spontaneous emission rates via the environment. The Purcell factor is governed by two central parameters of a photonic resonator: the quality factor (Q) and the modal volume (V).

While the cavity Q factor is nowadays well understood and extensively used to design and characterize resonators for applications such as all-optical signal processing, ultra-sensitive sensing, and low-threshold lasing, the modal volume V has remained more elusive. This is largely due to the absence of a consistent theoretical framework to normalize optical modes in open, leaky systems, particularly in disordered environments. Recent advances based on the non-Hermitian formalism of quasinormal modes (QNMs)—modes with complex eigenfrequencies—have profoundly reshaped the understanding of modal volume, introducing a complex-valued V whose imaginary part captures the intrinsic leakage of open systems.

Our group has experimentally validated this concept through near-field Q-perturbation experiments and through the design and characterization of coupled resonator systems exhibiting non-Lorentzian LDOS spectra, which confirm the non-Hermitian nature of realistic photonic structures.

In parallel, we have demonstrated that the QNM framework, combined with finite-element complex eigensolvers and a non-Hermitian perturbation theory for boundary deformations, constitutes a powerful and versatile toolbox for the automated optimization of the Q factor in both ordered and disordered photonic systems. We benchmarked this approach by optimizing the Q of:

• Engineered L3 photonic crystal cavities, showing that both in-plane and out-of-plane losses can be simultaneously minimized with minimal changes to the mode profile.

• Anderson-localized modes in disordered media, where we achieved a transformation of an initial low-Q mode (Q ≈ 200) into a completely different mode with a threefold reduction in modal volume and an impressive 1000-fold increase in Q.

These results open new perspectives in the inverse design of complex photonic systems, enabling robust and efficient control of light confinement down to the subwavelength scale—even in strongly disordered regimes.

Selected Publications

"Q-Factor Optimization of Modes in Ordered and Disordered Photonic Systems Using Non-Hermitian Perturbation Theory"

Nicoletta Granchi, Francesca Intonti, Marian Florescu, Pedro David García, Massimo Gurioli, Guillermo Arregui

ACS Photonics 10, 8, 2808 (2023)

“Non-Lorentzian Local Density of States in Coupled Photonic Crystal Cavities Probed by Near- and Far-Field Emission”
D. Pellegrino, D. Balestri, N. Granchi, M. Ciardi, F. Intonti, F. Pagliano, A.Y. Silov, F.W. Otten, T. Wu, K. Vynck, P. Lalanne, A. Fiore and M. Gurioli
Phys. Rev. Lett. 124, 123902 (2020)
“Mapping complex mode volumes with cavity perturbation theory”
K. G. Cognée, W. Yan, F. La China, D. Balestri, F. Intonti, M. Gurioli, A. F. Koenderink and P. Lalanne
Optica 6, 269 (2019)

Solid state dewetted dielectric ordered and disordered structures

Solid state dewetting is a natural shape instability occurring in thin solid films when heated at high temperature: it transforms a flat layer in isolated islands in a timeframe independent from the sample size. Solid state dewetting has been so far regarded as a drawback for microelectronic devices in silicon on insulator as it limits their further size reduction. However, its potential for applications based on complex pattern formation is still unexplored in spite of the manifold advantages it offers.

This research, in the field of photonics and material science, aims to develop photonic and opto-electronic devices realized by exploiting the natural instability of thin solid in order to form complex patterns and nano-architectures implemented over ultra-large scales and with cost-effective methods.

The research was financed by the European project FET Open NARCISO: “NAtuRal instability of semiConductors thIn SOlid films for sensing and photonic applications”. https://www2.mi.ifn.cnr.it/narciso/

Our group is prevalently focused on the spectroscopic investigation of nanostructures spatially distributed according both to complex disorder architectures, following a hyperuniform arrangement, and to ordered arrangements with high degree of symmetry. In the analysis of the spectroscopic results, particular emphasis will be given both at the optical characterization of the single nanostructure and at the correlation with the main morphological characteristics of the different architectures considered.

Selected Publications

"Assisted dewetting of pure Ge nanostructures for Mie-resonant all-dielectric photonics"

Sonia Freddi, Nicoletta Granchi, Michele Gherardi, Raffaele Giani, Gabriele Calusi, Camilla Gonzini, Leonardo Forcieri, Alexey Fedorov, Giovanni Isella, Francesca Intonti, Maria Antonietta Vincenti, and Monica Bollani
Optics Express 33, 18, 37509 (2025)

"Light scattering features induced by residual layers in dielectric dewetted nanoparticles"

Nicoletta Granchi, Luca Fagiani, Chiara Barri, Alexey Fedorov, Marco Abbarchi, Maria Antonietta Vincenti, Francesca Intonti, Monica Bollani

Optical Materials Express 13, 11, 3394 (2023)

"Engineering and detection of light scattering directionalities in dewetted nanoresonators through dark-field scanning microscopy"

Nicoletta Granchi, Luca Fagiani, Marco Salvalaglio, Chiara Barri, Andrea Ristori, Michele Montanari, Massimo Gurioli, Marco Abbarchi, Axel Voigt, Maria Antonietta Vincenti, Francesca Intonti, Monica Bollani

Optics Express 31, 5, 9007 (2023)

"Linear and nonlinear optical properties of dewetted SiGe islands"

Luca Fagiani, Nicoletta Granchi, Attilio Zilli, Chiara Barri, Francesco Rusconi, Michele Montanari, Erfan Mafakheri, Michele Celebrano, Mohammed Bouabdellaoui, Marco Abbarchi, Francesca Intonti, Anjam Khursheed, Paolo Biagioni, Marco Finazzi, Maria Antonietta Vincenti, Monica Bollani

Optical Materials: X, 13, 100116, (2022)

"Near-field hyper-spectral imaging of resonant Mie modes in a dielectric island"

Nicoletta Granchi, Michele Montanari, Andrea Ristori, Mario Khoury, Mohammed Bouabdellaoui, Chiara Barri, Luca Fagiani, Massimo Gurioli, Monica Bollani, Marco Abbarchi, Francesca Intonti

APL Photonics 6, 12, (2021)

“Hyperuniform Monocrystalline Structures by Spinodal Solid-State Dewetting”
Marco Salvalaglio, Mohammed Bouabdellaoui, Monica Bollani, Abdennacer Benali, Luc Favre, Jean-Benoit Claude, Jerome Wenger, Pietro de Anna, Francesca Intonti, Axel Voigt, and Marco Abbarchi
Phys. Rev. Lett. 125, 126101 (2020)
“Fabrication of spectrally sharp Si-based dielectric resonators: Combining etaloning with Mie resonances”
Dimosthenhs Toliopoulos, Mario Khoury, Mohammed Bouabdellaoui, Nicoletta Granchi, Jean-Benoît Claude, Abdennacer Benali, Isabelle Berbezier, Drisse Hannani, Antoine Ronda, Jérôme Wenger, Monica Bollani, Massimo Gurioli, Stefano Sanguinetti, Francesca Intonti, and Marco Abbarchi
Optics Express 28, 25, 37734 (2020)

Deep subwavelength imaging of confined optical fields

Tailoring the electromagnetic field at the nanoscale has led to artificial materials exhibiting fascinating optical properties unavailable in naturally occurring substances. Besides having fundamental implications for classical and quantum optics, nanoscale metamaterials provide a platform for developing disruptive novel technologies, in which a combination of both the electric and magnetic radiation field components at optical frequencies is relevant to engineer the light-matter interaction. Thus, an experimental investigation of the spatial distribution of the photonic states at the nanoscale for both field components is of crucial importance.

We developed different techniques, both in near-field and in far-field, to completely characterize the photonic modes localized in the optical cavities. Scanning-near field optical microscopy allows to image the spatial distribution of the photonic modes with a spatial resolution smaller than 100 nm. This has been done for optically active samples, collecting the photoluminescence signal, while for sample without internal light sources we combine near-field microscopy with resonant scattering. In this way the photonic modes appear as Fano resonance. In a complementary way, confocal microscopy permits to characterize also the far-field emission of nanoresonators, by mapping the angular emission pattern in k space.

We are currently investigating, with subwavelength spatial resolution, the LDOS of ordered and disordered dielectric systems as photonic crystal cavities, silicon ring resonators, dielectric Mie scatters and hyperuniform geometries, by means of scanning-near-field optical microscopy, dark-field microscopy, and finite difference time domain simulations.

Selected Publications

“Coupled Photonic Crystal Nanocavities as a Tool to Tailor and Control Photon Emission”
A. Gerardino, G. Pettinari, N. Caselli, S. Vignolini, F. Riboli, F. Biccari, M. Felici , A. Polimeni, A. Fiore, M. Gurioli and F. Intonti
Ceramics 2, 34 (2019)
“Generalized Fano lineshapes reveal exceptional points in photonic molecules”
N. Caselli, F. Intonti, F. La china, F. Biccari,F. Riboli, A. Gerardino, L.H. Li, E.H. Linfield, F. Pagliano, A. Fiore and M. Gurioli
Nature Communications 9, 396 (2018)
“Near-field speckle imaging of light localization in disordered photonic systems”
N. Caselli, F. Intonti, F. La China, F. Biccari, F. Riboli, A. Gerardino, L.H. Li, E.H. Linfield, F. Pagliano, A. Fiore and M. Gurioli
Appl. Phys. Lett. 110, 081102 (2017)
Near-Field Fano-Imaging of TE and TM Modes in Silicon Microrings
F. La China, F. Intonti, N. Caselli, F. Lotti, F. Sarti, A. Vinattieri, A. Noury, X. Le Roux, W. Zhang, E. Cassan, C. Alonso Ramos, E. Durán Valdeiglesias, N. Izard, L. Vivien, and M. Gurioli
ACS Photonics 2, 1712 (2015).
Vectorial near-field imaging of a GaN based photonic crystal cavity
F. La China, F. Intonti, N. Caselli, F. Lotti, A. Vinattieri, N. Vico Triviño, J.-F. Carlin, R. Butté, N. Grandjean and M. Gurioli
Appl. Phys. Lett. 107, 10110 (2015).
Ultra-subwavelength phase sensitive Fano-imaging of localized photonic modes
N. Caselli, F. Intonti, F. La China, F. Riboli, A. Gerardino, W. Bao, A. Weber-Bargioni, L.H. Li, E.H. Linfield, F. Pagliano, A. Fiore, and M. Gurioli
Light: Sci. Appl. 4, e326 (2015).
Deep-subwavelength imaging of both electric and magnetic localized optical fields by plasmonic campanile nanoantenna
N. Caselli, F. La China, W. Bao, M. Gurioli, F. Riboli, A. Gerardino, L.H. Li, E.H. Linfield, F. Pagliano, A. Fiore, P.J. Schuck, S. Cabrini, A. Weber-Bargioni, and F. Intonti
Scientific Reports 5, 9606 (2015).
Tailoring the Photon Hopping by Nearest-Neighbor and Next-Nearest-Neighbor Interaction in Photonic Arrays
N. Caselli, F. Riboli, F. La China, A. Gerardino, L.H. Li, E.H. Linfield, F. Pagliano, A. Fiore, F. Intonti, and M. Gurioli
ACS Photonics 2, 565 (2015).
Engineering of light confinement in strongly scattering disordered media
F. Riboli, N. Caselli, S. Vignolini, F. Intonti, K. Vynck, P. Barthelemy, A. Gerardino, L. Balet, L.H. Li, A. Fiore, M. Gurioli, and D.S.Wiersma
Nature Materials 13, 720 (2014).

Metalenses for Photovoltaic Applications

Metalenses are flat optical components composed of "meta-atoms", arrays of dielectric nanostructures, that can manipulate light at the nanoscale. Unlike conventional lenses, they are lightweight, compact, and capable of precise control of wavefronts, making them highly attractive for renewable energy technologies.

The research aims to improve the efficiency of multijunction solar cells, which require optical concentrators to direct sunlight across a broad spectral range. Traditional concentrators, such as Fresnel lenses or parabolic mirrors, are bulky and less suitable for applications where size and weight are critical, such as in space.

Metalenses, by contrast, offer the potential for much more compact and efficient designs. Using advanced numerical simulations based on the FDTD method, our group investigates how geometric parameters and phase profiles influence the performance of metalenses. The development of “relaxed” phase profiles enable higher focusing efficiency and improved light concentration across multiple wavelengths, addressing one of the main challenges in photovoltaic optics: chromatic dispersion.

Preliminary results show that optimized designs maintain higher concentration factors, over a broad spectral range, offering robustness against misalignments and enhancing overall system performance. Future work will focus on achromatic designs through phase dispersion engineering and on scaling lenses to practical sizes for real devices.

This activity is carried out in collaboration with RSE SPA – Ricerca sul Sistema Energetico.

Post-fabrication mode tuning

Nano resonators are the building blocks of many advanced optical, optoelectronic, and quantum optics devices. For most of these applications, a fundamental requisite is the design and control of the resonator modes at the target wavelengths, within an accuracy which is not directly obtainable due to the fabrication tolerances. The opportunity to control the spectral response of such nano-cavities after the growing process is a crucial issue in the progress of the field. The possibility to control and tune the optical modes, in fact, has a big impact both for the technological point of view, in order to realise devices that operate at a specific frequency, and also for fundamental physics, like the case of solid state cavity quantum electrodynamics experiments where the modes of nano-cavities have to be tuned into resonance with emission of sources.

We developed different post-fabrication processing methods, able to tune, in a reversible way, the nanocavity modes for compensating the fabrication imperfections, based on local micro-infiltration and mechanical actuation.

Selected Publications

“Multimode photonic molecules for advanced force sensing”
N. Granchi, M. Petruzzella, D. Balestri, A. Fiore, M. Gurioli and F. Intonti
Optics Express 27, 7579 (2020)
“Mechanical and Electric Control of Photonic Modes in Random Dielectrics”
D. Balestri, M. Petruzzella, S. Checcucci, F. Intonti, N. Caselli, F. Sgrignuoli, F.W.M. van Otten, A. Fiore and M. Gurioli
Advanced Materials 31, 1807274 (2019)
“Nanoscale mechanical actuation and near-field read-out of photonic crystal molecules”
M. Petruzzella, F. La China, F. Intonti, N. Caselli, M. De Pas, F. W. M. van Otten, M. Gurioli, and A. Fiore
Phys. Rev. B 94, 115413 (2016)
“Spatial steadiness of individual disorder modes upon controlled spectral tuning”
N. Caselli, F. Riboli, F. Intonti, F. La China, Francesco Biccari, A. Gerardino, and M. Gurioli
APL Photonics. 1, 041301 (2016).

Quantum light emitters

Semiconductor quantum dots (QDs), carbon nanotubes (CNTs), 2D semiconductors materials, are the basic materials for the realization of high-performance optoelectronic devices, like single photon sources, and they can be also useful as building blocks for spintronic devices and for the creation and manipulation of quantum bits.

We are currently addressing several frontier topics with QDs and CNTs, either realized with traditional methods or with site controlled fabrication techniques based on nano-photonics, by means of low-temperature confocal spectroscopy and photon coincidence measurements.

Selected Publications

“Site-Controlled Single-Photon Emitters Fabricated by Near-Field Illumination”
F. Biccari, A. Boschetti, G. Pettinari, F. La China, M. Gurioli, F. Intonti, A. Vinattieri, M. Sharma, M. Capizzi, A. Gerardino, L. Businaro, M. Hopkinson, A. Polimeni and M. Felici
Advanced Materials 30, 1705450 (2018)

“Near-field imaging of single walled carbon nanotubes emitting in the telecom wavelength range ”
F. La China, N. Caselli, F. Sarti, F. Biccari, U. Torrini, F. Intonti, A. Vinattieri, E. Durán-Valdeiglesias, C. Alonso Ramos, X. Le Roux, M. Balestrieri, A. Filoramo, L. Vivien, and M. Gurioli
Journal of Applied Physics 120, 123110 (2016).

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