arXiv subjects

Luca Perfetti

Publications and source records attributed to Luca Perfetti.

16 recordsLinked to original sources

arXiv2025

Plasmonic metamaterial time crystal

Periodically driven optical materials and metamaterials have recently emerged as a promising platform for realizing photonic time crystals (PTCs), which are systems whose optical properties are strongly and periodically modulated on timescales comparable to the optical cycle of light. These time-varying structures are the temporal counterparts of spatial photonic crystals (SPCs), for which a large and periodic dielectric contrast is achieved spatially on wavelength scales. Just as SPCs have revolutionized control over light-matter interactions by engineering the photonic density of states in space, PTCs promise comparable breakthroughs from a fundamentally new perspective: a temporal one. However, harnessing such phenomena all-optically poses severe experimental challenges, as it requires order-unity modulation depths of a material's optical properties on ultrafast timescales comparable to the light cycle, a regime that has remained elusive to date. Here, we demonstrate the first all-optical realization of a photonic time crystal, achieved with a surface plasmon cavity metamaterial operating at Terahertz (THz) frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the carriers' kinetic energy and effective mass - reaching up to 80% of their rest mass. Our spectroscopic measurements reveal a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% and we predict plasmonic lasing to be within experimental reach. These results pave the way for temporal engineering of losses and light-matter interactions in plasmonic systems, and establish a robust new platform for time-domain photonics.

physics.optics↗

arXiv2025

THz-Driven Coherent Phonon Fingerprints of Hidden Symmetry Breaking in 2D Layered Hybrid Perovskites

Metal-halide perovskites (MHPs) emerged as a family of novel semiconductors with outstanding optoelectronic properties for applications in photovoltaics and light emission. Recently, they also attract interest as promising candidates for spintronics. In materials lacking inversion symmetry, spin-orbit coupling (SOC) leads to the Rashba-Dresselhaus effect, offering a pathway for spin current control. Therefore, inversion symmetry breaking in MHPs, which are characterized by strong SOC, has crucial implications. Yet, in complex low-dimensional hybrid organic-inorganic perovskites (HOIPs), the presence of and structural contributions to inversion symmetry breaking remain elusive. Here, employing intense THz fields, we coherently drive lattice dynamics carrying spectroscopic fingerprints of inversion symmetry breaking in Ruddlesden-Popper (PEA)$_2$(MA)$_{n-1}$PbnI${3n+1}$ perovskites, which are globally assigned to a centrosymmetric space group. We demonstrate coherent control by THz pulses over specific phonons, which we assign to either purely inorganic or highly anharmonic hybrid cage-ligand vibrations. By developing a general polarization analysis for THz-driven phonons, we pinpoint linear and nonlinear driving mechanisms. From this, we identify simultaneous IR- and Raman-activity of inorganic cage modes below 1.5 THz, indicating mode-selective inversion symmetry breaking. By exploring the driving pathways of these coherent phonons, we lay the groundwork for simultaneous ultrafast control of optoelectronic and spintronic properties in 2D HOIPs.

cond-mat.mtrl-sci↗

arXiv2024

Chiral TeraHertz surface plasmonics

Chiral engineering of TeraHertz (THz) light fields and the use of the handedness of light in THz light-matter interactions promise many novel opportunities for advanced sensing and control of matter in this frequency range. Unlike previously explored methods, this is achieved here by leveraging the chiral properties of highly confined THz surface plasmon modes. More specifically, we design ultrasmall surface plasmonic-based THz cavities and THz metasurfaces that display significant and adjustable chiral behavior under modest magnetic fields (B<500mT). For such a prototypical example of non-hermitian and dispersive photonic system, we demonstrate the capacity to magnetic field-tune both the poles and zeros of cavity resonances, the two fundamental parameters governing their resonance properties. Alongside the observed handedness-dependent cavity frequencies, this highlights the remarkable ability to engineer chiral and tunable radiative couplings for THz resonators and metasurfaces. The extensive tunability offered by the surface plasmonic approach paves the way for the development of agile and multifunctional THz metasurfaces as well as the realization of ultrastrong chiral light-matter interactions at low energy in matter with potential far-reaching applications for the design of material properties.

physics.optics↗

arXiv2024

Direct observation of electronic band gap and hot carrier dynamics in GeAs semiconductor

Germanium arsenide (GeAs) is a layered semiconductor with remarkably anisotropic physical, thermoelectric and optical properties, and a promising candidate for multifunctional devices based on in-plane polarization dependent response. Understanding the underlying mechanism of such devices requires the knowledge of GeAs electronic band structure and of the hot carrier dynamics in its conduction band, whose details are still unclear. In this work, we investigated the properties of occupied and photoexcited states of GeAs in energy-momentum space, by combining scanning tunneling spectroscopy (STS), angle-resolved photoemission spectroscopy (ARPES) and time-resolved ARPES. We found that, GeAs is an indirect gap semiconductor having an electronic gap of 0.8 eV, for which the conduction band minimum (CBM) is located at the Gamma point while the valence band maximum (VBM) is out of Gamma. A Stark broadening of the valence band is observed immediately after photoexcitation, which can be attributed to the effects of the electrical field at the surface induced by inhomogeneous screening. Moreover, the hot electrons relaxation time of 1.56 ps down to the CBM which is dominated from both inter-valley and intra-valley coupling. Besides their relevance for our understanding of GeAs, these findings present general interest for the design on high performance thermoelectric and optoelectronic devices based on 2D semiconductors.

cond-mat.mtrl-sci↗

arXiv2023

Dynamics of electronic states in the insulating Intermediate surface phase of 1T-TaS$_2$

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide 1T-TaS$_2$. When heating the sample, the surface displays an intermediate insulating phase that persists for $\sim 10$ K on top of a metallic bulk. The weaker screening of Coulomb repulsion and stiffer Charge Density Wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time resolved ARPES and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low temperature phase and weaker in the intermediate one.

cond-mat.str-el↗

arXiv2023

Nonlinear THz Control of the Lead Halide Perovskite Lattice

Lead halide perovskites (LHPs) have emerged as an excellent class of semiconductors for next-generation solar cells and optoelectronic devices. Tailoring physical properties by fine-tuning the lattice structures has been explored in these materials by chemical composition or morphology. Nevertheless, its dynamic counterpart, phonon-driven ultrafast material control, as contemporarily harnessed for oxide perovskites, has not been established yet. Here we employ intense THz electric fields to obtain direct lattice control via nonlinear excitation of coherent octahedral twist modes in hybrid CH3NH3PbBr3 and all-inorganic CsPbBr3 perovskites. These Raman-active phonons at 0.9 - 1.3 THz are found to govern the ultrafast THz-induced Kerr effect in the low-temperature orthorhombic phase and thus dominate the phonon-modulated polarizability with potential implications for dynamic charge carrier screening beyond the Froehlich polaron. Our work opens the door to selective control of LHP's vibrational degrees of freedom governing phase transitions and dynamic disorder.

cond-mat.mtrl-sci↗

arXiv2022

Electronic dispersion, correlations and stacking in the photoexcited state of 1T-TaS$_2$

Here we perform angle and time-resolved photoelectron spectroscopy on the commensurate Charge Density Wave (CDW) phase of 1T-TaS$_2$. Data with different probe pulse polarization are employed to map the dispersion of electronic states below and above the chemical potential. Upon photoexcitation, the fluctuations of CDW order erase the band dispersion near to the chemical potential and halve the charge gap size. This transient phase sets within half a period of the coherent lattice motion and is favored by strong electronic correlations. The experimental results are compared to Density-Functional Theory (DFT) calculations with a self-consistent evaluation of the Coulomb repulsion. Our simulations indicate that the screening of Coulomb repulsion depends on the stacking order of the TaS$_2$ layers. The entanglement of such degrees of freedom suggest that both the structural order and electronic repulsion are locally modified by the photoinduced CDW fluctuations.

cond-mat.str-el↗

arXiv2021

Spin injection efficiency at metallic interfaces probed by THz emission spectroscopy

Terahertz (THz) spin-to-charge conversion has become an increasingly important process for THz pulse generation and as a tool to probe ultrafast spin interactions at magnetic interfaces. However, its relation to traditional, steady state, ferromagnetic resonance techniques is poorly understood. Here we investigate nanometric trilayers of Co/X/Pt (X=Ti, Au or Au0:85W0:15) as a function of the 'X' layer thickness, where THz emission generated by the inverse spin Hall effect is compared to the Gilbert damping of the ferromagnetic resonance. Through the insertion of the 'X' layer we show that the ultrafast spin current injected in the non-magnetic layer defines a direct spin conductance, whereas the Gilbert damping leads to an effective spin mixing-conductance of the trilayer. Importantly, we show that these two parameters are connected to each other and that spin-memory losses can be modeled via an effective Hamiltonian with Rashba fields. This work highlights that magneto-circuits concepts can be successfully extended to ultrafast spintronic devices, as well as enhancing the understanding of spin-to-charge conversion processes through the complementarity between ultrafast THz spectroscopy and steady state techniques.

cond-mat.mes-hall↗

arXiv2019

Photoinduced renormalization of Dirac states in BaNiS$_2$

By means of pump-probe time- and angle-resolved photoelectron spectroscopy, we provide evidence of a sizeable reduction of the Fermi velocity of out-of-equilibrium Dirac bands in the quasi-two-dimensional semimetal BaNiS$_2$. First-principle calculations indicate that this band renormalization is ascribed to a change in non-local electron correlations driven by a photo-induced enhancement of screening properties. This effect is accompanied by a slowing down of the Dirac fermions and by a non-rigid shift of the bands at the center of the Brillouin zone. This result suggests that other similar electronic structure renormalizations may be photoinduced in other materials in presence of strong non-local correlations.

cond-mat.mtrl-sci↗

arXiv2019

Evidence of Direct Electronic Band Gap in two-dimensional van der Waals Indium Selenide crystals

Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Metal mono-chalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers and stacking-order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photo-response. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy (2PPE), we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Gamma point of the Brillouin zone (BZ). STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction band minimum (CBM) and a very sharp one near the maximum of the valence band (VMB). This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle resolved photoemission spectroscopy (ARPES) investigation. technologies. In fact, a hole effective mass of about m/m0 = -0.95 gammaK direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic

cond-mat.mes-hall↗

arXiv2018

Ultrafast electrons dynamics reveal the high potential of InSe for hot carrier optoelectronics

We monitor the dynamics of hot carriers in InSe by means of two photons photoelectron spectroscopy (2PPE). The electrons excited by photons of 3.12 eV experience a manifold relaxation. First, they thermalize to the electronic states degenerate with the $\bar M$ valley. Subsequently, the electronic cooling is dictated by Fr\"ohlich coupling with phonons of small momentum transfer. Ab-initio calculations predict cooling rates that are in good agreement with the observed dynamics. We argue that electrons accumulating in states degenerate with the $\bar M$ valley could travel through a multilayer flake of InSe with lateral size of 1 micrometer. The hot carriers pave a viable route to the realization of below-bandgap photodiodes and Gunn oscillators. Our results indicate that these technologies may find a natural implementation in future devices based on layered chalcogenides.

cond-mat.mtrl-sci↗

arXiv2018

Non-local Coulomb correlations in pure and electron-doped ${\mathrm{Sr}}_{2}{\mathrm{IrO}}_{4}$: spectral functions, Fermi surface and pseudogap-like spectral weight distributions from oriented cluster dynamical mean field theory

We address the role of non-local Coulomb correlations and short-range magnetic fluctuations in the high-temperature phase of Sr$_2$IrO$_4$ within state-of-the-art spectroscopic and first-principles theoretical methods. Introducing a novel cluster dynamical mean field scheme, we compute momentum-resolved spectral functions, which we find to be in excellent agreement with angle-resolved photoemission spectra. We show that while short-range antiferromagnetic fluctuations are crucial to account for the electronic properties of the material even in the high-temperature paramagnetic phase, long-range magnetic order is not a necessary ingredient of the insulating state. Upon doping, an exotic metallic state is generated, exhibiting cuprate-like pseudo-gap spectral properties, for which we propose a surprisingly simple theoretical mechanism.

cond-mat.str-el↗

arXiv2017

Time resolved photoemission spectroscopy of electronic cooling and localization in CH$_3$NH$_3$PbI$_3$ crystals

We measure the surface of CH$_3$NH$_3$PbI$_3$ single crystals by making use of two photon photoemission spectroscopy. Our method monitors the electronic distribution of photoexcited electrons, explicitly discriminating the initial thermalization from slower dynamical processes. The reported results disclose the fast dissipation channels of hot carriers (0.25 ps), set a upper bound to the surface induced recombination velocity ($<4000$ cm/s) and reveal the dramatic effect of shallow traps on the electrons dynamics. The picosecond localization of excited electrons in degraded CH$_3$NH$_3$PbI$_3$ samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. Minimizing the density of shallow traps and solving the aging problem may boost the macroscopic efficiency of solar cells to the theoretical limit.

cond-mat.mtrl-sci↗

arXiv2016

Stable topological insulators achieved using high energy electron beams

Topological insulators are transformative quantum solids with immune-to-disorder metallic surface states having Dirac band structure. Ubiquitous charged bulk defects, however, pull the Fermi energy into the bulk bands, denying access to surface charge transport. Here we demonstrate that irradiation with swift ($\sim 2.5$ MeV energy) electron beams allows to compensate these defects, bring the Fermi level back into the bulk gap, and reach the charge neutrality point (CNP). Controlling the beam fluence we tune bulk conductivity from \textit{p}- (hole-like) to \textit{n}-type (electron-like), crossing the Dirac point and back, while preserving the Dirac energy dispersion. The CNP conductance has a two-dimensional (2D) character on the order of ten conductance quanta $G_0 =e^2/h$, and reveals, both in Bi$_2$Te$_3$ and Bi$_2$Se$_3$, the presence of only two quantum channels corresponding to two topological surfaces. The intrinsic quantum transport of the topological states is accessible disregarding the bulk size.

cond-mat.mtrl-sci↗

arXiv2015

Ultrafast photocurrents at the surface of the three-dimensional topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$

Topological insulators constitute a new and fascinating class of matter with insulating bulk yet metallic surfaces that host highly mobile charge carriers with spin-momentum locking. Remarkably, the direction and magnitude of surface currents can be controlled with tailored light beams, but the underlying mechanisms are not yet well understood. To directly resolve the "birth" of such photocurrents we need to boost the time resolution to the scale of elementary scattering events ($\sim$ 10 fs). Here, we excite and measure photocurrents in the three-dimensional model topological insulator $\mathrm{Bi}_2\mathrm{Se}_3$ with a time resolution as short as 20 fs by sampling the concomitantly emitted broadband THz electromagnetic field from 1 to 40 THz. Remarkably, the ultrafast surface current response is dominated by a charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to have orders of magnitude smaller magnitude than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are also of direct relevance for optoelectronic devices based on topological-insulator surface currents.

cond-mat.mtrl-sci↗

arXiv2013

Non-trivial Surface-band Dispersion on Bi(111)

We performed angle-resolved photoelectron spectroscopy of the Bi(111) surface to demonstrate that this surface support edge states of non-trivial topology. Along the $\barΓ\bar{M}$-direction of the surface Brillouin zone, a surface-state band disperses from the projected bulk valence bands at $\barΓ$ to the conduction bands at $\bar{M}$ continuously, indicating the non-trivial topological order of three-dimensional Bi bands. We ascribe this finding to the absence of band inversion at the $L$ point of the bulk Bi Brillouin zone. According to our analysis, a modification of tight-binding parameters can account for the non-trivial band structure of Bi without any other significant change on other physical properties.

cond-mat.mtrl-sci↗