arXiv subjects

Ioannis Chatzakis

Publications and source records attributed to Ioannis Chatzakis.

12 recordsLinked to original sources

arXiv2026

Fr\"ohlich-type Polarons in Isotopically Enriched Hexagonal Boron Nitride

Exciton-phonon interactions play a central role in defining the optical response of hexagonal boron nitride (hBN), yet their quantitative determination has remained incomplete. Here, we reveal the Fr\"ohlich-type exciton-phonon coupling in boron-10-enriched hBN using low-temperature cathodoluminescence. We resolve the indirect exciton 5.95$\pm$0.02 eV together with its longitudinal optical (LO) phonon replica detuned by 184$\pm$56 meV, enabling the extraction of a Fr\"ohlich coupling constant $\alpha$=0.159 and a larger exciton binding energy of 161 meV, larger than previously reported values for natural-abundance hBN, which is attributed to isotope enrichment. The inferred polaron radius exceeds the lattice constant, indicating large-polaron behavior. We deduced an exciton scattering time ~of 97 fs, corresponding to a homogeneous linewidth of ~6.76 meV. We further obtain a polaron binding energy of ~48 meV and an effective mass of 1.045 $m_0$. These results provide a direct quantitative characterization of exciton-phonon coupling in isotopically engineered hBN and establish a foundation for tailoring its phonon-polaritonic and quantum-optical properties.

cond-mat.mes-hall↗

arXiv2026

Excitation-Energy-Selective Control of Hot-Carrier Cooling via a Resonant Optical-Phonon Bottleneck in Graphene

Understanding and controlling hot-carrier relaxation in graphene is crucial for advancing ultrafast optoelectronic and terahertz technologies. Here, we investigate carrier cooling dynamics in monolayer and bilayer graphene using mid-infrared pump pulses (0.22-0.73 eV) and terahertz probe pulses. We uncover a pronounced, reproducible, and non-monotonic dependence of the carrier relaxation time on excitation photon energy. Remarkably, within a narrow spectral window (0.42 to 0.48 eV), the carrier lifetime increases by an order of magnitude compared to a few picosecond-scale cooling observed at other energies. We show that this anomalous slowdown originates from a resonant enhancement of the optical-phonon lifetime, causing accumulation and reabsorption of hot optical phonons that suppress energy transfer to the lattice. All observed behaviors are captured within a unified carrier-phonon energy-balance framework, where excitation-energy-dependent variations of the effective optical-phonon decay pathway govern the cooling dynamics. These findings demonstrate excitation-energy-selective control of hot-carrier relaxation in graphene and provide new insight into non-equilibrium carrier-phonon interactions near the optical-phonon bottleneck.

cond-mat.mes-hall↗

arXiv2023

Ultrafast Hot-Carrier cooling in Quasi Freestanding Bilayer Graphene with Hydrogen Intercalated Atoms

We perform a femtosecond-THz optical pump-probe spectroscopy to investigate the cooling dynamics of hot carriers in quasi-free standing bilayer epitaxial graphene. We observed longer decay time constants, in the range of 2.6 to 6.4 ps, compared to previous studies on monolayer graphene, which increase nonlinearly with excitation intensity. The increased relaxation times are due to the decoupling of the graphene layer from the SiC substrate after hydrogen intercalation which increases the distance between graphene and substrate. Furthermore, our measurements do not show that the supercollision mechanism is related to the cooling process of the hot carriers, which is ultimately achieved by electron-optical phonon scattering.

cond-mat.mes-hall↗

arXiv2018

Strong Confinment of optical fields using localised surface phonon polaritons in cubic Boron Nitride

Phonon polaritons (PhPs) are long-lived electromagnetic modes that originate from the coupling of infrared photons with the bound ionic lattice of a polar crystal. Cubic-Boron nitride (cBN) is such a polar, semiconductor material, which due to the light atomic masses can support high frequency optical phonons. Here, we report on random arrays of cBN nanostructures fabricated via an unpatterned reactive ion etching process. FTIR reflection spectra suggest the presence of localized surface PhPs within the Reststrahlen band, with quality factors in excess of 38 observed. These can provide the basis of next generation infrared optical components like antennas for communication, improved chemical spectroscopies, and enhanced emitters, sources and detectors.

physics.app-ph↗

arXiv2018

Controlling the Infrared Dielectric Function through Atomic-Scale Heterostructures

Surface phonon polaritons (SPhPs) - the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons - offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. Here, we present a novel approach to overcome the major limitation of SPhPs, namely the narrow, material-specific spectral range where SPhPs can be supported, called the Reststrahlen band. We use an atomic-scale superlattice (SL) of two polar semiconductors, GaN and AlN, to create a hybrid material featuring layer thickness-tunable optic phonon modes. As the IR dielectric function is governed by the optic phonon behavior, such control provides a means to create a new dielectric function distinct from either constituent material and to tune the range over which SPhPs can be supported. This work offers the first glimpse of the guiding principles governing the degree to which the dielectric function can be designed using this approach.

cond-mat.mtrl-sci↗

arXiv2017

Ultra-low-loss Polaritons in Isotopically Pure Materials: A New Approach

Conventional optical components are limited to size-scales much larger than the wavelength of light, as changes in the amplitude, phase and polarization of the electromagnetic fields are accrued gradually along an optical path. However, advances in nanophotonics have produced ultra-thin, co-called "flat" optical components that beget abrupt changes in these properties over distances significantly shorter than the free space wavelength. While high optical losses still plague many approaches, phonon polariton (PhP) materials have demonstrated long lifetimes for sub-diffractional modes in comparison to plasmon-polariton-based nanophotonics. We experimentally observe a three-fold improvement in polariton lifetime through isotopic enrichment of hexagonal boron nitride (hBN). Commensurate increases in the polariton propagation length are demonstrated via direct imaging of polaritonic standing waves by means of infrared nano-optics. Our results provide the foundation for a materials-growth-directed approach towards realizing the loss control necessary for the development of PhP-based nanophotonic devices.

cond-mat.mtrl-sci↗

arXiv2017

Broadband Terahertz Modulation in Electrostatically-doped Artificial Trilayer Graphene

We report a terahertz optical modulator consisting of randomly stacked trilayer graphene (TLG) deposited on an oxidized silicon substrate by means of THz- Time Domain Spectroscopy (THz-TDS). Here, the gate tuning of the Fermi level of the TLG provides the fundamental basis for the modulation of THz transmission. We measured a 15% change in the THz transmission of this device over a broad frequency range (0.6-1.6THz). We also observed a strong absorption >80% in the time-domain signals and a frequency independence of the conductivity. Furthermore, unlike previous studies, we find that the underlying silicon substrate, which serves as a gate electrode for the graphene, also exhibits substantial modulation of the transmitted THz radiation under applied voltage biases.

cond-mat.mtrl-sci↗

arXiv2015

Ultrafast terahertz probes of interacting dark excitons in chirality-specific semiconducting single-walled carbon nanotubes

Ultrafast terahertz spectroscopy accesses the {\em dark} excitonic ground state in resonantly-excited (6,5) SWNTs via internal, direct dipole-allowed transitions between lowest lying dark-bright pair state $\sim$6 meV. An analytical model reproduces the response which enables quantitative analysis of transient densities of dark excitons and {\em e-h} plasma, oscillator strength, transition energy renormalization and dynamics. %excitation-induced renormalization. Non-equilibrium, yet stable, quasi-1D quantum states with dark excitonic correlations rapidly emerge even with increasing off-resonance photoexcitation and experience a unique crossover to complex phase-space filling of %a complex distribution between both dark and bright pair states, different from dense 2D/3D excitons influenced by the thermalization, cooling and ionization to free carriers.

cond-mat.mes-hall↗

arXiv2014

One- and two-dimensional photo-imprinted diffraction gratings for manipulating terahertz waves

Emerging technology based on artificial materials containing metallic structures has raised the prospect for unprecedented control of terahertz waves through components like filters, absorbers and polarizers. The functionality of these devices is static by the very nature of their metallic or polaritonic composition, although some degree of tunability can be achieved by incorporating electrically biased semiconductors. Here, we demonstrate a photonic structure by projecting the optical image of a metal mask onto a thin GaAs substrate using a femtosecond pulsed laser source. We show that the resulting high-contrast pattern of photo- excited carriers can create diffractive elements operating in transmission. With the metal mask replaced by a digital micromirror device, our photo-imprinted photonic structures provide a route to terahertz components with reconfigurable functionality.

physics.optics↗

arXiv2014

Broadband terahertz generation from metamaterials

The terahertz spectral regime, ranging from about 0.1 to 15 THz, is one of the least explored yet most technologically transformative spectral regions. One current challenge is to develop efficient and compact terahertz emitters/detectors with a broadband and gapless spectrum that can be tailored for various pump photon energies. Here we demonstrate efficient single-cycle broadband THz generation, ranging from about 0.1 to 4 THz, from a thin layer of split-ring resonators with few tens of nanometers thickness by pumping at the telecommunications wavelength of 1.5 micrometer (200 THz). The terahertz emission arises from exciting the magnetic-dipole resonance of the split-ring resonators and quickly decreases under off-resonance pumping. This, together with pump polarization dependence and power scaling of the terahertz emission, identifies the role of optically induced nonlinear currents in split-ring resonators. We also reveal a giant sheet nonlinear susceptibility $\sim$10$^{-16}$ m$^2$V$^{-1}$ that far exceeds thin films and bulk non-centrosymmetric materials.

cond-mat.mes-hall↗

arXiv2013

Electron temperature dependence of the electron-phonon coupling strength in double-wall carbon nanotubes

We applied Time-Resolved Two-Photon Photoemission spectroscopy to probe the electron- phonon (e-ph) coupling strength in double-wall carbon nanotubes. The e-ph energy transfer rate G(Te,Tl) from the electronic system to the lattice depends linearly on the electron (Te) and lattice (Tl) temperatures for Te>ΘDebye. Moreover, we numerically solved the Two-Temperature Model. We found: (i) a Te decay with a 3.5 ps time constant and no significant change in Tl; (ii) an e-ph coupling factor of 2x10^16 W/m3; (iii) a mass-enhancement parameter, λ, of (5.4=+/- 0.9)x10e-4; and (iv) a decay time of the electron energy density to the lattice of 1.34+/-0.85 ps.

cond-mat.mtrl-sci↗

arXiv2011

Temperature dependence of the anharmonic decay of optical phonons in carbon nanotubes and graphite

We report on the temperature dependence of the anharmonic decay rate of zone-center (G mode) optical phonons in both single-walled carbon nanotubes and graphite. The measurements are performed using a pump-probe Raman scattering scheme with femtosecond laser pulses. For nanotubes, measured over a temperature range of 6 K-700 K, we observe little temperature dependence of the decay rate below room temperature. Above 300 K, the decay rate increases from 0.8 to 1.7 ps-1. The decay rates observed for graphite range from 0.5 to 0.8 ps-1 for temperatures from 300 K-700 K. We compare the behavior observed in carbon nanotubes and graphite and discuss the implications of our results for the mechanism of the anharmonic decay of optical phonons in both systems.

cond-mat.mtrl-sci↗