120 research outputs found
Optoelectronic properties of hexagonal boron nitride
École thématiqueI will discuss here our results on phonon-assisted recombination in hBN, from bulk crystals to few-layer samples. First, I will focus on hBN crystals where the combination of isotopic purification, Raman scattering and polarized-resolved photoluminescence allowed us to identify the different phonon modes involved in the efficient phonon-assisted recombination in bulk hBN. In a second part, I will discuss photoluminescence experiments in epilayers grown by high-temperature molecular beam epitaxy. These epilayers have a thickness of few monolayers of hBN, and I will show that phonon-assisted recombination displays a distinct phenomenology with a PL spectrum different from the one of bulk crystals
Ultra-coherent single photon source
We present a novel type of single photon source in solid state, based on the
coherent laser light scattering by a single InAs quantum dot. We demonstrate
that the coherence of the emitted single photons is tailored by the resonant
excitation with a spectral linewidth below the radiative limit. Our
ultra-coherent source opens the way for integrated quantum devices dedicated to
the generation of single photons with high degrees of indistinguishability
Spontaneous emission of color centers at 4eV in hexagonal boron nitride under hydrostatic pressure
The light emission properties of color centers emitting in 3.3-4 eV region
are investigated for hydrostatic pressures ranging up to 5GPa at liquid helium
temperature. The light emission energy decreases with pressure less sensitively
than the bandgap. This behavior at variance from the shift of the bandgap is
typical of deep traps. Interestingly, hydrostatic pressure reveals the
existence of levels that vary differently under pressure (smaller increase of
the emission wavelength compared to the rest of the levels in this energy
region or even decrease of it) with pressure. This discovery enriches the
physics of the color centers operating in the UV in hBN.Comment: 16 pages, 3 figure
Dephasing processes in a single semiconductor quantum dot
International audienceWe discuss the decoherence dynamics in a single semiconductor quantum dot and analyze two dephasing mech- anisms. In the ¯rst part of the review, we examine the intrinsic source of dephasing provided by the coupling to acoustic phonons. We show that the non-perturbative reaction of the lattice to the interband optical transition results in a composite optical spectrum with a central zero-phonon line and lateral side-bands. In fact, these acoustic phonon side-bands completely dominate the quantum dot optical response at room temperature. In the second part of the paper, we focus on the extrinsic dephasing mechanism of spectral di®usion that determines the quantum dot decoherence at low temperatures. We interpret the variations of both width and shape of the zero- phonon line as due to the °uctuating electrostatic environment. In particular, we demonstrate the existence of a motional narrowing regime in the limit of low incident power or low temperature, thus revealing an unconventional phenomenology compared to nuclear magnetic resonance
Optically-gated resonant emission in single quantum dots
We report on the resonant emission in coherently-driven single semiconductor
quantum dots. We demonstrate that an ultra-weak non-resonant laser acts as an
optical gate for the quantum dot resonant response. We show that the gate laser
suppresses Coulomb blockade at the origin of a resonant emission quenching, and
that the optically-gated quantum dots systematically behave as ideal two-level
systems in both regimes of coherent and incoherent resonant emission
Giant optical anisotropy in a single InAs quantum dot in a very dilute quantum-dot ensemble
We present the experimental evidence of giant optical anisotropy in single
InAs quantum dots. Polarization-resolved photoluminescence spectroscopy reveals
a linear polarization ratio with huge fluctuations, from one quantum dot to
another, in sign and in magnitude with absolute values up to 82%. Systematic
measurements on hundreds of quantum dots coming from two different laboratories
demonstrate that the giant optical anisotropy is an intrinsic feature of dilute
quantum-dot arrays.Comment: submitted to Applied Physics Letter
Ultrafast carrier dynamics in single-wall carbon nanotubes
Time-resolved carrier dynamics in single wall carbon nanotubes is investigated by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photo-bleaching observed on the first interband transition of the semi-conducting tubes. The carrier dynamics takes place on a one picosecond time scale which is one order of magnitude slower than in graphite. Transient photo-induced absorption is observed with exactly the same dynamics for non-resonant probe conditions and is interpreted as a global red shift of the plasmon resonance. We show that the opening of the band gap in semi-conducting carbon nanotubes determines the non-linear response dynamics over the whole visible and near-infrared spectrum
Overtones of interlayer shear modes in the phonon-assisted emission spectrum of hexagonal boron nitride
We address the intrinsic optical properties of hexagonal boron nitride in deep ultraviolet. We show that the fine structure of the phonon replicas arises from overtones involving up to six low-energy interlayer shear modes. These lattice vibrations are specific to layered compounds since they correspond to the shear rigid motion between adjacent layers, with a characteristic energy of about 6-7 meV. We obtain a quantitative interpretation of the multiplet observed in each phonon replica under the assumption of a cumulative Gaussian broadening as a function of the overtone index, and with a phenomenological line broadening taken identical for all phonon types. We show from our quantitative interpretation of the full emission spectrum above 5.7 eV that the energy of the involved phonon mode is 6.8±0.5 meV, in excellent agreement with temperature-dependent Raman measurements of the low-energy interlayer shear mode in hexagonal boron nitride. We highlight the unusual properties of this material where the optical response is tailored by the phonon group velocities in the middle of the Brillouin zone. © 2017 American Physical Society.Peer reviewe
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