Resonant Laser Excitation Research Articles

Mobility and conductivity of laser-generated e-h plasmas in direct-gap nanowires

This article examines the role of field strength, frequency, and many-body scattering during the ultrafast optoelectronic response in a direct-gap semiconductor nanowire using numerical simulation. Following resonant laser excitation, an AC or bias DC field perturbs the 1D e-h plasma as it relaxes by carrier-phonon and Coulomb scattering. For bias DC fields, the laser-excited carrier distributions evolve to a static non-equilibrium from which a stable DC mobility is calculated. Carrier-phonon collisions contain the e-h carriers near energy minima for fields of 0.5 kV/cm or less, while the Coulomb collisions redistribute some electrons across the Brillouin zone where they drift into other band structure energy minima and are there contained by phonon scattering. This behavior results in carrier mobilities with a field-strength dependence specific to a 1D solid. For AC probe fields, the analyze the resulting frequency-dependent conductivity for frequencies between the plasmon frequency and interband resonance. In all cases, we compare results to standard-conductivity models by calculating distribution-averaged collision rates and times, and show how, unlike in the bulk, these quantities for the nanowire are strongly dependent on both field magnitude and frequency.

Vacuum-ultraviolet laser source for spectroscopy of trapped thorium ions

A tunable vacuum-ultraviolet (VUV) laser source based on four-wave frequency mixing in xenon is presented. Using seed radiation from two continuous-wave lasers, the system allows for precise control of the VUV frequency and is developed for the resonant laser excitation of the Th-229 nucleus to its low-energy isomeric state. The system is prepared to operate in a wide scanning range from 148 nm to 155 nm. The source produces pulses of 6–10 ns duration with up to 40 µJ energy and is coupled via a vacuum beamline to a linear radiofrequency ion trap. In a first implementation of VUV laser spectroscopy of trapped Th+ ions we excite three previously unknown resonance lines near 149 nm wavelength to electronic levels that are close to the Th-229 isomer energy. The resonances are detected and analyzed via fluorescence of the excited Th+ ions. An analysis of the lineshape is used to estimate the linewidth of the VUV radiation to be in the range of 6 GHz, dominated by phase noise that is enhanced in harmonic generation and in the four-wave mixing process. The prospects for the use of the system in nuclear laser spectroscopy of Th-229 are discussed.

Investigating the fast spectral diffusion of a quantum emitter in hBN using resonant excitation and photon correlations

The ability to identify and characterize homogeneous and inhomogeneous dephasing processes is crucial in solid-state quantum optics. In particular, spectral diffusion leading to line broadening is difficult to evidence when the associated timescale is shorter than the inverse of the photon detection rate. Here, we show that a combination of resonant laser excitation and second-order photon correlations allows to access such fast dynamics. The resonant laser drive converts spectral diffusion into intensity fluctuations, leaving a signature in the second-order coherence function $g^{(2)}(\tau)$ of the scattered light that can be characterized using two-photon coincidences -- which simultaneously provides the homogeneous dephasing time. We experimentally implement this method to investigate the fast spectral diffusion of a color center generated by an electron beam in the two-dimensional material hexagonal boron nitride. The $g^{(2)}(\tau)$ function of the quantum emitter measured over more than ten orders of magnitude of delay times, at various laser powers, establishes that the color center experiences spectral diffusion at a characteristic timescale of a few tens of microseconds, while emitting Fourier-limited single photons ($T_2/2T_1 \sim 1$) between spectral jumps.

Excitation wavelength dependent photoluminescence and photochromic studies of Yb3+/Er3+ doped β−KYF4 and β−NaYF4 nanoparticles

We report both resonant and off-resonant laser excitation of Er3+ upconversion fluorescence and colour tunability in Er3+/Yb3+ co-doped KYF4 and NaYF4 nanoparticles. The nanoparticles were prepared using a hydrothermal method with particle sizes of 74 ± 20 nm and 345 ± 91 nm for β−KYF4 and β−NaYF4, respectively. The Yb3+ 2F7/2 → 2F5/2 absorption spectra exhibit absorption maxima at 10237 cm−1 (977 nm) for β−NaYF4 and 10267 cm−1 (974 nm) for β−KYF4 nanoparticles. The Er3+ upconversion fluorescence spectra consist of the 2H11/2,4S3/2, and 4F9/2 → 4I15/2 transitions for either 974, 977 or 980 nm laser excitation in both materials. We observed an enhancement in the upconversion intensity by a factor of up to 20 for β−KYF4 and 1.5 fold for β−NaYF4 under resonant excitation compared with off-resonant excitation at 980 nm. Tuneable upconversion fluorescence wavelength was achieved in β−KYF4:Yb/Er nanoparticles by adjusting the excitation wavelength. The CIE chromaticity coordinates are (0.6836, 0.3151) with a highest red colour purity of 99.70% for β−KYF4 (4700 K) and (0.3953, 0.5915) with a colour purity of 96.84% for β−NaYF4 (4533 K) nanoparticles. The respective absolute upconversion quantum yields are 1.18% and 2.34% under low power density (1.65 W⋅cm−2).

A field ionizer for photodetachment studies of negative ions.

In this paper, we present an apparatus for studies into the photodetachment process of atomic negative ions. State-selective detection of the residual atom following the initial photodetachment step is achieved by combining resonant laser excitation of the photo-detached atom with electric field ionization. The resonance ionization technique in combination with a co-linear ion-laser beam geometry gives an experimental apparatus that has both high selectivity and sensitivity. In addition to measurements of a single selected partial photodetachment channel, the apparatus also can be used to study a manifold of photodetachment channels in which the residual atom is left in a high-lying Rydberg state and for investigation of the double electron-detachment process. Ion-optical simulations in SIMION are used to illustrate the operation of the apparatus for studying such processes. Successful performance of the apparatus against the simulation is demonstrated by a high resolution study of the photodetachment of cesium, where the sharp s-wave threshold of the photodetachment processes leaving the residual atom in the excited 6p state was investigated.

Observation of Collisional De-Excitation Phenomena in Plutonium

A program of research towards the high-resolution optical spectroscopy of actinide elements for the study of fundamental nuclear structure is currently ongoing at the IGISOL facility of the University of Jyväskylä. One aspect of this work is the development of a gas-cell-based actinide laser ion source using filament-based dispensers of long-lived actinide isotopes. We have observed prominent phenomena in the resonant laser ionization process specific to the gaseous environment of the gas cell. The development and investigation of a laser ionization scheme for plutonium atoms is reported, focusing on the effects arising from the collision-induced phenomena of plutonium atoms in helium gas. The gas-cell environment was observed to greatly reduce the sensitivity of an efficient plutonium ionization scheme developed in vacuum. This indicates competition between resonant laser excitation and collisional de-excitation by the gas atoms, which is likely being enhanced by the very high atomic level density within actinide elements.

Dipole-dipole-interaction–driven antiblockade of two Rydberg atoms

Resonant laser excitation of multiple Rydberg atoms are prohibited, leading to Rydberg blockade, when the long-range van der Waals interactions are stronger than the laser-atom coupling. Rydberg blockade can be violated, i.e., simultaneous excitation of more than one Rydberg atom, by off-resonant laser excitation, causing an excitation antiblockade. Rydberg antiblockade gives rise to strongly correlated many-body dynamics and spin-orbit coupling and also finds quantum computation applications. Instead of commonly used van der Waals interactions, we investigate antiblockade dynamics of two Rydberg atoms interacting via dipole-dipole exchange interactions. We study typical situations in current Rydberg atom experiments, where different types of dipole-dipole interactions can be achieved by varying Rydberg state couplings. An effective Hamiltonian governing underlying antiblockade dynamics is derived. We illustrate that geometric gates can be realized with the Rydberg antiblockade which is robust against the decay of Rydberg states. Our study may stimulate new experimental and theoretical exploration of quantum optics and strongly interacting many-body dynamics with Rydberg antiblockade driven by dipole-dipole interactions.

Unvailing the spin–phonon coupling in nanocrystalline BiFeO3 by resonant two-phonon Raman active modes

We report on temperature dependence of two-phonon Raman spectra in BiFeO3 nanocrystals, above and below the Néel temperature TN using a resonant laser excitation line (λ=532 nm). Two-phonon modes exhibited anomalous frequency hardening and deviation from the anharmonic decay below TN. Such behavior strongly supported the existence of spin-two-phonon interaction, because these modes are known to be very sensitive to the antiferromagnetic ordering. Within the mean-field theory for the nearest-neighbor interaction, the linear relationship between spin–spin correlation function and observed two-phonon frequency shift below TN was obtained. This approach enabled to quantify the spin–phonon interaction by spin–phonon coupling strength for both two-phonon modes and justified the application of mean-field approach. Magnetic measurements revealed the coexistence of antiferromagnetic and weak ferromagnetic phases below TN, which were found non competitive, additionally supporting the mean-field approach from which we deduced that the two-phonon modes in BiFeO3 are correlated with antiferromagnetic ordering below TN.

Generalized uncertainty principle corrections on atomic excitation

In this paper, we investigate the effect of minimal length on Einstein model of a gas of two-level atoms in thermal equilibrium with blackbody radiation at temperature T. We reformulate the classical Hamiltonian describing the interaction of atomic electrons with the electromagnetic field of blackbody radiation by employing Kempf Algebra to obtain the Hamiltonian with a minimal length. In the electric dipole approximation, we obtain the modified transition rate relations, and modified Einstein coefficients for broadband excitation. We utilize the modified quantities to obtain the modified cross-section in the resonant laser excitation. We deduce the upper bounds for the deformation parameter and minimal length.

Low‐Temperature Silicon‐Vacancy Luminescence of Individual Chemical Vapor Deposition Nanodiamonds Grown by Seeding and Spontaneous Nucleation

Diamond nanoparticles with optically active color centers can be used in nanoscale devices for quantum networking and sensing. A particular interest is focused on nanodiamonds (NDs) containing a small ensemble of negatively charged silicon‐vacancy (SiV) centers. Such NDs can be integrated with on‐chip optical structures. Here, chemical vapor deposition (CVD) is used to synthesize diamond particles of less than 100 nm in size containing 10–20 centers per ND. Two types of NDs grown on a Si substrate are studied, seeded with detonation NDs and without any seeding. The SiV luminescence under off‐resonant and resonant laser excitation at the temperature of 15 K is investigated. The emission lines of SiVs of individual NDs are well resolved and found to be spread in the range of 730–750 nm. The typical SiV linewidth is within 1–2 GHz for spontaneous NDs, which is only a factor of 2.5–5 broader than the lifetime limited linewidth of the SiV at 15 K. The emission of a single SiV is highly stable for both types of samples. This work is a critical step toward the synthesis of SiVs suitable for the realization of integrated quantum photonic devices.

Experimental observation of a negative grey trion in an electron-rich WSe2 monolayer

We measure the evolution of low temperature photoluminescence in a WSe2 monolayer with increasing electron concentration level. By comparing non-resonant and resonant laser excitation, we find that the formation of negative trions is facilitated by very efficient phonon emission. The most prominent line in photolumienscence spectra in the intermediate range of carrier concentrations (below cm−2) is found to be 66 meV below the bright negative trion. Its measured properties, including low oscillator strength and the temperature dependence point to an interacting bright intervalley and dark intervalley trion state as the origin of the line.

Electronic level structure of Th+ in the range of the Th229m isomer energy

Using resonant two-step laser excitation of trapped $^{232}\mathrm{Th}^+$ ions, we observe 166 previously unknown energy levels of even parity within the energy range from 7.8 to 9.8 eV and angular momenta from $J=1/2$ to $7/2$. We also classify the high-lying levels observed in our earlier experiments by the total angular momentum and perform ab-initio calculations to compare their results with the observed level density. The observed levels can be relevant for the excitation or decay of the $^{229m}\mathrm{Th}$ isomeric nuclear state which lies in this energy range. The high density of electronic levels promises a strongly enhanced electronic bridge excitation of the isomer in $^{229}\mathrm{Th}^+$.

Resonance Raman Scattering Spectroscopy of Four-Monolayer Thick MoS2 Films

We studied the Raman scattering in bulk and four-monolayer thick MoS2 films under the resonance laser excitation of A, B, and C excitons. We demonstrate the fine structure agility of a phonon mode associated with out-of-plane vibration of sulfur atoms (A1g) when passing from bulk to atomically thin film. The C exciton resonance excitation allows us to register the line corresponding to optical-phonon (E1g) scattering and the set of peaks with a 660 – 740 cm−1 Stokes shift associated with resonance scattering by two optical (Eu) phonons.

Nonreciprocal Transmission and Reflection of a Chirally Coupled Quantum Dot

We report strongly nonreciprocal behavior for quantum dot exciton spins coupled to nanophotonic waveguides under resonant laser excitation. A clear dependence of the transmission spectrum on the propagation direction is found for a chirally coupled quantum dot, with spin up and spin down exciton spins coupling to the left and right propagation directions, respectively. The reflection signal shows an opposite trend to the transmission, which a numerical model indicates is due to direction-selective saturation of the quantum dot. The chiral spin-photon interface we demonstrate breaks reciprocity of the system and opens the way to spin-based quantum optical components such as optical diodes and circulators in a chip-based solid-state environment.

Intrinsic and environmental effects on the interference properties of a high-performance quantum dot single-photon source

We report a joint experimental and theoretical study of the interference properties of a single-photon source based on a In(Ga)As quantum dot embedded in a quasiplanar GaAs microcavity. Using resonant laser excitation with a pulse separation of 2 ns, we find near-perfect interference of the emitted photons, and a corresponding indistinguishability of $\mathcal{I}=(99.6{\phantom{\rule{0.16em}{0ex}}}_{\ensuremath{-}\phantom{\rule{0.16em}{0ex}}1.4}^{+\phantom{\rule{0.16em}{0ex}}0.4})%$. For larger pulse separations, quasiresonant excitation conditions, increasing pump power, or with increasing temperature, the interference contrast is progressively and notably reduced. We present a systematic study of the relevant dephasing mechanisms and explain our results in the framework of a microscopic model of our system. For strictly resonant excitation, we show that photon indistinguishability is independent of pump power, but strongly influenced by virtual phonon-assisted processes which are not evident in excitonic Rabi oscillations.

Surface‐enhanced micro‐CARS mapping of a nanostructured cerium dioxide/aluminum film surface with gold nanoparticle‐bound organic molecules

AbstractHighly contrast epi‐surface‐enhanced coherent anti‐Stokes Raman scattering (SECARS) microimages of Au‐nanoparticle‐bound organic reporter molecule distributions at a surface of novel surface‐enhanced Raman scattering (SERS)‐active metamaterial junctions, based on nanoparticles spread over a nanostructured CeO2 faceted dielectric film, deposited on an Al sublayer, were recorded at two‐color picosecond excitation of the surface in the near‐infrared spectral range. For this, a scanning confocal laser‐based micro‐CARS spectrometer was employed. The investigations showed that at Raman resonant laser excitation of the molecules/Au‐NP conjugates immobilized on the surface strong SECARS signals can be generated with laser powers not deteriorating the conjugates. Coupling CARS with the plasmonic metamaterial structures under investigation provided excellent chemical imaging contrast (up to 400) for biochemically relevant 5‐thio(2‐nitrobenzoic acid) and 4‐mercaptophenylboronic acid reporter molecules. Taking into account easy handling and utmost long‐term stability of the investigated metamaterial junction at ambient conditions, it might be considered as a promising perspective for a single‐molecule‐sensitivity surface‐enhanced Raman scattering or SECARS biosensor.

New Lines and Levels in the UV Spectrum of NdI

The spectrum of transitions to high-lying odd states of the neodymium atom in the UV range from 317 to 323 nm is studied by resonance laser excitation and ionization. Fifty-five of the 72 transitions detected are observed for the first time. For the brightest lines (18), the lower levels of transitions are identified, the energies of the upper levels are determined (31023.813–33437.403 cm–1), and the isotope shifts between 142Nd and 144Nd are measured. The shifts for all stable even neodymium isotopes are measured for five transitions (142–150).

Method for generating a photonic NOON state with quantum dots in coupled nanocavities

We propose a method to generate path-entangled $N00N$-state photons from quantum dots (QDs) and coupled nanocavities. In the systems we considered, cavity mode frequencies are tuned close to the biexciton two-photon resonance. Under appropriate conditions, the system can have the target $N00N$ state in the energy eigenstate, as a consequence of destructive quantum interference. The $N00N$ state can be generated by the resonant laser excitation. This method, first introduced for two-photon $N00N$ state ($N=2$), can be extended toward higher $N00N$ state ($N>2$) based on our recipe, which is applied to the case of $N=4$ as an example.

Second Harmonic Excitation of Acetylene by the Optical Near Field Generated in a Porous Material

We perform first-principles calculations of the photoexcitation dynamics of an acetylene molecule in a metal organic framework, IRMOF-10. Resonant laser excitation of the IRMOF-10 generates an optical near field (ONF) around the two benzene rings that comprise the main framework of the IRMOF-10. Interestingly, along with the ONF of frequency ω, which synchronizes with the resonant frequency of the IRMOF-10, a second harmonic (=2ω) component of the ONF is generated. The second harmonic component effectively excites the acetylene molecule at a frequency of 2ω, although the incident laser field is in tune with ω. It is noteworthy that the second harmonic excitation is caused by spatial nonuniformity of the ONF interaction. It is completely different from the conventional two-photon excitation and is much more efficient than the two-photon excitation by several orders of magnitude.

On algorithms for calculating the SVL-fluorescence spectra of polyatomic molecules

A software package for modeling the fluorescence spectra from a single vibrational level (SVL) of excited electron states under resonance laser excitation, in the adiabatic approximation with allowance for the “confusion” of normal coordinates under electron excitation, i.e., the Dushinsky effect, is presented. To algorithmize the calculation of the intensities in resonance fluorescence spectra, thirteen modules for the calculating Franck–Condon integrals are created. To optimize the calculation process, some simpler expressions are derived for the common terms of nested sums in the formulas for calculating Franck–Condon integrals from tone and overtone levels. To accelerate calculations, some constraints in the form of inequalities are additionally imposed on the range of vibrational quantum numbers in the ground electron state. The proposed algorithm for the modeling of SVL-fluorescence spectra of polyatomic molecules is based on quantum models of molecules in the ground and excited electron states calculated within the framework of contemporary ab initio quantum-mechanical methods and methods based on density functional theory.