Strong Optical Excitation Research Articles

Interplay Between Optical Bianisotropy and Magnetism in Plasmonic Metamolecules.

The smallness of natural molecules and atoms with respect to the wavelength of light imposes severe limits on the nature of their optical response. For example, the well-known argument of Landau and Lifshitz and its recent extensions that include chiral molecules show that the electric dipole response dominates over the magneto-electric (bianisotropic) and an even smaller magnetic dipole optical response for all natural materials. Here, we experimentally demonstrate that both these responses can be greatly enhanced in plasmonic nanoclusters. Using atomic force microscopy nanomanipulation technique, we assemble a plasmonic metamolecule that is designed for strong and simultaneous optical magnetic and magneto-electric excitation. Angle-dependent scattering spectroscopy is used to disentangle the two responses and to demonstrate that their constructive/destructive interplay causes strong directional scattering asymmetry. This asymmetry is used to extract both magneto-electric and magnetic dipole responses and to demonstrate their enhancement in comparison to ordinary atomistic materials.

Chemical doping modulation of nonlinear photoluminescence properties in monolayer MoS2

We demonstrate a simple modulation technique of nonlinear optical properties in monolayer (1L) MoS2 via chemical doping. The strong nonlinear behavior of the exciton photoluminescence (PL) intensity is observed with increasing excitation power density for low-electron-density 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)-doped 1L-MoS2, whereas the exciton PL intensity of as-prepared, heavily electron-doped 1L-MoS2 exhibits weak sublinear behavior. These results are attributable to an enhanced exciton–exciton annihilation rate for the excitons in F4TCNQ-doped 1L-MoS2 as the dominant decay pathway under strong optical excitation conditions.

Optical memory bandwidth and multiplexing capacity in the erbium telecommunication window

We study the bandwidth and multiplexing capacity of an erbium-doped optical memory for quantum storage purposes. We concentrate on the protocol revival of a silenced echo because it has the largest potential multiplexing capacity. Our analysis is applicable to other protocols that involve strong optical excitation. We show that the memory performance is limited by instantaneous spectral diffusion and we describe how this effect can be minimized to achieve optimal performance.

Femtosecond Dynamics of the Collinear-to-Spiral Antiferromagnetic Phase Transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.

The Formation of Warm Dense Matter: Experimental Evidence for Electronic Bond Hardening in Gold

Under strong optical excitation conditions, it is possible to create highly nonequilibrium states of matter. The nuclear response is determined by the rate of energy transfer from the excited electrons to the nuclei and the instantaneous effect of change in electron distribution on the interatomic potential energy landscape. We used femtosecond electron diffraction to follow the structural evolution of strongly excited gold under these transient electronic conditions. Generally, materials become softer with excitation. In contrast, the rate of disordering of the gold lattice is found to be retarded at excitation levels up to 2.85 megajoules per kilogram with respect to the degree of lattice heating, which is indicative of increased lattice stability at high effective electronic temperatures, a predicted effect that illustrates the strong correlation between electronic structure and lattice bonding.

Photon Antibunching in the Photoluminescence Spectra of a Single Carbon Nanotube

We report the first observation of photon antibunching in the photoluminescence from single carbon nanotubes. The emergence of a fast luminescence decay component under strong optical excitation indicates that Auger processes are partially responsible for inhibiting two-photon generation. Additionally, the presence of exciton localization at low temperatures ensures that nanotubes emit photons predominantly one by one. The fact that multiphoton emission probability can be smaller than 5% suggests that carbon nanotubes could be used as a source of single photons for applications in quantum cryptography.

Population inversion of photoexcited electrons and holes in graphene and its negative terahertz conductivity

AbstractWe demonstrate that sufficiently strong optical excitation may result in the population inversion in graphene, so that the real part of the ac conductivity can be negative in the terahertz range of frequencies. We study also how the heating of the electron‐hole system influences the effect of negative ac conductivity. The effect of population inversion and negative ac conductivity might be used in graphene‐based coherent sources of terahertz electromagnetic radiation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Excitation-Density Dependence of Photoluminescence from Si-Doped AlGaN/AlGaN Multiple Quantum Wells at Low Temperature

To investigate the effect of Si doping on AlxGa1-xN/AlyGa1-yN multiple quantum wells (MQWs), photoluminescence (PL) measurements of Al0.15Ga0.85N/Al0.20Ga0.80N MQWs with an undoped Al0.20Ga0.80N barrier layer (undoped MQWs) and with a Si-doped layer (Si-doped MQWs) are compared in weak (1.8 ×10-4 to 5.2 ×10-3 W/cm2) and strong (360 to 1.28 ×104 W/cm2) optical excitation ranges at 10 K. Based on the results of PL measurement, the carrier injection into the undoped MQWs induced by light illumination in the strong optical excitation range and that into the Si-doped Al0.15Ga0.85N/Al0.20Ga0.80N MQWs in the weak optical excitation range show an increase in the PL intensity and PL peak energy, and a narrowing of the full width at half maximum (FWHM) of the PL peak. Although the internal polarization fields of the undoped and Si-doped MQWs are screened in the strong optical excitation range, the PL intensity of the undoped MQWs is higher than that of the Si-doped MQWs. The difference in the PL intensity is attributed to the formation of a nonradiative center due to Si doping.

Population Dynamics of Excitons in Silicon Nanocrystals Structures under Strong Optical Excitation

We report on the experimental and theoretical studies of population/depopulation dynamics of excitons in the structures with Si nanocrystals in SiO2 matrix (nc-Si/SiO2) under strong optical excitation. The experimental results are explained using a phenomenological model based on rate equations for coupled system of energy donors (excitons) and energy acceptors (erbium ions). Exciton luminescence is found to exhibit superlinear dependence for Er-doped samples. At the same time the Er-related luminescence at 1.5 μm shows a saturation of the intensity and shortening of the lifetime, which are attributed to the population inversion of the Er ions states. The obtained results demonstrate that nc-Si/SiO2:Er systems can be used for applications in Si-based optical amplifiers and lasers, compatible with planar Si-technology.

Optical gain in fluorenyl-thiophene co-oligomer thin films

Abstract We report on the emission properties of thin films of a fluorenyl-thiophene co-oligomer under strong optical excitation. A clear line narrowing is observed in the emission spectra as a function of the excitation density, due to a Stimulated Emission band attributed to Amplified Spontaneous Emission (ASE). The ASE threshold of about 150 μJ cm −2 is comparable with the best values reported for other oligomeric and/or co-oligomeric films or crystals. A maximum gain value of about 8 cm −1 is obtained for an excitation density of about 940 μJ cm −2 . These results propose fluorene-thiophene co-oligomers as interesting materials for high performance optically pumped laser devices.

Erbium Ion Luminescence of Silicon Nanocrystal Layers in a Silicon Dioxide Matrix Measured under Strong Optical Excitation

The photoluminescence (PL) spectra and kinetics of erbium-doped layers of silicon nanocrystals dispersed in a silicon dioxide matrix (nc-Si/SiO 2 ) are studied. It was found that optical excitation of nc-Si can be transferred with a high efficiency to Er 3+ ions present in the surrounding oxide. The efficiency of energy transfer increases with increasing pumping photon energy and intensity. The process of Er 3+ excitation is shown to compete successfully with nonradiative recombination in the nc-Si/SiO 2 structures. The Er 3+ PL lifetime was found to decrease under intense optical pumping, which implies the establishment of inverse population in the Er 3+ system. The results obtained demonstrate the very high potential of erbium-doped nc-Si/SiO 2 structures when used as active media for optical amplifiers and light-emitting devices operating at a wavelength of 1.5 μ m. © 2005 Pleiades Publishing, Inc.

Many-Body Interactions in the CdTe-Based Quantum Well under Strong Optical Excitation

We performed pump-probe time-resolved absorption measurements on a modulation p-doped (Cd,Mn)Te quantum well. The density of the 2D hole gas was controlled, in the 10 cm−2 range, by additional cw illumination. Interactions between photocreated neutral (X) and charged (X) excitons and the 2D hole gas were analyzed. We found that the strongest effect is the influence of the carriers on both X and X optical resonances. Neutral and charged excitons are screened by holes, which results in the decrease in their intensities. We conclude from polarization resolved experiments that this screening is spin-dependent: it is more efficient between holes with opposite spins. Binding holes into the charged excitons reduces the screening of the neutral excitons and leads to an enhancement of the neutral exciton intensity. We also analyzed weaker effects, due to exciton–exciton interaction, at a constant hole density. We found that the reduction of the neutral exciton intensity due to two different mechanisms (phase-space filling and biexciton formation) is almost equal. We observed a spin-dependent blue shift of the neutral exciton line in the presence of a population of neutral excitons created by the pump pulse. Due to the attractive interaction between excitons with opposite spins, the corresponding shift of the neutral exciton is smaller than that observed due to the interaction between excitons with the same spins.

Subpicosecond dynamical renormalization of spin-polarized electron–hole plasma in Cd 1− xMn xTe

Ultrafast dynamics of many-body effects induced by the spin-polarized electron–hole plasma in Cd 1− x Mn x Te epilayers were investigated by using femtosecond pump-probe spectroscopy in magnetic field. In the strong optical excitation limit, the renormalization of the energy gap between the spin-polarized conduction subbands and valence subbands induced by the exchange-correlation interaction, and the suppression of exciton formation due to the screening of Coulomb interaction were observed. The dynamical behaviour of these many-body effects suggests faster cooling of higher-lying spin-polarized electron–hole pairs in magnetic fields.

Intracenter transitions of iron-group ions in II–VI semiconductor matrices

A review is made of studies of intracenter optical transitions in 3d shells of iron-group divalent (magnetic) ions. Attention is focused on the emission spectra of Mn2+ ions in CdTe, ZnS, and ZnSe crystals. An analysis is made of the structure of intracenter absorption and luminescence and of the effect that the elemental matrix composition, magnetic-ion concentration, temperature, hydrostatic pressure, and structural phase transitions exert on the intracenter transitions. The mutual influence of two electronic excitation relaxation mechanisms, interband and intracenter, is considered. The specific features of the intracenter emission of magnetic ions embedded in two-dimensional systems and nanocrystals associated with a variation in sp-d exchange interaction and other factors are discussed. Data on the decay kinetics over the intracenter luminescence band profile are presented as a function of temperature, magnetic ion concentration, and excitation conditions. The saturation of the luminescence and the variation of its kinetic properties under strong optical excitation, which are caused by excitation migration and the cooperative effect, as well as the manifestation of a nonlinearity in intracenter absorption, are studied.

Enhanced coupling to vertical radiation using a two-dimensional photonic crystal in a semiconductor light-emitting diode

Enhanced coupling to vertical radiation is obtained from a light-emitting diode using a two-dimensional photonic crystal that lies entirely inside the upper cladding layer of an asymmetric quantum well structure. A sixfold enhancement in light extraction in the vertical direction is obtained without the photonic crystal penetrating the active material. The photonic crystal is also used to couple pump light at normal incidence into the structure, providing strong optical excitation.

Optically Induced Stark Shift in DFB Resonators

We design, as a possible device structure for optic–optic switching, an electronic three level system with distributed electronic wave function and localised hole function on the basis of the quantum mechanical tunnel effect between asymmetric coupled double quantum wells (ASC-DQWs). After strong optical excitation the ASC-DQW shows a strong dipole field which induces strong Stark shifts in the whole electronic structure. The dipole fields can be enhanced and used for coupling if the ASC-DQW is patterned in wire or dot structures. We demonstrate by periodic wire arrays light-induced Stark shifts of the electronic states and thus switching. Due to the short tunnelling times of the electrons in the three level system of the ASC-DQW time responses of the Stark shift in the ps range are possible.

Ultrafast nonlinear gain dynamics in semiconductor nanocrystals

II - VI semiconductor (CdS, CdSe) nanocrystals with an average size of approximately one bulk exciton Bohr radius are embedded in a glass matrix. Due to the quantum confinement effect, they act as a quasi zero-dimensional system (quantum dots). Under strong nanosecond and femtosecond optical excitation, these quantum dots exhibit optical amplification (gain). We investigate the ultrashort gain dynamics of the strongly confined CdSe quantum dots by femtosecond pump-probe spectroscopy. From multiple-beam pump-probe measurements, we conclude that the gain mechanism is governed by biexciton to exciton transitions. Femtosecond dephasing measurements reveal a constant scattering rate across the gain region and confirm the two-electron-hole pair gain model. Nanosecond pump-probe measurements on CdS quantum dots in sol-gel glasses show optical gain up to room temperature. In all cases, the gain region is broad and stretches below the fundamental absorption of the nanocrystals. The reason is the multitude of transitions involved in the gain formation. No phonon bottleneck did prevent any quick relaxation or any fast scattering that would replenish the spectral holes in the gain region. The relaxation rates are on the order of 10ps. The luminescence decay time is nonexponentiai with a 40–100 ps component for the stimulated emission.

Light amplification and laser emission in conjugated polymers

Seppo Honkanen, MEMBER SPIE Mahmoud Fallahi, MEMBER SPIE Bernard Kippelen Nasser Peyghambarian, MEMBER SPIE University of Arizona Optical Sciences Center Tucson, Arizona 85721 Abstract. We study optical gain and laser emission from semiconducting conjugated polymers after strong optical excitation with femtosecond laser pulses. Stimulated emission from few-hundred-nanometer thin films results in a sharp increase in emission intensity and emission line narrowing. Very large optical gain due to the stimulated emission is measured directly by pump-probe spectroscopy. Using the large light amplification in these conjugated polymers, we demonstrated planar and ring lasers of very small sizes that are suitable for applications in integrated optics. Vertical cavity surface emitting lasers, distributed feedback through surface relief grating, and whispering-gallery mode emission from ring resonators are demonstrated. © 1998 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(98)01204-5]

FTIR and FT Raman Spectra of B6O

The IR active phonon spectrum of B6O is typical for theα-rhombohedral boron structure group. The number of phonons corresponds exactly to that predicted by group theory. The phonon bands in the spectrum are weakly damped, indicating that the structure is largely undistorted. Contrary to the IR spectrum the FT Raman spectrum exhibits no narrow lines but considerably broadened bands. This leads to the conclusion that the strong optical excitation by the Nd:YAG laser with about 2 W irradiation power causes a considerable distortion of the structure by fluctuations in consequence of the interaction between optically excited electrons and intraicosahedral phonons.

Interband Transitions and Phonon Spectra of Si-Doped B12P2

Optical absorption and FT Raman spectra of polycrystalline B12P2prepared by CVD on silicon substrates are presented. The silicon content is rather high, and we assume that a ternary compound with the structure formula B12(P2)1−x(Si2)xis formed. The absorption edge yields indirectly allowed transitions at 1.62, 1.80, 2.17, 2.46, and 2.75 eV, the energies of which are considerably lower than those reported by G. A. Slacket al.(1983,J. Phys. Chem. Solids44, 1009) for pure B12P2. Six gap-state related transitions have ionization energies, which agree with those of electron traps inβ-rhombohedral boron. The number of one-phonon resonance frequencies agrees exactly with that predicted from group theory. The two-phonon spectra are quantitatively determined. Strong luminescence radiation superimposes the FT Raman spectrum, which differs qualitatively from that traditionally measured. This difference is explained by fluctuating distortions of the icosahedra related to the strong optical excitation of trapped electrons.