Optical Resonance Excitation Research Articles

Terahertz Stimulated Emission under the Optical Resonant Excitation of Germanium Doped with Shallow Donors

Terahertz Stimulated Emission under the Optical Resonant Excitation of Germanium Doped with Shallow Donors

Thermally activated resonant grating using a vanadium dioxide waveguide

In this work, we report on the design of a one-dimensional subwavelength resonant grating comprised of a fused silica substrate and a bi-layer waveguide, consisting of a solgel synthetized anatase TiO2 layer followed by a thin VO2 layer that is applied using pulsed laser deposition and rapid thermal annealing. A TE waveguide mode is excited under normal incidence in the VO2/TiO2 bi-layer via a positive photoresist based grating printed on top, leading to high resonant reflection at room temperature. Increasing the temperature to about 68°C causes the VO2 to undergo a dielectric to metallic transition accompanied by optical modifications in the IR region, canceling the resonance effect. This thermally triggered absorber/emitter tunable configuration enabling the on and off switching of optical resonant excitation in a reversible manner is proposed for passive Q-switching self-protecting devices for high power lasers in the IR wavelength range. Modeling of the optimized temperature dependent resonant waveguide and preliminary experimental results are presented.

1.56 µm and 1.93 µm synchronized mode-locked fiber laser with graphene saturable absorber

Ultrafast pulse with short duration ranging from picosecond to femtosecond has extensive industrial and scientific applications. The laser cavity generates this ultrafast pulse, however, was typically designed to emit the light at wavebands such as at approximately 1.5–1.6 µm or 1.9–2.0 µm. This could be limited by the bandgap of certain saturable absorber material to conduct ultra-broadband laser emission. Graphene, a 2D material with gapless band structure contributes to the optical resonant excitation to emit at any wavelength. The graphene was employed as the saturable absorber and positioned in a laser cavity consisting of both erbium and thulium-doped fiber laser. A synchronized mode-locked fiber laser was generated at a centre wavelength of 1563.5 nm and 1931.9 nm, giving a pulse duration of 700 fs and 1.77 ps at a constant pulse repetition rate of 12.905 MHz. The success of this work will provide a better insight by developing the optimum utilization of a saturable absorber in generating multiple mode-locked lasers with different yet far wavelength in near-infrared red region.

Vibrational Changes Induced by Electron Transfer in Surface Bound Azurin Metalloprotein Studied by Tip-Enhanced Raman Spectroscopy and Scanning Tunneling Microscopy.

The copper protein azurin, due to the peculiar coupling of its optical and vibronic properties with electron transfer (ET) and its biorecognition capabilities, is a very promising candidate for bioelectronic, bio-optoelectronic and biosensor applications. However, a complete understanding of the fundamental processes relating azurin ET and its optical and vibronic characteristics with the charge transport mechanisms occurring in proteins bound to a conductive surface, the typical scenario for a biosensor or bioelectronic component, is still lacking. We studied azurin proteins bound to a gold electrode surface by scanning tunneling microscopy combined with tip-enhanced Raman spectroscopy (STM-TERS). Robust TER spectra were obtained, and the protein's vibronic response under optical excitation in resonance with its ligand-to-metal charge transfer band was found to be affected by the tunneling parameters, indicating a direct involvement of the active site vibrations in the electron transport process.

Polarized quantum dot emission from photonic crystal nanocavities studied under moderesonant enhanced excitation

We study the linear polarization of the emission from single quantum dots embedded in an "L3" defect nanocavity in a two-dimensional photonic crystal. By using narrow linewidth optical excitation in resonance with higher-order modes, we are able to achieve strong quantum dot emission intensity whilst reducing the background from quantum dots in the surrounding lattice. We find that all the dots observed emit very strongly linearly polarized light of the same orientation as the closest mode, despite the fact that these quantum dots may be spectrally detuned by several times the mode linewidth. We discuss the coupling mechanisms which may explain this behavior.

Net Coherent Optical Gain in Strongly Pumped Semiconductor Microcavities

We study the subpicosecond gain dynamics in a high-finesse semiconductor microcavity containing quantum wells by means of degenerate pump–probe spectroscopy. Tuning the optical excitation in resonance with both the exciton and the cavity modes, at high pump fields the probe pulses are strongly amplified near the energy of the empty-cavity mode. At zero probe delay, integration of the probe spectrum yields a net gain of several percents. The coherent nature of the gain is demonstrated by its oscillatory behavior versus probe delay. A dynamical Maxwell-Bloch model for semiconductors reproduce well the experimental findings and gives insight into the microscopic origin of the coherent gain.

Effect of negative electron mobility in a mixture of argon, molecular nitrogen and optically excited lithium vapours

Parametric computations of the electron energy distribution function and of the electron mobility in a photoplasma of a lithium vapour mixture with argon and molecular nitrogen are presented. The possibility of the occurrence of a steady-state negative electron mobility when using a certain ratio of mix components under optical resonant excitation of the Li(2S-2P) transition is considered. The critical plasma parameters (the lithium excited atomic density, the temperature of the vibrational distribution of nitrogen molecules, the plasma ionization degree, the external electric field strength) determining this effect are revealed.

Intersubband transitions in InAs/AlSb quantum wells studied by resonant Raman scattering.

We have used resonant Raman scattering to study intersubband transitions in InAs/AlSb quantum wells. For optical excitation in resonance with the ${\mathit{E}}_{1}$ band gap of InAs, both high- and low-frequency coupled longitudinal-optical phonon-intersubband plasmon modes are observed. From the measured energies of these plasmon modes the single-particle transition energies between the first and second confined electron subband were deduced as a function of the InAs well width. Good agreement was found with subband spacings predicted by theory including the effects of strain and nonparabolicity. InAs/AlSb surface quantum wells, where a pseudomorphically strained InAs quantum well is grown on an AlSb buffer layer without an AlSb top barrier, also show well-resolved intersubband plasmon modes, indicating higher-electron mobilities than those typically found in the surface inversion region of thick InAs layers.

Resonant Raman scattering studies of multilayer (In, Ga, Al) (As, Sb) heterostructures with InAs quantum wells

Raman scattering effects have been used to study phonon properties of complex multilayer (In, Ga, Al) (As, Sb) heterostructures with InAs quantum wells grown by molecular beam epitaxy on GaAs and GaSb substrates. Raman scattering in the region of folded acoustic phonons gives information on the periodicity of cladding superlattices and the abruptness of their interfaces. Raman scattering by InAs phonon modes due to an InAs quantum well are strongly enhanced under optical excitation in resonance with the E 1 band gap of InAs. Interface modes of InSb are detected in resonant Raman scattering, in agreement with the intended interface bond controlled by the beam supply sequence during growth. Resonant excitation results also in the observation of Raman scattering by a collective intersubband plasmon mode due to a two dimensional electron gas in the InAs quantum well. It is shown that a detailed analysis of complex multilayer structures can be performed based upon resonant Raman experiments which agrees fairly well with double crystal x-ray diffraction and galvanomagnetic studies.

Mode structure effects in optical resonance excitation

The dependence of the excitation strength on the longitudinal mode structure of a pulsed, resonant excitation field is examined. As an experimental test case, resonance ionization signal fluctuations are studied with a two-step excitation of strontium. Successive ionization signals, the corresponding pulse energies, and the mode structures of the resonant step laser pulses are recorded. The large signal fluctuations, up to 120%, cannot be explained by the modest (1-3%) pulse-energy fluctuations of either the resonant or the photoionizing field. In the case of weak excitation, the fluctuations in the signal correlate strongly with the intensity in a narrow frequency band around the resonance. In this weak-field region, the experimental correlation curves, i.e., the correlation between the signal and the spectral intensity versus the frequency, agree well with calculations based on a simple linear-response model. With the aid of correlation analysis a resonance can be localized with a single-moderesolution. As the resonant field is increased the correlation between the signal and single-mode intensity diminishes and almost disappears at full saturation. However, also in the saturation region, the correlation technique can be applied to localize a resonance with a resolution much better than that determined by the laser linewidth.

Resonance effects in Raman scattering from InAs/AlSb quantum wells

Resonant Raman scattering has been used to study GaSb-capped AlSb/InAs/AlSb quantum wells, grown by molecular-beam epitaxy with a shutter sequence intended to promote the formation of an InSb-like InAs/AlSb heterointerface. For optical excitation in resonance with the E1 band gap of InAs, scattering by one-longitudinal-optical (LO) phonon and by two-LO phonons from the quantum well layer is strongly enhanced. Excitation close to the E1/E1+Δ1 gap resonance of GaSb, in contrast, enhances the one-LO scattering from the GaSb cap over that from the InAs quantum well. Thus resonance effects in LO phonon Raman scattering allow us to distinguish between scattering from these two materials despite the near-coincidence of the corresponding LO phonon frequencies. Scattering by an InSb-like mode is found to resonate at approximately the InAs E1/E1+Δ1 gap energy, supporting its assignment to an interface mode.

Raman depth profiling by in situ sputtering

Raman scattering has been combined with in situ sputtering for a depth resolved Raman spectroscopic characterization of semiconductor structures. Using optical excitation in resonance with higher lying band gaps, such as the E1 band gap of GaAs, a depth resolution of a few nm can be achieved along with an enhanced sensitivity due to resonance effects in the scattering efficiency. The present experimental technique is demonstrated by recording a composition profile from a GaAs/AlxGa1−xAs multiple quantum well structure. Furthermore, the ability of lattice site selective depth profiling of dopant distributions is shown for Si incorporated on Ga and As lattice sites, respectively, in Si+ implanted and annealed GaAs.

Effect of spatial localization of dopant atoms on the spacing of electron subbands in δ-doped GaAs:Si

Using Raman spectroscopy we have investigated the spacing of the electron subbands in nominally δ-doped GaAs structures which show a considerable spread of the silicon dopant atoms along the growth direction. For optical excitation in resonance with the E0+Δ0band gap, spin-density intersubband excitations are observed. For excitation in resonance with the E1 band gap we find a strong enhancement of scattering by collective intersubband plasmon-phonon modes. The measured energy spacings between the electron subbands deviate significantly from what is expected for ideal δ doping. Self-consistent electronic subband calculations taking into account the spread of the dopant atoms along the growth direction, in contrast, yield a good quantitative agreement between calculated and measured subband spacings. This demonstrates the potential of intersubband Raman spectroscopy for the analysis of the spatial localization of dopant atoms in δ-doped structures.

A method for the direct determination of the decay rate γ0for the population of an excited state

The method involves the application of a depolarizing magnetic field at the 'no-polarization' angle of 54.7 degrees to the electric vector of the optical resonance excitation. The decay of fluorescence following a pulse of excitation then gives gamma 0 directly. Application is made to the 53P1 state of cadmium.