High Excitation Power Research Articles

Conductive optical-fiber STM probe for local excitation and collection of cathodoluminescence at semiconductor surfaces

Luminescence imaging of semiconductor surfaces in nanometric resolution is a key to novel optoelectronic nano-devices, which requires local carrier excitation and local luminescence collection within the nanometric areas at the surfaces. However, there have not been a practical nanospectroscopies applicable to wide range of specimens. STM-cathodoluminescence (STM-CL) nanospectroscopy offers both high spatial resolution (of the order of 10 nm) and novel high carrier excitation power (up to ~1 mW), which enables local luminescence imaging of less-luminescent nano-structures. In this study, we advanced STM-CL technique by introducing a novel optical fiber probe with Cr thin film coating (Cr-FP), which was found to work as a STM probe, as an electron field-emitter for local carrier excitation, and as an alignment-free efficient local STM-CL collector which blinds luminescence after the minority carrier diffusion.

Tuning of the Landé g-factor in AlxGa1−xAs/AlAs single and double quantum wells

We report on the spin dynamics of a high mobility two-dimensional electron gas in a AlxGa1−xAs/AlAs double quantum well structure. For high electron density samples, the g-factor was measured using time-resolved Kerr rotation technique. The g-factor tuning capability was observed by changing the aluminum content x independently in each well. Experiments demonstrated an unusual spin dephasing time robustness for high excitation power. The effect of the interaction between wells was analyzed in samples with different tunneling barriers. Results were compared with experiments on single well systems demonstrating higher spin polarization generation, longer spin dephasing time, and coupling for the double structures.

Enhanced Terahertz Bandwidth and Power from GaAsBi‐based Sources

Terahertz photoconductive (PC) switches are popular for THz wave generation. Unlike nonlinear crystals, they don't require high optical excitation power and precise phase matching, but their output THz power is limited. Here, the conventional substrate is replaced with enhanced GaAsBi (growth recipe included) and its performance is compared to that of a LT-GaAs PC switch and a nanoplasmonic PC switch. The results show a 0.5 THz increase in bandwidth and 4-fold improvement in emission power for GaAsBi and GaAsBi nanoplasmonic PC switches, respectively As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Photocurrent Phenomena in Nanoribbon InAlN/GaN High Electron Mobility Transistors

High electron mobility transistors (HEMTs) based on InAlN/GaN nanoribbon (NR) structures appear as a new technology which can deliver record current densities. Ultraviolet (UV) photodetectors based on ungated InAlN/GaN NR HEMT structures have a gain in the range of the 105–107, a sublinear behaviour with the excitation power and a UV/visible contrast of more than three orders of magnitude. In spite of the larger surface-to-volume ratio, sublinearities and persistent photocurrent effects are less intense in NR structures compared to planar devices, and they decrease at higher excitation power. The asymmetric NR morphology does not induce a preferential coupling of polarized light in the NR structure; since light is mostly absorbed in the GaN buffer layer and photogenerated charges are efficiently collected by the HEMT channel.

Large photoluminescence redshift of ZnTe nanostructures: The effect of twin structures

In this work, ZnTe single crystalline nanowires and twinning nanobelts were synthesized to investigate the relationship between microstructure and optical properties. Power-dependent photoluminescence measurements reveal that the emission peaks of the single crystalline nanowires redshift with increasing the excitation power as a result of the band-gap renormalization effect. However, the photoluminescence redshift of the twinning nanobelts is larger than that of the nanowires, which is caused by both band-gap renormalization effect and electron–hole plasma effect induced by the increase of carrier density in the twin structure at high excitation power.

Functionalized Two-Photon Absorbing Diketopyrrolopyrrole-Based Fluorophores for Living Cells Fluorescent Microscopy

Two-photon excited microscopy has evolved as a routine technique for long-term cellular and in vivo imaging and is now available in most optical microscopy facilities. Classical dyes and fluorescent proteins, developed for epifluorescence or confocal microscopy, are used but unfortunately present a low efficiency upon two-photon excitation inducing the need of high excitation power (over 20 mW). To reduce this excitation power, new dyes need to be developed, allowing really low two-photon excitation power in the milliwatt or sub-milliwatt range. We report here the conception, synthesis, and physicochemical and photophysical properties of new functionalized diketopyrrolopyrrole (DPP) derivatives acting as fluorescent tags for biomolecules. They present high two-photon absorption cross-sections and bright luminescence around 600 nm. These two-photon optimized fluorophores were bioconjugated to HIV-I Tat (44-61), and their cellular localization was observed by two-photon excited microscopy using sub-milliwatt laser excitation power.

Towards nanoantenna electron switches

The electromagnetic ‘ponderomotive force’ was discovered in 1957 in a radio-frequency experiment by Boot and Harvie 1. The force arises due to gradients in an inhomogeneous oscillating electric field, and can be used to control and accelerate electrons, with applications in ion traps, plasma accelerators and high-harmonic generation. However, being a second-order nonlinear effect in the applied field, the ponderomotive force is relatively weak. The availability of high-power, short laser pulses has enabled new practical realizations of this effect 2. In a Letter in this issue, Batelaan and co-workers predict that metal nanoantennas can produce sufficient ponderomotive force to deflect electrons 4. The phenomenon relies on the surface plasmon resonance properties of the nanoantenna, which produces a high local field enhancement combined with large field gradients in a small volume around the nanoantenna. To achieve the required field strengths, they propose to use high energy laser pulses of femtosecond time duration, tuned to the surface plasmon resonance wavelength. These short laser pulses offer yet another advantage, as they allow producing an ultrafast modulation of an incident electron beam. Such an electron switch would be of interest for a range of applications, for example ultrafast electron microscopy 4. Ultrafast electron microscopy is a powerful tool for characterizing chemical and physical processes at nanometer length scales and ultrafast time scales. Currently, the generation of ultrafast electron pulses is done by directly exciting the electron emission source with a high-power UV laser pulse. Plasmonic electron switches may be a feasibly alternative route toward the development of ultrafast electron beams at higher repetition frequencies and using lower-power femtosecond oscillators. As a first step, the authors measured the coherence time of surface plasmon modes in an array of nanoparticles, so-called optical antennas 3. A short, resonant laser pulse can drive a coherent plasmon oscillation, however this oscillation rapidly dephases via collisions of the electrons (intraband) or through excitation of electron-hole pairs (interband). Surface plasmons in isolated metal nanospheres have a dephasing time of several femtoseconds, depending on the type of metal antenna under study and details of the damping mechanism. Detailed measurements of plasmon dephasing were pioneered in the 1990s by Franz Aussenegg and co-workers, who used high harmonic generation as a nonlinear process to make an autocorrelation of the plasmon pulse 5. High-harmonic generation in nanoparticles is complicated, since second-harmonic requires noncentrosymmetric structures, while third harmonic signals are comparably weak. Becker et al. use a different, more direct method of measuring both the amplitude and phase of the plasmon field using cross-correlation of the plasmon field with a reference pulse with known width taken directly from the laser. The prospect of electron beam modulation using ultrafast plasmon fields is promising as a fundamental phenomenon and may further open up the field of ultrafast electron microscopy. Both the coherence time and local field enhancement can be tailored by the geometry of the nanoantenna, allowing for a rational design of nanostructures optimized for specific applications 6. Of particular interest will also be the application of techniques from coherent control to achieve local plasmonic hot-spots in both space and time 7, potentially resulting in even stronger field gradients at shorter time scales. Several hurdles will have to be overcome, related to the high field strengths required and the breakdown of metal nanoantenna arrays under such high power excitation. Similar problems are encountered in higher-harmonic generation experiments, and require careful optimization of material properties of the antennas and substrate. However, if successful, the plasmonic antenna approach may result in a new generation of electron modulation devices.

Structural properties of ultra-low density nanoholes for the generation of well-separated GaAs quantum dots

We apply the local droplet etching (LDE) technique in a conventional molecular beam epitaxy system for self-organized nanohole formation on AlGaAs surfaces. By optimizing the arsenic flux during the Al droplet deposition the hole density can be reduced down to the 106cm−2 regime. The nanohole shape and density are tunable in a wide range by the amount of droplet material deposited and the initial holes are customized by overgrowth with AlGaAs of varied thickness. Nanoholes with controlled depths from 30 down to a few nanometers are fabricated, while the ultra-low density is sustained. These nanoholes are filled with GaAs to form well separated GaAs QDs, with size precisely controlled by the filling level. Studies of the optical emission of single dots exhibit sharp excitonic lines at low and a clear shell structure at high excitation power. In particular, the quantization energies in dependence of the QD size are measured.

High excitation carrier density recombination dynamics of InGaN/GaN quantum well structures: Possible relevance to efficiency droop

We report on the optical properties of InGaN/GaN quantum well structures measured at 10 K as a function of excitation density. At high excitation power densities we observe a component in the spectra that decays more rapidly than the localised carrier emission observed for low excitation power densities. We attribute this component to recombination involving weakly localised or delocalised carriers. At the high excitation power densities there is a reduction in the recombination internal quantum efficiency, so called efficiency droop. These observations are compatible with the model whereby efficiency droop is explained in terms of the non radiative loss of delocalised carriers.

Exciton delocalization and dynamics in helical π-stacks of self-assembled perylene bisimides

Whilst the excitonic properties of J-aggregates have been investigated in great detail, those of H-aggregates have not been systematically investigated yet. In this regard, we have explored the exciton dynamics and excited-species formation processes in columnar H-aggregates of planar PBI dyes that are stacked in a helical fashion by various spectroscopic techniques such as time correlated single-photon counting and femtosecond pump–probe measurements with anisotropy changes. The outcome of this study is that photogenerated excitons in helically stacked PBI dyes experience complicated relaxation processes that involve excited-state interactions such as exciton delocalization and excimer formation. To scrutinize the exciton dynamics in the helically stacked aggregates, we have also included distorted bay-substituted PBI dyes as reference molecules that exhibit either no or only relatively small-sized dimeric aggregate structures. The comparative study revealed that the excited-state interactions in the large-sized helically stacked aggregates extend beyond two PBI units, leading to a final excimer (here, excimer means not only an “excited dimer” but an “excited multimer”) trap state within ∼50 ps. Although in competition with this relaxation path into the excimeric trap state, exciton diffusion has been revealed by exciton–exciton annihilation processes, occurring at high excitation power. Whilst the excimer formation process interrupts the direct observation of exciton diffusion in these columnar PBI aggregates, the exciton migration distance could be estimated by the incorporation of non-fluorescent PBI quencher molecules. From this analysis we can conclude that the exciton diffusion can reach a length of about 10 monomer units. Although this value appears to be shorter than those values observed for J-aggregates, this result shows that columnar PBI stacks might still be useful for optoelectronic applications if the relaxation process leading to excimer traps is prevented, e.g. by structural modifications of the molecules.

Excitonic properties of wurtzite InP nanowires grown on silicon substrate

In order to investigate the optical properties of wurtzite (Wz) InP nanowires grown on Si(001) by solid source molecular beam epitaxy with the vapour–liquid–solid method, the growth temperature and V/III pressure ratio have been optimized to remove any zinc-blende insertion. These pure Wz InP nanowires have been investigated by photoluminescence (PL), time-resolved PL and PL excitation. Direct observation of the second and third valence band in Wz InP nanowires using PL spectroscopy at high excitation power have been reported and, from these measurements, a crystal field splitting of 74 meV and a spin–orbit interaction energy of 145 meV were extracted. Based on the study of temperature-dependent optical properties, we have performed an investigation of the thermal escape processes of carriers and the electron–phonon coupling strength.

Highly Enhanced Exciton Recombination Rate by Strong Electron–Phonon Coupling in Single ZnTe Nanobelt

Electron-phonon coupling plays a key role in a variety of elemental excitations and their interactions in semiconductor nanostructures. Here we demonstrate that the relaxation rate of free excitons in a single ZnTe nanobelt (NB) is considerably enhanced via a nonthermalized hot-exciton emission process as a result of an ultrastrong electron-phonon coupling. Using time-resolved photoluminescence (PL) spectroscopy and resonant Raman spectroscopy (RRS), we present a comprehensive study on the identification and the dynamics of free/bound exciton recombination and the electron-phonon interactions in crystalline ZnTe NBs. Up to tenth-order longitudinal optical (LO) phonons are observed in Raman spectroscopy, indicating an ultrastrong electron-phonon coupling strength. Temperature-dependent PL and RRS spectra suggest that electron-phonon coupling is mainly contributed from Light hole (LH) free excitons. With the presence of hot-exciton emission, two time constants (∼80 and ∼18 ps) are found in photoluminescence decay curves, which are much faster than those in many typical semiconductor nanostructures. Finally we prove that under high excitation power amplified spontaneous emission (ASE) originating from the electron-hole plasma occurs, thereby opening another radiative decay channel with an ultrashort lifetime of few picoseconds.

Competitive carrier interactions influencing the emission dynamics of GaAsSb-capped InAs quantum dots

The optical properties of InAs/GaAs quantum dots capped with a GaAsSb quantum well are investigated by means of power-dependent and time-resolved photoluminescence. The structure exhibits the coexistence of a type-I ground state and few type-II excited states, the latter characterized by a simultaneous carrier density shift of the peak position and wavelength-dependent carrier lifetimes. Complex emission dynamics are observed under a high-power excitation regime, with the different states undergoing shifts during specific phases of the measurement. These features are satisfactorily explained in terms of band structure and energy level modifications induced by two competitive carrier interactions inside the structure.

Crossover from photon to exciton-polariton lasing

We report on a real-time observation of the crossover between photon and exciton-polariton lasing in a semiconductor microcavity. Both lasing phases are observed at different times after a high-power excitation pulse. Energy-, time- and angle-resolved measurements allow for the transient characterization of carrier distribution and effective temperature. We find signatures of Bose–Einstein condensation, namely macroscoping occupation of the ground state and narrowing of the linewidth in both lasing regimes. The Bernard–Douraffourgh condition for inversion was tested and the polariton laser as well as the photon laser under continuous wave excitation were found to operate at estimated densities below the theoretically predicted inversion threshold.

Fabrication of Al doped ZnO films using atmospheric pressure cold plasma

Under atmospheric pressure, homogeneous non-equilibrium cold plasma was generated stably by high voltage pulsed power (1kV, 20kHz, 38W) excitation of a mixture of He and O2 gases produced by a dielectric barrier discharge setup. By feeding Bis (2 methoxy‐6-methyl‐3, 5-heptanedione) Zn (Zn-MOPD, C18H3O6Zn) and Tris (2-methoxy‐6‐methy l‐3, 5-heptanedione) Al (Al-MOPD, C27H45O9Al) into this plasma with He carrier gas, transparent flat Al-doped ZnO (ZnO:Al) films about 120–240nm thick were prepared on glass substrates directly under the slit made into the cathode. Deposition rates of the films were about 20–40nm/min. The concentration of Al was measured by inductively coupled plasma atomic emission spectroscopy. The composition ratio of Al to Zn was 7.8mol% when the carrier He gas flow rate of Al-MOPD was 30cm3. The average transmittance of all films was more than 85% in the wavelength range from 400 to 800nm. When the composition ratio of Al/Zn was between 1.1 and 7.8mol%, the optical band gap of the film increased from 3.28 to 3.40eV. The resistivity of ZnO:Al film was 2.96Ωcm at 1.3mol% of Al/Zn. In addition, the microstructure of the films was studied by X-ray diffraction measurement and field emission scanning electron microscope observation. It was revealed that doped Al is substituted onto the Zn site of the ZnO crystalline structure in ZnO:Al films.

Line-scanning detection instrument for photonic crystal enhanced fluorescence

A laser line-scanning instrument was developed to optimize the near-field enhancement capability of a one-dimensional photonic crystal (PC) for excitation of surface-bound fluorophores. The excitation laser beam is shaped into an 8 μm × 1 mm line that is focused along the direction of the PC grating, while remaining collimated perpendicular to the grating. Such a beam configuration offers high excitation power density while simultaneously providing high resonant coupling efficiency from the laser to the PC surface. Using a panel of 21 immunofluorescence assays on the PC surface in a microarray format, the approach achieves an enhancement factor as high as 90-fold between on-resonance and off-resonance illumination. The instrument provides a capability for sensitive and inexpensive analysis of cancer biomarkers in clinical applications.

The ultraviolet laser from individual ZnO microwire with quadrate cross section

The ZnO microwires with quadrate cross section were synthesized by chemical vapor deposition method. The ultraviolet laser with the Fabry-pérot cavity modes was realized from an individual ZnO microwire. Under the low excitation power densities, the amplified spontaneous emission was observed from the ZnO microwire, while the lasing action was observed under the high excitation power densities. The ZnO microwire exhibited low threshold excitation intensity of 58 kW/cm(2) and quality factor of 485. The characteristics and possible lasing mechanism were investigated in detail.

Fluidically Tunable Frequency Selective/Phase Shifting Surfaces for High-Power Microwave Applications

We examine fluidically tunable periodic structures acting as highly-selective frequency selective surfaces (FSSs) or spatial phase shifters (SPSs) capable of providing phase shifts in the range of 0°-360°. These devices are multi-layer periodic structures composed of non-rePsonant unit cells. The tuning mechanism is based on integrating small, movable liquid metal droplets with the unit cells of the periodic structure. By moving these liquid metal droplets by small distances within the unit cell, the structure's frequency response can be tuned continuously. Using this technique, a fluidically tunable FSS with a fifth-order bandpass response is designed and its tuning performance is examined for various incidence angles and polarizations of the incident EM wave. Additionally, electronically tunable counterparts of the same structure are also designed and their tuning performances are examined under short-duration high-power excitation conditions. It is demonstrated that such electronically tunable FSSs/PSSs demonstrate extremely nonlinear responses. Since the fluidically tunable structure examined in this work does not use any nonlinear devices, its response is expected to remain linear for such short-duration high-power excitation conditions. The tuning performances of these fluidically tunable periodic structures are also experimentally demonstrated by fabricating three prototypes and characterizing their responses in a waveguide environment.

Temperature dependent growth of InGaN/GaN single quantum well

InGaN/GaN single quantum well (SQW) structures under various InGaN growth temperatures have been grown by metal organic chemical vapor deposition (MOCVD), the surface morphologies and optical properties are investigated. The radius of the typical V-pits on the SQW surface is affected by the InGaN well-temperature, and the surface roughness decreased as the well-temperature reduced. Room-temperature photoluminescence (PL) and cathode luminescence (CL) shows the quantum well and quantum dot (QD)-like localized state light emission of the SQWs grown at 700 and 690°C, respectively, whereas the samples grown at 670 and 650°C present hybrid emission peaks. Excitation power dependent PL spectra indicates the QD-like localized state emission dominates at low excitation power and the quantum well emission starts to take over at high excitation power.

A Yagi-Uda Plasma Monopole Array

A surface wave driven plasma Yagi-Uda monopole array composed of several surface wave driven plasma monopole elements is proposed and studied with finite difference time domain simulation in this work. By varying the excitation power of plasma monopoles, a reconfigurable antenna is obtained for different frequencies. Plasma monopoles are fed by RF power at 500 MHz through an unequal power divider with a fixed power division ratio and a surface wave discharge at gas pressure of 0.4 mb. The results show that while the variations of the excitation power shift the Yagi-Uda resonant frequency between 0.6 to 1.6 of the initial center frequency, the array gain and directivity remain approximately unchanged in the new resonant frequency. Since efficiency is critical to communication systems, the total efficiencies of the reconfigurable array were analyzed from the least to the highest excitation power. At the least excitation power the efficiency was found to be 58%, compared with the 24% efficiency of the highest excitation power.