Search

In this paper, we study light emission from a tunnel contact between the Au film on a glass substrate and the Au-coated tungsten probe of the scanning tunneling microscope at ambient conditions. We...

International audience; A generic model of two dissimilar antenna-type resonators coupled both via near-field and farfield and embedded in active gain and/or loss medium is considered. Conditions required for hitting the superscattering regime corresponding to the threshold of lasing emission operation are established. It is shown that modulation of the medium gain and losses level within realistic parameters is an efficient approach for implementation of tunability. This two-antenna setup can serve as an inspiration for the implementation of high-contrast tunable metasurfaces.

International audience; We investigate the influence of a static electric field on the incoherent nonlinear response of an unloaded electrically contacted nanoscale optical gap antenna. Upon excitation by a tightly focused near-infrared femtosecond laser beam, a transient elevated temperature of the electronic distribution results in a broadband emission of nonlinear photoluminescence (N-PL). We demonstrate a modulation of the yield at which driving photons are frequency up-converted by means of an external control of the electronic surface charge density. We show that the electron temperature and consequently the N-PL intensity can be enhanced or reduced depending on the command polarity and the strength of the control static field. A modulation depth larger than 100% is observed for activation voltages of a few volts.

Electrically driven optical antennas can serve as compact sources of electromagnetic radiation operating at optical frequencies. In the most widely explored configurations, the radiation is generated by electrons tunneling between metallic parts of the structure when a bias voltage is applied across the tunneling gap. Rather than relying on an inherently inefficient inelastic light emission in the gap, we suggest to use a ballistic nanoconstriction as the feed element of an optical antenna supporting plasmonic modes. We discuss the underlying mechanisms responsible for the optical emission and show that, with such a nanoscale contact, one can reach quantum efficiency orders of magnitude larger than with standard light-emitting tunneling structures.

Background: Electrically controlled optical metal antennas are an emerging class of nanodevices enabling a bilateral transduction between electrons and photons. At the heart of the device is a tunnel junction that may either emit light upon injection of electrons or generate an electrical current when excited by a light wave. The current study explores a technological route for producing these functional units based upon the electromigration of metal constrictions.Results: We combine multiple nanofabrication steps to realize in-plane tunneling junctions made of two gold electrodes, separated by a sub-nanometer gap acting as the feedgap of an optical antenna. We electrically characterize the transport properties of the junctions in the light of the Fowler–Nordheim representation and the Simmons model for electron tunneling. We demonstrate light emission from the feedgap upon electron injection and show examples of how this nanoscale light source can be coupled to waveguiding structures.Conclusion: Electromigrated in-plane tunneling optical antennas feature interesting properties with their unique functionality enabling interfacing electrons and photons at the atomic scale and with the same device. This technology may open new routes for device-to-device communication and for interconnecting an electronic control layer to a photonic architecture.

Electrically Driven Optical Antennas represent promising issues for integrated plasmonic nanosources. However, their low quantum efficiency (QE) remains a major hurdle. To address this issue, we analyze the different light emission mechanisms in planar nanoscale devices. We found that the electrical properties of the device are dictating the processes at play. For low device conductance, photons are essentially emitted by inelastic tunneling events and the applied voltage sets the highest photon energy. For high conductance gap antennas, we show that the spectrum released by the device originates from the spontaneous emission of out-of-equilibrium electronic distribution. We then propose a novel route for increasing the QE. Estimation shows that if the nanoantennas are excited by electrons passing ballistically through mesoscopic contacts constituting the antenna, the excitation QE may reach ∼ 0.01–0.1. Finally, we explore an alternative approach based on charge injection in semiconductor Mie resonators.

We propose a single cavity dual comb source based on a diode pumped Yb:YAG laser modelocked by quantum dot semiconductor absorber mirror. Its output consists of two orthogonally polarized pulse trains with different repetition rate. We use two phase locked loops to simultaneously stabilize both dual comb repetition rates to an external microwave reference using an electrooptical KTP crystal and a piezoelectric transducer.

We model output characteristics of the 1645 nm 8 mJ 10 ns 100 Hz Q-switched Er:YAG DPSSL. The laser is end pumped at a wavelength of 1532 nm. Fiber-coupled diode laser module was 10 nm FWHM, 12 W CW, 200 μm, NA 0.22. Various tapering of the active rod has been considered for 1 mm diameter, 20 mm long and 0.5% Er doping. We discuss the heat deposition process, the energy storage efficiency and the average power limitations for Q-switched regime of generation and amplification, and find the system scalable for the high power operation.

Quantum emitters, such as q-dots and dye molecules, in the immediate vicinity of plasmonic nanostructures, resonantly excite surface plasmon-polaritons (SPPs) under incoherent pump. The efficiency in the excitation of SPPs per emitter increases with the number of the emitters, because the SPP field synchronizes emission of the coupled emitters, in analogy with the superadiance (SR) in free space. Using fully quantum mechanical model for two emitters coupled with a metal nanorod, we predict up to 15% increase in the emission yield of single emitter compared to only one emitter near the nanorod. Such emission enhancement is stationary and should be observable even with strong dissipation and dephasing under incoherent pump of emitters. Solid-state quantum emitters with blinking behaviors may be utilized to demonstrate such plasmonic SR emission enhancement. Plasmonic SR may find implications in the excitation of nonradiative modes in plasmonic waveguides, in lowing threshold of plasmonic nanolasers.

We demonstrate a pulse-bursting phenomenon in Yb:Er glass laser operating at 1.54 μm. Glass-ceramic material with a low value of saturation threshold based on Co2+:β-ZnSiO4 nanocrystals was used as a passive gate for pulse-burst operation. The bursts of pulses were 1.5 ms long, each burst consisted of 40-55 pulses with 9-30 μJ energy per pulse and 0.2-3 μs pulse width. Bursting outputs arise via a coupling between slow switching arising via a slow pump modulation and fast pulsations resulting from Q-switch mechanism. We show that absorption cross-section strongly affects the mode of laser operation ranging from relaxation oscillations corresponding to low cross-section values to bursting and conventional Q-switch operation in the case of their higher values.

We theoretically study the characteristics of photoelectron emission in plasmonic nanoparticle arrays. Nanoparticles are partially embedded in a semiconductor, forming Schottky barriers at metal/semiconductor interfaces through which photoelectrons can tunnel from the nanoparticle into the semiconductor; photodetection in the infrared range, where photon energies are below the semiconductor band gap (insufficient for band-to-band absorption in semiconductor), is therefore possible. The nanoparticles are arranged in a sparse rectangular lattice so that the wavelength of the lattice-induced Rayleigh anomalies can overlap the wavelength of the localized surface plasmon resonance of the individual particles, bringing about collective effects from the nanoparticle array. Using full-wave numerical simulations, we analyze the effects of lattice constant, embedding depth, and refractive index step between the semiconductor layer and an adjacent transparent conductive oxide layer. We show that the presence of refractive index mismatch between media surrounding the nanoparticles disrupts the formation of a narrow absorption peak associated with the Rayleigh anomaly, so the role of collective lattice effects in the formation of plasmonic resonance is diminished. We also show that 5 to 20-times increase of photoemission can be achieved on embedding of nanoparticles without taking into account dynamics of ballistic electrons. The results obtained can be used to increase efficiency of plasmon-based photodetectors and photovoltaic devices. The results may provide clues to designing an experiment where the contributions of surface and volume photoelectric effects to the overall photocurrent would be defined., Comment: 9 pages, 7 figures

Inelastic electron tunneling in a tunnel junction may be used as an electrical nanosource of surface plasmon polaritons and photons. In this work, we investigate emission from tunnel contact between the Platinum/Iridium tip and a thin Au film on glass. The experiment has shown that the intensity of this emission can be enhancement by use a gold nanoantenna located in the vicinity of tunnel contact.

In this paper we investigate light emission in a tunnel junction between a thin gold film on a glass substrate and a gold-coated tungsten tip of a scanning-tunneling microscope probe. The experiments show that the size of grains in the gold film surface dramatically affects the intensity of emissions. We demonstrate that a decrease in the grain aspect ratio provides an increase in the quantum yield of the tunnel-junction emission.

Numerical simulations show that the introduction of aluminum nanoparticles into one layer of a bulk-heterojunction organic solar cell leads to an increase in the rate of exciton generation in the active layer of the cell. According to calculations of the optical absorption in the cell, which have been performed in the effective refractive index approximation using the Maxwell-Garnet model, a maximum relative increase in the rate of exciton generation due to plasmonic nanoparticles is about 4%.

We propose a novel physical mechanism for the excitation of optical nanoantenna which utilizes the dissipative instability of DC electric current in the quantum well. Realization of this approach in nanoplasmonics can lead to a new device - nanoresotron.

A rigorous theory of photon emission accompanied inelastic tunnelling inside the gap of plasmonic nanoantennas has been developed. The disappointingly low efficiency of the electrical excitation of surface plasmon polaritons in these structures can be increased by orders of magnitude when a resonant tunnelling structure is incorporated inside the gap. Resonant tunnelling assisted surface plasmon emitter may become a key element in future electrically-driven nanoplasmonic circuits., 7 pages, 6 figures

We model the electron photoemission from metal nanoparticles into a semiconductor in a Schottky diode with a conductive oxide electrode hosting the nanoparticles. We show that plasmonic effects in the nanoparticles lead to a substantial enhancement in photoemission compared with devices with continuous metal films. Optimally designed metal nanoparticles can provide an effective mechanism for the photon absorption in the infrared range below the semiconductor bandgap, resulting in the generation of a photocurrent in addition to the photocurrent from band-to-band absorption in a semiconductor. Such structure can form the dais of the development of plasmonic photoemission enhanced solar cells. Copyright © 2012 John Wiley & Sons, Ltd.

We study numerically the photon input efficiency of silicon solar cells due to gold plasmonic nanoparticles deposited on the cells. At low densities, when collective effects in light scattering by the nanoparticle ensemble are negligible, the absorption dependence increases linearly for a significant range of the solar spectrum. Collective effects lead to the input efficiency saturates, reaches its maximum and then decreases with nanoparticle density. The maximal input efficiency depends on the photon wavelength, nanoparticle shape and size, their distance to the cell, and the cell thickness, and can reach ~ 95% in thick solar cells. Finally, we show that aluminum nanoparticles improve the input efficiency in comparison with gold nanoparticles.

Nanoscale electronics and photonics are among the most promising research areas providing functional nano-components for data transfer and signal processing. By adopting metal-based optical antennas as a disruptive technological vehicle, we demonstrate that these two device-generating technologies can be interfaced to create an electronically-driven self-emitting unit. This nanoscale plasmonic transmitter operates by injecting electrons in a contacted tunneling antenna feedgap. Under certain operating conditions, we show that the antenna enters a highly nonlinear regime in which the energy of the emitted photons exceeds the quantum limit imposed by the applied bias. We propose a model based upon the spontaneous emission of hot electrons that correctly reproduces the experimental findings. The electron-fed optical antennas described here are critical devices for interfacing electrons and photons, enabling thus the development of optical transceivers for on-chip wireless broadcasting of information at the nanoscale.

Quantum emitters, such as q-dots and dye molecules, in the immediate vicinity of plasmonic nanostructures, resonantly excite surface plasmon-polaritons (SPPs) under incoherent pump. The efficiency in the excitation of SPPs increases as the number of the emitters, because the SPP field synchronizes emission of the coupled emitters, in analogy with the superadiance (SR) of coupled emitters in free space. Using fully quantum mechanical model for two emitters coupled to a single gold nanorod, we predict up to 15% increase in the emission yield of single emitter compared to only one emitter coupled to the nanorod due to plasmonic SR. (XW: I use emission yield because the quantum efficiency is one for emitters in free space and there is no room for enhancement). Such emission enhancement is stationary and should be observable even with strong dissipation and dephasing under incoherent pump. Solid-state quantum emitters with blinking behaviors may be utilized to demonstrate such plasmonic SR emission enhancement. Plasmonic SR may find implications in the excitation of non-radiative modes in plasmonic waveguides; and lowing threshold of plasmonic nanolasers.

Photoconductive and photovoltaic properties of metamaterials comprising plasmonic nanocone arrays embedded in a semiconductor matrix are studied. Under uniform plane-wave illumination, directed photocurrent and electromotive force arise near asymmetrically shaped nanocones. The resulting giant photogalvanic effect is a plasmonic analogue of the bulk photovoltaic effect in ferroelectrics.

Photoelectric properties of metamaterials comprising asymmetrically shaped, similarly oriented metallic nanoparticles embedded in a homogeneous semiconductor matrix are theoretically and numerically studied. The asymmetric shape of the nanoparticles is found to result in the existence of a preferred direction where “hot” photoelectrons are emitted from the nanoparticle surface under the action of the localized plasmonic resonance excited in the nanoparticles. The resulting directional photocurrent flow occurring when nanoparticles are uniformly illuminated by a homogeneous plane wave is the direct analogy of the photogalvanic effect known to exist in naturally occurring non-centrosymmetric media. This plasmonic bulk photovoltaic effect is intermediate between the inner photoelectric effect in bulk media and the outer photoelectric effect at macroscopic interfaces. The results obtained are valuable for characterizing photoemission and photoconductive properties of plasmonic nanostructures. They can find many uses for photodetection-related and photovoltaic applications.

Numerical modeling has shown that carrier cooling and carrier heating strongly influence saturation dynamics and pulse shaping in bulk semiconductor absorbers. With no electric field in the absorbing region, carrier cooling leads to strong additional fast saturation of absorption. The saturation causes a substantial decrease in the saturation energies for subpicosecond pulses in comparison with picosecond pulses. Comparison of bulk and quantum-well absorbers shows that fast saturation can be stronger in a bulk absorber, so bulk saturable absorbers may be interesting for usage in mode-locked solid-state lasers. Applying a nonzero electric field to a bulk absorber leads to strong carrier heating, which in turn suppresses absorption saturation. In this ease, transition to absorption saturation involves new mechanisms such as screening of the electric field by photogenerated carriers, as well as carrier cooling due to carrier-phonon interaction and by generated cold carriers. Carrier heating by the electric field causes the saturation energy of the absorber to increase with the applied electric field. The increased saturation energy allows one to shorten high-energy picosecond and subpicosecond pulses without increasing the length of the saturable absorber, which could be useful for the generation of high energy pulses with mode-locked and Q-switched semiconductor lasers.

In contrast to absorbers, where the delay time via wave mixing saturates to its maximum with increasing device length due to decay of pump power along the device, in semiconductor optical amplifiers (SOAs) the delay time increases with the device length and is limited by the corresponding growth of the SOA gain, which can lead to unacceptable amplified stimulated emission in the SOA. The number of pulses which can be stored in an SOA, defined by the gain and group velocity dispersion, also is given ultimately by this SOA gain limitation, and is estimated between one and two.

We theoretically compare surface- and volume-based photoelectron emission from spherical nanoparticles, obtaining analytical expressions for the emission rate in both mechanisms. We show that the surface mechanism prevails, being unaffected by detrimental hot electron collisions.

Photoelectric properties of noncentrosymmetric, similarly oriented metallic nanoparticles embedded in a homogeneous semiconductor matrix are theoretically studied. Because of the asymmetric shape of the nanoparticle boundary, photoelectron emission acquires a preferred direction, resulting in a photocurrent flow in that direction when nanoparticles are uniformly illuminated by a homogeneous plane wave. This effect is a direct analogy of the photogalvanic (or bulk photovoltaic) effect known to exist in media with noncentrosymmetric crystal structure, such as doped lithium niobate or bismuth ferrite, but is several orders of magnitude stronger. Termed the giant plasmonic photogalvanic effect, the reported phenomenon is valuable for characterizing photoemission and photoconductive properties of plasmonic nanostructures and can find many uses for photodetection and photovoltaic applications.

It is shown that interferometers containing quantum-dot semiconductor optical amplifiers can be effective for ultrafast cross-phase modulation and digital signal processing with low dependence on the specific random data pattern.

We propose to use collective lattice resonances in plasmonic nanoparticle arrays to enhance photoelectron emission in Schottky-barrier photodetectors and solar cells. We show that the interaction of lattice resonances (the Rayleigh anomaly) and individual particle excitations (localized surface plasmon resonances) leads to stronger local field enhancement and significant increase of the photocurrent compared to the case when only individual particle excitations are present. The results can be used to design new photodetectors with highly selective, tunable spectral response, able to detect photons with the energy below the semiconductor bandgap, and to develop solar cells with increased efficiency., Comment: To appear in Plasmonics

Pump-probe measurements in reverse-biased GaAs-GaAlAs waveguide saturable absorbers used in mode-locked diode lasers reveal an ultrafast transient in the absorption recovery dynamics, and a second slower signal before the final recovery. In this paper, we present a numerical model of the absorption and carrier dynamics in these saturable absorbers. The model includes high-field and space-charge effects, in addition to the spectral hole burning and carrier heating typically considered in semiconductor laser amplifiers. The results of the model indicate that field-induced carrier heating strongly influences the absorption dynamics in saturable absorbers. Screening of the electric field by drifting photogenerated charges affects the sweepout of carriers from the absorbing region, and thus, influences the absorption dynamics as well. >

We introduce a new type of electroplasmonic interfacing component to electrically generate surface plasmons. Specifically, an electron-fed optical tunneling gap antenna is integrated on a plasmonic waveguiding platform. When electrical charges are injected in the tunneling barrier of the gap antenna, a broad-band radiation is emitted from the feed area by a process identified as a thermal emission of hot electrons. Part of the emitted photons couples to surface plasmon modes sustained by the waveguide geometry. The transducing optical antenna is thus acting as a localized electrical source of surface plasmon polaritons. The integration of electrically-activated optical antennas into a plasmonic architecture mitigates the need for complex coupling scheme and proposes a solution for realizing nanoscale units at the interface between nano-electronics and photonics.

In order to improve the photoconversion efficiency, we consider the possibility of increasing the photocurrent in solar cells exploiting the electron photoemission from small metal nanoparticles into a semiconductor. The effect is caused by the absorption of photons and generation of local surface plasmons in the nanoparticles with optimized geometry. An electron photoemission from metal into semiconductor occurs if photon energy is larger than Schottky barrier at the metal-semiconductor interface. The photocurrent resulting from the absorption of photons with energy below the bandgap of the semiconductor added to the solar cell photocurrent can extend spectral response range of the device. We study the effect on a model system, which is a Schottky barrier n-GaAs solar cell, with an array of Au nanoparticles positioned at the interface between the semiconductor and the transparent top electrode. Based on the simulations, we chose to study disk-shaped Au nanoparticles with sizes ranging from 25nm to 50nm using electron beam lithography. Optical characterization of the fabricated devices shows the presence of LSP resonance around the wavelength of 1250nm, below the bandgap of GaAs.

A.Brodsky and Yu.Gurevitch approach is discussed and generalized for photoemission from metal nano-particles taking into account the excitation of localized plasmon resonance (LPR) and changes of electromagnetic field (EMF) and conduction electron mass in the metal - environment interface. New result is the increase of photo-emission current several time respectively to the case of continues metal film due to increase of intensity of EMF near the surface of nanoparticles and also due to surface phenomena mentioned above. Results can be applied for development new photodetectors, photo energy converters (solar cells) and for more studies of photoemission from metal nanoparticles., Accepted for publication in Uspekhi Fizicheskikh Nauk, http://ufn.ru/en/articles/accepted/35575/, Citation: Protsenko I E, Uskov A V "Photoemission from metal nanoparticles" Phys. Usp., accepted

We report on an experimental and theoretical study of phase transients at 1.3µm in an InAs/GaAs QD SOA in response to pump pulses tuned to either the dot ground or excited state.

Carrier heating and spectral hole burning are shown to have a strong influence on the amplification of ultrashort pulses using semiconductor laser amplifiers. Approximate analytical expressions for the effective saturation energy in different pulsewidth regimes are derived. >

We present a novel scheme to increase the THz-bandwidth fast light effect in semiconductor optical amplifiers and increase the number of advanced pulses. By introducing a linear chirp to the input pulses before the SOA and recompressing at the output with an opposite chirp, the advance-bandwidth product reached 3.5 at room temperature, 1.55 microm wavelength. This is the largest number reported, to the best of our knowledge, for a semiconductor slow/fast light device.

Tunable delays in semiconductor optical amplifiers are achieved via four wave mixing between a strong pump beam and a modulated probe beam. The delay of the probe beam can be controlled both electrically, by changing the SOA bias, and optically, by varying the pump power or the pump-probe detuning. For sinusoidal modulated signal at 0.5 GHz, a tunable delay of 1.6 ns is achieved. This corresponds to a RF phase change of 1.6 pi. For 1.3 ns optical pulses propagating through the SOA a delay of 0.59 ns is achieved corresponding to a delay-bandwidth product exceeding 0.45. For both the cases, slow light and superluminal light are observed as the pump-probe detuning is varied.

We show that the various techniques commonly used to measure the linewidth enhancement factor can lead to different values when applied to quantum dot semiconductor lasers. Such behaviour is a direct consequence of the intrinsic capture/escape dynamics of quantum dot materials and of the free carrier plasma effects. This provides an explanation for the wide range of values experimentally measured and the linewidth re-broadening recently measured.

Chirped pulse scheme is shown to be highly effective to attain large tunable time shifts via slow and fast light for an ultra-short pulse through a semiconductor optical amplifier (SOA). We show for the first time that advance can be turned into delay by simply reversing the sign of the chirp. A large continuously tunable advance-bandwidth product (ABP) of 4.7 and delay-bandwidth product (DBP) of 4.0 are achieved for a negatively and positively chirped pulse in the same device, respectively. We show that the tunable time shift is a direct result of self-phase modulation (SPM). Theoretical simulation agrees well with experimental results. Further, our simulation results show that by proper optimization of the SOA and chirper design, a large continuously tunable DBP of 55 can be achieved.

A simple model is presented for carrier heating in semiconductor lasers from which the temperature dynamics of the electron and hole distributions can be calculated. Analytical expressions for two new contributions to the nonlinear gain coefficient, in are derived, which reflect carrier heating due to stimulated emission and free carrier absorption. In typical cases, carrier heating and spectral holeburning are found to give comparable contributions to nonlinear gain suppression. The results are in good agreement with recent measurements on InGaAsP laser diodes. >

We present theoretical study of fast and slow light in a chirp-compensate semiconductor optical amplifier scheme. We obtain excellent match with experimental results. Using this model, we predict that a delay-bandwidth product of 33 can be attained with properly chosen parameters.

We present excellent agreement of theoretical simulation and experimental results of THz fast and slow light using a novel chirped-pulse technique. A large timeshift of 8.7 pulses is demonstrated with minimum broadening and distortion.

Polarization dependence of fast light in semiconductor optical amplifiers is studied. For a 630 fs pulse propagating through the SOA, an advance of 2.7 pulses is achieved for TE polarisation while TM polarisation gives an advance of 2.3 pulses.

We demonstrate fast light advance of 2.7 pulse widths for sub-picosecond pulses in a single semiconductor optical amplifier. Two such pulses, separated by 7 ps, can also be advanced more than 1.8 pulse widths each.

Electrically tunable delays and advances for 600fs pulses are achieved using ultra-fast nonlinearities in SOAs. Feasibility of cascading multiple SOAs to achieve higher delays is confirmed using a novel scheme that uses a single SOA. (Slow and Fast Light, 2007) Article not available.

The change in group index in a semiconductor optical amplifier due to carrier heating and spectral hole burning dominates in short pulse shaping, resulting in fast/slow light.

Electrically tunable delays and advances for 600fs pulses are achieved using ultra-fast nonlinearities in SOAs. Feasibility of cascading multiple SOAs to achieve higher delays is confirmed using a novel scheme that uses a single SOA.

We analyze a self-assembled quantum-dot laser where lasing levels are pumped purely through Auger carrier capture. Analysis shows that Auger carrier capture and Auger transport of carriers between quantum dots can provide effective laser pumping and fast modulation at least up to 10 GHz. Auger carrier transport decreases spectral hole burning, which together with dot-barrier hole burning can damp relaxation oscillations.