Optical Pump Intensity Research Articles

Ultrafast, broadband, and configurable midinfrared all-optical switching in nonlinear graphene plasmonic waveguides

Graphene plasmonics provides a unique and excellent platform for nonlinear all-optical switching, owing to its high nonlinear conductivity and tight optical confinement. In this paper, we show that impressive switching performance on graphene plasmonic waveguides could be obtained for both phase and extinction modulations at sub-MW/cm2 optical pump intensities. Additionally, we find that the large surface-induced nonlinearity enhancement that comes from the tight confinement effect can potentially drive the propagating plasmon pump power down to the pW range. The graphene plasmonic waveguides have highly configurable Fermi-levels through electrostatic-gating, allowing for versatility in device design and a broadband optical response. The high capabilities of nonlinear graphene plasmonics would eventually pave the way for the adoption of the graphene plasmonics platform in future all-optical nanocircuitry.

Quantum networks

Researchers at University College Cork in Ireland have shown how the power budget of a passive optical network (PON) can be increased to support a higher number of customers and have longer reach without introducing active elements into the outside network plant. The bandwidth of upstream links in the PON is shared using time division multiplexing to allocate separate time-slots to the different users, which transmit bursts of data. Due to the non-uniform distribution of the outside network, plant loss, and differences in the user transmitters, the burst from the various users can present power levels with a high dynamic range. The burst-mode operation is ensured without the need to adopt additional high-speed electronic controls Optical fibre networks provide the backbone of today's global internet: the high bandwidth and long transmission reach of optical fibre make it the only technology capable of supplying the high speeds aggregate traffic capacities required for core and metro network links. As the bandwidth demand from end-users grows, optical fibre is becoming an economical solution for broadband access, where fibre-to-the-home solutions compete with other technologies, such as wireless and copper. To increase fibre efficiency and capability, various reach extenders can be used. These devices are available for passive optical networks including optical-electronic-optical repeaters and discrete optical amplifiers, based on either semiconductor optical amplifiers (SOA) or doped fibre amplifiers (xDFA). Unfortunately, they are usually placed in a powered enclosure or street cabinet, which increases installation and maintenance costs. Discrete amplifiers also need a high speed power control circuit for dealing with the upstream burst traffic. The solution proposed in the featured work maintains the passive structure of the network and does not need external control circuitry. It also offers a wider coverage of the optical spectrum used in PONs simply by changing the pump laser wavelength. Raman based reach extenders can also be used, and this method takes advantage of the Raman effect. The Raman effect is an inelastic scattering process in which an input optical photon is re-emitted at a different frequency with lower (or higher) energy, and the difference in energy is absorbed (or emitted) by molecular vibrations of the medium. In the silica optical fibre the Raman spectrum is broad due to the disorder of the silica molecules, and has a maximum at a frequency shift of 13.2 THz. Hence, if an intense optical pump source's frequency shifts by this amount when an optical data signal is introduced into the same optical fibre, stimulated Raman scattering will occur, effectively transferring part of the pump energy to the signal, leading to amplification. This can be used by reach extenders to amplify the signal. A number of factors have contributed to the recent development of compact reach extender modules that allow the network central office to extend the scale of already existing PON systems. These factors include the commercial availability of new high-power compact semiconductor quantum-dot lasers at the wavelengths of interest for this application, and the low-complexity structure of modern amplifiers. This opens up the possibility of compact reach extender modules for the network central office to extend the scale of already existing PON systems. Without such pump sources, more expensive and bulky fibre lasers would be required, and these offer lower potential for cost-effective integration. Increasing the span of fibre to home systems to 50 km opens the possibility of decreasing the number of electronic switching nodes in the network, thus reducing cost This technology is thus suitable for use in the public sphere, since the fabrication process of the employed pump lasers is established, making for reliable production. The 10 gigabit passive optical networks (XG-PON) standard is particularly suitable for this type of amplification, as only a relatively narrow spectral region is used for the upstream link, which can be covered by a single wavelength Raman pump module. Other standards, such as gigabit passive optical networks (G-PON), would require more stringent transmitter wavelength restrictions or the use of multiple wavelength Raman pump modules. This technology could be deployed by network operators and, if so, would benefit the general public through the wider availability of very high speed, fibre-enabled broadband.

On the Photoluminescence Linearity of Eu2+ Based LED Phosphors upon High Excitation Density

This work deals with the saturation behavior of Eu2+ doped luminescent materials. Eu2+ activated phosphors, excited by high radiance pump sources, like high power LEDs or Laser Diodes, offer considerable potential for high radiance conversion in Solid State Lighting. Remarkably, theoretical arguments suggest that the radiance of the luminescent spot should increase linearly with the excitation density of the incoming light source up to 1 kW/mm2. In practice, however, thermal quenching and (non-thermal) optical saturation limit the maximum attainable radiance of the luminescent material. We present experimental data of the widely applied red emitting LED phosphors (Ca,Ba)2SiO4:Eu2+ (OSE), Ba2Si5N8:Eu2+ (258:Eu), and CaAlSiN3:Eu2+ (CASN:Eu) in which these limits have been investigated. High excitation fluxes are achieved using laser pumping at 405 nm up to 800 W/mm2. In case of 258:Eu and CASN:Eu, optical pumping intensities have been shown to produce a significant efficiency depreciation already at low pump powers, which does not correlate with the relatively short Eu2+ luminescence lifetime (∼1 μs). We assume a de-excitation mechanism that depletes the excited state [Xe]4f 65d1 to the conduction band of the host structure, which bottlenecks the limit maximum pump intensities. We find that the investigated Eu2+ phosphors tend to show different efficiency depreciation. Finally, we use a rate equation model to simulate this nonlinear optical pumping.

Terahertz time domain spectroscopy for carrier lifetime mapping in the picosecond to microsecond regime.

We demonstrate a terahertz time-domain spectroscope for spatially resolved pump-probe experiments which are based on terahertz probe pulse generation with a photo-conductive switch synchronized to the pump pulse generation of a nanosecond laser. The accessible pump-probe delay ranges from 600 ps up to 200 μs. The spatial resolution of the spectroscope is better than 50 μm. We use the measurement system for spatially resolved lifetime mapping of photo-induced carriers in thin silicon in dependence on the photoexcitation intensity of the pump laser. At an optical pump intensity of 70mW/cm2, we measured a carrier lifetime of 15.7 μs for silicon with 200 μm thickness and a much shorter carrier lifetime of 233 ns for 30 μm thick silicon.

Threshold pump intensity effect on the refractive index changes in InGaN SQD: Internal constitution and size effects

In the present paper, internal composition and size-dependent threshold pump intensity effects on on-center impurity-related linear, third-order nonlinear and total refractive index changes are investigated in wurtzite (In,Ga)N/GaN unstrained spherical quantum dot. The calculation is performed within the framework of parabolic band and single band effective-mass approximations using a combination of Quantum Genetic Algorithm (QGA) and Hartree–Fock–Roothaan (HFR) method. According to the results obtained, (i) a significant red-shift (blue shift) is obtained as the dot size (potential barrier) increases and (ii) a threshold optical pump intensity depending strongly on the size and the internal composition is obtained which constitutes the limit between two behaviors.

Thermally induced whispering gallery mode laser in MEH-PPV solutions

We report on thermally activated whispering gallery laser modes of a solution of MEH-PPV conjugated polymer supported by a silica optical fiber. The viscosity of the polymer solution gives place to a thin shell of the gain solution around the fiber driven by capillary action. Whispering gallery modes (WGMs) are thermally induced by a decrease in the refractive index of the polymer solution under intense optical pumping. The laser emission is produced because the evanescent waves of the WGMs couple the surrounding gain medium. These results support the use of conjugated polymers in optofluidic laser systems and highlight the importance of physicochemical properties, such as viscosity and optically induced heating, on the performance of the devices.

Persistence of magnetic order in a highly excited Cu2+ state in CuO

We use ultrafast resonant x-ray diffraction to study the magnetic order in CuO under conditions of high electronic excitation. By measuring changes in the spectral shape of the Cu2+ magnetic (1/2 0 −1/2) reflection we investigate how an intense optical pump pulse perturbs the electronic and magnetic states. We observe an energy shift in the magnetic resonance at short times after the pump pulse. This shift is compared with expectations from band structure calculations at different electronic temperatures. This spectral line shift indicates that although the electrons are heated to effective electron temperatures far above TN on a time scale faster than the experimental resolution, magnetic order persists in this highly excited state for several hundred femtoseconds.Received 17 September 2013Revised 24 March 2014Corrected 25 June 2014DOI:https://doi.org/10.1103/PhysRevB.89.220401©2014 American Physical Society

Attenuation characteristics of the guided THz wave in parallel-plate ferroelectric-graphene waveguides

Guided THz wave characteristics in a parallel-plate waveguide (PPWG) consisting of ferroelectric film (LiNbO3 and LiTaO3) and multilayer graphene (MLG) is studied in this paper, with their low and tunable attenuation valley predicted. The electrical conductivity of MLG is calculated by a set of closed-form equations with the coupling effect between the bottom graphene layer (BGL) and its substrate taken into account carefully, while the dispersive behavior of ferroelectric film itself is described by the Lorentz model over an ultra-wide THz band. It is shown that the guided TM-mode propagation can be adjusted effectively by changing temperature, frequency, optical pumping intensity, MLG layer number, film thickness and its transverse optical-phonon frequency. Moreover, one low attenuation valley of TM-mode in such ferroelectric-graphene waveguide is captured, which can be exploited for developing some THz planar tunable waveguides with ultra-low loss.

Spin relaxation time dependence on optical pumping intensity in GaAs:Mn

We analyze the dependence of electron spin relaxation time on optical pumping intensity in a partially compensated acceptor semiconductor GaAs:Mn using analytic solutions for the kinetic equations of the charge carrier concentrations. Our results are applied to previous experimental data of spin-relaxation time vs. excitation power for magnetic concentrations of approximately 1017 cm−3. The agreement of our analytic solutions with the experimental data supports the mechanism of the earlier-reported atypically long electron-spin relaxation time in the magnetic semiconductor.

Terahertz wave responses of dispersive few-layer graphene (FLG) and ferroelectric film composite

Terahertz wave responses of few-layer graphene (FLG) and ferroelectric film (LiNbO3) composite are studied using our developed algorithm based on the spectral-domain exponential matrix (SDEM) technique. The electrical conductivity of such FLG is calculated by a set of closed-form equations with coupling effect between the bottom graphene layer (BGL) and the substrate treated in an appropriate way, while the dispersive properties of the LiNbO3 film are described by the Lorentz model over an ultra-wide THz band. It is shown that both transmittance and reflectance of the composite can be adjusted effectively by changing the number of layers in FLG, the optical pumping intensity and the transverse optical-phonon frequency of LiNbO3. Moreover, producing such FLG and ferroelectric film composite is technically feasible, which can be exploited for further developing some novel planar THz structures.

Effect of optical pumping on the momentum relaxation time of graphene in the terahertz range

The momentum relaxation time of a photoexcited graphene in the THz frequency range has been studied by using terahertz time domain spectroscopy under optical pumping at room temperature. It is found that the momentum relaxation time of the graphene as a function of the optical pumping intensity exhibits a threshold behavior. The features of the momentum relaxation time as a function of the optical pumping intensity are also investigated. The results are useful for understanding the basic underlying physics of graphene scattering as well as finding the possible applications in carbon-based electronics.

All-optical tuning of the Stokes shift in PbS quantum dots

The Stokes shift of colloidal 4.7 nm PbS quantum dots was measured between 5 and 300 K at incrementally increasing continuous laser intensities. The results demonstrate Stokes shift tuning by optical means only at stable given temperatures due to optically enforced electronic state alteration in the quantum dots. The tuning phenomenon is perfectly fit by a semi-empirical model, which provides a design tool for the chromaticity of quantum dots at different optical pump intensities.

Resolution enhancement in noise spectrum by using velocity selective optical pumping in cesium vapor

We demonstrate experimentally that the resolution of amplitude noise spectrum in Cs atomic vapor can be enhanced by narrowing the absorption using velocity selective optical pumping technique. It is found that the steep atomic dispersion accompanied by high absorption leads to more conversion of laser phase noise to amplitude noise, when the field propagates throughout the atoms, and meanwhile the spectral resolution is improved. The effect of optical pumping intensity on the spectrum resolution is experimentally discussed, and a theoretical explanation for this phenomenon is given, which shows that the phase-to-amplitude noise conversion is directly proportional to the dispersion of medium.

Optical magnetometer array for fetal magnetocardiography

We describe an array of spin-exchange-relaxation-free optical magnetometers designed for detection of fetal magnetocardiography (fMCG). The individual magnetometers are configured with a small volume with intense optical pumping, surrounded by a large pump-free region. Spin-polarized atoms that diffuse out of the optical pumping region precess in the ambient magnetic field and are detected by a probe laser. Four such magnetometers, at the corners of a 7 cm square, are configured for gradiometry by feeding back the output of one magnetometer to a field coil to null uniform magnetic field noise at frequencies up to 200 Hz. We present the first measurements of fMCG signals using an atomic magnetometer.

Tunneling recombination in optically pumped graphene with electron-hole puddles

We evaluate recombination of electrons and holes in optically pumped graphene associated with the interband tunneling between electron-hole puddles and calculate the recombination rate and time. It is demonstrated that this mechanism can be dominant in a wide range of pumping intensities. We show that the tunneling recombination rate and time are nonmonotonic functions of the quasi-Fermi energies of electrons and holes and optical pumping intensity. This can result in hysteresis phenomena.

Nonlinear Temperature Dependence of Resonant Pump Wavelengths in Optical Pumping Injection Cavity Lasers

Type-II W optical pumping injection cavity lasers emitting ~3 μm at room temperature are pumped resonantly by tuning with an optical parametric oscillator. The resonance pump wavelength consistently is found to vary quadratically with temperature. Linear corrections to the lattice constant and index of refraction with temperature do not adequately account for this trend. Using multilayer reflectivity modeling, this nonlinear temperature dependence is attributed to gain-induced changes in the refractive index of the active region and spacer layers that result from the increase in optical pumping intensity with increasing temperature.

Spontaneous and stimulated red luminescence of Lu2O3: Eu nanocrystals

The spectra of spontaneous and stimulated luminescence of Lu2O3: Eu (7 at %) nanopowders at different optical pumping intensities have been investigated. The obtained results—changes in the shape of the red luminescence spectra and in the lifetime of the 5 D 0 excited state of Eu3+ ions—indicate the onset of superluminescence with an increase in the excitation power. It has been found that an increase in the optical pumping intensity leads to a decrease in the luminescence decay time of the Lu2O3: Eu (7 at %) phosphor in the stimulated luminescence regime and to an increase in the quantum efficiency of red luminescence with a maximum at 611 nm.

Repetition of the shape of the ultrafast self-modulation of the optical absorption spectrum upon varying the energy of pulse of GaAs pumping

Ultrafast self-modulation of the fundamental optical absorption emerges during intense picosecond optical pumping of GaAs and, according to the main assumption, reflects self-oscillations of depletion of electron populations in the conduction band. In this study, the quantitatively confirmed explanation of previously experimentally found cyclic repetition of the form of ultrafast self-modulation of the absorption spectrum upon varying the energy of the pumping pulse and fixed delay between pumping and probing (the measurement of absorption) is given. Repetition of the shape is explained by varying the phase of self-oscillations of the optical absorption. The explanation is based on the previously found experimentally dependence of the frequency of self-oscillations of absorption on the pumping energy. Therefore, this is also a new confirmation of the mentioned dependence (which satisfactorily coincides with a similar calculated dependence of the frequency of self-oscillations of depletion of populations).

Biexciton emission from sol-gel ZnMgO nanopowders

We studied the power-dependent photoluminescence of Zn1−xMgxO nanopowders grown by sol-gel method, at temperature T=100 K. At moderate optical pumping intensity, a nonlinear emission band due to the radiative recombination of free biexcitons was detected. We found that the free biexciton binding energies of Zn1−xMgxO nanopowder (0.01≤x≤0.05) are nearly constant (13.5±1.5 meV).

Photochemistry in Photonic Crystal Fiber Nanoreactors

We report the use of a liquid-filled hollow-core photonic crystal fiber (PCF) as a highly controlled photochemical reactor. Hollow-core PCFs have several major advantages over conventional sample cells: the sample volume per optical path length is very small (2.8 nL cm(-1) in the fiber used), long optical path lengths are possible as a result of very low intrinsic waveguide loss, and furthermore the light travels in a diffractionless single mode with a constant transverse intensity profile. As a proof of principle, the (very low) quantum yield of the photochemical conversion of vitamin B(12), cyanocobalamin (CNCbl) to hydroxocobalamin ([H(2)OCbl](+)) in aqueous solution was measured for several pH values from 2.5 to 7.5. The dynamics of the actively induced reaction were monitored in real-time by broadband absorption spectroscopy. The PCF nanoreactor required ten thousand times less sample volume compared to conventional techniques. Furthermore, the enhanced sensitivity and optical pump intensity implied that even systems with very small quantum yields can be measured very quickly--in our experiments one thousand times faster than in a conventional cuvette.