Threshold Pump Intensity Research Articles

Superradiant Topological Peierls Insulator inside an Optical Cavity.

We consider a spinless ultracold Fermi gas tightly trapped along the axis of an optical resonator and transversely illuminated by a laser closely tuned to a resonator mode. At a certain threshold pump intensity, the homogeneous gas density breaks a Z_{2} symmetry towards a spatially periodic order, which collectively scatters pump photons into the cavity. We show that this known self-ordering transition also occurs for low field seeking fermionic particles when the laser light is blue detuned to an atomic transition. The emergent superradiant optical lattice in this case is homopolar and possesses two distinct dimerizations. Depending on the spontaneously chosen dimerization, the resulting Bloch bands can have a nontrivial topological structure characterized by a nonvanishing Zak phase. In the case where the Fermi momentum is close to half of the cavity-mode wave number, a Peierls-like instability here creates a topological insulator with a gap at the Fermi surface, which hosts a pair of edge states. The topological features of the system can be nondestructively observed via the cavity output: the Zak phase of the bulk coincides with the relative phase between laser and cavity field, while the fingerprint of edge states can be observed as additional broadening in a well-defined frequency window of the cavity spectrum.

Long-wavelength stimulated emission and carrier lifetimes in HgCdTe-based waveguide structures with quantum wells

The interband photoconductivity and photoluminescence in narrow-gap HgCdTe-based waveguide structures with quantum wells (QWs) (designed for long-wavelength stimulated emission under optical pumping) are investigated. The photoconductivity relaxation times in n-type structures reach several microseconds, due to which stimulated emission at a wavelength of 10.2 μm occurs at a low threshold pump intensity (~100 W/cm2) at 20 K. In the p-type structures obtained by annealing (to increase the mercury vacancy concentration), even spontaneous emission from the QWs is not detected because of a dramatic decrease in the carrier lifetime with respect to Shockley–Read–Hall nonradiative recombination.

All-optical high performance graphene-photonic crystal switch

The all-optical switch is realized based on nonlinear transmission changes in Fano resonance of 2D photonic crystals (PhC) which enhances the light intensity on the graphene in PhC; and in this study, the graphene layer is used as the nonlinear material. The refractive index change of graphene layer leads to a shift in the Fano resonance frequency due to the input light intensity through the Kerr nonlinear effect. Through finite-difference time-domain simulation, it is found that the high performance of all-optical switching can be achieved by the designed structure with a threshold pump intensity as low as MW/cm2. This structure is featured by optical bistability. The obtained results are applicable in micro optical integrated circuits for modulators, switches and logic elements for optical computation.

Diode-Pumped Organo-Lead Halide Perovskite Lasing in a Metal-Clad Distributed Feedback Resonator.

Organic-inorganic lead halide perovskite semiconductors have recently reignited the prospect of a tunable, solution-processed diode laser, which has the potential to impact a wide range of optoelectronic applications. Here, we demonstrate a metal-clad, second-order distributed feedback methylammonium lead iodide perovskite laser that marks a significant step toward this goal. Optically pumping this device with an InGaN diode laser at low temperature, we achieve lasing above a threshold pump intensity of 5 kW/cm(2) for durations up to ∼25 ns at repetition rates exceeding 2 MHz. We show that the lasing duration is not limited by thermal runaway and propose instead that lasing ceases under continuous pumping due to a photoinduced structural change in the perovskite that reduces the gain on a submicrosecond time scale. Our results indicate that the architecture demonstrated here could provide the foundation for electrically pumped lasing with a threshold current density Jth < 5 kA/cm(2) under sub-20 ns pulsed drive.

Energy transfer and energy level decay processes of Er3+ in water-free tellurite glass

This report details the fundamental spectroscopic properties of a new class of water-free tellurite glasses studied for future applications in mid-infrared light generation. The fundamental excited state decay processes relating to the 4I11/2→4I13/2 transition in singly Er3+-doped Tellurium Zinc Lanthanum glass have been investigated using time-resolved fluorescence spectroscopy. The excited state dynamics was analyzed for Er2O3 concentrations between 0.5mol% and 4mol%. Selective laser excitation of the 4I11/2 energy level at 972nm and selective laser excitation of the 4I13/2 energy level at 1485nm has established that in a similar way to other Er3+-doped glasses, a strong energy-transfer upconversion by way of a dipole–dipole interaction between two excited erbium ions in the 4I13/2 level populates the 4I11/2 upper laser level of the 3μm transition. The 4I13/2 and 4I11/2 energy levels emitted luminescence with peaks located at 1532nm and 2734nm respectively with luminescence efficiencies of 100% and 8% for the higher (4mol.%) concentration sample. Results from numerical simulations showed that a population inversion is reached at a threshold pumping intensity of ∼57kWcm−2 for a CW laser pump at 976nm for [Er2O3]=2mol.%.

Multilayer-WS2:ferroelectric composite for ultrafast tunable metamaterial-induced transparency applications

An ultrafast and low-power all-optical tunable metamaterial-induced transparency is realized, using polycrystalline barium titanate doped gold nanoparticles and multilayer tungsten disulfide microsheets as nonlinear optical materials. Large nonlinearity enhancement is obtained associated with quantum confinement effect, local-field effect, and reinforced interaction between light and multilayer tungsten disulfide. Low threshold pump intensity of 20 MW/cm2 is achieved. An ultrafast response time of 85 ps is maintained because of fast carrier relaxation dynamics in nanoscale crystal grains of polycrystalline barium titanate. This may be useful for the study of integrated photonic devices based on two-dimensional materials.

Multilayer-MoS2-microsheet/(Nano-Au:LiNbO3) for all-optical tunable metamaterial-induced transparency

All-optical tunable metamaterial-induced transparency is realized using polycrystalline lithium niobate doped with gold nanoparticles and multilayer molybdenum disulfide microsheets as a third-order nonlinear optical material. A low threshold pump intensity of 90 kW cm−2 is obtained based on nonlinearity enhancement associated with the quantum confinement effect, the local-field enhancement effect, and reinforced interaction between photons and the multilayer molybdenum disulfide microsheets. An ultrafast response time of 27.4 ps is maintained owing to the fast relaxation dynamics of bound electrons in the polycrystalline lithium niobate. This work may pave a way for the realization of ultrahigh speed information processing chips based on metamaterials.

Terahertz lasers based on intracentre transitions of group V donors in uniaxially deformed silicon

This paper presents a brief overview of available experimental data on the characteristics of stimulated terahertz emission from optically excited neutral group V donors (phosphorus, antimony, arsenic and bismuth) in crystalline silicon subjected to uniaxial compressive strain along the [100] axis. Strain is shown to have a significant effect on the characteristics in question. Optimal strain depends on the dopant and may reduce the threshold pump intensity and improve lasing efficiency. We discuss possible mechanisms behind this effect and estimate the limiting output emission parameters.

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.

Measurement of the threshold of nonfocusing-pumped stimulated Brillouin scattering based on temporal characteristic of the reflected pulse

We present a novel method for measuring the threshold of stimulated Brillouin scattering with large-aperture super-Gaussian-shaped laser pulses transmitting in a nonlinear medium. From the differences between the waveforms of reflected light below and above the SBS threshold and the “energy in the bucket” method, threshold pump intensity can be determined clearly. Experiment is carried out with a high-power Nd:glass laser system of 527 nm wavelength, used as the light source that delivers 3 ns super-Gaussian-shaped pulses, and the nonlinear medium FC-40. The experimental results agree well with the numerically calculated results, and the SBS threshold in this case is 780 MW/cm2.

Multilayer Graphene:Polycrystalline ITO for Ultralow-Power Active Control of Polarization-Insensitive, Metamaterial-Induced Transparency

An ultralow power polarization-insensitive metamaterial-induced transparency with ultrafast all-optical control is realized by adopting gold nanoprism trimers as the meta-molecules and using multilayer graphene microsheet/polycrystalline indium-tin oxide composite as the nonlinear medium. Compared with previous reports, the threshold pump intensity is reduced by six orders of magnitude, while an ultrafast response time of picoseconds order is maintained.

Design of a terahertz parametric oscillator based on a resonant cavity in a terahertz waveguide

We demonstrate ns-pulsed pumping of terahertz (THz) parametric oscillations in a quasi-triply resonant cavity in a THz waveguide. The THz waves, down converted through parametric interactions between the pump and signal waves at telecom frequencies, are confined to a GaP single mode ridge waveguide. By combining the THz waveguide with a quasi-triply resonant cavity, the nonlinear interactions can be enhanced. A low threshold pump intensity for parametric oscillations can be achieved in the cavity waveguide. The THz output power can be maximized by optimizing the quality factors of the cavity so that an optical to THz photon conversion efficiency, ηp, of 0.35, which is near the quantum-limit level, can be attained. The proposed THz optical parametric oscillator can be utilized as an efficient and monochromatic THz source.

Light amplification in DNA-surfactant complex films stained by hemicyanine dye with immersion method

Organic dye-doped deoxyribonucleic acid (DNA)-surfactant complex films for dye lasers were fabricated by immersing non-doped complex films into a solution of hemicyanine dye. The threshold pumping intensity for amplified spontaneous emission was found to be 0.3 mJ/cm2, the value was one order smaller than those obtained for the samples made by conventional methods. Durability under pumping was also significantly improved and laser oscillation under optical excitation was observed. Dye concentration of the final products was estimated to be 10 wt% and there were no deformation of the samples, suggesting that dye molecules in the complex did not necessarily intercalate in DNA strand but replace with surfactant molecules.

All-optical sensitive phase shifting based on nonlinear out-of-plane coupling through 1-D slab photonic crystal with a layer of graphene.

We realize all-optical sensitive phase shifting based on nonlinear out-of-plane coupling to a slab waveguide through Fano resonance of a slab 1-D photonic crystal (PhC). We use a graphene layer as the nonlinear material and change its refractive index by the input light intensity through Kerr nonlinear effect to obtain a shift in the Fano resonance frequency. The Fano resonance and self-focusing effect lead to light-intensity enhancement on the graphene in the PhC, reinforcing the nonlinear effect of refractive index in the graphene. Through finite-difference time-domain simulation, we demonstrate that the phase changing sensitivity obtained can be 4 orders higher than that by a single graphene under the same input light intensity. Moreover the threshold pump intensity for all-optical sensitive phase shifting in the coupled light to the waveguide is as low as ~4 MW per square centimeter. The results are applicable in micro optical integrated circuits for phase shifters, phase modulators, power limiters, and phase logic elements for optical computation, digital phase shift keying in communication systems, and non-contact sensitive signal detectors.

Generation of continuously tunable 5 µm coherent radiation by second harmonic conversion of the emission of a multi-atmosphere TE CO2 laser in AgGaSe2

Operation of a continuously tunable coherent source in the 5 micron region of the mid-infrared by frequency doubling of the continuously tunable emission of a multi-atmosphere TE CO2 laser in an AgGaSe2 crystal is reported. The performance has been characterized with regard to energy and power conversion efficiency as well as the threshold pump intensities at which the onset and termination of the second harmonic (S H) conversion process occurs. The absence of a gain switched peak in the emission of the pump laser that stemmed from its operation with a lean CO2 gas mixture enabled error-free threshold measurement for the onset of the S H generation process. Further, the excellent phase and amplitude synchronization of the SH emission with the pump pulse, that could be established by taking advantage of the rich modulation in the emission of the multimode pump laser in nanosecond time scale, bore experimental testimony to the instantaneous nature of the SH generation process.

Ultralow-power all-optical tunable double plasmon-induced transparencies in nonlinear metamaterials

An all-optical tunable double plasmon-induced transparency is realized in a photonic metamaterial coated on the surface of a nanocomposite layer made of polycrystalline indium-tin oxide doped with gold nanoparticles. The local-field effect, quantum confinement effect, and hot-electron injection ensure a large optical nonlinearity for the nanocomposite. A shift of 120 nm in the central wavelength of transparency windows is reached under excitation with a weak pump laser with an intensity of 21 kW/cm2. Compared with previous reports, the threshold pump intensity is reduced by five orders of magnitude, while an ultrafast response time of 34.9 ps is maintained.

Low-power all-optical tunable plasmonic-mode coupling in nonlinear metamaterials

All-optical tunable plasmonic-mode coupling is realized in a nonlinear photonic metamaterial consisting of periodic arrays of gold asymmetrically split ring resonators, covered with a poly[(methyl methacrylate)-co-(disperse red 13 acrylate)] azobenzene polymer layer. The third-order optical nonlinearity of the azobenzene polymer is enormously enhanced by using resonant excitation. Under excitation with a 17-kW/cm2, 532-nm pump light, plasmonic modes shift by 51 nm and the mode interval is enlarged by 30 nm. Compared with previous reports, the threshold pump intensity is reduced by five orders of magnitude, while extremely large tunability is maintained.

Steady State and Transient Gain Characteristics of the Stimulated Brillouin Scattered Mode in Quantum Semiconductor Plasmas

Using the quantum hydrodynamical model, the steady state and transient gain characteristics of the stimulated Brillouin scattered mode in semiconductor plasmas have been studied. The third-order Brillouin susceptibility due to a nonlinear current density and electrostriction of the medium have been determined using the coupled mode theory. The effect of the Bohm potential on the steady state and transient Brillouin gain coefficient was studied through the quantum corrections in the classical hydrodynamic equations. It was found that the Bohm potential in the electron dynamics enhances the steady-state and transient Brillouin gain coefficients. Reduction in the threshold pump intensity of the said process has been realized as a consequence of the inclusion of quantum correction term.

Ultralow-power all-optical tunable dual Fano resonances in nonlinear metamaterials

Dual Fano resonances are realized in a nonlinear photonic metamaterial consisting of periodic arrays of asymmetrical meta-molecules etched in a gold film coated with azobenzene polymer layer made of poly[(methyl methacrylate)-co-(disperse red 13 acrylate)]. Enormously enhanced photoisomerization associated with resonant excitation brings about a large refractive index variation in the azobenzene polymer. Under excitation of a weak pump light as low as 0.61 kW/cm2, a large shift of 50 nm in the Fano resonance wavelength is obtained. Compared with previous reports, the threshold pump intensity is reduced by seven orders of magnitude while a large tunability is maintained simultaneously.

Ultralow-power and ultrafast all-optical tunable plasmon-induced transparency in metamaterials at optical communication range

Actively all-optical tunable plasmon-induced transparency in metamaterials paves the way for achieving ultrahigh-speed quantum information processing chips. Unfortunately, up to now, very small experimental progress has been made for all-optical tunable plasmon-induced transparency in metamaterials in the visible and near-infrared range because of small third-order optical nonlinearity of conventional materials. The achieved operating pump intensity was as high as several GW/cm2 order. Here, we report an ultralow-power and ultrafast all-optical tunable plasmon-induced transparency in metamaterials coated on polycrystalline indium-tin oxide layer at the optical communication range. Compared with previous reports, the threshold pump intensity is reduced by four orders of magnitude, while an ultrafast response time of picoseconds order is maintained. This work not only offers a way to constructing photonic materials with large nonlinearity and ultrafast response, but also opens up the possibility for realizing quantum solid chips and ultrafast integrated photonic devices based on metamaterials.