Threshold Pump Intensity Research Articles

Excitation of pure quartic platicon in optical microresonators.

We analytically derive the formulas of the threshold pump intensity and the range of possible detuning for the initiation of the pure quartic platicon (PQP) in the presence of multiphoton absorption, free-carrier absorption, and free-carrier dispersion. Theoretical investigations demonstrate a feasible approach for the excitation of PQP in the normal quartic dispersion regime via the free-carrier effects in platforms such as silicon, germanium, and their derivates. Due to the time-variant nonlinear loss related to free-carrier absorption or additional nonlinear detuning induced by free-carrier plasma dispersion, PQP can be generated through turn-key or laser frequency scanning schemes in both the three- and four-photon absorption regimes. Parameter spaces associated with varying detuning and free-carrier lifetime for the different dark, oscillating, and bright PQP states are revealed in the turn-key generation case. Such solitonic pulses are more energetic than conventional quadratic solitons with the same pulse width, especially for shorter pulse operations. In the spectral domain, PQP microcombs in the mid-infrared are desired for spectroscopy. These findings will facilitate the generation and manipulation of PQP and present promising potential for Kerr microcomb related applications.

Tunable backward terahertz-wave parametric oscillator centered at a high frequency of 0.87 THz with injection seeding.

We report the first demonstration of a frequency tunable backward THz-wave parametric oscillator (BW-TPO) centered at a high frequency of 0.87 THz using a slant-stripe-type magnesium oxide-doped periodically poled lithium niobate (PPLN) crystal as the nonlinear medium. Down-converted THz and idler beams generate upon excitation of the PPLN with a sub-nanosecond pulsed source of λ = 1064.44 nm. The resulting first idler has a wavelength of 1067.75 nm, equivalent to an oscillation frequency of 0.872 THz as per the spectral line separation from the pump. We also present angle tuning of the BW-TPO frequency ranging from 0.836-0.905 THz through PPLN rotation. The threshold pump intensity for BW-TPO is determined to be 5.6 GW/cm2 while obtaining a conversion efficiency as high as 12.3% at a pump energy (intensity) of 15.25 mJ (8.90 GW/cm2). A reduction of the BW-TPO threshold energy and improved pump-to-idler energy conversion efficiency resulted from injection seeding with a CW laser at the same wavelength as the first idler. The THz output is also directly proportional to seed power.

Balancing the Number of Quantum Wells in HgCdTe/CdHgTe Heterostructures for Mid-Infrared Lasing.

HgCdTe-based heterostructures with quantum wells (QWs) are a promising material for semiconductor lasers in the atmospheric transparency window (3-5 μm) thanks to the possibility of suppressing Auger recombination due to the no-parabolic law of carrier dispersion. In this work, we analyze the thresholds of stimulated emission (SE) under optical pumping from heterostructures with a different number of QWs in the active region of the structure. Total losses in structures are determined from the comparison of thresholds for the different number of QWs in the active region. It is shown that, thanks to the increased modal gain, a higher number of QWs results in lower threshold pumping intensity and, consequently, higher temperature of SE. These results indicate that improvements to the modal gain can result in a moderate uplift in the temperature of SE from mid-infrared HgCdTe-based heterostructures. On the other hand, at a high enough QW count threshold, the intensity no longer depends on the number of the QWs and is determined by the transparency concentration of a single QW.

Simulation of far-infrared HgTe/HgCdTe quantum-well vertical-cavity surface-emitting laser

We propose an original design for a far-infrared vertical-cavity surface-emitting laser based on a heterostructure with ten 4.7-nm HgTe / Hg0.3Cd0.7Te quantum wells grown on a [013] GaAs substrate. Its characteristics were calculated, and it was shown that the quality factor of the proposed resonator design can reach 248 at a wavelength of 23.25 μm, which is sufficient for the occurrence of laser generation at this wavelength at a temperature of 20 K and a concentration of nonequilibrium carriers of 1.4 × 1010 cm − 2. The estimate of the threshold pump intensity using CO2 laser radiation at a wavelength of 10.6 μm is 1 W / cm2, which makes it possible to expect continuous wave mode lasing. Such a low-threshold pump intensity should also lead to lasing at substantially higher temperatures.

Steady-state and transient Raman gain coefficients of weakly-polar magnetoactive doped semiconductors

Assuming that the origin of stimulated Raman scattering (SRS) lies in third-order effective (Raman) susceptibility arising due to nonlinear induced polarization, we obtained expressions for steady-state and transient Raman gain coefficients of weakly-polar magnetoactive doped semiconductors under various geometrical configurations. The threshold pump intensity and optimum pulse duration for the onset of transient SRS are estimated. The dependence of Raman gain coefficients on doping concentration, external magnetostatic field, magnetostatic field inclination, scattering angle and pump pulse duration are explored in detail with aim to determine suitable values of these controllable parameters to enhance Raman gain coefficients at lower threshold intensities, and to search the feasibility of efficient semiconductor nonlinear devices based on Raman nonlinearities. Most expectedly, the technological potentiality of weakly-polar magnetoactive doped semiconductors as hosts for compression of scattered pulses and fabrication of efficient nonlinear devices such as frequency converters, Raman amplifiers and oscillators etc. based on Raman nonlinearities are established.

Parametric oscillation of acoustical phonon mode in magnetized doped III–V semiconductors

Using the hydrodynamic model of semiconductor plasmas and coupled mode theory of interacting waves, we develop a theoretical formulation followed by numerical analysis to study parametric oscillation of acoustical phonon mode in magnetized doped III–V semiconductors. The origin of nonlinear interaction is assumed to lie in effective second-order optical susceptibility ( $$\chi^{(2)}$$ ) arising due to nonlinear induced current density and acousto-optic polarization of the semiconductor medium. Expressions are obtained for threshold pump intensity ( $$I_{{0,{\text{th}}}}$$ ) and operational characteristics such as conversion efficiency ( $$\eta_{k}$$ ) and single pass signal power gain ( $$g_{{{\text{sp}}}}$$ ) of the proposed single resonant optical parametric oscillator. Numerical analysis is made for a representative n-InSb crystal irradiated by 10.6 µm pulsed CO2 laser. The effects of some important parameters including external magnetic field ( $$B_{0}$$ ), mirror reflectivity ( $$R$$ ), crystal length ( $$L$$ ), etc. on $$I_{{0,{\text{th}}}}$$ , $$\eta_{k}$$ and $$g_{{{\text{sp}}}}$$ of the single resonant optical parametric oscillator are analyzed in detail. The analysis establishes the technological potentiality of magnetized doped III–V semiconductors as the hosts for fabrication of highly efficient and widely tunable single-resonant optical parametric oscillators.

THz laser generation on a hybrid surface plasmon in a HgCdTe-based structure

The possibility of amplifying a THz hybrid surface plasmon in a structure with an Hg0.82Cd0.18Te epitaxial film grown on a GaAs substrate and covered with a metal layer is investigated. It is shown that for a film thickness of 100 nm and a temperature of 80 K, the hybrid surface plasmon mode gain can be greater than external losses at a pump radiation intensity with a wavelength of 2.3 μm, exceeding 850 kW cm−2. Additional doping of the Hg0.82Cd0.18Te layer with a donor impurity having a concentration of 4 × 1017 cm−3 will lead to a 1.5-fold decrease in the threshold pump intensity.

Fluorescence and Time-Delayed Lasing during Single Laser Pulse Excitation of a Pendant mm-Sized Dye Droplet.

Fluorescence and lasing emission that are produced separately in time during excitation laser pulse for an mm-sized Rhodamine 6G dye-water droplet are reported. The droplet acts as a quasi-spherical closed optical resonator and due to multiple internal reflections, the resonant amplified emission is delayed with respect to fluorescence emission. Measurements of the temporal evolution of the droplet’s emission were performed by varying the signal acquisition gate width and gate delay with respect to the pumping pulse. The droplet emission spectra are structured in two bands which appear one after the other in time: first, the fluorescence emission band which follows pumping laser pulse time shape and then a second band, the lasing band, placed at shorter wavelengths and formed in time after the peak of the pumping laser pulse intensity, on the pulse tail. The lasing threshold pumping intensity is much lower than those for typical dye lasers.

Nonlinear phase matching in parametric four-wave mixing process of Cs vapor

Phase matching plays an important role on the nonlinear process. The linear phase matching in parametric four-wave mixing (FWM) process of Cs vapor is firstly studied and we find out that linear phase matching can be implemented within narrow range of frequency offsets for involved pumping light. Considering the high pumping power intensity, we innovatively take the influence of nonlinear refractive index to phase matching into account. The relationship between threshold pump intensity and frequency offsets is discovered, and it makes up a complicated curved surface. The threshold has the minimum value when the frequency offset satisfies the condition of v1 + v2 = 0. In addition, these simulation results could provide support for further study and pumping parameters selection in FWM process of Cs vapor.

Transient amplification characteristics of frequency modulated wave in semiconductor plasmas

We perform an analytical investigation of modulational interaction of an electromagnetic pump wave in a transversely magnetized weakly polar semiconductor plasma medium. Based on the hydrodynamic model, a critical examination of frequency modulational amplification is made in a moderately doped III–V semiconductor crystal (n-GaAs) duly shined by pulsed 10.6 µm CO2 laser. The effect of electron-LO phonon interaction and coupled magnetoplasma excitations on the nonlinear interaction of a laser beam adds new dimension to the analysis. Steady state and transient gain coefficients, threshold pump intensity and optimum-pulse duration for the onset of modulational instability are estimated. The paper concentrates on the study of gain characteristics mainly in transient region. Effect of doping and applied magnetic field on the threshold pump intensity and growth rate has been significantly analysed. Graphical analysis interprets strong dependence of transient amplification characteristics on these parameters. So a judicious choice of physical parameters may develop growth at much lower pump intensity improving the cost effectiveness of the system. These results may help to be important for understanding and improving the performance of modulator.

Free-carrier-driven Kerr frequency comb in optical microcavities: Steady state, bistability, self-pulsation, and modulation instability

Continuous-wave pumped optical microresonators have been vastly exploited to generate a frequency comb (FC) utilizing the Kerr nonlinearity. Most of the nonlinear materials used to build photonic platforms exhibit nonlinear losses such as multiphoton absorption, free-carrier absorption, and free-carrier dispersion which can strongly affect their nonlinear performances. In this work, we model the Kerr FC based on a modified Lugiato-Lefever equation (LLE) along with the rate equation and develop analytical formulations to make a quick estimation of the steady state, bistability, self-pulsation, and modulation instability (MI) gain and bandwidth in the presence of nonlinear losses. The analytical model is valid over a broad wavelength range as it includes the effects of all nonlinear losses. Higher-order $(g3)$ characteristic polynomials of intracavity power describing the steady-state homogeneous solution of the modified LLE are discussed in detail. We derive the generalized analytical expressions for the threshold (normalized) pump detuning that initiates the optical bistability when nonlinear losses are present. Free-carrier dispersion-led nonlinear cavity detuning is observed through the reverse Kerr tilt of the resonant peaks. We further deduce the expressions of the threshold pump intensity and the range of possible cavity detuning for the initiation of the MI considering the presence of nonlinear losses. The proposed model will be helpful in explaining several numerical and experimental results which have been previously reported and thereby will be able to provide a better understanding of the comb dynamics.

Four wave mixing and compensating losses in metamaterials

The conditions under which losses can be compensated in the case of propagation of an backward signal wave under parametric four-wave interaction are analyzed. It is shown that, in the presence of a parametric coupling between forward and backward waves, compensation of signal wave losses by losses of direct waves will allow the parametric amplification and generation of the backward wave to be realized at the threshold pump amplitude. An analytical expression is obtained for the threshold pumping amplitude under phase-matching conditions. It is shown that the threshold pump amplitude increases with increasing losses, the input intensity of the idler wave, and with increasing nonlinear coupling coefficients for pump waves. As a consequence, it is possible to control the threshold pump intensity. and it is possible to obtain significant amplification in the metamaterial, as well as parametric generation of the backward wave.

Ultrabright Fluorescent and Lasing Microspheres from a Conjugated Polymer

AbstractFluorescent microspheres are used for biomarkers, assay substrates, chemical diagnostics, flow cytometry, and biological imaging. These applications demand the highest fluorescence intensity achievable; however, concentration quenching limits the amount of dye that can be practically incorporated in conventional fluorescent microspheres. Conjugated polymers (CPs) can be less susceptible to concentration quenching, suggesting that they can be excellent candidates for a new class of light‐emitting microspheres. Due to their long‐chain‐conjugated backbone, however, CPs can be resistant to forming smoothly curved or spherical structures. Here, strongly fluorescent CP microspheres as large as 100 µm in diameter are synthesized. Whispering gallery modes (WGMs) appear in the fluorescence spectra, and the microspheres show clear evidence of lasing above a threshold pump intensity. These conjugated polymer beads are up to 50 times larger than CP microspheres obtained by other methods, and they exceed the emission intensity of conventional fluorescent microspheres by more than an order of magnitude.

A comprehensive dual beam approach for broadband control of ultrafast optical nonlinearity in reduced graphene oxide

A unique delay-dependent dynamic switching of optical nonlinearity in terms of saturation absorption (SA) and excited state absorption (ESA) in graphene oxide (GO) and reduced graphene oxide (RGO2 and RGO100) is achieved with an optical pump and white light super-continuum probe only above a threshold pump intensity. Infrared and chemical reduction are used to obtain RGO2 and RGO100 respectively. The switching regime of probe wavelength can be modulated by varying either pump wavelength or degree of reduction. When pumped at 415 nm, the threshold pump intensity to obtain switching property decreases to 9 GW/cm2 for RGO2 from 18 GW/cm2 in GO and the tunability range shifts from 471 to 526 nm for as grown GO to 519–623 nm in maximally reduced RGO2. Though the saturation intensity of intrinsic non-degenerate two photon absorption (nd-TPA) is found to be lower in GO (4.3 GW/cm2) than RGO2 (18.2 GW/cm2), nd-TPA coefficient increases from 0.0015 cm/GW (GO) to 0.0026 cm/GW (RGO2) with increasing reduction. The detailed kinetics of the scattering processes show variation as a function of pump power, probe wavelength and degree of reduction. A model band diagram based on amorphous-carbon model and a Tauc analysis are used to explain the unusual nonlinear optical properties.

Nonlinear digital out-of-plane waveguide coupler based on nonlinear scattering of a single graphene layer

A new mechanism for out-of-plane coupling into a waveguide is presented and numerically studied based on nonlinear scattering of a single nano-scale Graphene layer inside the waveguide. In this mechanism, the refractive index nonlinearity of Graphene and nonhomogeneous light intensity distribution occurred due to the interference between the out-of-plane incident pump light and the waveguide mode provide a virtual grating inside the waveguide, coupling the out-of-plane pump light into the waveguide. It has been shown that the coupling efficiency has two distinct values with high contrast around a threshold pump intensity, providing suitable condition for digital optical applications. The structure operates at a resonance mode due to band edge effect, which enhances the nonlinearity and decreases the required threshold intensity.

Investigation of HgCdTe waveguide structures with quantum wells for long-wavelength stimulated emission

The photoluminescence and stimulated emission during interband transitions in quantum wells based on HgCdTe placed in an insulator waveguide based on a wide-gap CdHgTe alloy are studied. Heterostructures with quantum wells based on HgCdTe are of interest for the development of long-wavelength lasers in the range of 25–60 μm, which is currently unattainable for quantum-cascade lasers. Optimal designs of quantum wells for attainment of long-wavelength stimulated emission under optical pumping are discussed. It is shown that narrow quantum wells from pure HgTe appear to be more promising for long-wavelength lasers in comparison with wide (potential) wells from the alloy due to the suppression of Auger recombination. It is demonstrated that molecular-beam epitaxy makes it possible to obtain structures for the localization of radiation with a wavelength of up to 25 μm at a high growth rate. Stimulated emission is obtained for wavelengths of 14–6 μm with a threshold pump intensity in the range of 100–500 W/cm2 at 20 K.

Enhanced absorption and cavity effects of three-photon pumped ZnO nanowires

Semiconductor nanowire (NW) lasers attract a lot of attention as potential elements of nanophotonic circuits and lab-on-a chip devices. Here, we report on the experimental investigation of stimulated near ultraviolet (NUV) emission, pumped by three-photon absorption from near infrared femtosecond laser pulses, from ZnO NW arrays of different morphologies and compare it to the bulk. The spectrally and temporally resolved measurements of the NUV emission show both strong enhancements in the absorption and emission properties of the nanowire arrays compared to bulk samples. Thus, we determine a many times higher three-photon absorption in the nanostructure morphology compared to the bulk material. Furthermore, the threshold pumping intensity for stimulated emission in a vertically oriented nanowire array is twice lower and the emission onset time is shorter than in randomly oriented arrays, revealing strong influence of the macroscopic nanowire arrangement.

All-optical tunable dual Fano resonance in nonlinear metamaterials in optical communication range

Low-power, ultra-fast all-optical tunable dual Fano resonance was realized in a metamaterial coated with a non-linear nanocomposite layer composed of gold nanoparticle-doped polycrystalline barium strontium titanate and multilayer tungsten disulphide microsheets. A high non-linear refractive index of −2.148 × 10−11 m2/W was achieved in the nanocomposite material that originated in the non-linearity enhancement associated with the quantum confinement effect, the local-field enhancement effect, and reinforced interactions between photons and the multilayer tungsten disulphide microsheets. An ultra-low threshold pump intensity of 600 kW/cm2 was obtained. An ultra-fast response time of 25.4 ps was maintained because of the fast relaxation dynamics of the bound electrons in the nanoscale polycrystalline barium strontium titanate grains. The large third-order non-linear responses of the metamaterial were confirmed with a high third harmonic generation conversion efficiency of 5.4 × 10−5. This work may help to pave the way towards realization of ultra-high-speed information processing chips and multifunctional integrated photonic devices based on metamaterials.

Phase mismatched optical parametric generation in semiconductor magnetoplasma

Optical parametric generation involves the interaction of pump, signal, and idler waves satisfying law of conservation of energy. Phase mismatch parameter plays important role for the spatial distribution of the field along the medium. In this paper instead of exactly matching wave vector, a small mismatch is admitted with a degree of phase velocity mismatch between these waves. Hence the medium must possess certain finite coherence length. This wave mixing process is well explained by coupled mode theory and one dimensional hydrodynamic model. Based on this scheme, expressions for threshold pump field and transmitted intensity have been derived. It is observed that the threshold pump intensity and transmitted intensity can be manipulated by varying doping concentration and magnetic field under phase mismatched condition. A compound semiconductor crystal of n-InSb is assumed to be shined at 77 K by a 10.6μm CO2 laser with photon energy well below band gap energy of the crystal, so that only free charge carrier influence the optical properties of the medium for the I.R. parametric generation in a semiconductor plasma medium. Favorable parameters were explored to incite the said process keeping in mind the cost effectiveness and conversion efficiency of the process.

Parametric interactions of acoustic waves in semiconductor quantum plasmas with strain dependent dielectric constants

Using quantum hydrodynamic model (QHD) of semiconductor plasma for a one-component we present an analytical investigation on parametric interaction of a laser radiation in an unmagnetised material with a strain-dependent dielectric constant. The nonlinear current density and third order susceptibility are analyzed in different wave number regions in presence and absence of quantum effect. We present the qualitative behavior of threshold pump intensity with respect to wave number in presence and absence of quantum effect. The numeric estimates are made for n-BaTiO3 crystals at 77k duly irradiated by pulsed 10.6μm CO2 laser. It is found that the quantum correction through Fermi temperature and Bohm potential terms modifies the threshold characteristics.