Pump-induced Refractive Index Changes Research Articles

All-optical high-contrast femtosecond switching using nonlinearity from an epsilon-near-zero effect in plasmonic metamaterials.

Metamaterials opened a new realm to control light-matter interactions at sub-wavelength scale by engineering meta-atoms. Recently, the integration of several emerging nonlinear materials with metamaterial structures enables ultra-fast all-optical switching at the nanoscale and thus brings enormous possibilities to realize next-generation optical communication systems. This Letter presents a novel, to the best of our knowledge, design of plasmonic metamaterials for high-contrast femtosecond all-optical switching. We leverage magnetic plasmon (MP) resonance combined with the nonlinear effects of an epsilon-near-zero (ENZ)-material. The proposed design comprises a periodic array of two closely spaced Au-nanogratings deposited on an optically thick Au-substrate to excite MP-resonance. To enable a dynamically tunable resonance, the nanogrooves in meta-atoms are filled with an ENZ-material, cadmium-oxide (CdO). The intraband transition-induced optical nonlinearities in the ENZ-medium are studied using a two-temperature model. The MP-resonance ensures strong light-matter interactions enabling enhancement of the nonlinearities of the proposed structure. We observe that the pump-induced refractive index change in the CdO layer causes a redshift of the MP-resonance dip wavelength in the reflectance spectrum, leading to a high modulation depth of 0.83 at 1.55 µm. With an ultra-fast response time of 776 fs while maintaining a low pump-fluence of 75 µJ/cm2, the proposed metamaterial could help in realizing switches for next-generation optical computation systems.

Time-resolved study of pump-induced refractive index changes in Tb[formula omitted]-doped phosphate glasses: Discrimination of electronic and thermal contributions

Time-resolved Z-scan measurements were performed in Tb3+-doped Zinc Phosphate (PZABP) glasses. The electronic and thermal contributions to the Z-scan signal were unequivocally discriminated. The electronic component was shown to increase linearly with the Tb3+ concentration in the glasses and explained by the polarizability difference (Δαp) between excited and ground states of the Tb3+ ions. Open aperture Z-scan measurements indicate the presence of excited state absorption due to a spin-forbidden transition from the metastable (5D4) 4f level of Tb3+ to the 4f75d1 state. The Δαp value obtained in the PZABP glass matrix is two orders of magnitude smaller than the observed in aluminosilicate glasses, indicating that Δαp of Tb3+-doped materials is very sensitive to the host crystal field, as compared to other rare-earth ions such as Nd3+ and Yb3+. These results are justified by the host dependence of energy position of the 4f75d1 levels of Tb3+.

All-optical switching in epsilon-near-zero asymmetric directional coupler

We propose an all-optical switch based on an asymmetric directional coupler structure with epsilon-near-zero (ENZ) layer. The nonlinear optical properties the of ENZ layer are analyzed by hot-electron dynamics process, and the all-optical operating performance of the switch on the silicon nitride platform is investigated. It is found that the pump-induced refractive index change in ENZ layer gives rise to a transfer of signal light in the optical system. We demonstrate that the proposed switch design features an insertion loss of < 2.7 dB, low crosstalk of < − 18.93 dB, and sub-pico-second response time at the communication wavelength of 1.55 μm. With ultrafast response, high performance, and simple structure, the device provides new possibilities for all-optical communication and signal processing.

Pump-induced refractive index changes in Tb3+ doped glasses

It now well known in laser materials, that a refractive index change appears when the active ions are pumped from ground to excited state due to the polarizability difference between ground and excited states (metastable). In this paper this effect was investigated in Tb3+ doped glasses: calcium alumino phosphate (CAP), low-silica calcium aluminosilicate (LSCAS) and calcium aluminosilicate (CAS). The measurements were performed using the time resolved Z-scan technique, with an Ar+ laser at 488nm, close to the resonance of 7F6→5D4 absorption line, where 5D4 is a metastable state. We obtained for low-silica calcium aluminosilicate glass Δαp~10−24cm3 which is the highest value ever reported for a RE doped material.

Reconstruction-free sensitive wavefront sensor based on continuous position sensitive detectors

We propose a new device that is able to perform highly sensitive wavefront measurements based on the use of continuous position sensitive detectors and without resorting to any reconstruction process. We demonstrate experimentally its ability to measure small wavefront distortions through the characterization of pump-induced refractive index changes in laser material. In addition, it is shown using computer-generated holograms that this device can detect phase discontinuities as well as improve the quality of sharp phase variations measurements. Results are compared to reference Shack-Hartmann measurements, and dramatic enhancements are obtained.

Baryscan: a sensitive and user-friendly alternative to Z scan for weak nonlinearities measurements

A new and efficient technique for measuring weak optical nonlinearities is reported. Like Z scan, its implementation is basic, both experimentally and theoretically, but leads to an improved sensitivity of λ/5.10⁴, which represents, to date, one of the highest observed enhancements. With this technique, which is based upon the use of a position sensitive detector, nonlinear properties are deduced by monitoring the barycentric position of a truncated pump-probe laser beam as the sample is moved along the optical axis. The technique is experimentally validated by measuring the pump-induced refractive index change and the underlying polarizability variation resulting from the excitation of the Cr(3+) ions in ruby.

Ultrafast gain dynamics in InP quantum-dot optical amplifiers

We measured the gain dynamics at the ground-state transition in an electrically pumped InP/AlGaInP quantum-dot optical amplifier at room temperature by femtosecond differential transmission. The gain shows an ultrafast recovery within 200 fs, even faster than in state-of-the-art InAs/GaAs quantum-dot amplifiers. This finding, likely to be due to the less confined and more closely spaced hole levels in InP dots, is promising for optical signal processing at high bit rates. We furthermore measured the pump-induced refractive index changes and deduced a linewidth enhancement factor similar to the one in InAs/GaAs quantum dots.

Detailed characterization of pump-induced refractive index changes observed in Nd:YVO_4, Nd:GdVO_4 and Nd:KGW

The refractive index changes which can be induced in the Nd:YVO(4), Nd:GdVO(4) and Nd:KGW high gain laser crystals, when their Nd(3+) laser active ions are pumped from their ground- to excited- energy levels, have been carefully measured and characterized. By using two complementary optical techniques based on pump-probe interferometry and transient diffraction grating, the electronic and thermal contributions to the observed refractive index variations have been accurately determined and successfully exploited to derive various parameters such as polarizability changes, thermo-optic coefficients and thermal diffusivities.

Effect of Coupled Double-Quantum-Well Design on Pump-Induced Refractive-Index Change in AlAsSb–InGaAs–AlAs Optical Waveguides

Optimization of the AlAsSb-InGaAs-AlAs coupled double-quantum-wells in optical waveguide core for optimal pump-induced refractive-index change indicates that thin AlAs center barrier leads to larger refractive-index change, and the InGaAs double-quantum-wells have an optimum thickness of which maximum refractive-index change is obtained. Analysis shows that the cross phase-modulation efficiency of these optical waveguides is highly dependent on optical confinement and is limited to approximately 0.05 rad/pJ in the 1550 nm region as far as the cross phase-modulation based on free-carrier mass dispersion effect is concerned.

Dynamics of pump-induced refractive index changes in single-mode Yb-doped optical fibers

We quantify the refractive index changes (RIC) in single-mode ytterbium-doped optical fibers induced by optical pulses at 980 nm and, for the first time, report details of the effect dynamics. The RIC dynamics is shown to follow that of the population of the excited/unexcited ion states with a factor proportional to their polarizability difference (PD). The absolute PD value is evaluated in the spectral range of 1460-1620 nm for different fiber samples and is found to be independent on the fiber geometry and on the ion concentration. The PD dispersion profile indicates to a predominant far-resonance UV rather than near-resonant IR transitions contribution to the RIC.

DFB Waveguide All-Optical Switching Devices Employing Pump-Induced Refractive Index Change in GaInAsP

A pump-induced refractive index change was measured in a passive GaInAsP waveguide together with its temporal response. Also, the temporal response of distributed feedback (DFB) waveguide transmittance modulated by the pump-induced index change was experimentally investigated for an application to all-optical switching devices.

Measurement of pump-induced refractive index changes in erbium-doped optical fibre

A novel technique for measuring the pump induced change in refractive index in erbium-doped optical fibre is reported. A saturated peak-to-trough change in index of 6.7 × 10-8 /(dB/m) of absorption was measured and the technique was verified by showing excellent agreement with a Kramers-Kronig transform of the measured change in absorption.