Reflective vertical cavity quantum-well saturable absorber as an all-optical nonlinear phase-shifting element

In Marangoni–Bénard convection with mass transfer from gas phase to liquid phase, it is found that solitary waves, excited and sustained by Marangoni stresses, are very stable during interactions with others, keeping their original shapes and celerities after interactions, thus experiencing at most a phase shift. Both positive and negative phase shifts have been observed for different types of oblique interaction. A ‘‘critical’’ angle, at which zero phase shift occurs as a result of the interaction, has been defined to delineate regions of negative and positive phase shifts.

We present the modeling and experiment on the intensity-dependent phase shift of a nonlinear optical phase-shifter based on vertical micro-cavity saturable absorber for high-speed response. We demonstrated the 25ps transient response with a large negative nonlinear phase shift of over -0.7 radian. Our device would enable us to control the phase of high speed optical pulses including the compensation of fiber nonlinear effects in optical domain.

Materials with vanishing real part of permittivity, also known as epsilon-near-zero (ENZ), have emerged as a new paradigm to obtain large optical nonlinearities. Transparent conducting oxides such as indium tin oxide (ITO) naturally exhibit an ENZ condition in the near-infrared and close to their plasma frequency. The peculiarity of ENZ materials is their highly dispersive nature explained by Drude model, allowing to obtain reduced group velocity at frequencies where the phase velocity is enhanced. This condition can dramatically increase the phase sensitivity caused by any given changes in material permittivity. To accurately address the NLO mechanism in such materials, we present a generalized formalism for the nonlinear phase shift for a dispersive material, where it includes contributions from both group velocity and phase velocity. We experimentally demonstrate that the nonlinear phase can reach to the values close to π/2 when the probe beam is at ENZ, while the index change in the conventional definition is not enhanced necessarily. In this work, we present nonlinear optical measurements using the nondegenerate Beam Deflection (BD) methodology at normal incident together with cross-phase modulation experiments. BD is a pump-probe method that directly characterizes the ultrafast response of the nonlinear phase-shift, hence nonlinear refraction. We see no polarization dependence proving that bound electronic third-order nonlinearities are negligible compared to the sub-picosecond fast carrier effects. We also present that frequency shift is highly sensitive to the temporal dynamics of the nonlinear phase shift suggesting that short pulses may help to improve the magnitude of frequency shift.

In order to achieve precise control of phase shift amplitude and position in the echo spectrum of phase-shifted fiber Bragg grating (PSFBG) sensor, the method to control PSFBG sensor using mechanical micro-strain was proposed. There were micrometer screws, strain beams, phase shifting fiber gratings, demodulators, etc. in this method. The micro-strain control was performed by a micrometer screw, and the strain was transmitted to the PSFBG by the beam, thereby changing the echo spectrum. Finally, a functional model of spectrum offset and microdisplacement Δy was constructed. Experimental results show that stretching and compression can cause positive phase shift and negative phase shift of PSFBG, and the phase shift fit curve is linear, and its comprehensive fit slope is 0.9914. The system can achieve continuous tuning of the phase shift by bending the micro-strain of the beam. The magnitude of the phase shift can be linearly controlled by changing the load displacement. The method is simple, high precision, large control range and passive. It has certain application value.

Pulses of a fluorinated analog of leucine, 5',5',5'-trifluoroleucine, reset the phase of the circadian rhythm of K(+) uptake in Lemna gibba G3 under continuous light conditions. The trifluoroleucine pulse caused the largest delay phase-shifts during the early subjective phase but it caused only small phase advances. The action of trifluoroleucine was investigated and the following results were obtained. (a) The uptake of trifluoroleucine was essentially the same at all circadian phases, even though phase shifting was dramatically different at different phases. At effective phases, the magnitude of phase shifting was well correlated with the amount of trifluoroleucine taken up by the duckweed. (b) The trifluoroleucine pulse lowered the endogenous content of valine and leucine but these decreases did not correlate with phase shifting. (c) Protein synthesis was not affected by trifluoroleucine pulses which caused large phase shifts. (d) Pulses of 4-azaleucine, a different structural analog of leucine, also caused phase shifting. However, neither the direction nor the effective times of phase shifting were similar to those of trifluoroleucine. Taken together, these results negate the proposition that trifluoroleucine and azaleucine caused phase shift by disturbing amino acid metabolism and/or inhibiting protein synthesis, but they suggest instead that these analogs are incorporated into some protein(s) which are necessary for normal clock operation.

A phase shift keying (PSK) modulator system integrating structured light and Kerr induced effects on TiO 2 -based materials is proposed. TiO 2 and Ag–TiO 2 thin films were prepared by a sol–gel approach and a spin coating deposition method. Morphology and composition analysis confirmed the successful introduction of Ag dopants into the TiO 2 matrix. A remarkable nonlinear refractive index sign change was highlighted from the nonlinear optical characterization by Z-scan experiments on comparable thin films. The induced birefringence generates a fast phase shift in response to high intensity irradiation, which is transformed into a helical wavefront by an optical component, resulting in a very attractive property for precise control over signal modulation for applications in PSK systems. A standard two-wave mixing was implemented to induce Kerr nonlinearities in the studied materials allowing polarization changes and a phase shift. The nonlinearities together with a phase wave plate were employed to generate structured light and an all-optical PSK modulation system. The ability to contrast and tune the phase response in the samples from the Kerr effect results in an interesting research opportunity for direct applications in all-optical telecommunication systems and a variety of nonlinear nanophotonic potential applications.

Chemical shifts of N 14 in the NMR spectra of nitrates, nitrites, and nitro-compounds

Semiconductor multiple quantum wells (QWs) have saturable absorption and saturable refractive index change properties. When it is embedded within a reflective Fabry–Perot cavity, the device shows forward and inverse saturation reflectivity for a radiation at cavity resonance wavelength with input intensity around the impedance matching point. Consideration of non-linear index change increases the extinction of forward saturation characteristics of the device. However its application in logic devices is very limited. Whereas, the inverse characteristics for which a number of logic devices can be proposed is severely affected in its extinction because of the consideration of non-linear index change. Here the authors optimise the parameters of vertical cavity semiconductor saturable absorber for the static and dynamic device performance of all-optical logic operations based on inverse saturation characteristics with both dispersive and absorptive saturation into consideration. The authors obtain an extinction of almost 77.5% with input intensity of the order of saturation intensity of a multiple QWs. The novelty of this work lies in the observation that the saturation of non-linear index of refraction has enormous effect on the characteristics of logic operations, which is not addressed in earlier reports. It is also observed that the detrimental effect of non-linear index saturation can be overcome by proper absorber parameters.

A nonlinear phase shift of low-power spin waves (SWs) induced by a high-power pumping SW excited both inside and outside the magnonic band-gaps of a magnonic crystal has been studied. The magnonic crystal with spatially periodic thickness is fabricated from an yttrium iron garnet film by chemical etching. The results show that the phase shift of the low-power SWs can be effectively controlled by variation of power level of the pumping SW. This induced nonlinear phase shift is weakened if the pump frequency lies in the magnonic bandgap. The data obtained are well explained by contradirectional coupling of the high-power forward and reflected spin waves. A theoretical model for this effect is presented. Our findings are important for the further progress in SW computing.

The mode locking of a cw Nd:YAG laser by translation of one of the cavity mirrors is reported. The mode-locking effect is greatly enhanced when an intracavity nonlinear KTP crystal is used. Stable pulse trains with a 60-ps average pulse width are obtained. Theoretical analysis shows that the combined effect of linear phase shift imposed by the moving mirror and the nonlinear phase shift in the Kerr medium gives rise to a saturable-absorber-like behavior in a homogeneously broadened laser. The theory agrees well with the experimental results as well as with results obtained with a Ti:sapphire laser.

We study the problem of estimating the phase shift due to the general relativistic time dilation in the interference of photons using a non-linear Mach-Zender interferometer setup. By introducing two non-linear Kerr materials, one in the bottom and one in the top arm, we can measure the non-linear phase $\phi_{NL}$ produced by the space-time curvature and achieve a scaling of the standard deviation with photon number ($N$) of $1/N^{\beta}$ where $\beta > 1$, which exceeds the conventional Heisenberg limit of a linear interferometer ($1/N$). The non-linear phase shift is an effect that is amplified by the intensity of the probe field. In a regime of high number of photons, this effect can dominate over the linear phase shift.

We discuss, theoretically, employing formalism based on the transfer matrix method and the so-called sheet model, the linear and nonlinear (saturated) optical phase shifts associated with intersubband transitions in multiple quantum well (MQW) structures. A commonly used total internal reflection geometry is considered. Numerical simulations are performed for two-level GaAs∕AlxGa1−xAs MQWs. They show that the above-mentioned phase shift can be greatly modified by optical interference effects and radiative coupling between QWs particularly at the grazing angle of incidence. For the restricted range of parameters even the bistable behavior of the nonlinear phase shift and reflectivity are possible.

We demonstrated a novel two-dimensional photonic crystal (PC) based Symmetric-Mach-Zehnder-type all-optical switch (PC-SMZ) with InAs quantum dots (QDs) acting as a nonlinear phase-shift source. The 600-μm-long PC-SMZ with integrated wavelength-selective PC-based directional couplers and other PC components exhibited a 15-ps-wide switching-window with a 2-ps rise/fall time at a wavelength of 1.3μm. Nonlinear optical phase shift in the 500-μm-long straight PC waveguide was also achieved at sufficiently low optical-energy (e.g., π phase shift at ~100-fJ control-pulse energy) due to the small saturation energy density of the QDs, which was enhanced in the PC waveguide, without having to use conventional measures such as SOAs with current-injected gain. These results pave the way to achieving novel PC- and QD-based photonic integrated circuits including multiple PC-SMZs and other novel functional devices.

Development of modern optical fiber networks puts an increasing demand on the optical hardware. All-optical signal processing components enable the highest switching rates and allow all-optical regeneration of pulse streams without converting the optical signal into electrical current. The subject of our research is self-switching in a Mach-Zehnder interferometer (MZI) and its applications in optical telecommunication networks. In this device, light injected in one of the input ports is unequally distributed over the two interferometer arms. Due to the intensity dependent refractive index in the interferometer arms, there can be a nonlinear phase shift induced between the optical signals of unequal intensities. The two signals therefore interfere destructively or constructively depending on the input power: in this way we obtain nonlinear self-switching. Two mechanisms of nonlinear phase shifting were considered: active, based on semiconductor optical amplifiers (SOAs), and passive, based on quantum dots (QDs). Interferometers of two types were developed: 2-to-2 (two input ports and two output ports) and 2-to-1 (two input ports and one output port). The 2-to-2 SOA-MZIs based on self-switching have been investigated for two applications. One of them is the pattern effect compensator. If SOAs are employed for all-optical signal amplification, e.g. in optical access networks, an unwanted pulse distortion (known also as the pattern effect) takes place, as a result of the SOA gain saturation. Our component allows pattern-free amplification of the optical signals at bitrates up to 20 Gb/s. At 10 Gb/s it shows an extended input power range (up to 7 dB improvement) and comparable gain, which makes it suitable to be used as an optical amplifier. Another application of the 2-to-2 SOA-MZI is a 2R-regenerator. Optical amplifiers used in long distance optical links add noise to the optical signal, causing signal degradation. The signal can be regenerated by passing it through an optical gate with a nonlinear transfer function. The 2-to-2 SOA-MZI has such a nonlinear transfer function. The regeneration capabilities were demonstrated at 2.5 Gb/s by an improvement of the receiver sensitivity of about 2.5 dB. The dynamic characterization of the chips was carried out in a close cooperation with the research group COM at the Technical University of Denmark within the ePIXnet "network of excellence". The 2-to-1 MZI based on self-switching can be used as a low-loss optical combiner. An essential function in optical fiber networks is the combining of optical signals. A serious disadvantage of the conventional type of combiners used in the networks is that they let only half of the power (3 dB) through. In order to compensate for this loss, passive optical combiners are often used in combination with an in-line SOA. The first realization of the low-loss optical combiner uses SOAs as active phase shifters. Such an active low-loss combiner shows an improvement of transmission of over 2 dB compared to a conventional combiner with an in-line SOA. It is therefore expected that the output optical signal-to-noise ratio of the self-switching SOA-MZI is more than 2 dB better than that of a conventional combiner with an in-line SOA. While for the pattern effect compensator and the 2R-regenerator SOAs are used not only for inducing the nonlinear phase shift, but also for sufficient amplification, for the low-loss optical combiner the preferred nonlinear effects should be passive: combination of the signal is a passive function. Therefore, the second realization is based on a novel material, quantum dots. QDs provide improved all-optical nonlinearities resulting in a very small switching energy and large refractive index changes. Such a passive 2-to-1 QD-MZI based on self-switching showed an improvement up to 1.7 dB with respect to a conventional combiner. These improvements have a huge effect on e.g. the power budget in passive optical networks, where a large number of splitting stages are required. The Mach-Zehnder interferometers were realized in the InP/InGaAsP semiconductor material system, which is perfectly suitable for the integration of the photonic integrated circuits for the telecommunication applications. In order to realize both active components (such as e.g. SOAs) and passive components (such as e.g. waveguides, couplers), an active-passive integration technique was applied. This integration was realized in a close cooperation between JDS Uniphase Eindhoven and the COBRA Research Institute. It employs a three-step metal-organic vapor-phase epitaxy regrowth process. The quantum dot material was grown within the COBRA Research Institute. Our MZIs use a ridge waveguide design, for which a reactive ion etching process was developed in the COBRA cleanroom. As a result of photonic integration our integrated Mach-Zehnder interferometers have very small dimensions: less than a square millimeter.

Third-order Kerr nonlinearities and Raman gain are studied experimentally in high-purity As2Se3 optical fibers for wavelengths near 1.55 μm. Kerr nonlinear coefficients are measured to be nearly 1000 times higher than those for silica fibers. In pulsed mode, nonlinear phase shifts near 1.2-π rad are measured in fibers only 85 cm long with peak pulse powers near 3 W. However, there are nonlinear losses near 20% for nonlinear phase shifts near π. By use of a cw optical pump, large Raman gains nearly 800 times that of silica were measured. In the cw case there were losses in the form of index gratings formed from standing waves at the exit face of the fiber. Discrete Raman amplifiers and optical regenerators are discussed as possible applications.