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The adiabatic theorem, a corollary of the Schr\"odinger equation, manifests itself in a profoundly different way in non-Hermitian arrangements, resulting in counterintuitive state transfer schemes that have no counterpart in closed quantum systems. In particular, the dynamical encirclement of exceptional points (EPs) in parameter space has been shown to lead to a chiral phase accumulation, non-adiabatic jumps, and topological mode conversion [1- 8]. Recent theoretical studies, however, have shown that contrary to previously established demonstrations, this behavior is not strictly a result of winding around a non-Hermitian degeneracy [9]. Instead, it appears to be mostly attributed to the non-trivial landscape of the Riemann surfaces, sometimes because of the presence of an exceptional point in the vicinity [9- 11]. In an effort to bring this counterintuitive aspect of non-Hermitian systems into light and confirm this hypothesis, we provide here the first set of experiments to directly observe the field evolution and chiral state conversion in an EP-excluding cycle in a slowly varying non- Hermitian system. To do so, a versatile yet unique fiber-based photonic emulator is realized that utilizes the polarization degrees of freedom in a quasi-common path single-ring arrangement. Our observations may open up new avenues for light manipulation and state conversion, while providing a foundation for understanding the intricacies of the adiabatic theorem in non-Hermitian systems.

Integrated optical signal processors, in combination with conventional electrical signal processors, are envisioned to open a path to a new generation of signal processing hardware platform that allows for significant improvement in processing bandwidth, latency, and power efficiency. With its well-known features and potential, silicon photonics is considered as a favorable candidate for the device implementation, particularly with high circuit complexity, and hence has been the focus of the study. As an outlook from the previous discussions on such processors, we are considering new building blocks in the silicon photonics platform for further extending the processor capabilities and adding practical features, particularly the miniaturized devices that enable ultra-dense integration of complex circuits into such processor chips. As enlightening examples, we review here our recent contribution together with representative works from other groups of compact designs of silicon photonics devices that enrich functionalities of processor building blocks such as multiplexing, polarization handling, and optical I/Os. The results shown in this review reflect the significance and maturity of the state-of-the-art photonic fabrication technology and contribute to the implementation of high-capacity, general-purpose optical signal processing functionalities on the chip scale.

A 1 × 2 integrated four-mode power splitter based on multimode interference is proposed and experimentally demonstrated for the first time. A three-dimensional simulation is used to optimize the core/cladding refractive index difference and the input waveguide position for the TE00, TE01, TE10, and TE11 input waveguide modes. Minimum insertion loss (IL) and mode-dependent loss (MDL) of 0.1 and 0.2 dB are predicted, respectively. IL and MDL less than 1 dB can be maintained over a bandwidth of approximately 50 and 25 nm, respectively. The device was fabricated on SiO2 and experimentally characterized. The variation of the power splitting ratio for each mode is less than 0.4 dB over the full C-band, and the excess losses of the splitter for all four modes exceed the IL of a straight waveguide by less than 1.85 dB.

Few-mode semiconductor optical amplifiers (FM SOAs) allow more than one transverse modes to propagate inside of the waveguide region. They are useful in optical signal processing of distinct spatial mode signals when applied to spatial division multiplexing in an optical fiber communication. A model for FM SOAs including carrier rate and coupled-mode equations, which govern linear and nonlinear effects, is derived for the first time. The model reveals two distinctions of parametric process in FM SOAs through carrier gratings. First, the carrier gratings with transverse variation patterns provide unique mode selectivity, which only allows efficient interaction between certain modes. This mode selectivity is summarized as a parallelogram rule, a simple method to find the interacting modes and categorize them. Second, degenerate four wave mixing (FWM) via static carrier grating diffraction distinguishes itself from other types of FWMs for the highest wave interaction efficiency. Theoretical analysis and numerical simulation show that optical signal processing based on the degenerate FWM features high power efficiency, broadband operation, phase sensitivity, and mode selectivity. The processing can be done either in the spatial mode domain or in the frequency domain, or by combing them together. These features bestowed by the introduction of spatial degree of freedom, make FM SOAs promising in optical signal processing.

A planar and broadband cholesteric liquid crystal (CLC) lens is designed, fabricated, and hybridized with a refractive lens to form a catadioptric pancake lens for virtual reality (VR) displays. Due to their opposite optical dispersion behaviors, the chromatic aberration of the assembled pancake lens is dramatically suppressed, as verified by both ray-tracing analysis and experimental results. The demonstrated catadioptric pancake lens has great potential for next-generation VR displays.

The dynamical behavior of broken symmetric coupled cavity lasers is theoretically investigated. The frequency response of this class of lasers is obtained using small signal analysis under direct modulation. Our model predicts a modulation bandwidth enhancement as a broken symmetric laser, operating in the parity-time (PT) symmetry and non-PT symmetry domains. This theoretical prediction is numerically examined in a laser system based on an InGaAs quantum dot platform. Our results clearly show that in these structures, in addition to the injection current, the gain-loss contrast can be used as a new degree of freedom in order to control the characteristic poles of the frequency response function.

We demonstrate how the presence of gain-loss contrast between two coupled identical resonators can be used as a new degree of freedom to enhance the modulation frequency response of laser diodes. An electrically pumped microring laser system with a bending radius of 50 μm is fabricated on an InAlGaAs/InP MQW p-i-n structure. The room temperature continuous wave (CW) laser threshold current of the device is 27 mA. By adjusting the ratio between the injection current levels in the two coupled microrings, our experimental results clearly show a bandwidth improvement by up to 1.63 times the fundamental resonant frequency of the individual device. This matches well with our rate equation simulation model.

We present a resonant magnetic field sensor based on microelectromechanical systems technology with optical detection. The sensor has single resonator composed of two orthogonal silicon beams (600 µm × 26 µm × 2 µm) with an integrated mirror (50 µm × 34 µm × 0.11 µm) and gold tracks (16 µm × 0.11 µm). The resonator is fabricated using silicon-on-insulator wafer in a simple bulk micromachining process. The sensor has easy performance that allows its oscillation in the first bending vibration mode through the Lorentz force for monitoring in-plane magnetic field. Analytical models are developed to predict first bending resonant frequency, quality factor, and displacements of the resonator. In addition, finite element method models are obtained to estimate the resonator performance. The results of the proposed analytical models agree well with those of the finite element method models. For alternating electrical current of 30 mA, the sensor has a theoretical linear response, a first bending resonant frequency of 43.8 kHz, a sensitivity of 46.1 µm T−1, and a power consumption close to 54 mW. The experimental resonant frequency of the sensor is 53 kHz. The proposed sensor could be used for monitoring in-plane magnetic field without a complex signal conditioning system.

In this paper, we experimentally analyze the intensity and phase dependence of the electric current in an active long-range surface plasmon polariton (LRSPP) device. It consists of a metallic waveguide embedded in a dielectric material where we have included external electric contacts. Experimental results show that the intensity beam decays almost 15 dB when we apply 150 mW, as we can obtain a phase change of 15p for the same value of electric power.

We report on our initial attempt to characterize the intrinsic frequency response of metal-clad nanolasers. The probed nanolaser is optically biased and modulated, allowing the emitted signal to be detected using a high-speed photodiode at each modulation frequency. Based on this technique, the prospect of high-speed operation of nanolasers is evaluated by measuring the D-factor, which is the ratio of the resonance frequency to the square root of its output power (fR/P_out^2). Our measurements show that for nanolasers, this factor is an order of magnitude greater than that of other state-of-the-art directly modulated semiconductor lasers. The theoretical analysis, based on the rate equation model and finite element method simulations of the cavity is in full agreement with the measurement results., Comment: Optics Express

Dynamically varying system parameters along a path enclosing an exceptional point is known to lead to chiral mode conversion. But is it necessary to include this non-Hermitian degeneracy inside the contour for this process to take place? We show that a slow enough variation of parameters, even away from the system's exceptional point, can also lead to a robust asymmetric state exchange. To study this process, we consider a prototypical two-level non-Hermitian Hamiltonian with a constant coupling between elements. Closed form solutions are obtained when the amplification/attenuation coefficients in this arrangement are varied in conjunction with the resonance detuning along a circular contour. Using asymptotic expansions, this input-independent mode conversion is theoretically proven to take place irrespective of whether the exceptional point is enclosed or not upon encirclement. Our results significantly broaden the range of parameter space required for the experimental realization of such chiral mode conversion processes., 4 pages, accepted for publication

We present a novel long-range surface plasmon polariton (LRSPP) device consisting of a suspended dielectric matrix in which an electrically active, millimeter-long metallic waveguide is embedded. We show that, by opening an air gap under the lower cladding, the influence of the substrate is suppressed and the symmetry of the thermo-optical distribution around the LRSPP waveguide is preserved over extended ranges of applied electrical current with minimal optical losses. Experimental results show that, compared to a standard nonsuspended structure, our device allows either the induction of a phase change that is three times larger, for a fixed electrical power, or, equivalently, a scaling down of the device to one-tenth of its original length, for a fixed phase change.

Interest in polarization properties of rendered materials is growing, but so far discussions on polarization have been restricted only to surface reflection, and the reflection due to subsurface scattering is assumed to be unpolarized. Findings from other fields (e.g. optics and atmospheric science) show that volumetric interaction of light can contribute to polarization. We thus investigated the polarized nature of the radiance field due to subsurface scattering as a function of the thickness of the material layer for various types of materials. Though our computations show negligible polarization for material layers of high thickness, light reflected on thin layered materials show significant degrees of polarization. Consequently, polarization cannot be ignored for subsurface component of reflection from painted surfaces (particularly painted metal surfaces) or from coated materials. Reflection from such surfaces is computed by solving for light transport in the different paint layers. In this paper we employ the vector radiative transfer equation (VRTE), which is the polarized version of the radiative transfer equation, inside the material. We use and detail a discrete ordinate based method to solve the VRTE and use the solution to compute the polarized radiance field at the surface of the material layer. We generate the polarimetric BRDF from the solutions of the VRTE for incident irradiance with different polarizations. We validate our VRTE solution against a benchmark and demonstrate our results through renderings using the computed BRDF. Graphical abstractDisplay Omitted HighlightsWe model polarized light transport using the vector radiative transfer equation.We present a DOM solution to that equation to solve polarized subsurface scattering.We apply this solution to paint layers, and compute their polarized BRDF.We show that polarization is important for subsurface BRDF through an experiment.We compute polarized BRDF for a variety of paint layers on different base materials.

We report the selective area bandgap tuning of multiple quantum well structures by an impurity free vacancy induced quantum well intermixing technique. A 3㏈ waveguide directional coupler was fabricated in the disordered section of an intermixed quantum well sample as a demonstration of photonic device applications.

A technique of impurity‐free vacancy‐induced disordering of GaAs/AlGaAs multiquantum wells (MQW) that is area selective, very reliable, and highly reproducible, has been developed. The localized compositional disordering is induced by rapid thermal annealing of the sample after it has been coated with a thin film of ‘‘spin‐on’’ glass and prebaked at 400 °C in a high purity nitrogen:oxygen (78:22) atmosphere. In order to self‐consistently determine the diffusion coefficient of the Al and Ga atoms, the photoluminescence peak is fitted to the n=1 electron to heavy hole transition that corresponds to an error function potential profile caused by the diffusion. The process has been used to integrate two optical devices on a MQW structure. One is a nonlinear directional coupler all‐optical switch, and the other is an integrated Mach–Zehnder all‐optical modulator. The switching characteristics of the devices were measured using the conventional pump‐probe measurement technique.

We demonstrate the first few-mode semiconductor optical amplifier (FM SOA) that supports up to four waveguide modes. We show that each of the modes are confined to the waveguide, overlapping the quantum wells with approximately the same amount, leading to equalized gain for each of the four waveguide modes.

A monolithically integrated multi-wavelength LED based on selective dielectric cap intermixing is investigated experimentally. The proposed LED emits radiation with multiple wavelength peaks from one compact easy to fabricate quantum well (QW) structure. Each wavelength has an independent emission power control, allowing the LED to radiate one or more wavelengths simultaneously. The LED material is an AlGaAs/GaAs QW p-i-n heterostructure. The device is divided into three selectively intermixed regions using an impurity-free vacancy induced intermixing technique creating localized intermixed areas. Each region is intermixed to varying extent resulting in different luminescence peaks and by separately addressing each section with its electrical current, the net emission spectrum can be fully controlled. The fabrication process starts with the growth of a 400nm thick layer of SiO 2 over the whole sample using plasma enhanced chemical vapor deposition. Three regions with different SiO 2 thicknesses are defined via two photolithographic and subsequent reactive ion etching steps. The sample is then annealed at 975°C for 20s to activate the intermixing of the constituent atoms of the quantum well and barrier materials. The degree of intermixing is determined by the thickness of the SiO 2 cap. After removal of the SiO 2 cap, contact stripes are evaporated on each region to act as an independent intensity power control for that region. Experimental results have shown that a controllable 10nm, 21nm and 33nm blue shifts of the peak wavelength of emission from that of the as-grown sample corresponding to 0, 100nm, and 400nm thick SiO 2 caps respectively.

A novel (to the best of our knowledge) liquid level sensor based on multimode interference (MMI) effects is proposed and demonstrated. By using a multimode fiber (MMF) without cladding, known as no-core fiber, liquids around the MMF modify the self-imaging properties of the MMI device and the liquid level can be detected. We show that the sensor exhibits a highly linear response with the sensing range and multiplexed operations easily controlled by just modifying the length of the no-core fiber. At the same time, we can measure the refractive index of the liquid based on the maximum peak wavelength shift. We can also use the sensor for continuous and discrete liquid level sensing applications, thus providing a liquid level sensor that is inexpensive with a very simple fabrication process.

A stable optofluidically tunable fiber laser based on Multimode Interference (MMI) effect is experimentally demonstrated. The tuning mechanism relies on modifying the effective width of the multimode fiber (MMF) using a liquid with a specific refractive index, which in turns tunes the peak wavelength of the filter. We easily demonstrate a tunability of almost 40 nm with a side-mode suppression ratio (SMSR) of 47 dBm, and a 3dB bandwidth of 0.4 nm. The laser was operated at room temperature without any thermal control and the wavelength peak was very stable.

A widely tunable erbium-doped all-fiber laser has been demonstrated. The tunable mechanism is based on a novel tunable filter using multimode interference effects (MMI). The tunable MMI filter was applied to fabricate a tunable erbium-doped fiber laser via a standard ring cavity. A tuning range of 60 nm was obtained, ranging from 1549 nm to 1609 nm, with a signal to noise ratio of 40 dB. The tunable MMI filter mechanism is very simple and inexpensive, but also quite efficient as a wavelength tunable filter.

We report on a novel scheme to fabricate a simple, cheap, and compact tunable fiber laser. The tuning is realized by splicing a piece of single-mode fiber to one end of an active double-clad fiber, while the other end of the single-mode fiber is spliced to a 15 mm long section of 105/125 multimode fiber. The fluorescence signal entering into the multimode fiber will be reproduced as single images at periodic intervals along the propagation direction of the fiber. The length of the multimode fiber is chosen to be slightly shorter than the first re-imaging point, such that the signal coming out from the single mode fiber is obtained in free space, where a broadband mirror retroreflects the fluorescence signal. Since the position of the re-imaging point is wavelength dependent, different wavelengths will be imaged at different positions. Therefore, wavelength tuning is easily obtained by adjusting the distance between the broadband mirror and the multimode fiber facet end. Using this principle, the tunable fiber laser revealed a tunability of 8 nm, ranging from 1088–1097 nm, and an output power of 500 mW. The simplicity of the setup makes this a very cost-effective tunable fiber laser.

We demonstrate bandgap tuning of InGaAsP multiple quantum well structures by utilizing an impurity-free vacancy diffusion technique. Substantial modification of the bandgap energy toward the red and blue parts of the spectrum has been observed using SiO2/SiOyNx/SiNx capping layers and by controlling the associated oxygen and nitrogen content. The resulting degree of tuning, up to 120 nm red shift and 140 nm blue shift of the band-to-band wavelength emission, has been studied using room-temperature photoluminescence, in agreement with the emission spectra obtained from semiconductor optical amplifier waveguide strips.

A significantly improved recovery time of 130 ps has been achieved in an all‐optical zero‐gap directional coupler containing multiple quantum wells. The mechanism for the all‐optical switching is due to free‐carrier induced refractive nonlinearities at near‐band‐gap resonant frequencies. The large reduction in the switch recovery time was obtained by the application of an external dc bias which sweeps out the carriers from the quantum wells. From our experimental results on the laser pulse width limited switch‐up time of 2 ps, we deduce that the contribution of self‐electro‐optic effect nonlinearity is negligible in this case.

A time‐resolved pump‐probe configuration employing 10 ps laser pulses has been used to investigate the all‐optical switching characteristics of zero‐gap directional couplers containing a single quantum well. All‐optical switching of the weak probe pulses from one port into the adjacent one was obtained at pump input pulse energies of 150 pJ. The switching takes place within the duration of the laser pump pulses. The recovery time of the device was found to be ∼1.5 ns which indicates that carrier diffusion plays a major role in the device switching speed.

A monolithic tunable laser on a quantum well (QW) structure is demonstrated by integrating an optical beam-steering section with an area-selectively intermixed QW gain section. Wavelength tuning is achieved by guiding an amplified optical beam over an optical gain medium that consists of three laterally adjacent regions containing a quantum well that has been selectively intermixed to varying extents. The laser light output can be tuned over a total of a 17-nm wavelength range by separately injecting electrical current pulses to each of the two parallel contacts in the steering region and to the optical amplifier contact.

Investigations of interaction of laser beams with liquid crystals reveal new fundamental features of nonlinear dynamic systems. Those features are connected with the giant values of optical nonlinearity, multitude of control parameters, specific properties of light propagation in highly anisotropic and inhomogeneous media, and multidimensional (vector) character of light-induced modulations of liquid crystals. Exceptional opportunities to visualize complex spatio-temporal phenomena in real time, in simple experimental situations, and with utilization of low-power lasers and thin layers of materials, provide a great potential for the design of low-cost and portable set-ups for modeling, demonstrations, and general educational purposes.

A novel type of integrated optical surface plasmon resonance (SPR) sensor is proposed, for which the operational principle is based on the launching efficiency of eigenmodes in the sensor head. The sensor comprises an inverted-rib-type dielectric waveguide, a portion of which is covered with a thin gold layer. Eigenmodes in the sensor head are coupled modes of a surface plasmon polariton and a dielectric guided wave. The excitation efficiency of the coupled modes varies significantly depending on the refractive index of the analyte medium on the sensor head. Following this principle, the transmission coefficient of light through the sensor head can be used as a sensitive measure of the variation in the refractive index of the analyte medium.

A three-channel optical waveguide switch has been designed using carrier-induced refractive index effects. Switching between individual or a combination of channels was achieved by controlling electrical currents injected through two parallel electrodes. A crosstalk of -19 dB between adjacent channels has been measured over a spectral range spanning 40 nm.

An all-optical integrated Mach-Zehnder interferometer switch is implemented using selective area disordering of a multiple quantum well structure. Ultrafast all-optical switching with an adjustable switching window ranging from 2 to 100 ps is demonstrated. The switch contrast ratio in this preliminary device is measured to be 9 dB.

Self-guiding of an optical beam at energies close to and below the bandgap has been demonstrated in an active semiconductor optical amplifier under quasi CW conditions with a laser intensity of /spl sim/1.1 MW/cm/sup 2/. A second beam at a 60 nm longer wavelength was guided and steered by the self-guided beam.