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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.

Microscale and nanoscale electrically pumped lasers are expected to play an indispensable role in future photonic integrated circuits. Small footprint, low threshold, high efficiency, and large side-mode suppression ratio (SMSR) are among the most important characteristics that such on-chip emitters should exhibit—allowing for large-scale integrability, low energy consumption, and stable output power. Microring resonators are one of the main contenders for the realization of such compact light sources. However, the use of microring laser cavities has been so far hindered by their tendency to operate in multiple modes. Quite recently, several studies have shown that notions from parity-time (PT)-symmetry and non-Hermitian physics can be utilized to effectively enforce single-mode operation in semiconductor microring laser arrangements. However, as of now, most works in this area were mainly devoted to the proof of concept demonstrations, while the feasibility of practical implementation has remained largely unexplored. In this article, we demonstrate the first regrowth free, electrically pumped microscale PT-symmetric laser. Our results can pave the way toward the utilization of exceptional points in more diverse applications ranging from on-chip light sources and modulators to laser beam steering systems and active sensors.

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.

We report on the first implementation of a photonic emulator platform upon which dynamic encirclement of an exceptional point can be observed. Using two polarization states, an omnipolarizer is realized based on chiral mode conversion.

We propose a reconfigurable broadband space-mode router using multiplane light conversion. For a 3-mode and 3-channel router, realizing a routing pattern similar to the AWG, insertion loss and mode dependent loss below 2dB was achieved over a 60 nm bandwidth using 7 phase plates.

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 dynamical encirclement of an exceptional point in a laser cavity. By continuously varying the detuning and coupling between a pair of PT-symmetric waveguides, the mode morphs into a different eigenmode as it emerges from opposing facets.

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.

Two watt-level broad-area laser diodes were simultaneously locked into a common external cavity made using a surface grating in a Littrow configuration. The spectral linewidth of the combined laser beam was narrowed down from over a nanometer to 10–15 pm (FWHM), and the output power was the sum of the power of the individually locked laser diodes. The spectrum of the combined laser beam can be tuned over a range of 2–3 nm by changing the tilt angle of the grating and varying the injection currents of each laser diode.

A two-section single stripe laser diode has been fabricated from a strained InGaAs/GaAs single quantum well heterostructure grown on GaAs substrate. The two sections have different band gap energies owing to selective area intermixing that is achieved by rapid thermal annealing of the sample with the two sections capped by silicon oxynitride (SiO x N y ) and silicon dioxide (SiO 2 ), respectively. The device is capable of producing laser emission at either 911 or 953 nm wavelengths depending on the current applied to either section of the laser stripe.

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.

A time-dependent trace of the reflection at the end of a two-hole fiber (THF) is use to measure surface tension. Gas is injected through the THF channels while reflection is measured through the fiber core.

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

We propose low-loss, low-crosstalk slab waveguide-to-fiber coupling using multiplane light conversion. For coupling between a 6-mode waveguide and fiber, insertion loss and mode-dependent loss are 0.38 dB and 0.23 dB, respectively, with 7 phase plates

In this work, we have demonstrated an optical surface Plasmon resonance (SPR) sensor head that is based on an inverted rib dielectric waveguide, in which the resonance wavelength of the surface plasmon excited at the gold metal-dielectric interface changes in relation to changes of the environment at the top metal surface. The inverted-rib waveguide of the SPR sensor head is made of a layer of SU-8 polymer with a refractive index of 1.5 while the lower cladding layer consists of silicon oxynitride (SiO x N y ) with a refractive index of 1.526. The top surface is coated with a 50 nm thick layer of gold. The SPR sensor head was designed to allow monitoring of analyte media with a refractive index ranging from 1.44 to the 1.502. Using a set of reference liquids representing the analyte medium, the sensitivity of the SPR sensor was measured using a broadband light source and a optical spectrum analyzer. It was found that with a liquid of 1.442 refractive index in contact with the gold metal, a sharp resonance dip in the transmission spectrum occurred at 1525 nm and its position shifted to 1537 nm when a liquid of 1.502 was used. From these measurements, the sensitivity of the sensor devices wasdetermined to be S = 232 nm.RIU -1 . We demonstrate that this device can potentially be totally integrated with a wavelength tunable light source, a photodetection unit as well as a liquid delivery system via microfluidic channels making it an extremely compact unit.

In this work, a two-section wavelength tunable laser diode is demonstrated using an InGaAsP multiple quantum well heterostructure. The laser diode consists of two sections with different bandgap energies achieved using selective area intermixing of the MQW. Using plasma enhanced chemical vapor deposition (PECVD), half of the sample is coated with a 30nm silicon nitride (SiNx) film followed by a 200nm thick overlay of silicon oxynitride (SiOxNy) film over the entire sample. The whole sample is then thermally annealed at 750°C for 30s, and that results in the SiOxNy covered section experiencing a narrowing of the bandgap energy, while leaving the SiNx covered section practically unchanged. A laser stripe is fabricated that passes through both MQW sections. The wavelength of laser operation can then be tuned by varying the injected current levels applied separately to the two sections. The obtained tuning range was 40 nm spanning from 1538 nm to 1578 nm.

A grating coupler for coupling vertically emitted laser light into planar silicon nitride waveguides is presented. The waveguide coupling efficiency was measured at 30% with a 3-dB bandwidth of 100 nm using an optical fiber.

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 an electrically pumped parity-time-symmetric coupled microring laser. Using the interplay between gain and loss, single mode operation is demonstrated with no penalty in terms of output power or threshold pump intensity.

InGaAsP quantum well structures are intermixed to varying degrees when rapidly annealed at elevated temperatures while capped with films of SiN x , and SiO y N x of different compositions. Laser diodes are fabricated with both blue and red shifted samples and their performances are reported.

A single waveguide laser with two separately addressed sections is fabricated using selective area intermixing of InGaAsP multiple quantum well grown on InP substrate. The selective intermixing of quantum wells is achieved by capping the two sections with PECVD grown silicon nitride and silicon dioxide respectively followed by rapid thermal annealing the device at 750°C for 30s prior to the fabrication of the quantum well laser. The fabricated device is capable of producing laser emission that can be tuned continuously from 1523 nm to 1556 nm by applying separate electrical currents into each 400 μm long section.

In this work, we report the micro fabrication of several types of resonant micro magnetic field sensors where we can use an optical technique to read the magnetic variations (MOEMS). The fabrication process we have used, is a simple process in a SOI wafer with 6 μm of active layer and 5 μm of sacrificial SiO2.

Heptamer arrays of coupled metallic nanolasers are demonstrated. The lasers operate in a single transverse and longitudinal mode, have low thresholds, and are capable of generating high output powers.

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.

We demonstrate an InGaAsP multiple quantum well tunable laser diode consisting of two gain sections with different bandgap energy levels. This tunable laser is used as a probing source for an optical sensor that makes use of surface plasmon resonance (SPR). Lasers fabricated on a single monolithic substrate can have widely differing output wavelengths. Therefore, by controlling the extent of the intermixing, the bandgap energy of the material can be accurately tuned over a broad spectral range. The regions of different bandgap energies are achieved using selective area intermixing of the multiple quantum wells, through impurity-free vacancy-induced disordering. By varying the current combination that was injected to each section, the laser wavelength can be effortlessly tuned from 1538 to 1578 nm with relatively constant output power. The free spectral range of the tunable laser is found to be 0.25 nm. This tunable laser was launched into an optical SPR sensor head that was fabricated on an inverted rib dielectric waveguide to provide an input light source for the SPR sensor. The average sensitivity of the SPR sensor devices was determined to be S = 334 nm RIU − 1.

We demonstrate a monolithic tunable laser on a QW structure by integrating an optical beam steering section with an area-selectively intermixed QW gain section. Wavelength switching is achieved by steering the laser beam over an optical gain medium that consists of two laterally adjacent quantum well regions selectively intermixed to varying extents. The device output had a total of 10 nm wavelength tuning that was controlled by injecting separate electrical current values to each of the steering contacts and the amplifier contact.

We report the first demonstration of a four-mode semiconductor optical amplifier (FM-SOA), with strategically positioned quantum-well layers to provide modal-gain control with uniform electrical pumping. On-off gain of 11 dB was demonstrated for all four modes (E 11 , E 12 , E 21 and E 22 ). The difference in on-off gain among all four modes is less than 1 dB in the range of 2-11 dB without any deliberately pumping profile control. Partic ular challenges with FM-SOA that must be overcome in comparison with traditional single mode SOA are discussed. Keywords: spatial mode division-multiplexing (SDM), semiconductor optical amplifier, quantum well 1. INTRODUCTION Mode-division multiplexing (MDM) can significantly increase per fiber transmission capacity by exploiting the new dimension of fiber modes 1-3 . Few-mode amplifiers are one of the mo st important technologies utilized in MDM 4,5 . While most research interests are focused on few-mode Erbium-doped fiber amplifiers (FM-EDFA), a complicated design, necessitating intricate ion doping concentration

The second-order coherence function is measured for a metallic coaxial nanolaser using a modified Hanbury Brown-Twiss technique. The results indicate that such nanoscale lasers can indeed generate coherent radiation.