Faraz Monifi

Parity–time-symmetric whispering-gallery microcavities

It is now shown that coupled optical microcavities bear all the hallmarks of parity–time symmetry; that is, the system’s dynamics are unchanged by both time-reversal and mirror transformations. The resonant nature of microcavities results in unusual effects not seen in previous photonic analogues of parity–time-symmetric systems: for example, light travelling in one direction is resonantly enhanced but there are no resonance peaks going the other way.

Loss-induced suppression and revival of lasing

Controlling and reversing the effects of loss are major challenges in optical systems. For lasers, losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. In this work, we show how to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the loss-induced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss. Introducing loss into a coupled optical system can result in an enhancement of the optical properties. [Also see Perspective by Schwefel] Achieving gain despite increasing loss When energy is pumped into an optically active material, the buildup (or gain) of excitations within the material can reach a critical point where the emission of coherent light, or lasing, can occur. In many systems, however, the buildup of the excitations is suppressed by losses within the material. Overturning conventional wisdom that loss is bad and should be minimized, Peng et al. show that carefully tweaking the coupling strength between the various components of a coupled optical system can actually result in an enhancement of the optical properties by adding more loss into the system (see the Perspective by Schwefel). The results may provide a clever design approach to counteract loss in optical devices. Science, this issue p. 328; see also p. 304

Highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman microlaser

Recently optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms to achieve label-free detection of nanoscale objects and to reach single molecule sensitivity. The ultimate detection performance of WGMRs are limited by energy dissipation in the material they are fabricated from. Up to date, to improve detection limit, either rare-earth ions are doped into the WGMR to compensate losses or plasmonic resonances are exploited for their superior field confinement. Here, we demonstrate, for the first time, enhanced detection of single-nanoparticle induced mode-splitting in a silica WGMR via Raman-gain assisted loss-compensation and WGM Raman lasing. Notably, we detected and counted individual dielectric nanoparticles down to a record low radius of 10 nm by monitoring a beatnote signal generated when split Raman lasing lines are heterodyne-mixed at a photodetector. This dopant-free scheme retains the inherited biocompatibility of silica, and could find widespread use for sensing in biological media. It also opens the possibility of using intrinsic Raman or parametric gain in other systems, where dissipation hinders the progress of the field and limits applications.Significance To date, loss compensation in optical microresonators has been done using rare-earth ions, which requires additional processing steps and costs and raises biocompatibility concerns. An alternative to integrating rare-earth ions for loss compensation is the use of intrinsic gain mechanisms such as Raman and parametric gain present in the materials from which resonators are fabricated. Here, we report the first implementation to our knowledge of Raman gain-induced loss compensation in silica whispering-gallery-mode (WGM) resonators for improved detection and the first demonstration to our knowledge of mode splitting in a WGM Raman microlaser for detecting and counting single nanoparticles down to 10 nm. This intrinsically self-referenced, self-heterodyned, and biocompatible scheme has enabled achieving record-high polarizability sensitivity (down to 3.82 × 10−6 μm3) without using plasmonic effects, passive or active stabilization, or frequency locking. Optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms for label-free detection of nano-objects. The ultimate sensitivity of WGMRs is determined by the strength of the light–matter interaction quantified by quality factor/mode volume, Q/V, and the resolution is determined by Q. To date, to improve sensitivity and precision of detection either WGMRs have been doped with rare-earth ions to compensate losses and increase Q or plasmonic resonances have been exploited for their superior field confinement and lower V. Here, we demonstrate, for the first time to our knowledge, enhanced detection of single-nanoparticle-induced mode splitting in a silica WGMR via Raman gain-assisted loss compensation and WGM Raman microlaser. In particular, the use of the Raman microlaser provides a dopant-free, self-referenced, and self-heterodyned scheme with a detection limit ultimately determined by the thermorefractive noise. Notably, we detected and counted individual nanoparticles with polarizabilities down to 3.82 × 10−6 μm3 by monitoring a heterodyne beatnote signal. This level of sensitivity is achieved without exploiting plasmonic effects, external references, or active stabilization and frequency locking. Single nanoparticles are detected one at a time; however, their characterization by size or polarizability requires ensemble measurements and statistical averaging. This dopant-free scheme retains the inherited biocompatibility of silica and could find widespread use for sensing in biological media. The Raman laser and operation band of the sensor can be tailored for the specific sensing environment and the properties of the targeted materials by changing the pump laser wavelength. This scheme also opens the possibility of using intrinsic Raman or parametric gain for loss compensation in other systems where dissipation hinders progress and limits applications.

A Robust and Tunable Add–Drop Filter Using Whispering Gallery Mode Microtoroid Resonator

We fabricated and theoretically investigated an add-drop filter (ADF) using an on-chip whispering gallery mode (WGM) microtoroid resonator with ultrahigh-quality factor (Q) side coupled to two taper fibers, forming the bus and drop waveguides. The new device design incorporates silica side walls close to the microresonators which not only enable placing the coupling fibers on the same plane with respect to the microtoroid resonator but also provides mechanical stability, leading to an ADF with high drop efficiency and improved robustness to environmental perturbations. We show that this new device can be thermally tuned to drop desired wavelengths from the bus without significantly affecting the drop efficiency, which is around 57%.

Three output port channel-drop filter based on photonic crystals

We present a photonic-crystal (PC) channel-drop filter (CDF) design based on 3x3 PC ring resonators. The normalized transmission spectra for single-ring and dual-ring configurations have been investigated using two-dimensional finite-difference time-domain (FDTD) technique in a square-lattice dielectric-rod PC structure. First, we investigate a single ring and we show that backward and forward dropping is possible in the third communication window. Then we add another ring and waveguide to develop a new CDF. This filter consists of an input and three outputs. Our FDTD simulation yields more than 85% efficiency over each output port.

Tunable add-drop filter using an active whispering gallery mode microcavity

An add-drop filter (ADF) fabricated using a whispering gallery mode resonator has different crosstalks for add and drop functions due to non-zero intrinsic losses of the resonator. Here, we show that introducing gain medium in the resonator and optically pumping it below the lasing threshold not only allows loss compensation to achieve similar and lower crosstalks but also tunability in bandwidth and add-drop efficiency. For an active ADF fabricated using an erbium-ytterbium co-doped microsphere, we achieved 24-fold enhancement in the intrinsic quality factor, 3.5-fold increase in drop efficiency, bandwidth tunability of 35 MHz and a crosstalk of only 2%.

Photonic crystal power dividers using L-shaped bend based on ring resonators

We propose a new type of two-dimensional photonic crystal power dividers based on ring resonators and directional couplers that can be applicable to photonic integrated circuits. The proposed power dividers mechanism is analogous to that of conventional waveguide directional couplers, utilizing coupling between guided modes supported by line defect waveguides. Based on the calculated position, a photonic crystal power divider is designed and verified by finite-difference time-domain computation. With low-loss bends based on ring resonators, a total transmission up to 99% is achieved. Different output power levels are achieved by changing the coupling length. Also the power in each branch can easily be further divided.

Photonic crystal power splitter and wavelength multi/demultiplexer based on directional coupling

In this paper, first by applying a theoretical approach (coupled-mode theory and one-dimensional scattering theory model) as well as a numerical approach, the transmission properties of different right angle bend geometries are investigated. Then using optimized bends, a two-dimensional photonic crystal power splitter based on directional coupling is analyzed. Effects of changing coupling length and distance between parallel waveguides on output transmissions are investigated. By further splitting the power in each branch a power splitter with four output branches is proposed. Our numerical simulation with a finite-difference time-domain (FDTD) technique shows that total transmission up to 96% and 92% is obtained for power splitters with two and four output branches, respectively, throughout the calculated coupling lengths. As another application a compact size wavelength multi/demultiplexer composed of optimized 90° bent waveguides and directional couplers is designed.

Heterostructure photonic crystal channel drop filters using mirror cavities

In this paper, a heterostructure channel drop filter in two-dimensional photonic crystals based on mirror cavities is investigated. First a channel drop filter using mirror cavities is presented. Then in order to achieve a three output port filter, we present a heterostructure built on a hybrid two-dimensional (2D) photonic crystal that includes two different refractive indices. Our numeric simulation with the finite-difference time-domain (FDTD) technique shows that the heterostructure has acceptable output efficiency over 80%. Finally, by applying a theoretical approach (coupled mode theory), the physical mechanism behind these structures is explained.

Design of efficient photonic crystal bend and power splitter using super defects

We propose a new scheme for an efficient photonic crystal 90° bend and power splitter. First we design a 90° bend by placing a super defect at the junction of two orthogonal photonic crystal waveguides. We changed the parameters of the super defect to optimize the transmission coefficient of the bent structure. Our two-dimensional (2D) simulations show more than 85% efficiency over a wide range of wavelengths. Then using this new scheme and adding another waveguide we design a new power splitter and optimize it with the same procedure. We used the coupled mode theory to analytically investigate the structures and finite difference time domain to simulate and optimize the structures performances.