Photonic Crystal Defect Cavity Research Articles

Transmissible topological edge states based on Su–Schrieffer–Heeger photonic crystals with defect cavities

Abstract Topological photonic crystals have great potential in the application of on-chip integrated optical communication devices. Here, we successfully construct the on-chip transmissible topological edge states using one-dimensional Su–Schrieffer–Heeger (SSH) photonic crystals with defect cavities on silicon-on-insulator slab. Different coupling strengths between the lateral modes and diagonal modes in photonic crystal defect cavities are used to construct the SSH model. Furthermore, two photonic SSH-cavity configurations, called α and β configurations, are designed to demonstrate the topological edge states. Leveraging the capabilities of photonic crystal transverse electric modes with on-chip transmission, we introduced a waveguide to excite a boundary defect cavity and found that the transmission peak of light, corresponding to the topological edge state, can be received in another boundary defect cavity, which is caused by the tunnel effect. Moreover, the position of this peak experiences a blue shift as the defect cavity size increases. Therefore, by tuning the size of the SSH defect cavity, on-chip wavelength division multiplexing function can be achieved, which is demonstrated in experiments. The ultrafast response time of one operation can be less than 20 fs. This work harmonizes the simplicity of one-dimensional SSH model with the transmissibility of two-dimensional photonic crystals, realizing transmissible on-chip zero-dimensional topological edge states. Since transmission peaks are highly sensitive to defect cavity size, this configuration can also serve as a wavelength sensor and a reconfigurable optical device, which is of substantial practical value to on-chip applications of topological photonics.

Realization of spinning mode with maximum chirality in photonic-crystal defect cavity at exceptional point

Optical modes spinning with maximum chirality in cavities at the chip level are essential for quantum and biomedical applications. The coalescent chiral mode at the exceptional point (EP) due to non-Hermicity is the one in demand. Yet, approaching EPs require fine balances between the loss/gain and the dielectric perturbation, which are typically nontrivial. In this paper, by elevating the reliability of fine-tuning, we realize circularly-polarized-like lasing modes with maximum chirality at the EP of photonic-crystal one-hole cavities. Such an EP is of intracavity type in the subwavelength scale and reaching it requires more reliable strategies than those for its forerunners. We adopt the in-plane tunneling loss that is well controlled with the layer number of air holes in the photonic-crystal slab. By removing and relocating holes in blocks, we systematically elevate the chirality of the radiation field. The collective variations of holes render the EP robust against the uncertainty in fabrications. Without auxiliary non-Hermitian and chiral structures, our works promote coherent chiral light sources at the chip level.

Highly boosted trace-amount terahertz vibrational absorption spectroscopy based on defect one-dimensional photonic crystal.

Although terahertz (THz) spectroscopy demonstrates great application prospects in the fields of fingerprint sensing and detection, traditional sensing schemes face unavoidable limitations in the analysis of trace-amount samples. In this Letter, a novel, to the best of our knowledge, absorption spectroscopy enhancement strategy based on a defect one-dimensional photonic crystal (1D-PC) structure is proposed to achieve strong wideband terahertz wave-matter interactions for trace-amount samples. Based on the Fabry-Pérot resonance effect, the local electric field in a thin-film sample can be boosted by changing the length of the photonic crystal defect cavity, so that the wideband signal of the sample fingerprint can be greatly enhanced. This method exhibits a great absorption enhancement factor, of about 55 times, in a wide terahertz frequency range, facilitating the identification of different samples, such as thin α-lactose films. The investigation reported in this Letter provides a new research idea for enhancing the wide terahertz absorption spectroscopy of trace samples.

Slow light in topological coupled-corner-state waveguide

We theoretically propose a uide (CCSW), which is composed of a zigzag edge-like structure based on C-4 symmetrical lattice. CCSW mode is achieved by weak coupling between a sequence of higher order topological corner state (TCS). Based on the tight-binding approximation, the flat dispersion relation of CCSW mode is obtained, and suitable for slowing down light. The characteristics of slow light, including the group index, group velocity dispersion, normalized bandwidth and normalized delay-bandwidth product, are discussed in detail. At the Eigen frequency of individual TCS, the group velocity dispersion of CCSW mode is zero. Importantly, the CCSW mode shows strong robustness when introducing disorders, compared with the conventional Coupled-Resonator-Optical Waveguide based on photonic crystal defect cavities. Our findings may find topological slow light applications such as optical buffers, the processing of optical signals, optical delay lines and so on.

Coupling of photonic crystal cavity and interlayer exciton in heterobilayer of transition metal dichalcogenides

The advent of van der Waals heterostructures marks the emergence of a new class of synthetic materials with novel properties that are unattainable in their constituent materials. The 2D architecture of these layered materials makes them naturally suited for integration with a wide variety of planar nanophotonic cavities for next-generation low-power optoelectronic devices and explorations of fundamental physical effects in these new systems. Here, we report the coupling of the interlayer exciton in a transition metal dichalcogenide heterobilayer with a gallium phosphide photonic crystal defect cavity. The exciton-cavity coupling is found to be in the weak regime, resulting in ~15-fold increase in the photoluminescence intensity for interlayer exciton in resonance with the cavity. Simulation results suggest that the increased intensity stems from a Purcell enhancement of ~60. The order of magnitude enhancement of the photoluminescence yield offsets the low oscillator strength of the interlayer exciton, adding a new tool for probing the underlying physics of this excitonic system.

Benchmarking five numerical simulation techniques for computing resonance wavelengths and quality factors in photonic crystal membrane line defect cavities.

We present numerical studies of two photonic crystal membrane microcavities, a short line-defect cavity with a relatively low quality (Q) factor and a longer cavity with a high Q. We use five state-of-the-art numerical simulation techniques to compute the cavity Q factor and the resonance wavelength λ for the fundamental cavity mode in both structures. For each method, the relevant computational parameters are systematically varied to estimate the computational uncertainty. We show that some methods are more suitable than others for treating these challenging geometries.

Proposal and numerical study of a flexible visible photonic crystal defect cavity for nanoscale strain sensors

A flexible photonic crystal cavity, consisting of a III-V active layer embedded in a flexible medium, with a line-defect by removing three air holes for nanoscale structural deformation detection is proposed and optimized. The cavity can hold the photonic band-gap modes with the fundamental mode located at approximately 686 nm, overlapping with the photoluminescence spectrum of the InGaP/InGaAlP quantum wells. Results of finite-difference time-domain simulations indicate that the L3 cavity features an ultra-compact mode volume of 10-3 µm3 and high quality factor of 104 at a sub-micron footprint within the studied visible wavelength. Theoretical optical strain sensitivities of approximately 4.5 and 3 nm per ε (1% strain for both) for the x and y directions are predicted, respectively. When the cavity is under large bending curvatures, the Q factor rapidly decreases from 8000 to 2000.

Curie’s Symmetry Principle for Selection Rule of Photonic Crystal Defect Modes

Symmetry, which defines invariant properties under a group of transformations, provides a frame of generalization uncovering regularities from given quantitative descriptions. Based on the Curie’s symmetry principle, connecting between causality and symmetry, we formulate the intuitive but formal selection rules and apply to determine the excitable resonant modes of a photonic crystal defect cavity, which is an important element for plasmonic applications. Quantitative agreement with the numerical simulations demonstrates the effectiveness of the fundamental principle in finding the critical symmetry conditions for the available localized defect states within photonic crystals. Moreover, the principle facilitates analysis of the higher-order or even forbidden modes in the asymmetric excitation configurations regarding the polarizations or positions of the light source, which typically require heavy computations. Our results may be extended similarly to develop the qualitative selection rules in other physical systems with a geometric symmetry.

Modelling defect cavities formed in inverse three-dimensional rod-connected diamond photonic crystals

Defect cavities in 3D photonic crystal can trap and store light in the smallest volumes allowable in dielectric materials, enhancing non-linearities and cavity QED effects. Here, we study inverse rod-connected diamond (RCD) crystals containing point defect cavities using plane-wave expansion and finite-difference time domain methods. By optimizing the dimensions of the crystal, wide photonic bandgaps are obtained. Mid-bandgap resonances can then be engineered by introducing point defects in the crystal. We investigate a variety of single spherical defects at different locations in the unit cell focusing on high-refractive-index-contrast (3.3:1) inverse RCD structures; quality factors (Q-factors) and mode volumes of the resonant cavity modes are calculated. By choosing a symmetric arrangement, consisting of a single sphere defect located at the center of a tetrahedral arrangement, mode volumes < 0.06 cubic wavelengths are obtained, a record for high-index cavities.

Light Emission Characteristics of Nonpolar &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$a$ &lt;/tex-math&gt; &lt;/inline-formula&gt;-Plane GaN-Based Photonic Crystal Defect Cavities

In this paper, the light emission performance from nonpolar $a$ -plane gallium nitride (GaN)-based photonic crystal (PC) slabs with line-type defect cavities has been investigated. Via the focused ion beam milling technique, thin membrane structures of the nonpolar GaN PC defect cavities were engineered, exhibiting one dominated resonant mode occurred at 419.3 nm with a high quality factor of $1.9\times 10^{3}$ in the photoluminescence emission characterizations. In the far-field region, light emission performance shows a high polarization degree of 50% with the beam direction perpendicular to the defect cavity. Compared with the PC defect cavity patterns fabricated on $c$ -axial oriented GaN materials, the $a$ -plane PC defect cavities display not only the linear light output–input power dependence, but also an invariant resonant mode of constant light emission behavior with increasing the injection carriers, therefore providing a new perspective for the future development of promising optoelectronic devices.

Topological defect lasers

We introduce a topological defect to a regular photonic crystal defect cavity with anisotropic unit cell. Spatially localized resonances are formed and have high quality factor. Unlike the regular photonic crystal defect states, the localized resonances in the topological defect structures support powerflow vortices. Experimentally we realize lasing in the topological defect cavities with optical pumping. This work shows that the spatially inhomogeneous variation of the unit cell orientation adds another degree of freedom to the control of lasing modes, enabling the manipulation of the field pattern and energy flow landscape.

Integrated III-V Photonic Crystal--Si waveguide platform with tailored optomechanical coupling.

Optomechanical systems, in which the vibrations of a mechanical resonator are coupled to an electromagnetic radiation, have permitted the investigation of a wealth of novel physical effects. To fully exploit these phenomena in realistic circuits and to achieve different functionalities on a single chip, the integration of optomechanical resonators is mandatory. Here, we propose a novel approach to heterogeneously integrate arrays of two-dimensional photonic crystal defect cavities on top of silicon-on-insulator waveguides. The optomechanical response of these devices is investigated and evidences an optomechanical coupling involving both dispersive and dissipative mechanisms. By controlling the optical coupling between the waveguide and the photonic crystal, we were able to vary and understand the relative strength of these couplings. This scalable platform allows for an unprecedented control on the optomechanical coupling mechanisms, with a potential benefit in cooling experiments, and for the development of multi-element optomechanical circuits in the framework of optomechanically-driven signal-processing applications.

Origin and reduction of wakefields in photonic crystal accelerator cavities

Photonic crystal (PhC) defect cavities that support an accelerating mode tend to trap unwanted higher-order modes (HOMs) corresponding to zero-group-velocity PhC lattice modes at frequencies near the top of bandgaps. The effect is explained quite generally by photonic band and perturbation theoretical arguments. Transverse wakefields resulting from this effect are observed (via simulation) in a 12 GHz hybrid dielectric PhC accelerating cavity based on a triangular lattice of sapphire rods. These wakefields are, on average, an order of magnitude higher than those in the 12 GHz waveguide-damped Compact Linear Collider copper cavities. The avoidance of translational symmetry (and, thus, the bandgap concept) can dramatically improve HOM damping in PhC-based structures.

Cavity-stimulated Raman emission from a single quantum dot spin

Solid-state quantum emitters have shown strong potential for applications in quantum information, but the spectral inhomogeneity of these emitters poses a significant challenge. We address this issue in a cavity–quantum dot system by demonstrating cavity-stimulated Raman spin flip emission. This process avoids populating the excited state of the emitter and generates a photon that is Raman shifted from the laser and enhanced by the cavity. The emission is spectrally narrow and tunable over a range of at least 125 GHz, which is two orders of magnitude greater than the natural linewidth. We obtain the regime in which the Raman emission is spin dependent, which couples the photon to a long-lived electron spin qubit. This process can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic-crystal quantum network. Cavity-stimulated Raman spin-flip emission is demonstrated by coupling a negatively charged InAs/GaAs quantum dot to a photonic crystal defect cavity. The emission is spectrally narrow and tunable over a range of about 125 GHz. The process can be made spin selective by tuning the scattered photons to be in resonance with the cavity.

Enhanced photodetection in graphene-integrated photonic crystal cavity

We demonstrate the controlled enhancement of photoresponsivity in a graphene photodetector by coupling to slow light modes in a long photonic crystal linear defect cavity. Near the Brillouin zone (BZ) boundary, spectral coupling of multiple cavity modes results in broad-band photocurrent enhancement from 1530 nm to 1540 nm. Away from the BZ boundary, individual cavity resonances enhance the photocurrent eight-fold in narrow resonant peaks. Optimization of the photocurrent via critical coupling of the incident field with the graphene-cavity system is discussed. The enhanced photocurrent demonstrates the feasibility of a wavelength-scale graphene photodetector for efficient photodetection with high spectral selectivity and broadband response.

A chip-scale integrated cavity-electro-optomechanics platform

We present an integrated optomechanical and electromechanical nanocavity, in which a common mechanical degree of freedom is coupled to an ultrahigh-Q photonic crystal defect cavity and an electrical circuit. The system allows for wide-range, fast electrical tuning of the optical nanocavity resonances, and for electrical control of optical radiation pressure back-action effects such as mechanical amplification (phonon lasing), cooling, and stiffening. These sort of integrated devices offer a new means to efficiently interconvert weak microwave and optical signals, and are expected to pave the way for a new class of micro-sensors utilizing optomechanical back-action for thermal noise reduction and low-noise optical read-out.

Decimated Photonic Crystal Defect Cavity Lasers

We present the design, fabrication, and characterization of decimated lattice photonic crystal membrane lasers. Based on our simulated designs, decimated photonic crystal lattices have reduced number of holes, which enhance the current and heat conductive paths from the optical cavity with a minimal impact on the cavity quality factor and resonance. Our calculations indicate lower operating temperature using the decimation lattice designs. Two decimated cavity designs are demonstrated and compared to conventional photonic crystal defect cavity lasers in InP-based membranes. The decimated designs show improved laser performance with a small reduction of measured cavity quality factor. Reduction of the photonic crystal holes around the defect cavity to as few as two periods still enables photopumped operation of photonic crystal defect lasers. The improvement in the thermal properties is characterized by varying the photopumping duty cycle.

Shaking optical nanocavities

Surface acoustic waves can actively modulate the optical mode of a photonic crystal defect cavity. Fast tuning rates, compared with the typical lifetimes of optical emitters and photons inside the cavity, pave the way for deeper insights into cavity quantum electrodynamics.

Optomechanical Coupling in a Two-Dimensional Photonic Crystal Defect Cavity

Periodically structured materials can sustain both optical and mechanical modes. Here we investigate and observe experimentally the optomechanical properties of a conventional two-dimensional suspended photonic crystal defect cavity with a mode volume of ~3(λ/n)³. Two families of mechanical modes are observed: flexural modes, associated to the motion of the whole suspended membrane, and localized modes with frequencies in the GHz regime corresponding to localized phonons in the optical defect cavity of diffraction-limited size. We demonstrate direct measurements of the optomechanical vacuum coupling rate using a frequency calibration technique. The highest measured values exceed 80 kHz, demonstrating high coupling of optical and mechanical modes in such structures.

Spontaneous-emission control by local density of states of photonic crystal cavity

The local density of states (LDOS) of two-dimensional square lattice photonic crystal (PhC) defect cavity is studied. The results show that the LDOS in the centre is greatly reduced, while the LDOS at the point off the centre (for example, at the point (0.3a, 0.4a), where a is the lattice constant) is extremely enhanced. Further, the disordered radii are introduced to imitate the real devices fabricated in our experiment, and then we study the LDOS of PhC cavity with configurations of different disordered radii. The results show that in the disordered cavity, the LDOS in the centre is still greatly reduced, while the LDOS at the point (0.3a, 0.4a) is still extremely enhanced. It shows that the LDOS analysis is useful. When a laser is designed on the basis of the square lattice PhC rod cavity, in order to enhance the spontaneous emission, the active materials should not be inserted in the centre of the cavity, but located at positions off the centre. So LDOS method gives a guide to design the positions of the active materials (quantum dots) in the lasers.