Research

Photonic Integrated Circuits with Photonic Crystal Elements

As the number of integrated transistors per chip continues to obey Moore’s law as a function of time, the communication speed is also expected to increase. As the communication speed increases over 10 Gb/s, the cooper wires inside tiny chips will distort data streams because small imperfections in the copper lines or slice irregularities in the circuit boards. Fundamentally, the photon is a better choice for communication than the electron because the electron-electron interaction is much stronger than the photon-photon interaction. Optical communication is projected to dominate in the next decade for high speed communication and photonic integrated circuits because a very important topic for implementing the optical communication systems in a tiny chip.

Photonic crystals, a type of artificial periodic dielectric structures, are designed to manipulate the behavior of photons. The photonic band gap, in which photons are forbidden to propagate within a certain frequency range, can be created with a periodic geometry of some crystal lattices. Photonic crustal devices have many applications as lasers, LEDs, optical waveguides, optical switches in optical communications, high quality factor cavities and single photon sources in quantum information system. One of the important applications in building dense chip-scale photonic integrated circuits with photonic crystal elements. The elements of circuit are lasers, waveguides and bending structures, modulators and photodectors. In other words, photonic crystal devices are applicable to chip-scale photonic integrated circuits because of their compact size and versatile functions.

Flexible Micro/Nano Lasers and Optical Curvature Sensors on a Polymer Substrate

Many microdisk lasers were demonstrated in a suspended membrane or on the dielectric substrates such as Si/SiO2, III-V and GaN materials. All of them work on the flat hard surfaces. However, for future applications such as the flexible photonic circuits on human skins, emitters/sensors for bio-systems or optical lasers/sensors on the airplane surfaces, the flexible compact lasers are in demand. In this topic, we study a flexible microdisk laser on a polydimethylsiloxane (PDMS) substrate. This flexible laser has the ability to operate on the non-flat for the desired applications. With a flexible platform, this type of micro/nano-lasers can function not only as a µm size light sources, but also benefit to the flexible laser arrays for the photonic integrated circuits in the non-flat surfaces. The compact microdisk array can also function as the compact optical curvature sensors with the tunable lasing wavelength.

Compact GaN-based Microdisk and Photonic Crystal Lasers

GaN-based materials have been attracted much attention since 1990s due to the large direct wide band gap and high potential doe the optoelectronic devices, including light emitting diodes (LEDs) and laser diodes (LDs). Today, blue LEDs are widely used in many applications such as illumination, lighting, display, traffic signals and etc. We have studied and integrated photonic crystal and microdisk cavities with GaN-based lasers for higher efficiency and better optical properties.

Plasmonic Devices in Photonic Circuitry

Surface plasmon polaritons (SPPs) are electromagnetic waves coupled to the oscillations of free electrons at the metal-dielectric interface. The coupling waves will propagate along the interface with a strong field enhancement and a decay of evanescent wave. By employing the unique property of SPPs, surface-plasmon circuits were proposed. Most attractive surface plasmon components are focused on reflectors, resonators, and subwavelength waveguides with the guiding and confinement properties of electromagnetic radiation in light. The localization of surface plasmon polaritons (LSPPs) at the metallic/dielectric interface has also been reported. The non-propagating LSPP property offers a potential in many applications such as sensors, solar cells, spasers, and thermal emitters. Using those plasmonic properties, a number of passive and active surface plasmon-based elements were developed, bringing the dream of a surface plasmon based photonic integrated circuit into reality. For communication systems, nm-sized electronic (metallic) circuits are inherently slow due to resistor-capacitor (RC) delay time limiting its data-transmission speed, whereas optical (dielectric) circuits having high data-transmission speed are μm-sized due to optical diffraction limit. However, it is fascinating to note that surface plasmon based photonic circuits can combine the compactness of the electronic circuits with the broad bandwidth of the optical circuits. A surface plasmon based photonic integrated circuit includes a range of elements on a chip such as waveguides, modulators, filters, detectors, emitters and lasers.

We have studied the difference between symmetric and asymmetric T-shaped gratings and found that the symmetric structure has a momentum gap in the photonic band structure, which can be avoided in the asymmetric structure. A gap introduced in the post of T-shaped plasmonic gratings plays an important role in controlling the surface plasmon-polariton band gap and group velocities. This asymmetric T-shaped plasmonic grating is expected to have applications in surface plasmon polariton (SPP) based optical devices, such as filters, waveguides, splitters and lasers, especially for applications requiring large photonic band gap.