By Yvonne Carts-Powell
Kostadin Djordjev and others in P. Daniel Dapkus? group at the University of Southern California (USC; Los Angeles) have reported making active switching devices from semiconductor microdisks.1 The devices have high quality factor (Q; more than 5000), high finess (of about 40), and high power-extraction efficiency. The researchers believe this is the first experimental demonstration of gain and loss trimming of the transmission characteristics of a microdisk cavity.
Microdisks are compact structures (typically less than 20 μm in diameter) that can be used to make a number of different components in a photonic integrated circuit. Passive microdisk devices include add/drop filters, demultiplexers, and notch filters. Because active devices could provide gain, however, they could compensate for losses from manufacturing imperfections, as well as tuning or otherwise adjusting the resonant frequency.
The USC group made an indium phosphide (InP)-based vertically coupled active device (see figure) using a wafer-bonding approach they developed in 1999 for making passive semiconductor microdisks.2 The vertical coupling stacks the disk above the bus waveguide. The fabrication method allows the disk and waveguides to be made from different materials, and allows the components in different levels to be optimized separately. The InP-based system included indium gallium arsenide phosphide (InGaAsP) waveguides and a disk core incorporating four InGaAsP quantum wells with an emission wavelength of 1.55 μm.
By using a semiconductor for the microdisk material, the researchers were able to create a device that can provide both gain and electroabsorption effects. These allow the user to trim the transmission properties of the coupler. By reverse-biasing the device, the researchers were able to explore loss trimming using the quantum-confined Stark effect and by forward-biasing the device to explore gain trimming.
When no voltage is applied (and at wavelengths far from the bandgap), a microdisk with a radius of 10 μm showed a high Q of 5700, a finess of 40, and a transmission at resonance of 0.1 around a wavelength of 1.584 μm. At wavelengths closer to the bandgap (at 1.55 μm) the quality decreases because of increased absorption.
Applying a negative bias shifts the absorption edge to longer wavelengths, thus decreasing the quality and increasing transmission. At -3 V, the resonant peak shifts 0.2 nm, the Q lowers to 2500, and transmission at resonance increases to 0.55. This effect could be used to turn the microdisk into a switch or modulator. Applying a forward current to the microdisk results in gain, which could be used to compensate for loss
For more information contact Dan Dapkus at firstname.lastname@example.org.
1. K. Djordjev et al., Optical Fiber Conference 2002, postdeadline paper FA2.
2. D. V. Tishinin et al., IEEE Photon. Tech. Lett. 11(8), 1003 (1999).