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Silicon photonics goes organic

21 Aug 2007

A combination of organic semiconductors and silicon photonics results in a hybrid red surface-emitting laser.

A hybrid silicon-polymer laser that emits in the red could provide a cost-effective route to chip-to-chip optical interconnects, say researchers at the University of St Andrews, UK. The device combines microstructured silicon resonators with light-emitting organic semiconductors to overcome silicon's intrinsically poor light emission. (Applied Physics Letters 91 051124)

"Silicon is a very inefficient light emitter and integrating light-emitting semiconductors on silicon is a new way to tackle this problem," Graham Turnbull of St Andrews' Organic Semiconductor Centre told optics.org. "Our hybrid lasers can be fabricated very simply using standard CMOS processing and emit at wavelengths easily detected by conventional silicon photodetectors."

Silicon is the dominant material of modern electronics and mass-fabrication of wavelength-scale structures is now well established. However, there is a growing need for high-bandwidth interconnects and one possible solution is to integrate optoelectronic components directly with on silicon processors.

There are two significant challenges when it comes to fabricating a hybrid silicon-polymer laser. "The first problem is silicon's substantial absorption at the visible wavelengths of the polymer photoluminescence," explained Turnbull. "The second is the high refractive index of silicon, which complicates how laser emission is confined within a polymer waveguide."

The team's solution was to create a distributed Bragg reflector resonator design based on a silicon-on-insulator substrate. Mirrors come in the form of two periodically-poled microstructured silicon segments that are 50 microns apart. The silicon epilayer between the mirrors is removed, exposing a buried SiO2 layer, before the whole structure is covered in an organic amplifying medium.

"A polymer waveguide is formed between the two silicon mirrors with a confinement factor limited only by the thickness of the polymer layer," explained Turnbull. "The area between the two microstructured silicon reflectors also minimizes the interaction of the laser with the silicon and addresses the high absorption coefficient of silicon."

Turnbull and colleagues use 1.2 ns pulses from a frequency-doubled microchip emitting at 532 nm to optically excite their hybrid laser at room temperature. These pulses are focused down to an area 85 microns in diameter and the resulting surface-emission is collected by a fiber-coupled spectrometer. "The lasers in this work were tunable from 608 to 628 nm, were pulsed at 5kHz and typical pulse energies were in the pJ range," added Turnbull.

The next problem is moving to electrical excitation. "We envisage using the underlying silicon chip to apply a field or inject charge to modulate the light emission and encode information," concluded Turnbull. "For interconnects, a move to higher repetition rates is necessary and in the longer term, electrically-pumped lasing using p-doped silicon as a hole injection layer is also a possibility."

Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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