Two-photon absorption speeds optical switching
Research at Trinity College Dublin suggests that two-photon absorption in semiconductor microcavities supports multi-gigahertz switching and demuxing.
By John Donegan Director, Semiconductor Photonics Group, Trinity College, Dublin
To accommodate future broadband optical networks it is necessary to design and develop new optical signal processing technologies. At Trinity College, we have developed an innovative photonic device based on two-photon absorption (TPA) in semiconductor micro-cavities for optical processing functions.
Optical time division multiplexing (OTDM) is used in conjunction with wavelength division multiplexing (WDM) to maximise the data capacity of installed optical fibre. Using OTDM, it is possible to achieve data rates of 100Gbit/s and beyond on a single wavelength channel. However, at these data rates it is not possible for standard optoelectronic receivers to demultiplex the data. High-speed optical demultiplexers are required for such systems. In addition, optical oscilloscopes will ne needed to display high-speed data channels.
To develop both high-speed optical demulti-plexers and optical oscilloscopes one must take advantage of ultra-fast optical non-linearities in optical fibre, semiconductors, or optical crystals. The optical nonlinearity that is normally employed for developing optical demultiplexers is the nonlinear refractive index in fibre or semiconductor waveguides.
To develop optical oscilloscopes for monitoring the performance of high-speed systems, the optical nonlinearity employed is that of second harmonic generation (SHG) in optical crystals. However, the optical nonlinearity is very inefficient, resulting in the need for high-power optical input signals.
The phenomenon of TPA is a nonlinear optical-to-electronic conversion process where two photons are absorbed in the generation of a single electron-hole carrier pair in the semiconductor. The TPA nonlinearity has an ultra-fast response time t (as it uses virtual states, t ~ 2h/Eg ~ 10-14s at 1.5m) thus making it suitable for the development of high-speed devices.
We have developed a novel approach to take advantage of the TPA time response while giving a markedly increased efficiency. We place the active semiconductor material within a semiconductor microcavity. The microcavity device length is designed to be an integral of the absorption wavelength to enhance the light-matter interaction that in our case is the enhancement of the TPA efficiency. Bragg-stack mirrors placed at either end of the cavity lead to very strong optical fields within the cavity, which can be viewed as an enhancement of the interaction length within the cavity. The device structure is shown in Fig. 1.
We have demonstrated this effect both theoretically and experimentally in specially fabricated devices designed for TPA operation at 890nm. Our results have shown an enhancement of more than four orders of magnitude in the TPA photocurrent by using a microcavity structure compared with a standard semiconductor (Fig 2).
The impact of the cavity on the dynamic response of the TPA microcavity device has also been investigated using autocorrelation measurements and the results compared with a BBO crystal. As can be seen in Fig. 3, the cavity lifetime elongates the measured autocorrelation pulse width slightly due to the high reflectivity of the Bragg mirrors: 2.39ps was recorded for the BBO crystal compared with 2.89ps for the TPA device.
These results strongly suggest that TPA in semiconductor microcavities is a key candidate for demultiplexing in optical network systems and for monitoring the network timing through the use of optical oscilloscopes in which pulse widths on the order of 1-2 ps will be in use at multi-gigahertz repetition rates.
We are now working with devices operating at 1.55µm and expect to see the enhancement factor of about 104 reproduced for structures operating at this telecomms wavelength. The next stage will be to develop a prototype TPA switch to demultiplex a 100GHz data signal.
We will also work on developing an optical oscilloscope where we have replaced the BBO crystal and its SHG process with the very fast and efficient TPA process in our semiconductor systems. We have a licence agreement with Eblana Photonics, a start-up that has developed from our research group, to develop this technology beyond this R&D stage (see LWE March 2002).
This research has been carried out in collaboration with Dr. Liam Barry of the Electronic Engineering Dept. of Dublin City University where the network aspects of the TPA devices are being investigated. The TPA structures were grown in the Univ. of Sheffield in the group led by Prof. John Roberts.
Dr John Donegan
Semiconductor Photonics Group
Department of Physics, Trinity College Dublin