World’s First Ultrafast All-Optical Room Temperature Transistor

Illustration of a room-temperature organic polariton transistor. (Source: A. V. Zadesatelev, Skoltec)

A team at the IBM Research Lab in Zurich together with our partners from the research lab of Pavlos Lagoudakis at Skolkovo Institute of Science and Technology and Southampton – a colla­boration established within the framework of the European Horizon-2020 training network SYNCHRONICS –, has succeeded in building the first ever cascadable, all-optical transistor capable of operating at room temperature. The team achieved this by exploiting the material properties of an organic semi­conducting polymer. Based on this material, a micro­cavity was engineered in which an incoming optical signal can be switched on and off or amplified by another laser beam.

All-optical components that manipulate infor­mation with light alone could enable much faster switching and logical opera­tions as well as provide building blocks for new appli­cations like routing flying qubits from quantum microwave optical trans­duction or blind quantum computing. But such all-optical components are very difficult to build. And in fact, efforts to make all-optical computers have been around for about 50 years.

In order to switch or amplify an optical signal with another optical signal, a material that mediates the inter­action is needed. It’s just in the quantum nature of light beams that they don’t interact with each other in vacuum. In the new transistor, the mediating part is done by exciton-polaritons. They arise in an organic semi­conductor (methyl-substituted ladder-type poly- [para­phenylene] or MeLPPP) provided by our long-time partner Ullrich Scherf from the Wuppertal University. We placed a 35 nanometer-thin layer of MeLPPP between two highly reflective mirrors to form an optical cavity in which exciton-polaritons were produced using a laser. An exciton-polariton consists in the super­position of an exciton – an electron-hole pair – and a photon. That’s why our device falls in the category of organic polariton tran­sistors.

The transistor not only is the first of its kind to work at ambient conditions, it also provides an unpre­cedented 6500-fold optical signal amplification with a device length of just a few micrometers. That is 330 times higher than the amplification attained by its inorganic counterpart and allows for casca­dability, which is a necessary condition to use the transistor for logic gates. In experiments, our device also exhibited the highest net optical gain ever observed for an optical transistor (~10 dB/micrometer). Further­more, the transistor features ultrafast switching in the sub-picosecond range, which makes it comparable in terms of multi-terahertz switching speed to some previous all-optical devices with the added advantage that our device doesn’t require cryogenic cooling to operate.

Impor­tantly, the organic polariton transistor gets rid of another limitation present in its inorganic counterparts that is relevant for practical purposes. In inorganic polariton micro­cavities, the pump laser used to trigger the transistor response must be directed at the device under certain angles only. In the organic device, there’s no specific requirement on the angle of the pump laser which gives much greater flexibility in the geometry of the setting and allows for fiber pig-tailing of the optical device or creating integrated planar circuits with it.

In the material, the energy states of exciton-polaritons are given by several polariton branches, which arise from strong light-matter inter­action of the cavity photons with the excitons. The strategy consisted in using the bosonic nature of exciton-polaritons and the occurrence of strong vibra­tional excitations in the organic semi­conductor to trigger an avalanche-like relaxation of the excitons to the lowest polariton branch. This vibron-mediated relaxation channel was strong enough to outcompete the multiple internal conversion channels in the material. All expec­tations were thoroughly confirmed by the experiments.

In a first step, a pump laser produced a large population of hot excitons. By tuning the wavelength of this laser the researchers produced excitons with an energy exactly one vibronic energy quantum above the lower polariton branch in the micro­cavity. The vibronic mode corresponds to a breathing mode in which ring-shaped aromatic units within the polymer shrink and expand in a way that resembles a breathing lung. As stated above, the researchers only had to care about the energy of the pump laser photons, but not their in-plane momentum component. This is possible because of the wide spread in the momentum distribution of the strongly localized excitons in the material. That means that the stringent phase-matching requirement typical of inorganic micro­cavities is irrelevant in our system, and it can be pumped at almost any angle.

With increasing pump excitation density, we observed a transition from the linear to the non-linear regime, with the threshold density lying at roughly 82 μJ cm−2. To lower the threshold and further accelerate the relaxation of excitons to the polariton ground state the researchers seeded this ground state with a control beam. This seeding proved very effective in speeding up the relaxa­tion process, in spite of the fact that the excitation density of the control beam was kept constant at about 20 nJ cm−2, more than three orders of magnitude weaker than the non-resonant pump. By seeding the ground polariton state, they observed nearly twice lower threshold for polariton conden­sation, while the exciton to polariton relaxation rate was increased by a factor of 50 under the same non-resonant optical excitation density.

Sub-picosecond switching times were achieved thanks to the combination of ultrafast exciton relaxation dynamics, inherent to organic semiconductors, and the sub-picosecond cavity lifetime of the device. In the setup, the pump beam formed the address state that was gated by the control beam. Keeping the switching energy of the control beam at 1 pJ, the researchers attained a maximum extinction ratio of 17 dB. The response time for switching between the two logical states was roughly 500 femtoseconds. Finally, they demonstrated the potential of organic polariton transistors for casca­dability by imple­menting two-stage cascaded amplification. In the scheme, the condensate emission of the first stage is redirected onto the chip and amplified further by a second pump. Furthermore, they employed the concept of cascaded ampli­fication to demonstrate OR and AND logic gate operation by coupling three polariton transistors on the same “chip” utilizing a single pump-double probe optical setup.

The experiments demonstrate vibron-mediated, dynamic polariton conden­sation in an organic microcavity at ambient conditions, enabling all-optical polariton amplification, switching at sub-picosecond time scales as well as casca­dability and OR and AND logic gate operation. Efficient control over the address state permits reliable switching between low and high logic levels with ultrafast transient response, while the giant net gain of the structure gives rise to record optical ampli­fication at the micrometer scale. The developed principles of dynamic polariton conden­sation in combination with the recently observed frictionless polariton flow in organic microcavities, pave the way for on-chip circuitry with ultrafast, all-optical, logic operability. If one could furthermore exploit strong polariton-polariton inter­actions, where important progress has just been shown earlier this year with inorganic micro­cavities, such transistors would be able to operate with only a few photons, and thereby drastically lowering the required switching energy to the attojoule regime. (Source: IBM Research)

Reference: A. V. Zasedatelev et al., A room-temperature organic polariton transistor, Nat. Photonics 13, 378 (2019); DOI: 10.1038/s41566-019-0392-8

Link: IBM Research Zurich, Rüschlikon, Switzerland

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