Organic microcavity optical transistors open up opportunities for real-world optical switching at room temperature. Now, an all-optical switch at room temperature, using an organic exciton medium with high quantum yield, brings us a step closer to all-optical logical networks.
Photonics scientists and engineers have worked for at least three decades to try to produce an optical transistor, or optical switch, in which one light beam can switch another beam on and off, the same way that one electrical signal can switch another on and off in an electrical transistor. This type of light switch could be the basic building brick for optical circuits and even optical computers.
In principle, such a device could work at terahertz frequencies or higher, and the dissipation of power could be far less. As discussed in an earlier review1, there are five crucial requirements for a practical device: (1) amplification (a weak beam is used to switch a strong beam); (2) small size, preferably small enough to make millions on a chip; (3) fast on/off time; (4) the possibility of cascading devices, that is, the output of one device can act as the input for a subsequent device; and (5) room-temperature operation.
Substantial progress was already made by 2013 on the first four goals, using the nonlinear optical effects of polaritons in III–V semiconductor (GaAs-based) microcavities at cryogenic temperatures2. The polariton effect uses strong coupling of light and matter; in the typical scenario, an exciton in a semiconductor is strongly coupled to a photon in a Fabry–Pérot cavity. The coupling of the two leads to the appearance of a hybrid quasiparticle known as a polariton, which has some of the properties of both: like a photon, a polariton has long wavelength and long propagation length, and like an exciton, a polariton interacts strongly with other particles, leading to large nonlinearities. The pioneering work used for demonstrating microcavity polariton formation and the associated dynamic behaviour of the polaritons was done by Weisbuch and colleagues in 19923, leading to an explosion of research in the following decade culminating in the observation of Bose–Einstein condensation of polaritons4.
Although the work in III–V semiconductors2 was promising, it was limited to low temperatures due to the small exciton binding energy and weak oscillator strength. In subsequent years, researchers have turned to look for materials with better excitonic properties, and organic light emitters have emerged as a promising candidate5.
Now Anton Zasedatelev and co-workers have demonstrated an all-optical switch at room temperature, using an organic exciton medium with high quantum yield6. They report a net gain of 10 dB μm–1 and sub-picosecond switching time in ambient conditions. Optical injection at two different wavelengths was used to create transient Bose–Einstein condensates of polaritons, which had a threshold density for nonlinear gain that depended sensitively on the power of both input beams. Cascading the light output of one condensate into a second, they were then able to demonstrate AND and OR logical operations using only optical inputs (Figs. 1 and 2). A key step forward was to tune the wavelength of one of the beams to an upper state that could relax efficiently to the lower state by vibron emission. Although there are strong nonlinear effects in this system, the input optical intensities needed for the switching are low, of the order of 10–100 nJ cm–2.
Zasedatelev and co-workers fabricated the organic microcavity with a conjugated methyl-substituted ladder-type poly(paraphenylene) (MeLPPP) polymer, which has pronounced singlet-related optical transitions and a high photoluminescence quantum yield because of a relatively rigid backbone, owing to a methylene bridge between phenyl rings. The dynamic polariton condensation was realized via non-resonant excitation with photon energy of 2.8 eV, 200 meV higher than the ground state of polaritons. Because a vibron exists in this organic material with energy of 200 meV, the ‘hot’ excitons created by this excitation efficiently pumped the polariton ground state. Owing to the ultra-strong oscillator strength of the MeLPPP, this single-step ‘cool’ down process can overcome the competing internal conversion process from the pumped state to other states of the exciton. This method with single-step vibron-mediated relaxation achieves condensation of polaritons with a pump power threshold about an order of magnitude smaller than previous attempts.
Various other optical methods of optical switching have been explored, but none have yet made the net gain as high as that of the organic microcavity reported here. A vertical-cavity semiconductor optical amplifier7 with a net gain of 1.94 dB μm–1 needed a high Q-factor of the cavity with a bandwidth of about 1 nm. A quantum-dot semiconductor optical amplifier8 has broader bandwidth operation, covering all of the telecommunication range, but the net gain was as low as 0.006 dB μm–1. Also, the sensitive dependence of the exciton energy relaxation with temperature in semiconductors resulted in the poor thermal stability of such devices. In contrast to a semiconductor optical amplifier, an erbium-doped waveguide amplifier9 has more robust thermal stability, while the net gain is about 0.01 dB μm–1 and the bandwidth tunability is relatively small. Although a plasmon-polariton amplifier10 can reduce the size of a device down to the nanometre scale, the high loss accompanied with Joule heating in metals results in a low net gain of 0.0085 dB μm–1 and significantly hinders the application of such a structure.
The effect demonstrated by Zasedatelev and colleagues does not yet allow full logical circuits, because there is no equivalent of a NOT gate yet. This is because the effect works by amplifying a weak signal up to a large one; as yet there is no process by which a polaritonic optical signal is reduced, or switched off, by the presence of another optical beam. It would also be good to increase the Q-factor of the cavities since a higher Q-factor allows longer polariton lifetime and lower losses. But synthesizing organic microcavities with a high Q-factor is still limited by the deposition methods. Also, electrical pumping incorporated in the optical transistors could significantly reduce the chip size and be more applicable for real-world devices.
Nevertheless, the work of Zasedatelev and colleagues is an important step forward in showing the utility of using polariton condensates for real-world optical switching at room temperature. The organic microcavity optical transistors open up exciting opportunities for novel device applications and are a further step toward all-optical logical networks.
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Sun, Z., Snoke, D.W. Optical switching with organics. Nat. Photonics 13, 370–371 (2019). https://doi.org/10.1038/s41566-019-0445-z
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