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All-optical polariton transistor
Dario Ballarini, Milena De Giorgi, Emiliano Cancellieri, Romuald Houdré, Elisabeth Giacobino, Roberto Cingolani, Alberto Bramati, Giuseppe Gigli, Daniele Sanvitto
TL;DR
The paper tests whether polariton fluids can provide a controllable optical switch whose output propagates to a spatially separated device. It uses a weak Control state to switch a nonlinear Address state, demonstrating gain modulation and propagation-based transistor cascading.
Problem
The paper addresses whether polariton fluids can switch a spatially separated transistor through propagation within a microcavity.
Source: Results · p. 8, Discussion · p. 11Method
The experiment uses a Control polariton population to tune the switching threshold of a detuned Address state, then uses propagating Address polaritons as the next transistor's Control.
Source: Results · pp. 3, 6, 8Results
Gain varies from 4 to 19 with energy detuning, and the propagating on-state activates a second spatially separated polariton fluid.
Source: Results · p. 8, Discussion · p. 11Takeaways & Limitations
The results demonstrate an all-optical transistor based on polariton propagation in the microcavity plane, with an on-state able to activate a second fluid.
Source: Discussion · p. 11The experiments use a relatively wide laser spot, so total switching energy reaches a few femtojoules; the authors suggest waveguides and mesa structures could reduce it.
Source: Results · p. 7
Abstract
from arXiv · showhide
While optical technology provides the best solution for the transmission of information, optical logics still calls for qualitative new concepts to be explored. Exciton-polaritons are composite particles, resulting from the strong coupling between excitons and photons, which have recently demonstrated exceptional properties like huge non-linearities, long range coherence and suppression of scattering. Here we demonstrate a switching scheme for polaritons moving in the plane of a microcavity which satisfy all the requirements for an all-optical transistor. Under resonant excitation, the power threshold for the nonlinear increase of the polariton density is varied by a weak control beam, obtaining up to 19 times amplification with switching energies in the range of attojoule per square micron. Polariton propagation in the plane of the microcavity is then used to control the switching of a second, spatially separated transistor, opening the way to the implementation of polariton integrated circuits.
Results
The experiments use polariton interactions and energy shifts to switch an Address state with a weaker Control state. They demonstrate gain, propagation between spatially separated transistors, and AND/OR gate operation.
Blueshift mechanism
- Repulsive polariton interactions blueshift the resonance, bringing a detuned Address state into resonance with the pump above a density threshold and sharply increasing its population.For the resonant Control state, population instead rises linearly until pump-induced blueshift saturates absorption.Source: Results · p. 3
Address and Control state
- The Address threshold is measured without a hysteresis power cycle, a condition the authors identify as important for controlling switching.Increasing- and decreasing-power measurements overlap around the threshold.Source: Results · pp. 4–5, Figure 1 · p. 5
Transistor Operation
- A Control beam switches the Address on or off by lowering the threshold for its nonlinear population increase.The Address population rises abruptly above threshold, while the resonantly excited Control population increases linearly with power.Source: Results · pp. 3, 6
- At threshold, the Address density exceeds the Control density by more than one order of magnitude, with 5–20 polaritons/µm2 in the Control and switching energies of a few attojoules/µm2.Gain is defined from the ratio of the Address and Control emission intensities at threshold.Source: Results · p. 6
- Gain can be modulated from 4 to 19 by changing energy detuning, with higher gain at larger detuning requiring increased Address power.The reported discriminator regime spans a detuning range that permits this modulation.Source: Results · pp. 7–8
Propagation and Cascadability
- With the first Control focused on Address A, its propagating polariton fluid activates Address B 40 µm away; lowering the first Control below threshold switches both off.A and B propagate in different directions, and B becomes activated by the fluid from A.Source: Results · p. 8, Figure 3 · p. 9
AND/OR gates
- The third transistor operates as an OR gate when either input activates it, and as an AND gate when both are required.The AND configuration uses a 10% lower Address power for C than the OR configuration.Source: Results · pp. 9–10
Discussion
The experiments establish polariton amplification and cascadability as bases for all-optical circuits. They also demonstrate a three-terminal AND/OR operation, while identifying planar fluid guidance as a design challenge.
Discussion
- The transistor’s propagating on-state can trigger a spatially separated transistor, demonstrating cascadability, and two outputs can drive one transistor for AND/OR logic.Above threshold, output intensity is almost independent of input intensity, allowing multiple inputs to connect to one gate while restoring the output level.Source: Discussion · p. 11
- The polariton transistor combines amplification and switching with propagation in the chip plane, providing a basis for integrating all-optical circuits.The authors report switching times of about 10 ps and activation energies of 1 fJ.Source: Discussion · p. 11
- Guiding and coupling polariton fluids in the planar microcavity remain design challenges; etched one-dimensional wires in high-quality samples are proposed to improve propagation and footprint.The discussion also proposes localized 0D pillars for exploiting energy quantization and related effects.Source: Discussion · p. 11
Methods
The methods model resonantly driven polaritons with coupled cavity and exciton fields, inject address and control states using two laser beams, and numerically solve the dynamics to steady state.
Methods
- The model uses a Gross-Pitaevskii equation for coupled cavity and exciton fields, extended to include resonant pumping and decay.The cavity dispersion is expressed as a function of in-plane wave vector; the model also uses a flat exciton dispersion and assumes zero detuning at normal incidence.Source: Methods · p. 14
- Two coherent monochromatic laser fields inject the address and control states with a shared smooth top-hat spatial profile and different in-plane momenta.The exciton-exciton interaction strength is set to one by rescaling the fields and pump intensities.Source: Methods · p. 14
- The theoretical results come from numerically solving the model on a two-dimensional grid with an adaptive-step fifth-order Runge-Kutta algorithm until steady state after 1000 ps.The grid is 256x256 in a 140x140 box.Source: Methods · p. 14