One of the research groups using the Montana Instruments Cryostation has developed an optical polariton transistor capable of amplification, cascadability, and logic operations. Polaritons are quasiparticles arising from strong coupling of excitons and photons in an optical microcavity. Advantages of polaritons include their light mass, which is 10000 times lighter than an electron, fast switching times, and strong nonlinear effects at low energy thresholds.
The experiment was done at 10K in a transmission configuration. The microcavity was composed of GaAs/AlAs, and is positioned between Distributed Bragg Reflectors (DBR), and 3 InGaAs quantum wells embedded in the microcavity plane. Only frequencies matching the polariton dispersion travel through the DBR. The polariton intensity is proportional to the emission intensity and is read out with a CCD which is coupled to a spectrograph and a power meter.
Figure 1: Experimental Setup: Microcavity between two DBR. Incident beam comes in at an angle and is partially reflected. The polariton propagates in the microcavity plane.
Two states are created with external optical injection, the address and control state. The laser beam is split into two paths and is incident on the microcavity plane at different angles to resonantly excite the two states of the lower polariton energy level. The control and address energy levels are the same, but there is a different finite momenta in the microcavity plane. Figure 2 below shows the momentum of the control and address states at saturation. The lower portion of the figure shows the polariton dispersion for Ky=0 at non-resonant, lower power excitation.
Figure 2: Upper panel demonstrates saturated momentum of control and address, lower portion shows the Ky =0 dispersion non-resonant
It was found that the power threshold occurred at 5mW, increasing the power beyond the threshold causes saturation due to blueshift. The detuning between the exciting energy and the polariton resonance was intentionally set to eliminate the effects of any hysteresis. The power was cycled backwards and forwards to demonstrate no hysteresis effects, which is an important property to control switching.
For a fixed value of input power the address can be switched on and off by changing the polariton density in the control. The address is on for intensities greater than 30uW and off for intensities less than 30uW. Gain is the ratio between the polariton density in the address above the threshold and the polariton density in the control state necessary for the switching operation. In this instance a gain of 15 is measured, which corresponds to a control density of 5-20 polaritons/μm2, and a control energy of a few attoJoules/μm2. The range of detuning for which the system stays in the optical regime allows a gain between 4 and 19, although the address power has to be increased as the gain increases. Higher gain can be achieved with microcavities with higher finesse.
The experiment demonstrating propagation and cascadability shows that one transistor, which is spatially separated from a second, can trigger the one state of the second transistor. The experiment starts with two beams, A and B, which are both address beams, spatially separated. The control beam, C, begins below threshold, as shown in Figure 3a, c, and d. Figure 3d shows the momentum space, where A and B are below threshold and inside the laser elastic scattering ring. Once the density of C is increased, A switches on, which then brings beam B above threshold and propagates in the perpendicular direction, as shown by the red arrows in 3b, and as a real image in 3e. In the momentum space in 3f, it can be seen that A and B are on laser resonance and have increased in intensity, and the momentum intensity of C has decreased.
Figure 3: Experimental demonstration of propagation and cascadability, a:Beam A and B below threshold, b: Once the control reaches a certain density, A goes above threshold, which causes B to then go above threshold and propagate along the red arrows. c: Real image below threshold d: Momentum space below threshold e: Real image above threshold, f: Momentum space above threshold.
In this case there are two controls, A and B, and one address, C. Two transistors are used to control the output of the third transistor. Here A and B are again spatially separated by about 70um, and C is at the intersection of A and B. If the threshold is reached when A and B are on, then transistor C works as an AND gate. If the threshold for C is reached when A or B is on, then the transistor, C, works as an OR gate. The power of the address is decreased by 10% from the OR case to the AND case. The off state occurs when the controls, A and B, are below threshold.
Figure 4: Input beams A and B, with C acting as the AND/OR operator. top: A and B are below threshold therefore C is off, left column: demonstration of the OR gate, right column: demonstration of the AND gate. The intensity of the OR gate is 10% higher than the AND gate.
This research group demonstrated optical amplification and switching through the nonlinear properties of microcavity polaritons. The switching times were 10ps, the total activation energy required was 1fJ. The cascade effect was shown, as well as AND/OR logic operations. The goal for this research is to be able to incorporate the transistor into all optical circuits. Since transistors are a very important part of circuits, and revolutionized electronic circuits, this is an exciting step forward for optical circuitry.
This article, including all figures, was based on the following paper:
D. Ballarini, M. De Giorgi, E. Cancellieri, R. Houdre, E. Giacobino, R. Cingolani, A. Bramati, G. Gigli, D. Sanvitto, Nature Communications 4 1778 (2013)
This work should not be considered an endorsement of any product.