Quantum light sources integrated on-chip
In their experiment, the researchers used the light emitted by an electrically excited driving LED to excite quantum dots in the neighbouring diode. They were able to tune the wavelength of the quantum dot emission from the neighbouring driven diode via the quantum confined Stark effect.
The idea here is to generate on-demand entangled photon pairs for quantum computing applications, through an on-chip in-plane excitation structure that could readily be integrated into semiconductor devices and photonic cavities.
In their paper, the researchers demonstrated a method of producing electrically triggered anti-bunched light from an electrically tuneable source. To do so, they designed 16 individually tuneable diode structures on a single chip. The devices consisted of 180×210μm planar microcavity LEDs containing a layer of InAs quantum dots embedded in a 10nm GaAs quantum well with Al0.75Ga0.25As barriers.
Multiple Distributed Bragg Reflectors (DBRs) grown above and below the InAs quantum dot layer and quantum well were used to form a half-wavelength cavity to increase the portion of QD light emitted vertically while acting as a horizontal waveguide for optical emission from the InAs wetting layer. A diode structure suitable for electrical excitation was formed out of the top DBR and the bottom DBR, doped p-type and n-type, respectively.
“The key idea”, they wrote, “is to use light produced by one LED to excite the QDs in the neighbouring diode”. One LED is ran in forward bias, whose broadband light emission from the InAs wetting layer is guided horizontally by the Bragg reflectors above and below the wetting layer. As the neighbouring LED is hit by a portion of the emitted light, part of that light is absorbed by the wetting layer, generating excitons which can then be captured by the quantum dots in that neighbouring diode, resulting in quantum light emission.
Because the cavity mode of the planar microcavity matches the emission wavelength of the neighbouring quantum dots, this increases the proportion of QD emission directed upwards into the collection optics. By varying the bias across the second diode, one can tune the wavelength by Stark shifting the transitions and the intensity of light emission from the neighbouring diode can be controlled by varying the voltage across the first diode
The researchers also demonstrated that they could tune the fine structure splitting of an exciton in the second diode as a function of the electric field across it, making it possible to use such devices as sources of entangled photon pairs.
In future work, the researchers hope to improve the efficiency of the device, imparting more emission directionality from one to the next diode, possibly using a unidirectional antenna or a waveguide between the LEDs to increase the cross-coupling efficiency. In principle, one driving LED could excite many tuneable LEDs. Combined with fast electronics and low RC constant devices, they may be able emit entangled photons “on-demand”, by either varying the bias to diode 1 to modulate the “pump” or by varying the bias to diode 2 to modulate the wavelength.
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