Saleeb-Mousa, Bruce
(2022)
Near-infrared semiconductor photonic switches for quantum sensors.
PhD thesis, University of Nottingham.
Abstract
The promising performance of atom interferometry in determining precise values of fundamental constants, lends itself to applications in many areas of modern physics and engineering. In particular, the cold-atom gravimeter has proven itself to be a useful tool in determining the local acceleration due to gravity with a precision of delta g/g = 10^-9, with long term stability and accuracy. In recent years, there has been a major incentive to develop these mainly laboratory-based experimental systems towards more compact, portable and eventually commercialised systems. Part of this development involves the design, fabrication and testing of bespoke underpinning technologies such as high performance, low power integrated active optical components which would help to reduce the size, weight and power of these sensors.
In this thesis, the design, fabrication and testing of a novel integrated optical switch on semi-insulating gallium arsenide substrates is detailed. A standard double heterostructure with highly doped capping layers for ohmic metal contacts is grown on these substrates to form a PIN diode structure, consisting of a core of high refractive index and cladding layers of lower refractive index. The refractive index of each layer is selected by growing AlxGa(1-x)As (0<x<1) with differing fractional aluminium concentrations, x. This structure provides both electrical confinement of injected carriers and optical confinement of a guided optical wave in the core. The switch structure is then fabricated on these wafers, consisting of a beam-steering section and a waveguiding section. An optical beam enters the steering section where it is guided into one of two single mode waveguides delineated at the end of the steering section. This action provides the beam switching effect. This switching mechanism is based on a previously reported phenomena of optical beam steering, whereby an injection of a sufficient density of carriers into the core of a planar waveguide produces regions of depressed refractive index due to the Moss-Burstein effect. This change forms a channel waveguide within the steering section, and the direction of the guided beam can be controlled by adjusting the amount of current injection either side of the guided optical wave. These optical switches are designed to be used in cold-atom gravimeter experiments, intended for both fast (us) and high extinction (~80 dB) switching. These devices could be used for switching of the trapping/cooling beams of the magneto-optical trap, used to form the gas of cold atoms, and for the precise switching of the beams used to perform the interferometery sequence. Three separate optical switching designs are presented and tested, which are intended to switch optical beams in wavelength ranges 890-920 nm, 750-780 nm and 745-775 nm, for use in caesium, rubidium and potassium based atom interferometers respectively.
Characterisation of the fabricated devices revealed observation of the beam steering effects at low current injections (<20 mA), but suppression of the mechanism at higher injection currents due to excessive heating. The heating is attributed to current crowding due to the mesa geometry of the switch design on semi-insulating substrates. The switching was observed to be strongly wavelength-dependent, as predicted by the theory, with a maximum extinction of 21.42+-0.03 dB at 734 nm achieved for a single device. Devices can be cascaded in series on a single chip to provide an enhanced extinction. This was observed using the wafer designed for 767 nm. The extinction achieved at this wavelength for this wafer was measured to be 6.69+-0.03 dB. The switches designed for operation at 904 nm and 780 nm achieved measured extinction ratios of 13.40+-0.03 dB and 2.39+-0.03 dB respectively. As a result of these observations, a new epitaxial design is proposed for switching at 780 nm. The fabricated devices were also characterised electrically. Current-voltage measurements revealed that the p-i-n structure devices behave as expected with low power requirements - sufficiently low to be able to be sourced by commercially available circuits for packaged optoelectronics. Optical propagation loss measurements showed that the fabricated devices can offer propagation losses as low as ~1 dB for all epitaxial designs, due to their small dimensions of $\sim$1 mm in the optical propagation axis. Finally, the measured optical rise and fall times of one of the fabricated switches was measured to be 670+-5 ns and 675+-5 ns respectively, meeting the initial design requirements and confirming suitability of the speed of the switching mechanism for the intended application.
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