Diamond membrane devices for efficient coupling to vacancy centres

Lead Research Organisation: University of Strathclyde
Department Name: Physics


The efficiency of coupling between optical modes and defect centre based qubits is crucial to the performance of the solid state diamond quantum systems targeted in NQIT. The FP scheme under development targets Purcell enhancement of the emission centres. By using ultra-thin diamond membranes, the vacancy centre can be located in a compact cavity that can be in turn coupled to free space and fibre optical components. Low coupling efficiency to the external optics effectively reduces any benefit of cavity enhancement of the defect centre emission from a system viewpoint. Therefore, this PhD project proposes to investigate the joint effects of cavity type, (i.e. internal emission enhancement into cavity modes), and coupling mechanisms to free space optics and single mode fibre, required for the interconnection of nodes in a distributed system. This proposed work will focus on 2 key areas:
1. Creation of nanopillar diamond emitters for lateral confinement of the optical mode
2. High resolution pick-and-place techniques for integration of nano-diamond based emitters with free space and fibre based cavity systems
Objective (1) will involve the numerical simulation, micro-fabrication and optical characterisation of efficient integrated defect centres in micro-pillar diamond devices to open FP cavities. The designs will be complementary to the ultra-thin membrane technique, enhancing mode coupling through lateral control of the optical mode in the diamond. Accurate micro-fabrication schemes will be designed for co-location of the targeted defect centres and topological diamond structures.
Objective (2) will involve the design and micro-fabrication resonant cavity devices coupled to nano-diamond emitters. This work will leverage the expertise in Strathclyde in micro-assembly techniques to align single nano-diamond emitters with free-space, integrated or fibre based resonator devices.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1952672 Studentship EP/N509711/1 01/10/2017 30/06/2021 Gemma Quinn
Description Nanoscale accurate spatial mapping of emission centres
Micro-photoluminescence system
Native silicon vacancy centres (SiV) are embedded within the diamond crystal lattice at random locations. To utilise these vacancy centres as single photon quantum emitters placed at precise deterministic positions within devices, a system has been built which can enable vacancies to be found, characterised, and spatially registered with nanometric precision.
A homebuilt confocal microscope set-up was designed and constructed as part of this project, to perform high-speed micro-photoluminescence mapping of the diamond defect centres. This is a flexible set-up with nanoscale precision.
The target performance of this system is required to be nanoscale for accurate measurement of the nanodiamond vacancy centres and precise location at target sites within devices. A key component of this performance is the motorised XY stage. Following review of the available options from various suppliers, the most suitable stage from three quotes obtained was selected (SmarAct microscopy stage).
The chosen stage uses SmarAct piezo positioners with crossed roller bearings and integrated nanosensors, to give closed loop resolution down to 1nm and travel ranges of 31 mm x 31 mm with repeatability of +/- 50 nm over the complete travel range (31 mm).
The system comprises; a green pulsed laser at 532 nm wavelength for optical excitation of the diamond vacancy centres. The laser beam is converted from circular polarisation to linear polarisation via a waveplate and polarising beamsplitter cube. The beam is then directed via a periscope, up and into the entrance of the microscope.
The microscope is the Leica DM2700 M model, used in this application as point confocal microscope. This single point confocal system provides the best resolution and out-of-focus suppression and can also achieve high multispectral flexibility. Within the microscope the laser beam is directed through the light source pinhole aperture where a dichroic mirror then guides the incident laser light onto the sample.
The fluorescent sample is illuminated with a focused point of light from the incident light source pinhole via a high numerical aperture causing the SiV centres to fluoresce. The emitted fluorescent light from the in-focus point is focused at the detector pinhole and reaches the detector, any emitted light from the out-of-focus point is out-of-focus at the detector pinhole and is therefore largely excluded from the detector.
A specialised two camera mount was mounted between the microscope detector and the camera to house the beam splitter plate which will split the laser beam 10:90 T/R (transmitted / reflected) to enable simultaneous imaging and spectral analysis of the fluorescent nanodiamonds. The set-up also required a laser shield to be constructed to ensure safe operation.
In the first step markers were fabricated on the diamond chip with the native silicon vacancy centres, the known position of the marker was then characterised and the parameters recorded as a reference point for spatial registration of the fluorescent vacancy centres relative to the marker position (step 2).
This mapping technique will be employed for locating and spatially registering both nanodiamonds containing native SiV centres and SiV centres embedded within ultrathin diamond membranes.
Exploitation Route Efficient coupling is crucial to the performance of diamond as a solid state platform for networked quantum technologies.

Integration of Nanodiamonds:

Once identified and spatially registered, pick and place techniques, for example equivalent spatial registration systems for transfer printing will enable positioning of vacancy centre emitters at specific locations with ultrahigh accuracy.
Micro-assembly techniques will achieve precise alignment of single nanodiamond emitters with free space, integrated or fibre-based resonator devices.

Vertical Structures:

The development and vertical integration of cavity mirrors, diamond films and spacer layers will maximise coupling efficiencies within a mechanically robust package.

Ultrathin Diamond Membrane Technology:

Work will be carried out using ultrathin diamond membrane technology in conjunction with a compact free space open cavity to achieve good spatial overlap between the vacancy centres in the ultrathin diamond membranes and the cavity mode, resulting in maximum coupling efficiency, together with isolation of the system from mechanical vibrations.
Sectors Digital/Communication/Information Technologies (including Software),Security and Diplomacy

URL https://nqit.ox.ac.uk/