4K Cryostat-mounted THz Scattering Near Field Optical Microscope System.

Descriptions

The University of Surrey, acting through its Advanced Technology Institute, has been funded to establish a highly integrated facility for single ion implantation; quality assurance of silicon substrates with single impurity implants, and coherent control of the orbital state of the single silicon impurities for quantum technology applications. The primary atomic species of interest for these applications in silicon is phosphorus, but also include bismuth, selenium and others. The implanter is already under construction. A microscope is required for the single atom in silicon substrate quality assurance and for the coherent control of single silicon atom coherence.Based on their long experience of research in this area, the Investigators responsible for delivering the program believe that for imaging single, sub-surface dopant atoms in silicon with species specificity and THz control there is only one solution that will meet the objectives: a Scanning Near-field Optical Microscope (SNOM). The microscope system design must enable tip-enhanced optical measurements by elastic light scattering-SNOM (s-SNOM), as opposed to aperture-based-SNOM (due to the resolution and signal sensitivity requirements given). The s-SNOM must be mounted in a cryogen-free cryostat with very low vibration relative to the optical bench-mounted customized laser sources.The system is based on a Scanning Near-field Optical Microscope produced by Neaspec and a 4K optical table-integrated cryostat made by its parent company Attocube.1. Cryostat.The system will be based on a dry cryostat (requirement vii) which is integrated into a standard optical table (i.e. Newport) to allow the customizable laser system configurations with different THz bands described (xiii,ix). Having the cryostat actually integrated into the optical table, not sitting next to the optical table minimises the movement of the lasers and optics relative to the sample. The cryostat will provide = 2 views of optical access (xi) through room-temperature windows to the microscope focus via windows that can be changed to suit the laser wavelength. Windows are blankable for improved base temperature. The electrical access provided includes 66 DC lines (plus 6 internal ones) and 2 coaxial microwave lines for 2GHz (xi). The time required for evacuation and cooldown to 10K after a sample change will be less than 7 hours and the warm-up time from 10K will be approx. 3.5 hours (xii).2. Atomic-Force Microscopy (AFM).The cryostat design will provide sufficient vibration isolation for high-quality AFM measurements of the surface features with topographic RMS noise <2-3nm without degrading the optical access (v) and without degrading the near-field imaging quality in s-SNOM experiments. The AFM measurements will be done in tapping-mode configuration to provide means for efficient background suppression in s-SNOM by higher order signal demodulation. AFM read-out will be achieved by reflection of a deflection laser from the cantilever with optics outside of the cryostat.3. Mid-infrared and THz scattering-Scanning Near-field Optical Microscope (s-SNOM).The system performance will be guaranteed for s-SNOM measurements (x) at cryogenic conditions of T<20K (vii) (expected base temperature ca. 10K at the sample, and best efforts to achieve 5K at the sample) with existing, proven technology. A 10mW CO2 laser will be provided for mid-IR wavelengths of ca. 10µm (xii) and at this wavelength <50nm lateral resolution s-SNOM will be guaranteed at room temperature and at <20K (i,ii). Interferometric signal detection will be provided for complete background suppression (a patented pseudo-heterodyne detection technique for highest imaging sensitivity possible, as required for applications with single atom sensitivity. In addition pseudo-heterodyne detection provides the fastest imaging-speed possible (<4ms acquisition time per pixel) for fast acquisition of large field of view, megapixel images) and amplitude- & phase-resolved near-field measurements for clear optical differentiation of different species of defects (iii). 2 optical input ports will be provided for alternative sources for other wavelengths (iii,ix), The s-SNOM technology will have proven functionality for measurements in the THz spectral range (e.g. frequencies around 2THz for impurities in germanium) as well as for nanoscale resolved near-field photo-current measurements at THz frequencies.4. Positioning stability and control.The AFM tip holder, the sample stage and the light focusing unit (the parabolic-mirror objective with its 3D positioning unit) will be mounted on cryostat cold plate, i.e. all three at the same temperature, for optimized near-field measurement performance and minimized thermal drift effects (viii). All three will be independently adjustable for signal optimization (ii). The positioner for the objective will enable near-field signal optimization with wide adjustment range (5mm in x,y,z in parabola) with precision and position stability of 1 micron (viii). Megapixel images will be standard e.g. 10,000nm x 10,000nm at 10nm resolution (vi).5. Sample illumination.For the high throughput required for very high sensitivity (ii) and wide spectral range required from mid-infrared to THz (ix), the s-SNOM tip (x) illumination will be achieved by a reflective parabolic-mirror objective (with no fibre or aperture to limit the wavelength coverage or throughput). The parabolic-mirror objective will provide a patented means to measure and align its optical axis to avoid aberrations in the THz spot for ultimate throughput (ii). In addition the objective will provide a patented dual-beam access to the s-SNOM tip resulting in two independent optical beam paths for back-scatter and forward-scatter, for direct integration of THz-‘TDS’ systems for 1-3THz (ix). The system will provide visible wavelength sample inspection optics by an upright optical microscope outside of the cryostat with a demonstrated spatial resolution <2µm in both x and y in-plane axes, that does not interfere with the mid-infrared and THz optics and alignment (iv).