BACKGROUND:
Notice is hereby given by the University of Ottawa of the intent to purchase a closed-cycle cryostat from Attocube System AG to perform low temperature optical spectroscopy resolved in frequency and time. The cryostat will be used mainly to study the optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) when coupled to nanophotonic structures. Spectroscopy includes confocal micro photoluminescence, photoluminescence excitation, laser Raman spectroscopy, and photon statistics.
PROCESS:
Suppliers that consider their equipment functional, successfully tested, readily available and fully compliant to the ACAN minimum requirements may submit, in writing, a statement of specifications to the contact person identified in this Notice, on or before the closing date of this Notice. In the statement of specifications, the supplier must unequivocally demonstrate how their equipment, at minimum, equals, or exceeds the stated requirements.
If no other supplier submits a statement of specifications, on or before the closing date of this Notice, the competitive requirements of the University of Ottawa will be considered having been met. Following notification to any suppliers not successful in unequivocally demonstrating that their statement of specifications equals or exceeds the requirements set out in this Notice, the contract may then be awarded to the pre-identified supplier.
Date of issue: January 22, 2019
Closing Date: February 18, 2019, at 3:00:00 P.M. Eastern Time
INTENDED USE:
The cryostat will be integrated into an existing laser spectroscopy set-up. The entire setup allows us to perform laser spectroscopy in the visible at varying temperature. The 2D materials samples are excited by a laser system that comprises a high-power (25 W) CW green pump laser pumping a Ti:Sa ring CW tunable laser (680 nm < λ < 1020 μm) with linewidth < 30 kHz rms. The samples are mounted into an ultra-low vibration, closed-cycle cryostat enabling continuous temperature change (ranging from 6 K to 320 K). The signal from the excited sample is collected and sent to a 750 mm focal length spectrometer equipped with three interchangeable gratings and coupled to a liquid nitrogen cooled, back illuminated, deep-depletion Si-based CCD camera.
FUNCTIONALITY:
The equipment proposed by Attocube Systems AG conforms to the following minimum requirements:
- Closed-cycle (helium-free) cryostat.
- Integration of the cryostat into an optical table.
- Customizable vacuum shroud and possibility to switch to other vacuum shrouds configurations.
- Ultra-high spatial (position) stability of the sample relative to the optical fibers and the objective, with less than 5 nm peak-to-peak in the vertical direction.
- Ultra-low temperature fluctuations and gradients (< 15 mK peak-to-peak with damped sample mount).
- At low temperature, low degassing and consequent negligible unintentional gas physisorption on the sample.
- Fully automated temperature control between 6 K and 320 K.
- Low temperature apochromatic objective with NA > 0.8.
- 9 degrees of freedom to independently position sample, focus, and 2 fiber probes.
- Feedthroughs for electrical (DC, HF) wires, optical fibers, gas capillary.
- Both temperature and spatial stability must be preserved when a small amount of gas (nitrogen) is injected into the cold area via capillary.
Justification of Pre-Selected Supplier:
Supplier: Attocube Systems AG
Justification:
One essential characteristic of the requested cryostat is the ultra-high spatial (position) stability of the sample: low-intensity optical signals often imply long acquisition times (hours) and thus require conditions as stable as possible over extended measurement periods. The cryostat thereby is required to have ultra-low mechanical vibrations and to be closed-cycle (avoiding to switch helium dewars during measurements). Typically, closed-cycle cryostats suffer from severe vibrations at the sample location, which usually are in the range of many microns. Thanks to a proprietary vibration isolation technique of the Attocube cryostat, the residual vibrations on the cold plate were measured to be around 2.6 nm peak-to-peak (vertical direction). Such a low vibration will allow us to perform extremely sensitive measurements. Being a closed-cycle cryostat, it also adds the advantage that it requires no liquid cryogens and thus minimizing running costs. In addition, a fully automated temperature control between 6 K and 320 K conveniently enables unattended long measurement cycles.
Other essential specifications are temperature fluctuations and gradients as low as possible. Mismatched thermal expansion coefficients among the parts, can jeopardize the acquisition of data over a range of temperatures or longer periods of time. Both temperature and spatial stability must also be preserved when a small amount of gas (nitrogen) is injected into the cold area in order to tune the nanophotonic structures via a capillary. The cryostat has a sample holder with 12 optional electrical contacts; temp. sensor & heater included.
The cryostat is fully integrated into an optical table to give more mechanical stability. The cryocooler assembly is located in the space underneath the table ensuring a free workspace and unobstructed optical access to the sample. The position of the cryocooler assembly in the optical table give us the possibility in the future to switch the top unit (vacuum shroud) with smaller units that can result more suitable for faster and simpler characterizations.
The cold sample is optically accessible from one lateral and one top window. An apochromatic cold objective with high numerical aperture is integrated into the cryostat in the vertical position and coupled to the top optical entrance.
Cold objective:
Many high spectroscopy measurements, performed in this setup require cryogenic temperatures where conventional room temperature objectives cannot be employed. In this cryostat, the objective is apochromatic from 700 nm to 985 nm, with high numerical aperture (NA=0.82) and is integrated into the cryostat—directly into the vacuum shield (cold area). The objective does not crack under thermal cycles and vacuum operation. Because it is apochromatic it keeps the focal plane within one depth of focus and thus ensure both uniform spot size and constant collection efficiency for all colors within the apo-range. Because it is designed for the use at low temperature and in vacuum and mounted directly into the cryogenic space and thermally coupled with the cold plate of the cryostat, it does not impact the necessary temperature and mechanical stability of the system. Indeed, this objective configuration minimizes thermal drifts significantly compared to non-thermalized optics such as the one placed outside the cryostat.
Fiber coupling:
The cold sample space is designed to host several patented nano-positioners able to spatially align the sample to two independent side lensed optical fibers, to perform x-y translation of the block constituted by sample and fibers probes, and to independently focus the cold objective on the sample in the z-direction maintaining the thermal link between objective and cold plate.
