Applications

Systems for Photoluminescence

Photoluminescence (PL) is a purely optical experiment. The sample is cooled to cryogenic temperatures under a vacuum and exposed to radiation, usually in the visible spectrum. This radiation causes the atoms in the sample to enter an excited state. When the atoms return to a lower energy state, they emit radiation, which is measured by a detector. This luminescence yields a great amount of information about the electronic structure of the sample.

While PL measurements can be performed at any temperature range, performing PL measurements at cryogenic temperatures can yield information about low energy states that would otherwise be hidden by thermal effects at room temperature.

For the purpose of selecting a cryostat, it is useful to separate PL measurements into two categories: Macro-PL (or just PL) and Micro-PL.

Standard macro-PL measurements are performed on bulk samples. The optics used to collect the PL signal do not necessarily need to be very close to the sample and the vibration levels with our standard open and closed cycle systems are generally acceptable.

Most standard Janis optical cryostats that cool the sample in a vacuum environment are suitable for PL measurements. For customers that wish to use a closed cycle cryostat, the 10 K CCS-150 and CCS-100/202 are the most affordable choices. The model SHI-4-2 is the most popular 4 K closed cycle system for PL measurements. Closed cycle systems with larger cooling capacities are available if necessary, but the experimental heat load in PL systems is usually very small. The standard CCS-150 and SHI-4-2 usually have adequate cooling power.

For customers that prefer a lower-cost system that uses LHe, the continuous flow ST-100 is the most affordable open-cycle LHe system.

Click image to enlarge view of CCS-150 10 K closed cycle refrigerator from Janis Research Click image to enlarge view of SHI-4-2 4 K closed cycle refrigerator from Janis Research Click image to enlarge view of ST-100 continuous flow open-cycle cryostat from Janis Research
Janis 10 K closed cycle
CCS-150
Janis 4 K closed cycle
SHI-4-2
Janis continuous flow open cycle
ST-100

The excitation beam and the PL signal are usually in the visible spectrum, so UV grade fused silica is an ideal window material for this measurement. If fluorescence in the UV range (below 300 nm) is not a concern, less expensive fused Quartz windows may also be acceptable.

When a cryostat is combined with a commercially available spectrometer, a more compact window block may be necessary to fit the cryostat in the sample compartment. In this case, the 10 K CCS-350 and a special compact version of the 4 K SHI-4S-1 is recommended. The ST-300 would be the correct open-cycle cryostat.

Click image to enlarge view of CCS-350 compact optical cryocooler from Janis Research Click image to enlarge view of ST-300 compact optical cryostat from Janis Research
Janis CCS-350 compact optical system Janis ST-300 compact optical system

When performing micro-PL measurements on very small samples such as quantum dots, the experimental requirements are more demanding. High positional stability and low vibration are absolutely necessary in order to keep the optics focused on the sample throughout the course of the measurement.

The Janis ST-500 is the most popular choice among our open-cycle systems for micro-PL measurements. With a vibrational amplitude guaranteed to be ±15 nm or less (with proper mounting), the ST-500 has the lowest vibrational amplitude of all Janis open-cycle systems. A specially designed heat exchanger with the cryogen inlet and vent ports pulling in opposite directions results in the lowest positional drift upon cooldown of any Janis system.

The optical signal in micro-PL is typically collected using a microscope objective with a high numerical aperture and short working distance of a few millimeters at most. Therefore, the sample must also be placed very close to the window.

The distance from the sample holder to the inner surface of the window is specified when ordering the ST-500. Additional sample holders may be ordered for different sample-to-window distances if necessary. Janis recommends a distance of at least 2 mm between the sample and the window to prevent the window from cooling and collecting condensation. However, some microscope objectives require a shorter working distance. We will make the sample holder as close to the window as necessary, as long as our customers are aware that the window may get cold if the sample is less than 2mm from the window.

Some commercially available PL systems use a microscope for the sample mounting stage. The
ST-500 will fit in many popular microscopes, but for some models it may be too high and/or too large in diameter.

When a cryostat with a more compact profile is required, Janis offers the ST-500UC (Ultra Compact). The ST-500UC is built using the same design as the ST-500, so it should have similar specifications for vibrational amplitude and positional drift. The ST-500UC does consume more LHe than the standard ST-500, so the long-term operating costs will be higher.

Click image to enlarge view of ST-500 microscopy cryostat from Janis Research Click image to enlarge view of ST-500UC ultra compact microscopy cryostat from Janis Research
Janis ST-500 Janis ST-500UC

Scanning the surface of a sample may be useful in micro-PL measurements. Janis offers special configurations of the ST-500 that include attocube stepper and scanner stages for nano-positioning of the sample in the optical path.

Click image to enlarge view of mechanical drawing of Janis Research ST-500 with 3-axis attocube stepper and 3-axis attocube scanner stages. Click image to enlarge view of Janis Research ST-500 with 3-axis attocube stepper and 3-axis attocube scanner stages.
Janis ST-500 with 3-axis attocube stepper and 3-axis attocube scanner stages

The ST-500 is a very versatile system. The basic ST-500 can be easily converted from a micro-PL setup into a different configuration for other experiments simply by changing sample holders, electrical feedthroughs, and top plates.

Please refer to the "other configurations" section of the ST-500 page for more information on the various options available for this system.

Micro-PL in a closed cycle cryostat presents additional challenges. The vibration amplitude of all closed cycle cryocoolers is of the order of microns or tens of microns, which is usually unacceptable for micro-PL measurements. The Janis model CCS-204-XG-M system is a special vibration-isolated cryostat including a 10 K or 7 K closed cycle cryocooler. While the vibration levels are known to be less than ±45nm, the sample drift during cooldown is not as low as in the ST-500. The sample holder in the CCS-204-XG-M is connected to the cooling stage by an OFHC Copper bar. This copper bar will contract towards the crycooler during cooldown and expand away from it upon heating. 

Click image to enlarge view of Janis Research vibration-isolated CCS-204-XG-M with mounting stand.
Janis vibration-isolated CCS-204-XG-M with mounting stand

Contact Janis today for details of how our systems can be integrated into your laboratory.

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