Experimental Facility

Our lab houses a unique photoemission study experimental facility in which several thin-film deposition and surface preparation techniques are connected in ultra-high-vacuum (UHV) to atomic-scale surface characterization techniques, photoemission diagnostics, and a 200 kV DC electron gun with several beam diagnostics. It also houses a 150 fs pulsed laser with a wavelength continuously tunable from 200 nm to 2000 nm. The picture above shows a model of the experimental facility. It occupies nearly 1200 sqft space in Wexler Hall basement room WXLR A3 Potential collaborators interested in using this facility please contact Prof. Karkare at [email protected]. Below are the details of the various experimental capabilities:

Thin-film Growth and Surface Prep

We have a UHV thin-film deposition chamber capable of simultaneous deposition of up to 3 elements onto a substrate. This deposition chamber is currently be used to grow alkali-antimonide thin films, however, the evaporation sources can be changed easily for growth of other thin films. A new growth chamber with the capability to perform RHEED and EDX is under development and will be commissioned shortly. Other surface preparation facilities include Argon ion bombarding, high-temperature annealing up to 900K or 1500K and Cesium activation of surfaces for work-function reduction. The growth and surface prep chambers use omicron flag-style sample holders which can be transferred into other surface characterization and photoemission diagnostic chambers in UHV.

Image of a new growth chamber in development

Photoemission Electron Microscope

The FOCUS Photoemission Electron Microscope can obtain energy-resolved images of electrons emitted from a surface with a resolution of better than 40 nm and an energy resolution of 60 meV. It can also provide energy-resolved k-space images and hence the mean transverse energy (MTE). The photoemission light source can be the CW Hg lamp (at 5.2 eV photon energy) or the 150 fs pulse OPA laser with the tunable wavelength. The laser can be incident on the sample at various angles - normal (4 deg), 45 deg, 60 deg, back incidence. The PEEM is uniquely suited for measuring spatial non-uniformities in electron emission and for the development of nanostructured photoemission electron sources. It can also be used for ultrafast surface science in the pump-probe configuration.

Atomic Scale Surface Diagnostics

Our facility houses a UHV scanning probe microscopy chamber capable of performing Atomic Force Microscopy (AFM), Kelvin Probe Force Microscopy (KPFM), and Scanning Transmission Microscopy. Information on the sample topography down to sub-0.3 nm scale and surface potential variations down to sub-5 meV scale can be obtained at transverse spatial resolutions of better than 25 meV using AFM and KPFM techniques. Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES) are available in the Electron Energy Analyser vacuum chamber. They can be used to characterize the surface ordering of atoms and identity the atomic-scale surface composition. Reflective High Energy Diffraction (RHEED) and Energy Dispersive X-Ray Spectroscopy (EDX) are additional capabilities developed within the thin film growth chambers. Samples can be transferred from the thin-film growth and surface preparation chamber into the surface characterization chambers in UHV.

Electron Energy Analyzer

The Electron Energy Analyzer is a uniques instrument designed and developed in-house. It uses a time-of-flight-based technique to obtain the 3-D energy-momentum distribution of very low energy meV scale electrons emitted from surfaces via photoemission. With a potential sub-meV energy resolution, this instrument is ideal for performing ARPES measurements very close to the photoemission threshold. The sample can be cooled to cryogenic temperatures using liquid helium. The tunable wavelength laser can be directed to excited photoemission using any desired wavelength. More details about this instrument can be found here: Kakare et al, Review of Scientific Instruments 90, 053902 (2019)

Cryocooled 200 kV DC electron gun

We are developing a 200 kV DC electron gun with the capability to cool the cathode to 30K. The cathode can be transferred out of the gun and into the rest of the surface preparation, characterization and photoemission characterization chambers and vice-versa. We are also developing 4-D phase space measurement diagnostics and response time measurement capabilities on the electron beam-line connected to this gun. In the future, this gun can be used for UED applications.

Tunable Wavelength Laser

We have a 'Light Conversion Pharos' which drives an Optical Parametric Amplifier, the 'Light Conversion Orpheus'. The Pharos laser outputs 20 W of 1030 nm light in 150 fs pulses at a tunable repetition rate with 500 kHz max. 10 W of this 1030 nm light is used to drive the Orpheus. Using white light generation along with 2nd, 3rd and 4th harmonic generation processes the Orpheus can generate any desired wavelength between 200 nm to 2000 nm in a fully automated fashion. This system is regularly used to perform automated QE and other spectral response scans over several 100 nm wavelengths. The laser is also directed into the PEEM, the electron energy analyzer and the DC electron gun to study photoemission at any desired wavelength.

Helium Liquifier

We have a Cryomech Helium Liquification system capable of generating up to 24 liters/day of liquid Helium. The system has a 150-liter dewar with a cold head attached. The dewar has 2 extraction ports. Currently, the system is set up to enable LHe transfer into the continuous flow cryostat in the Electron Energy Analyzer of the DC electron gun. The He exhausts are collected and pumped back into the dewar for liquification using a scroll pump. Using this setup, any continuous flow cryostat can be operated continuously without any He loss.