Angle-Resolved Photo-Emission Spectroscopy with a Laser
Laser-based ARPES (Angle-Resolved Photo-Emission Spectroscopy) is a form of ARPES that uses a laser as an excitation source. Laser-based ARPES is operated with a narrow bandwidth UV laser source and offers several advantages over synchrotron radiation: A laser fits on a table (ok… a large table), and experiments can be performed at any time and independently of rare beam times in synchrotron facilities.
More technically speaking, a narrow-band UV laser offers a contantly high photon flux with an extremely high energy resolution of up to 0.1 meV. For comparison, the energy resolution of synchrotron radiation is typically in the range of 5 – 20 meV – and is therefore worse than the resolution of a good energy analyzer with about 1 meV.
The photon energy of the laser source can be varied between 5.77 eV and 6.5 eV. As these are low-energy photons, the Angle-Resolved Photo-Emission Spectroscopy benefits because the bulk sensitivity is significantly enhanced and the momentum resolution increased (compared to synchrotron radiation).
Low-energy photons are sufficient to induce photoemission in many samples. However, it should be noted that due to the lower photon energy compared to synchrotron radiation, Laser-ARPES is particularly suitable for the measurement of electronic states close to the Fermi level.
Laser ARPES/ARUPS with a High-Power UV Source
Tunable deep-UV pulses from 190 … 215 nm / 6.5 … 5.77 eV are available with APE’s ARPES-laser source, constiting of a HarmoniXX FHG in combination with Levante Emerald HP.
The energy-tunable system provides high laser power in the range of several mW. Its brillant and monochromatic laser light is ideally suited for high-resolution ARPES (Angle-resolved Photoemission Spectroscopy) or ARUPS (Angle-resolved Ultraviolet Photoemission Spectroscopy).
- Tunable (Vacuum)-UV from 190 … 215 nm / 6.5 … 5.77 eV
- Extremely narrow spectral bandwidth of < 0.2 meV
- High photon flux of > 10^15 photons/s
- Exceptional beam quality of M2 < 1.2
- Automated wavelength tuning via software
- Ideally suited for table-top laser-based ARPES / ARUPS
Laser ARPES/ARUPS with a Narrow Spectral Bandwidth Down to 0.1 meV
ARPES on Graphene/Ir(111)
The following measurement were performed with APE’s 190 nm / 6.5 VUV system. The ARPES measurement were taken on a SPECS GmbH system with a PHOIBOS 150 hemispherical analyzer and a 2D-CCD detector.
ARPES maps of graphene/Ir(111) sample measured with laser around normal emission geometry showing the Rashbasplit of graphene/Ir(111) surface state. Two different entrance slits of the Electron Analyzer were used, resulting in different energy resolution (marked in the figure).
hv = 6.31 eV
Lens mode: WAM
Nr of energy points: 251 (Sweep mode)
Nr of 0-angle channels: 300
Dependence on Temperature. 3D ARPES data sets for graphene/Ir(111) sample measured with laser (lens mode: WAM). Measurements were performed at T=300 K (RT, see image) and T=120 K (LT, not shown here).
polar β-scan around NE
hv = 6.31 eV
Nr of energy points: 251
Nr of Θ-angle channels: 300
Nr of β-angle steps: 101
Total acquisition time: 50 min
Turning Complexity into a Turn-Key System
This light source enables spectroscopy and pump probe measurements in the deep UV range < 200 nm. The system consists of the following sub-components:
Creating a 190 nm / 6.5 eV Table-top System
The HarmoniXX allows access to the wavelength range of 190 … 215 nm (6.5 … 5.77 eV) by mixing the THG with a fundamental wavelength to create the fourth harmonic. The harmonic pulses are of narrow bandwidth nature, enabling a high resolution across the entire tuning range.
Levante Emerald HP OPO
The generation of high power tunable UV radiation is based on a widely tunable picosecond OPO with computer controlled tuning, pumped by an industrial high power green laser and with subsequent frequency conversion to reach UV wavelengths below 200 nm.
Paladin 532 – 20000 from Coherent Inc. is ideal for pumping the OPO. It is enabling a wide tuning range and high-average powers of the OPO output. Of course, other pump lasers can be incorporated into the setup as well.
|Levante Emerald + HarmoniXX FHG (incl. THG, SHG)|
|Wavelength Range||192.5 … 210 nm | 6.44 … 5.9 eV (FHG 3+1)|
|210 … 230 nm | 5.90 … 5.39 eV (FHG 2+2)|
|235 … 330 nm | 5.28 … 3.76 eV (THG)|
|350 … 480 nm | 3.54 … 2.50 eV (SHG)|
|< 690 … 990 nm (Levante Emerald OPO Signal)|
|Power Output||> 0.25 mW at 200 nm | 6.20 eV (FHG 3+1)|
|> 0.15 mW at 220 nm | 5.64 eV (FHG 2+2)|
|> 14 mW at 267 nm | 4.64 eV (THG)|
|> 52 mW at 400 nm | 3.01 eV (SHG; THG components removed)|
|> 1750 mW at 800 nm|
|Pulse Width||approx. 7 ps|
|Spectral Bandwidth||10 pm | 0.32 meV at 196 nm | 6.33 eV|
|Repetition Rate||80 MHz|
|M2||< 1.2 (typical 1.1)|
|Polarization||Linear / Horizontal|
|Pulse Width||Approx. 7 ps|
|Remote Control||Possible via USB / Ethernet TCP/IP / Serial RS232|
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