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Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field
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Image of FIG. 1.
FIG. 1.

Schematics of the experimental setup. Excitation light of two tunable lasers is coupled into a polarization-preserving fiber-based beam splitter (port IN) and one of the outputs is connected to the fiber that runs to the microscope. This fiber delivers excitation light to the sample, which is mounted on an xyz-stack of piezo-motors. The sample position can be tuned to be in or out of the focal spot of the two-lens microscope. The microscope is mounted in a tube, which is vacuum pumped and immersed in a Helium bath (4.2 K) or used in a dilution refrigerator. A superconducting coil provides magnetic fields up to 9 T. A silicon pin-photodetector is positioned right behind the sample for detection of the optical transmission. Both the sample and the detector are mounted on a Γ-shaped sample holder. The second output of the beam splitter is coupled to a photodetector for monitoring the optical powers. Signals that come from reflection on the sample, as well as emission by the sample, retraces the optical path through the fiber. After passing the beam splitter is can be diverted to a regular photodetector, or to a spectrometer. Inset: microscope components mounted on the copper frame that forms the cold finger.

Image of FIG. 2.
FIG. 2.

Photo of a home-built heat-sink for use in cryogenic coaxial lines, with SMA connectors and a gold-plated sapphire substrate in a copper housing unit.

Image of FIG. 3.
FIG. 3.

(a) Reflected and transmitted signal as a function of the lateral position (y-axis) of the sample. The spot size W of the beam on the sample surface (waist W 0 when in focus) is determined with the knife-edge technique (inset). (b) Reflected signal (solid line) and the spot size W (dots) as a function of the axial displacement (x-axis) of the sample.

Image of FIG. 4.
FIG. 4.

(a) Energy level diagram and optical transitions for the D 0D 0 X system in GaAs. (b) Pump-assisted spectroscopy with pumping V-polarized light at the A transition (at 8174.45Å). This results in enhanced absorption for H-polarized light at the A* transition (8173.55 Å). (c) Complementary to the observation of (b), pumping with H-polarized light at the A* transitions results in enhanced absorption for V-polarized light at the A transition. Similar cross-pumping effects are observed for the nearby B and B* transitions. Data taken at B = 8T.

Image of FIG. 5.
FIG. 5.

Photoluminescence spectrum of low-doped n-GaAs, showing luminescence by free excitons (X), excitons bound to neutral donor sites (D 0 X), and excitons bound to ionized donor sites in the sample's depletion layer (D + X). Data taken at , the resolution of the spectrometer is .

Image of FIG. 6.
FIG. 6.

Scanning-probe transmission spectra taken at (Reproduced with permission from Sladkov et al., Phys. Rev. B82, 121308–R (2010). Copyright ©2011 American Physical Society (see Ref. 30) (a) and (b). Traces were recorded with linear H or V polarization for the probe light (giving identical results at 0 T). For performing these experiments the microscope was defocussed to a spot diameter of about 16μm. The strong absorption due to free excitons (X) and much weaker features due to donor-bound excitons (D 0 X) are labeled. The data at 5 T shows a diamagnetic shift of about 10Å with respect to the data at 0 T.

Image of FIG. 7.
FIG. 7.

Electromagnetically induced transparency within the A* absorption dip, induced by a strong control field that addresses the A transition. Spectra are taken for different intensities I of the control field, with . Traces are offset vertically for clarity.


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Scitation: Polarization-preserving confocal microscope for optical experiments in a dilution refrigerator with high magnetic field