Magnetooptics
In the project we use conventional microstat MO2 from Oxford Instruments with superconductiong magnet cooled by liquid helium. It enables polarization-resolved measurements (linear as well as circular) both in Voigt and Faraday configuration in magnetic fields up to 5.0 T. Sample temperature can be varied in a range from 6 K to 300 K with stability of 0.1 K. This setup provides low sample vibration < 20 nm and drift ~ 4 nm/min. The other parameters are the same as in the case of the microphotoluminescence setup.
Microphotoluminescence
The high-resolution photoluminescence experiments are performed using spectrometer composed of FHR1000 monochromator from HORIBA Jobin Yvon (focal length of 1 m) and a liquid nitrogen cooled InGaAs NIR linear array detector operating in the spectral range from (800 - 1600) nm attached to the system output. The maximum spectral resolution provided by the experimental setup is approx. 25 μeV. A non-resonant excitation is provided by semiconductor laser diode emitting at 660 nm wavelength. The sample is held in cryogenic temperatures of 4.5 K using standard microscopy cryostat ST-500 from JANIS operated in a horizontal configuration. Varying in the temperature up till room temperature and stabilizing it with an accuracy of 0.1K is also possible. By using microscope objective (positioned with a 3D piezoelectric stage with 20 nm step) with high numerical aperture NA = 0.4 a spatial resolution in the order of 2 μm is achieved. The sample surface is imaged on a CCD camera to allow observation of patterning on the sample surface for identification of target nanostructures and measurements’ repeatability. Therefore, the microphotoluminescence setup is optimized for investigation of single quantum emitters (also in photonic structures) in the NIR and telecom spectral range with high spatial and spectral resolution.
Microphotoluminescence excitation setup
High-resolution photoluminescence excitation (μPLE) setup is optimized for NIR wavelengths and equipped with additional tunable, narrow-line continuous wave lasers from Toptica – DL100 series (covering spectral range of 910-985 nm and 1435-1535 nm) and self- made external cavity laser in Littman configuration tunable in the range 1210- 1310 nm. Achievable spectral resolution provided by a 1 m focal length monochromator (FHR1000 from Horiba Jobin Yvon) and InGaAs linear array detector (1024 x 25 mm pixel, 800-1600 nm sensitivity) is as high as 20 meV which makes the setup suitable for measurements of single QDs emitting in the telecommunication spectral range. For measurements of nanostructures the microscope objective with numerical aperture of 0.4 is used resulting in the spatial resolution of around 2 mm. Flow-cryostat cooled with liquid helium provides the possibility of carrying out the experiments at the temperature range from 4.2 to 300 K. It is also possible to analyze the polarization of nanostructures’ emission.
Two-photon interference experimental setup
The two-photon interference experimental setup is an extension of the correlation setup. It uses the same superconducting NbN nanowire single-photon detectors from Scontel company with efficiency better than 80% at 1550 nm and exceeding 65% at 1310 nm, temporal resolution better than 50 ps and dark counts < 100 cps cooled down to operating temperature (1.8 K) with closed-cycle refrigerator. The histogram of coincidences from the two detectors is obtained using Time-Correlated Single Photon Counting system from PicoQuant company – PicoHarp providing the time-bin width in the range of (4-512) ps. The distance between the two consecutive excitation pulses can be controlled by flexible self-made Mach-Zender Interferometer. Which can be modified to allow either for non-resonant excitation with semiconductor pulsed laser diode with pulse lengths < 50 ps (PicoQuant) or quasi-resonant excitation with < 3 ps pulses provided by combination of pulsed Ti:Sa laser and optical parametric oscillator from Coherent company. The excitation is focused on the sample surface and the signal detected through a long working distance (20 mm) high-numerical aperture (0.4) microscope objective providing spatial resolution of 2 μm. The signal is spectrally filtered (typical bandwidth - 0.4 nm) by iHR320 monochromator from HORIBA Jobin Yvon company and then after polarisation optics coupled to the single-mode fiber. The Hong-Ou-Mandel interferometer itself is fiber-based with single mode polarisation maintaining telecom fibers with the crucial 50:50 coupler in 2:2 configuration with splitting ratio tolerances < ±3% for both slow and fast axis and excess loss without connectors < 0.1 dB (Evanescent Optics).
Correlation spectroscopy setup
Correlation spectroscopy measurements are realized in free-space Hanbury Brown and Twiss configuration with non-polarizing 50/50 cube beam splitter
directing the signal into the two arms of the interferometer. In each arm the signal is spectrally filtered by HORIBA iHR320 monochromators with 320 mm focal length
equipped with three gratings with the maximum efficiencies covering the NIR wavelength range. The signal is further fiber-coupled to a NbN
superconducting nanowire single photon detectors (SNSPDs) from Scontel with operating temperature of 1.8 K achieved in the closed cooling cycle.
The SNSPDs detectors are optimized for the telecom wavelength with high efficiency (QE~90% at 1550 nm), low dark counts (~10 Hz)
and low time jitter (<35 ps). The total temporal resolution of the experimental setup is ~80 ps.
Inclusion of polarisation optics in both arms of the interferometer allows for entanglement fidelity measurements.
One of the monochromators is additionally equipped
with the LN-cooled linear array InGaAs detector, which allows for a standard μPL measurements with high spectral (~100 μeV) and spatial (on the
order of single μm) resolution provided by long working distance Mitutoyo M Plan-Apo NIR 20x infinity corrected microscopic objective with numerical
aperture of NA=0.4). The sample is held in a Janis ST-500 helium flow microscopic cryostat at a temperature < 5 K (can be changed up to room temperature)
providing low drift and vibrations.
A non-resonant excitation is realized by 787 nm continuous wave (cw)
semiconductor laser. If pulsed excitation is provided and only one arm of the interferometer is used extraction efficiency and carrier dynamics by means of
time-correlated single photon counting can be measured. The pulsed excitation can be either by pulsed semiconductor laser at 805 nm (< 50 ps pulse length
and controllable repetition rate up to 100 MHz) from PicoQuant or by Ti:Sa (Coherent) laser tunable from (700-1000) nm with either 3 ps
or 140 fs pulse length and 76 MHz repetition rate. The Ti:Sa laser can be combined with the optical parametric oscillator to broaden the spectral range
of excitation to cover both quasi- and resonant excitation for quantum dots emitting at telecom wavelength.
