Research

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 microphotoluminescence 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 microscopic cryostat with the base temperature <5 K (can be changed up to room temperature) cooled in a close cycle due to recirculating gas cooler providing low drift and vibrations.

A non-resonant excitation is realized by 640 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 tuneable 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.