Images of semiconductor 'dot in rods' and their small clusters are studied by measuring the second-order correlation function with a spatially resolving ICCD camera. This measurement allows one to distinguish between a single dot and a cluster and, to a certain extent, to estimate the number of dots in a cluster. A more advanced measurement is proposed, based on higher-order correlations, enabling more accurate determination of the number of dots in a small cluster. , and a noticeable probability of two-photon emission, are still one of the most promising types of singlephoton emitters as they can be operated at room temperature and can be synthesized relatively easily. Especially promising are dot-in-rods (DR) [10], which have a higher probability of single-photon emission, show reduced blinking [11], and emit single photons with a high degree of polarization [12]. DRs can also merge into clusters. A cluster of DRs is not a single-photon emitter but it can be used for quantum information purposes since it still emits nonclassical light. In this work we perform measurements that allow one to distinguish between a single DR and a cluster. As a criterion, we use Glauber's second-order correlation function (CF) and the brightness of emission. Another method of distinguishing between a single DR and a cluster is defocused microscope technique [12], based on the dipole-like angular distribution of the DR emission; however, this method only allows one to recognize clusters consisting of differently oriented DRs. As an extension of our correlation measurement, we propose the study of higher-order correlation functions. Such a measurement will allow one to resolve the number of DRs in a cluster and also observe nonclassical behavior of its emission.For our experiment we used an intensified CCD (ICCD) camera. This device is equivalent to an array of single-photon detectors and can be used for spatially resolved measurements of Glauber's CFs. Alternatively, the pixels can be used for photon-number, rather than spatial, resolution, so that higher-order CFs can be measured. The ICCD camera can be used in either analog or photon-counting mode. In the photon-counting mode, used in our experiment, a certain level of the dimensionless readout signal S (proportional to the integral electric charge acquired in a pixel) is chosen as a threshold, and any signal exceeding this value is interpreted as a single-photon event in a corresponding pixel. If the threshold S th is taken too high, the resulting quantum efficiency becomes low, which fortunately does not affect the normalized CF measured in our experiment [13]. If the threshold is too low, the noise is increased.

We report testing of the new absolute method of photodetector calibration based on the difference-signal measurement for two-mode squeezed vacuum by comparison with the traditional absolute method based on coincidence counting. Using low-gain parametric downconversion, we have measured the quantum efficiency of a counting detector by both methods. The difference-signal method was adapted for the counting detectors by taking into account the dead-time effect.

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