Two New Ccd Systems Since 2002

We chose 2D detector CCDs instead of CMOS image sensors. The main advantage of a CCD is its high quality. It is fabricated using a specialized process with optimized photodetectors, very low noise, and very good uniformity. The photodetectors have high QE (Quantum Efficiency) and low dark current. No noise is introduced during charge transfer. The drawback of CCDs include: they can not be integrated with other analog or digital circuits such as clocks generation, control and A/D conversion; they have high power consumption because the array is switching continuously at high speed; they have limited frame rate, especially for large sensors due to increased transfer speed while maintaining acceptable transfer efficiency. Note that CCD readout is destructive; the pixel charge signal can only be readout once. Reading discharges the capacitor and eliminates the data.

CMOS image sensors, however, generally suffer from lower dynamic range than CCDs due to their high read noise and non-uniformity. Moreover, as sensor design follows CMOS technology scaling, well capacity will continue to decrease, eventually resulting in unacceptably low S/N. However the most significant characteristics for us were low noise, high QE and low dark current. This consequently led to the choice of the CCD.

The first CCD system was built around an e2v CCD30-11 back illuminated device (1024x256) which was cooled in a liquid N2 cryostat (see Fig. 1). It was a classical system with a CDS (Correlated Double Sampling) readout of 100 kHz (16 bits) and a readout noise under 10 e- . We saw the phonon effect for the decoherence in InAs/GaAs QD (Quantum Dot) for the first time with this system (see Fig. 2) [2]. In fact, the sidebands around the QD line are due to the recombination of one electron-hole pair with the lattice emission (for lower energy) and absorption (for higher energy) of acoustic phonons.

Figure 1. The experimental set up on which the CCD30-11 is assembled [1].
Figure 2. Photoluminescence spectra of a single QD spectra from the CCD30-11 system.

In 2005, we saw another effect owing once again to the improved S/N: a "giant optical anisotropy in a single InAs quantum dot " [3].

We made a second CCD "camera" to replace the Hamamatsu system (with linear InGaAs photodiodes array). It is used to detect the photons emitted from semiconductor microcavities. In semiconductor microcavities, a strong light-matter coupling regime can be reached, leading to new Eigenstates known as polaritons. Polaritons are hybrid quasi-particles, partly photonic and partly electronic. As such, they exhibit strong nonlinear behavior as well as strong coherent effects. This makes microcavities ideal candidates for the development of new integrated sources of twin or entangled photons, for quantum optics applications such as cryptography. Experimentally, the CCD is used for imaging the emission pattern on the surface of the microcavity structure. Alternatively, using a modified optical set-up, it acquires the emission diagram in the momentum space. Lastly, the CCD is also used to measure the spectrum of the emission.

This camera was built around an ICX285AL from Sony. It is a low cost interline CCD image sensor with excellent characteristics for its price: 1360(H)*1024(V) number of active pixels, horizontal drive frequency up to 28.64 MHz, high sensitivity, low smear, low dark current, excellent antiblooming characteristics and continuous variable-speed shutter.

To have lower dark current the CCD for the moment is cooled by a single stage Peltier module at -20°C. In the future we will put a second TE cooling stage to reach -50°C for long exposures ( >500s).

We continuously acquire the frames via an USB2 direct link connecting the camera and the computer with an average transfer rate up to 11 MB/s. So we can reach (in full resolution) a frame rate of about 4 fps (frames per second) at 14 bits, 8 fps at 8 bits, and up to 20 fps at 1360*256 resolution. The exposure time varies from 1/4000s to 1000s.

There is CDS readout and with this first version we have a readout noise of 30e- (for readout frequency at 2 MHz). In the second version we will improve it. Figure 3 gives a spectrum record taken during camera tests with our acquisition software.

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