Scientific Applications Using High Quantum Efficiency Photocathodes

Detecting low-intensity signals down to a single photon is important in many medical and scientific applications. For this reason, PHOTONIS has developed a wide range of photon counters which include Multi-MCP photomultiplier tubes (PMT) and Hybrid Photo Diodes (HPD).

Photocathode can be considered as the first stage of the amplifier, and as such, its properties are highly significant when deciphering the device’s overall performance. The properties of the device are directly influenced by the quantum efficiency (QE), dark rate, and response time. For instance, the QE of the photocathodes mainly characterizes the photon detection probability.

A photocathode emits dark electrons that are amplified in the same manner as a photoelectron, which means a dark pulse cannot be separated from the real single-photon event. In the majority of cases, the timing jitter of the devices is restricted by the traveling time of a photoelectron in a cathode prior to escaping into the vacuum. In this article, the photocathode properties are discussed in detail and the overall performance of a PHOTONIS dual-MCP PMT is demonstrated using the latest Hi-QE photocathodes.

Quantum Efficiency Spectra

Figure 1 shows the QE spectra for conventional and Hi-QE series of S20 photocathodes within the of 200–700 nm spectral range. At 270 nm, the peak QE of approximately 25% is obtained for the std-S20 process. These photocathodes possess a broad sensitivity spectral range and have a QE that is still in the range of 3%–4% at 700 nm (Figure 1). Conversely, the wide sensitivity range comes at the cost of lower QE at the peak and higher dark rates.

Spectrum of quantum efficiency for newly developed Hi-QE photocathodes: Hi-QE-UV (dark blue), Hi-QE-blue (light blue), and Hi-QE-green (green) in comparison to Standard S20 photocathode (dashed red).

Figure 1. Spectrum of quantum efficiency for newly developed Hi-QE photocathodes: Hi-QE-UV (dark blue), Hi-QE-blue (light blue), and Hi-QE-green (green) in comparison to Standard S20 photocathode (dashed red).

It is important to note that fast gating photocathodes can be provided with mesh underlay so as to reduce the gating time to 3 ns, but simultaneously, it also reduces the averaged QE by roughly 10%–15%.

The Hi-QE series of photocathodes was developed by PHOTONIS to target specific spectral ranges according to customer requests. The Hi-QE photocathodes were made on a fused silica input window using a high-energy cutoff at 170 nm. These types of photocathodes exhibit much higher QE values (>30%) in the spectral ranges of interest (see Figure 1) and also demonstrate very low dark rates (see Figure 2).

Moreover, the response time is much lower than 100 ps. The Hi-QE-UV photocathodes are developed for UV range with an optimum QE at 270 nm, normally in the range of 31%–34%. These photocathodes can even be grown on sapphire cathode substrates, making it possible to extend the sensitivity spectral range down to 150 nm.

The Hi-QE-blue photocathode was developed by PHOTONIS to offer the highest QE in the 260–410 nm spectral range. The QE spectrum, in this range, shows a plateau with a normal QE value that is greater than 30%. However, below 260 nm, the reduced QE (compared to Hi-QE-UV) is at the cost of high sensitivity in the blue spectral range. Furthermore, the Hi-QE-green photocathodes display a QE value that is much higher than 30% in the 390–480 nm range. Yet, the QE value is about 25% at 500 nm.

When compared to other Hi-QE photocathodes, the Hi-QE-green photocathode boasts a much greater sensitivity at longer wavelengths up to 700 nm. It should be noted that the dark rate of these photocathodes stays very low, similar to other Hi-QE cathodes, regardless of their high sensitivity at longer wavelengths. This feature makes Hi-QE-green an exclusive photon counting device in this spectral range.

Dark Rate of Hi-QE Photocathodes

A classic characteristic of the high-band-gap photocathodes is low, dark currents, but reducing the dark rate to almost zero is essential for low-rate single photon detection.

Figure 2 shows the evolution of dark rate versus time at room temperature for std-S20 and the recently developed Hi-QE S20 photocathodes. Here, the positioning of the cathode is done at time zero in dark conditions. For all Hi-QE S20 series photocathodes, the dark rates are the same. As illustrated in Figure 2, the low-rate plateau is reached after two to three hours. The high dark rate measured initially appears to evolve from the population by ambition light of long-living surface and bulk states, lying above Fermi level.

Evolution of dark rate vs. time at ~23 °C for standard S20 photocathode (blue) and newly developed Hi-QE S20 (red). Extremely low dark rate < 30 cts/cm2 can be achieved with Hi-QE series photocathodes.

Figure 2. Evolution of dark rate vs. time at ~23 °C for standard S20 photocathode (blue) and newly developed Hi-QE S20 (red). Extremely low dark rate < 30 cts/cm2 can be achieved with Hi-QE series photocathodes.

More time is needed for these states to be discharged, with a decay time also being a vital parameter of the detector performance.

In order to maintain a low dark rate and to quickly discharge surface and bulk states, the growing photocathode process was modified for Hi-QE photocathodes. Conventional broad-range S20 photocathodes have a dark rate which is close to 1000–2000 cts/cm2, while the Hi-QE photocathodes normally have a dark rate of just 20–30 cts/cm2. Here, the dark electron rate reduces to less than 50 cts/cm2 after 10 minutes in the dark.

Single/Multi-Photon PHD Measurements with Hi-QE Dual MCP-PMT

Figure 3 shows the pulse height distribution (PHD) achieved with Hi-QE photocathode and dual MCP-PMT, revealing the potential of single photon detection as well as elucidating the means of measuring the dark rate.

A shaping amplifier CSA4 (gain = 10, shaping time of 250 ns), a multi-channel-analyzer MCA3 (scale = 0.89 mV/chn), and a charge sensitive preamplifier CSP10 (1.4 V/pC) were used for recording the PHD. 32 chn was the set threshold value. To perform the dark-rate measurements, the tubes were placed into dark conditions and the MCP-voltages were set to achieve a gain of 1-2E05. Measuring the count rates versus time was the next step.

Shown in Figure 3 is the PHD, which was determined with low-input background light illumination by keeping the count rate down to several hundred Hertz. In this example, an MCP voltage of 1625 V (for dual set) was applied to achieve 1.07E5 electron gain. For the threshold used, the MCP dark rate was found to be below 0.2 cps and photocathode dark rate was 30 cps/cm2.

Pulse height distribution recorded with PHOTONIS’ dual MCP-PMT and Hi-QE S20 photocathode with single photon illumination (blue); Gaussian curve (black) is a fit of experimental results.

Figure 3. Pulse height distribution recorded with PHOTONIS’ dual MCP-PMT and Hi-QE S20 photocathode with single photon illumination (blue); Gaussian curve (black) is a fit of experimental results.

In the PHD indicated in blue curve in Figure 3, the peak appears to be well separated with a low-energy valley and noise less than the threshold. The peak correlates well with Gaussian (solid black curve).

The gain point (“G”) indicates the PHD’s mean energy, which was somewhat above the peak position of the PHD. Characterization of photon counting tubes is normally done using the full-width half maximum/gain (W/G) ratio and the peak/valley (P/V) ratio. The quantified values of P/V ≈ 6 and W/G ≈ 0.86 are perfect for dual MCP PMTs.

Another instance is shown in Figure 4, demonstrating the excellent performance of PHOTONIS’ MCP-PMT. Here, the photocathode was illuminated using a 100-kHz short-pulse (100 ps) defocused laser beam. Due to the considerable reduction in the laser intensity, the bunch of photoelectrons that escaped into the vacuum consisted of just a few electrons.

Pulse height distribution recorded with PHOTONIS’ dual MCP-PMT and Hi-QE S20 photocathode (the same tube and settings as in Figure 3) with few photons illumination. The fitting curve is a sum (1-7) of Gaussian curves, corresponding multi (1->7) photoelectron amplification. Inset: the normalized area of each (1-7) Gaussian curve (blue dots). The red curve is calculated Poisson distribution with a λ ≈ 2.52 to be the expected value for the average number of electrons emitted from the photocathode." src="/images/Article_Images/ImageForArticle_16955(4).jpg" style="border-width: 0px; border-style: solid;">

Figure 4. Pulse height distribution recorded with PHOTONIS’ dual MCP-PMT and Hi-QE S20 photocathode (the same tube and settings as in Figure 3) with few photons illumination. The fitting curve is a sum (1->7) of Gaussian curves, corresponding multi (1->7) photoelectron amplification. Inset: the normalized area of each (1-7) Gaussian curve (blue dots). The red curve is calculated Poisson distribution with a λ ≈ 2.52 to be the expected value for the average number of electrons emitted from the photocathode.

As illustrated in Figure 3, the same tube and settings were used to carry out the measurements. The blue curve indicates the measured PHD, and the black curve indicates the fit. The fit is a sum of seven Gaussian curves (see Figure 4). The width (W1) and position (G1) of the first fitting Gaussian curve, in relation to a single-photon illumination, were acquired from Figure 3.

Other Gaussian curves corresponded with the amplification of two to seven photoelectrons released by the photocathode. The curves’ positions were set as GN = G1*N, and the width was scaled up according to the statistical rule as WN = W1*N1/2. The fitting parameters were only the amplitudes of the Gaussian peaks.

An excellent fit of the measured curve can be found with distinct separation of the first and second peaks, in relation to one- and two-electron photoemission. While a peak for three-electron photoemission can also be visualized, the resolution of additional peaks is not sufficiently good due to the statistical increase of the PHD width for short-pulse photoemission of multiple electrons.

In Figure 4, the insertion (blue points) demonstrates the intensity (corresponding to the area) of each individual peak. The red curve is a Poisson distribution fit with the value of λ predicted to be 2.52. The excellent agreement in the data proves that the average number of photoelectrons in the bunch discharged by the photocathode is approximately 2.5 for these types of measurements.

Conclusion

PHOTONIS devised high quantum efficiency UV, blue, and green S20 photocathodes that have higher than 30% QE in specified spectral ranges. The Hi-QE photocathodes, with a combination of fast response time (well below 100 ps) and extremely low dark count rates (down to 30 cts/cm2), are perfect for photon counting devices. The measurement of PHD with a single photon and with a few photon illuminations demonstrated the high quality of PHOTONIS’ MCP-PMT photon counters.

This information has been sourced, reviewed and adapted from materials provided by PHOTONIS Technologies S.A.S.

For more information on this source, please visit PHOTONIS Technologies S.A.S.

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