Novel Detectors For Low Light Detection
Nick Bertone
OEC
Montreal, Quebec, Canada
Roberto Biasi
MPD
Bolzano, Italy
Bruno Dion
CMC Electronics
Montreal, Quebec, Canada
Introduction:
New applications in bioscience, imaging and lidar require low light detection. For example; in
fluorescence correlation spectroscopy, using a confocal technique to examine properties of single
molecules as shown in figure 1, requires single photon detection. This is due to the minuscule
amount of fluorescence generated from a single molecule. In this paper, we will examine novel
detector technology that is well suited for bioscience and lidar applications.
Figure 1, Single molecule illustration using Confocal microscope, courtesy of Carl Zeiss
Linear mode or Geiger mode light detection:
The simplest method of detecting light is by converting light energy into a voltage output using a
detector-amplifier configuration as shown in Fig.2. In such a configuration, the voltage output is
directly proportional to the light intensity, and the lowest light levels detectable is when the SNR =
1, or when Vout equals the sqrt of the detector + amplifier noise. At this light intensity, the output
from the amplifier equals the noise of the detector/amplifier combination, this value is referred to
the Noise Equivalent Power or NEP.
When the light being detected is CW, cooling the detector decreases the detector noise, and thus
lower light detection is achieved. However, in many biomedical or lidar applications, the light that
is being detected is not CW, it is usually a short pulse or a short burst of light with a fast decay.
In this case, the amplifier noise is the dominant source of noise, whereas at low bandwidths, the
detector noise is the dominant source. For example, a detector module with an NEP of 30fW/Hz
will be able to detect 20ns pulses of light with a peak power of 0.2nW. To detect lower levels,
photon counting will be required.
is
RF
PO
A
+
Vout=R*PO*RF,
R = Responsivity of detector
RF= Feedback resistance
SNR =
Vout
 Detector + Amplifier noise
Figure 2. Detector + Amplifier SNR
In linear mode operation as shown in figure 2, the input power produces an output that is directly
proportional to the input light intensity. In photon counting mode, or geiger mode as it often
referred to – one photon which is detected, generates a output pulse that is counted. It is very
important to note that in this mode of operation, photons are counted, and there is a “dead-time”
before the next photon can be detected and counted. For example, if 10 6 photons arrive at the
detector at the exact same time – only one pulse will be generated indicating that only 1 photon
has been detected. However, if 106 photons were available for detection, photon counting would
not be required – linear mode detectors would do just fine. If on the other hand, only 25 photons
per second were available for detection – which corresponds to an intensity of 0.01fW, then
photon counting would be required.
Figure 3. SPAD vs APD operating conditions (left), and SPAD passive quenching operation (right)
The solid-state detector that is used in this mode of operation is a Single Photon Avalanche
Diode (SPAD). A SPAD is a semiconductor junction diode where a self-sustaining avalanche
multiplication process can be triggered. A SPAD is biased above the breakdown voltage –
referred to overvoltage (see Fig.3, left). SPADs are therefore remarkably different from ordinary
Avalanche Photodiodes (APD), which work in linear amplifying mode biased slightly below the
breakdown voltage.
At a defined overvoltage, the electric field within the depleted region of the device is very high, so
that a single photon triggers a multiplication process, where an avalanche current of tens of
milliamps flows through the junction. This current is then used to initiate a TTL output pulse to
indicate a photon has been detected, and the current rising edge signals the photon arrival time.
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In order to detect the subsequent photon, the avalanche current must be quenched. Lowering the
SPAD bias voltage below breakdown, and then resetting it back to its overload voltage
accomplish this. The time it takes to reset the SPAD to its overvoltage is know as the “dead-time”,
since in this period, the SPAD cannot detect any incoming photon. It is therefore desirable to
have the dead-time as short as possible. The simplest approach is the passive quenching shown
in Figure 3(right), however, this quenching method leads to very long dead times (200ns or
more). Active quenching which is commonly used, provides dead times of less than 50ns.
Single Photon Avalanche Diodes using CMOS Processing Technologies:
Photon counting with specialized avalanche photodiodes has been commercially available for
many years using SPAD devices with the reach-through structure as shown in Figure 4, left. The
advantages of this structure are the high photon detection efficiency in the visible and near IR
region, and the large active diameter. The disadvantages are the high bias voltage (250-500V),
and the dedicated fabrication process. In addition this process is not ideally suitable for the
production of monolithic arrays.
Figure 4: SPAD Reach-Through structure with thick depleted region (left), planar structure with thin depletion for
ultrafast timing response (right).
Recent progress has also allowed for the development of commercial modules based on SPAD’s
fabricarted in CMOS technology. The planar structure, which can be processed using CMOS
technology, is shown in figure 5, right.[1] These SPAD’s provide many advantages such as:
ultrafast timing resolution, high fabrication yield and thus low cost, low power consumption for
portable systems, and make possible the production of monolithic arrays for producing large area
photon counting arrays. The main disadvantage of the CMOS SPAD is the lower photon
detection efficiencies at longer wavelengths. The planar SPAD using CMOS technology has a
thinner depletion layer than the reach-trough APD, and at longer wavelengths (near infrared), this
thin layer greatly reduces the detection efficiencies. For shorter wavelengths (in the visible range)
a reduced thickness of the depletion layer can suffice; at wavelengths below 550nm the efficiency
of the planar SPAD is even greater than that of the reach-through devices because the neutral
layer at the top surface is thinner and absorbs a lower percentage of the incident light.
Figure 5 (left), illustrates the photon detection efficiencies at various overvoltages; detection
efficiencies of better than 55% can be achieved by applying an AR coating on the detector. Figure
5 (right), illustrates the fast timing resolution which has been demonstrated using these SPAD’s.
Recently 35ps-timing resolution with a 100um detector at room temperature has been
demonstrated.[2] In comparison, modules using the reach-through SPAD are achieving timing
resolutions in the order of 300ps.
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Figure 5: Photon detection efficiencies vs. wavelength (left) and timing resolution using planar SPAD (right)
Bioscience Applications requiring fast Timing Resolution:
Fluorescence resonance energy transfer (FRET) is used to follow proteins, enzymes and other
biological macromolecules as they twist, turn and take new shapes. As these molecules undergo
changes, multiple fluorescence lifetimes are detected because of the difference spacing between
the donor and acceptor as shown in figure 6.[3] Using time-correlated photon counting with 50ps
timing resolution, and a confocal set-up to examine single molecules, researchers are able to
measure conformational changes with angstrom resolutions.
Figure 6: The fluorescence lifetime reflects the distance fluctuation in a single enzyme molecule (left), apparatus used (right)
Using the apparatus set-up shown in figure 6, Dr. Sunney Xie from Harvard used this technique
to study changes in an enzyme system,. A frequency-doubled mode-locked laser was focused
down to a spot that only one molecule was being excited to ensure fluorescence from only one
molecule. Each excitation pulse generated a series of fluorescent photons, which were detected
with a photon counting detector module. Because the distances measured in this example were
very small, the photon counting detector module required timing resolutions in the ten’s of
picoseconds. The module used for this experiment used a planar SPAD, had timing resolution of
less than 50ps, and was developed by; Dr. Sergio Cova of Politecnico di Milano.
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LIDAR Applications requiring high sensitivity at 1064nm:
There are many applications for LIDAR(Laser Detection and Ranging), and the wavelength
which is used depends on what needs to be detected and ranged. For example, applications
requiring long distances, 1064nm is an ideal wavelength as high power lasers for this wavelength
readily exist. However, as illustrated in figure 7, detectors with high sensitivities are not readily
available. From 400 to 900nm, it is seen that photon counting detectors with good photon
detection efficiencies are available using CCD’s, planar SPAD’s and reach-through SPAD’s, but
the sensitivities of all these detectors roll-off in dramatically at about 950nm.
Reach-Trough
SPAD
Planar SPAD
Figure 7: Quantum Efficiency comparison for various Silicon and PMT based detectors.
The module described in this section uses a hybrid photomultiplier tube manufactured by Intevac,
Santa-Clara, CA.[4] The tube which is referred to as a intensified photodiode (IPD), is sensitive
from 900 to 1300nm and has an active diameter of 1mm. The IPD uses an active transferred
electron photocathode, and a GaAs schottky avalanche photodiode as an anode. Figure 13,
illustrates the outline of the package – first stage amplification consists of a very low noise
electron-hole multiplication due to energetic impact of photoelectrons on the top surface of the
APD. This is then followed by traditional APD multiplication. These two processes are sufficient to
overcome preamplifier noise and is therefore capable of photon counting.
Figure 8. Intensified photodiode, courtesy of Intevac, Santa Clara, CA.
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Since this module operates in linear mode with high gain, both modes of operation are possible –
Linear mode and Geiger mode. Modules with a have been produced with a 1mm active diameter
detector that have both photon counting and analog outputs, photon detection efficiencies of
30%, dark counts of 3x104 cps, and an NEP of xxxx when operating in linear mode have been
demonstrated.
Summary:
New silicon detectors using CMOS processing techniques have been demonstrated for photon
counting. One of the many advantages using this technology is, the ability to produce monolithic
arrays. Using state of the art iAQC circuits; low power consumption and fast timing resolution
modules have been developed and are currently available.
For high-end applications requiring high photon detection efficiencies at 1064nm, it is illustrated
that the a hybrid tube + APD provides the highest detection efficiency at 1064nm – with a photon
detection efficiency of xx at 1064nm.
References:
[1]
F. Zappa, S. Tisa, S. Cova, P. Maccagnani, D. Bonaccini, G. Bonanno, M. Belluso, R. Saletti
and R. Roncella, “Pushing technologies: Single-Photon Avalanche Diode Arrays”, SPIE Int.
Symposium on Astronomical Telescopes & Instrumentation, Glasgow 21-25 June 2004.
S. Cova, M. Ghioni, I. Rech, “Photon Counting and Timing Detector Modules for SingleMolecule Spetroscopy and DNA Analysis”.
[2]
[3]
Haw Yang, Guobin Luo, Pallop Karnchanaphanurach, Tai-Man Louie, Ivan Rech, Sergio Cova,
Luying Xun, “Protein Conformational Dynamics Probed by Single-Molecule Electron Transfer”.
[4]
Ross A. La Rue, Kenneth A. Costello, Gary A. Davis, John P. Edgecumbe and Verle W. Aebi,
“Photon Counting III-V Hybrid Photomultipliers Using Transmission Mode Photocathodes”, IEEE
Transactions on Electron Devices, Vol. 44, No. 4, April 1997.
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