Hi; The photon detection efficiency (PDE) of Silicon Photomultiplier (SPM) directly influences the signal-to-noise ratio and thus the significant parameters for isotope identification, such as FWHM. Optimization of PDE can be achieved b increasing the pixel size, thereby impoving the detector geometrical fill-factor and the sensitive area. 

Robert,

Your question is not absolutely clear.

1. If you mean, that you want to tune the dependence of your device output signal on wavelength, then, of course, it is possible. For example, put spectral filter or wavelength converter before device input to reduce or increase detector`s response at given wavelengths .

2. If you mean another technologies, used for light detectors, where spectral response can be tuned by electrical or magnetic field, or temperature, then also it is possible. But not at any spectral range. Example - microwave detectors, based on magnetic resonance, etc.

The appropriate material doping or oxidation, etc. also is well known way to improve the characteristics of photodetector, but , as you understand, it is not "tuning".

In photo sensing CCD imagers the spectral filters that cut IR radiation are added to improve the resolution of the imager. The maximum of spectral sensitivity in this case is shifted to shorter wavelengths. But it does not rises the sensitivity.

Do you mean efficiency as the number of photons converted to number of e-h pairs?

We should estimate the total efficiency: the number of e-holes pairs per the total number of photons falling on the outer surface of the photosensitive device, including all possible filters. But in CCD devices the IR photons penetrate too deep and produce the e-h pairs on the large depth. These pairs are useless, because the carriers come back to the semiconductor surface with a large spread, decreasing the resolution. So, to improve the resolution we should reject the IR radiation.

In 1993 I was invited by Samsung company to explain, why the best Japanese CCD camera had small output in their IR imager, but the Russian camera had an output much larger. I explained them that the high resolution cameras have a filter, cutting IR radiation for better resolution. So, their IR imager should not have this filter to improve its sensitivity, but sacrificing with resolution. They were surprised to hear that!