Optical Sensors for far-infrared and submillimeter astronomy

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Far-infrared and submillimeter astronomy is a hot research field. The relative weakness of this radiation requires telescopes to use cryogenically cooled superconducting bolometers and associated transmission lines and processing equipment. Literature examination suggests that considerable time and care is spent on wiring and interconnecting systems to avoid failure points.

We propose a different thermal sensor, covered by submitter's US patent 7,586,106: a low thermal mass multi-pixel thin planar film, unwired, which generates, when absorbing said radiation and interrogated by light of suitable wavelength, a fluorescence of wavelengths shorter than 1.0 micrometer. Refrigeration requirements would be drastically cut: processing light signals requires no cooling. Light is measurable at the photon level. Optical signals can be amplified, time integrated and background subtracted with simple, mature techniques.

FIG 1 is a simplified molecular energy diagram of a fluorescent temperature probe. Energy level 0 is the ground electronic level, levels 1-2 its sublevels and level 3 is the excited, fluorescent level. In a statistically large ensemble, the number N1 of molecules occupying level 1, with an energy E above level 0 follows the Boltzmann distribution 

N1 ≈ N0.exp(-E/kT) 

where k is the Boltzmann constant.

The fluorescence intensity If, proportional to absorption coefficient αT and N1/N0, is expressed as 

If = K.exp(-E/kT) --------------------- (1)

where K is an instrumental constant. 

For a small temperature increase ΔT, the relative fluorescence intensity increase ΔI is

ΔI/If = (E/kT)(ΔT/T) = (E/kT)(H/mCT) ---(2)

where H is the energy absorbed by the sensor, m its mass in grams and C its specific heat. 

Applying Debye's theory of specific heats, (ΔI/If) varies as T-4. Therefore, the proposed method is many orders of magnitude more sensitive at sub-kelvin temperatures than at ordinary temperatures. Similar trends are predicted from Bose-Einstein statistics. 

In a practical system using a 'spiderweb' micromesh radiation absorber, the probe mass need not be greater than 10-12 g..

The physical basis of this sensing method appeared in 1916, as shown in FIG 2. At low optical densities, interrogation of the ruby crystal at wavelengths of 470nm or 610nm should determine its temperature from below 98K to above 673K

Precautions.- We deal with low energy differences. Therefore single crystal probes are necessary . Otherwise a specific light interrogating wavelength could be absorbed by differently oriented molecules with slightly different energy levels. Furthermore, very monochromatic fluorescence excitation light is required for selectively exciting molecules occupying a defined sublevel 1. Fortunately, highly absorbing thin single crystals and high resolution (sub-Angstrom) laser tuning techniques exist.

Task of R&D program: Choose a strong visible light absorbing single crystal,. Cool it to liquid helium temperatures and find the optimum excitation wavelength for any chosen temperature. The validities of Boltzmann and Bose-Einstein statistics have became so established, there is a high degree of confidence for finding the sought for sensors.

Success may spawn a commercial industry of optical temperature sensors operable from cryogenic to high temperatures.using a single probe..

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  • Name:
    Marcos Kleinerman
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  • Marcos is inspired by:
    I believe, based on my knowledge of current problems in instrumentation for infrared and submillimeter astronomy, that my concepts could have a substantial positive impact.
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