Fast Variable-Temperature Cryogenic Blackbody Sources for Calibration of THz Superconducting Receivers

Round 1

Reviewer 1 Report

In this paper,  electrically heated blackbody radiation source comprising thin metal film on a dielectric substrate and an integrating cavity was studied from millimeter wave to terahertz wave band. Analytical and numerical modeling were also made to optimize the emissivity, spectral uniformity, and modulation frequency of the radiation source with the spherical integrating cavity and thin film absorber. Some interesting results are given in this paper.  Although this paper would merit publication, there are many revisions which must be made. I can not recommend it to be publishes in its present form. The comments are as follows.

1. R□ = Z0/2 = 188 Ω, □ maybe a distake, many similar errors in this paper need to be cottrected.

2.  Fig.1 (b) shows that there is a relatively deviation between the simulated and theoretical calculation results. Why?

3. In the text of Fig.2, what does the □ mean ?

4、The relative dieletric constant of sapphire, silicon and quartz should be given where they come from, such as giving the reference.

5. In Fig.3, the thckness of the substrate is set as 340um, why ?

6. The unit of NiCr film with resistivity  Ωm, not ΩXm

7. Fig.6 (b) should give the comparation of the simlution results and experimental results.

8. Fig.6 (a）can not reflect the real status of the model of rotating cryogenic absorber and reflector.

9. Fig.9 and 10 are unclear, they should be replaced by new clear figures.

10. What do the the 1 and 2 in Fig 1 represent?

11、More physical explanation should be included in this paper.

Author Response

please find attached answers

Author Response File: Author Response.docx

Reviewer 2 Report

Major comments:

1. About the design of the thin-film absorber that absorbs perpendicularly incident electro-magnetic waves (Page 2), the authors indicate that "the absorption coefficient will be A = 4Z_0\sigma(2+Z_0\sigma)^2." This equation is not obviously correct to the reviewer, in addition, "\sigma" is not defined in the text. In particular, it seems to be strange that the film thickness is not included in this equation. The maximum of A, according to this equation, is 0.5 when Z_0\sigma=2. How this is related to "sheet resistance R=Z_0/2=100 Ohm" ? According to the reviewer's limited knowledge, a resistive film cannot behave like a perfect matched layer to EM wave that is perpendicularly incident. The calculation of absorptance can be done if the resistivity, permittivity, permeability, and the thickness as well as substrate information are known, for example "R. G. Buckey, etc. "Absorptance of thin film," Applied Optics, vol 16, no. 9, pp. 2495, 1977." In Figure, 1, the author simulated the absorptance by using a commercial simulator. The simulation was made in a circular waveguide boundary condition, which is different from free space. The results obtained from this simulation is thus considered to be not fully applicable to the normal incidence case. 

 

2. Because a thin film is not a good blackbody due to partial transparency to incident wave, the calibration of this source become necessary if it is used a calibrator for any THz detector. This article does not address on the absolute calibration method and quality of the proposed source. 

 

 

3. The idea of making a blackbody emitter with a thin film deposited on thick substrate is problematic because the large thermal capacitance of the substrate will limited the temperature modulation depth as well as the modulation speed. Is there any consideration of the use of suspended silicon membrane instead of thick substrate, as applied in many THz and infrared detectors?

 

4. The response of the source to pulsed heating was investigated.  However, the ring-down time of the temperature of the source is not shown, which seems to be long inferred from Figure 12-Figure 16, probably due to weak thermal link between the substrate and the cold stage.  If this time is long, this source can hardly be used in fast modulation of kHz as claimed by the authors.

 

 

Minor comments:

Page 1 : "... a few picowatts in a band of more than 10% cannot be met..." -> "10% fractional bandwidth"

Page 2 : " Cullen-Welton" -> "Callen-Welton"

Page 4 : Figure 3, please add simple identifiers, such as "from film side" and "from substrate side" to each sub-figures to aid the reading. 

Page 6 Figure 6: in the figure caption it is not clear what response is measured. please add more information about the measurement. 

Page 7 Figure 8: What does "HFI" stand for?

Page 10: Topic sentences of each paragraph are missing. This causes some difficulties to catch the meaning.

 

Author Response

please find attached

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I have no further comments.

Author Response

Dear reviewer, thank you very much for your valuable work and advices.

Reviewer 2 Report

Thanks for the reply and corresponding modification of the manuscript.  Only one thing to confirm. The author said in the reply that any substrate becomes thermally insulating (due to low phonon population). However, because the film on substate is a heater, it will heat up the substrate and then the substrate becomes thermally conducting when temperature increases. The heating is effiecent because the thermal capacitance of substrate is also very little at extreamly low temperature. I think this dynamic effect should be taken into consideration in the estimation of response time of the device. 

Author Response

Dear reviewer, thank you very much for valuable comments and advices. Concerning your comment about the role of thermal conductivity of substrate, to our opinion such effect will be important in the case of strong overheating of substrate. In our case for fine calibration of SINIS detector when electron temperature of BB increase from 3 K to 4 K, such effect will be of minor importance compared to thermal conductivity via golden electrodes.