Photonic and Optomechanical Thermometry

Round 1

Reviewer 1 Report

The authors discuss two techniques for temperature sensing, which is an interesting and important topic. They considered photonics temperature sensors as well as opto-mechanical temperature sensors over a relatively large range of temperatures.

The manuscript is well written and organized. The results are solid and clearly presented in a comprehensive and clear way. Therefore I recommend its publication in Optics.

Author Response

We thank the reviewer for these compliments and the recommendation to accept our work for publication. 

Reviewer 2 Report

The article provides an overview of the results obtained with sensor prototypes that exploit photonic and optomechanical techniques for sensing temperatures over a large temperature range. The manuscript is written well and clearly organized. The English of the paper is also OK. My only comment to this article is that it only has 31 references. In my view, this number of prior work is not sufficient for a review paper. I recommend the authors to increase the number of referecnes. 

Author Response

We thank reviewer 2 for his/her positive appreciation of our work. The reviewer suggests that we consider to publish the article as a review paper and to include more references. We however prefer to submit the paper as a regular article, and not as a review work as it presents new and original results. For a regular article, the number of references is sufficient. 

Reviewer 3 Report

Briant et al. discuss sensor prototypes using photonic and optomechanical techniques to detect temperatures over large scales. Specifically, they discuss ring resonators, silicon nitride membranes, and GaP nanobeam structures – and their utilization for measurement of temperature. The authors discuss approaches towards calibration of these sensors, and some important limitations such as optothermal self-heating. I have minor comments listed below:

• Line 255: The authors mention that the silicon nitride membrane was patterned with holes for ease of fabrication. Can the authors comment on the dimensions of these holes? (1 µm?) And whether these structural features (holes) change the substrate susceptibility and thermal sensing sensitivity.
• Line 456: “With the photonic devices a very high temperature sensitivity of 81 pm/K is demonstrated…”
  Line 457: “…while accuracy is enhanced using a gas cell to determine the laser wavelength. Photomechanical noise-thermometry on nanomechanical devices was applied to measure temperatures over the range from 5K to 300K.”
  Can the authors discuss the resolution of temperature detection? What were the smallest temperature changes reliably measured using these sensors? There can be a theoretical limit for every sensor geometry and experimental observations based on the instrumentation used by different labs.

 

Author Response

We thank reviewer 3 for her/his careful reading of our manuscript and her/his judicious comments and questions.
Concerning her/his first point on the size of holes and the resulting change in the susceptibility and thermal conductivity: the holes have a radius of 450 nm and are spaced of 1438 nm. The holes change the volume of the resonator, thus reducing its effective mass and increasing the resonance frequency. The susceptibility (equation 4 of the manuscript) is then shifted in frequency and changes in amplitude. On the contrary the effective stiffness keff (omega^2*meff=keff) is not affected by the holes. Therefore the area under the mechanical noise spectrum (equation 6 of the manuscript) and the thermal sensitivity do not change.
We have modified the manuscript accordingly :
 “As shown in Fig. 7a, the membrane is patterned with holes of 450 nm radius and 1438 nm lattice constant, to allow the release of the structure from the silicon substrate underneath. Those holes affect the resonator’s mass-per-area reducing the effective mass, increasing the resonance frequency and keeping the effective stiffness keff (omega^2*meff=keff) unchanged."
Concerning the second point:
For noise thermometry with the optomechanical devices presented in the paper, the resolution of the temperature measurement with the 2D membrane is limited to several Kelvin by the signal to noise ratio (see error bars in Fig 9c) and on the 1D ladder, that has a higher thermal resistance, the accuracy is limited by the absorption of light (self heating) that can increase its temperature by several Kelvin (see fig 12). However these limitations are of technical origin and not fundamental in nature. More engineering efforts will be needed to achieve further resolution improvements e.g. by better optical coupling, stability, higher cavity finesse and lower laser power.
We have added a sentence to the manuscript: “The resolution of the temperature measurement (error bars in Fig. 9c), is limited by experimental factors like stability, self-heating and the accuracy by which the thermomechanical noise peak can be characterized.”
For photonic thermometry:
The achievable uncertainty and the resolution strongly depends on the method used to detect the resonances in the photonic thermometer, as discussed in the last paragraph of section 2.1. With the method described in this paper (the full spectrum scan and peak fit - see Fig. 2) the resolution of the peak determination is 300 fm or equivalent to 4.1 mK (for details see Ref. [4]). The laser can be locked to the side flank of a resonance, as reported by [12,13]. In this case, the spectral noise of the laser at a slowly tracked wavelength is key to determine the temperature resolution. Values below 10 µK*Hz^(-1/2) can be achieved in the range of typical measurement times for our setup (see Ref. [4]). However, resolution was not the focus of our research in this work, rather the spectral response of the resonant structures and their optimization. Therefore, a definitive statement on the achievable resolution needs further investigations. Based on the results gathered, values in the range of 10 µK to 1 mK will provide opportunities in practical applications. Here, the resolution will be limited by the measurement equipment and no fundamental limitations.

Reviewer 4 Report

The authors present self-calibrated, embedded optomechanical sensors with high resolution and high reliability to measure temperature at the nano and meso-scales. The presented photonic integrated thermometers and optomechanical sensors with different calibration approaches may replace the standard platinum resistant thermometers. The manuscript is well-written and the results seem interesting, however, without comparison with previous studies, the readers cannot grasp the advantages/disadvantages of the discussed methods. Moreover, the authors should highlight the novelty of this work compared to previous studies. The authors should also address the following comments before publication:

1. The authors should compare their results with previous studies in a table. The advantages and disadvantages should also be highlighted.
2. Some sentences require modification while some articles and punctuations (mostly commas) are missing such as:

- from the range from

- this methods enables

- Consequently -> Consequently,

- First of all -> First of all,

- An ladder-like

Author Response

We thank reviewer 4 for her/his careful proofreading of our manuscript and her/his comments.
Concerning point 1, the field of optomechanical and photonic thermometry is emerging and relatively few results exist in literature. Our main results are uncertainty, linearity of area vs temperature, optical absorption and temperature range, poorly documented. Moreover, most of the optomechanical thermometry experiments with optomechanics aim to cool down mechanical resonators rather than develop a primary thermometer except for Purdy et al. work cited in the manuscript. There are a few papers on thermometry with SiN and GaP by measuring the shift in frequency (optical and mechanical), but they require calibration so it is also not a fair comparison, so unless there are works we miss, we don’t have sufficient relevant references to compose a table. To our knowledge the work of Purdy et al. that we cite is currently state-of-the-art, so we choose to compare our results to that work to assess its novelty.
Concerning point 2, we have implemented all the suggested corrections.

Round 2

Reviewer 2 Report

The authors have addressed my previous comments, so I suggest the publication of the work at this stage