Peer review of: Light phase detection with on-chip petahertz electronic networks (2020)

<b>REVIEWER COMMENTS</B> 

Reviewer #1 (Remarks to the Author): 

Yang et al. present an experimental study of a nano-optical device capable of detecting the carrier-
envelope phase (CEP) of ultrashort laser pulses. When an ultrashort pulse irradiates a bow-tie 
antenna structure, the resulting photocurrent crossing the vacuum gap between two opposing 
nanoantennas is strongly sensitive to the waveform of the pulse. A small CEP-sensitive current 
results, which can be detected using a lock-in scheme. Using on-chip arrays of bow-tie antennas, 
the authors demonstrate that a sizable CEP-sensitive current results, up to an order of magnitude 
higher than a similar demonstration from the same research group (Putnam et al., 2017). 

The manuscript is well-written and technically sound. Its conclusions are based on strong 
experimental evidence. In particular, I commend the authors for presenting a very careful study, 
also discussing results, reasons and solutions when the device doesn’t operate at its best. Up-
scaling of nano-circuitry is challenging, and the authors describe many aspects of this challenge 
and how they were able overcome it, with several pathways towards improving the device 
performance in the future. The manuscript definitely deserves publication. It also shows novelty 
with respect to prior work: 

* Enhancement of the CEP-sensitive current by one order of magnitude. 
* Design and fabrication of a large array device. 
* Practical pathway to circum-navigate fabrication errors. 
* Assessment of laser-induced damage and its impact on device performance. 
* First discussion of noise characteristics in such a device, to the best of my knowledge. 

In my opinion, the novelty meets the readership and quality level of Nature Communications, so I 
recommend the publication of this manuscript. However, two minor things should be discussed 
explicitly In the manuscript: 

* The device does not measure the CEP of the laser pulses directly, but only up to an 
(experimentally unknown) value. Indeed, there is nothing like an absolute phase of a laser pulse, 
but a CEP can be defined locally for a point in space. This should be clarified in the text. 
* From the simulations of the plasmonic field, which CEP shift would be expected between the 
plasmonic field (in the presence of the nanostructure) and the incident field (around the same 
point of space, but in the absence of the nanostructure)? 

I also recommend to include the following (new) references related to the work: 
* Zimmermann et al., Nano Lett. 19, 1172 (2019) 
* Sederberg et al., Nature Communications 11, 430 (2020) 
* Ludwig et al., Nature Physics doi: 10.1038/s41567-019-0745-8 (2020) 
* Kubullek et al., Optica 7, 35 (2020) 



Reviewer #2 (Remarks to the Author): 

In this manuscript, the authors demonstrate CEP detection using optical-field-driven electron 
emission in an array of plasmonic nanoantennas. Compared with the authors’ previous report of 
Ref.15, the authors integrate plasmonic nanoantennas monolithically. This integration is essential 
for improving the overall SNR. In fact, they operate the nanoantenna devices near their shot-noise 
limit. The direct detection of the light phase with on-chip device has tremendous importance for 
future PHz electronics as well as CEP detection. In addition, the contents are highly innovative. 
Therefore the reviewer thinks that this paper may be accepted for publication after some revisions. 
Please refer to the comments shown below. 

1) The authors should show the power spectrum of the incident pulse in Fig.1c or Fig.1e, to 
compare it with the antenna resonances. 
2) The authors show in Fig.1e that the field enhancement spectrum differs significantly depending 
on the connecting wire position X_{wire} and that highest field-enhancement is obtained by 
placing the connecting wires near the center of the nanotriangles. The authors should explicitly 
write X_{wire} used for the successive experiments shown in Figs 2-5. 
3) In page 9, the second paragraph, the authors write that there is the phase variation of 42 
degree of the CEP-sensitive response across the entire array. It would be much informative if the 
authors could explain why there exist such phase variation. 
4) In page 9, the second paragraph, the authors also write “one could scale the CEP-sensitive 
current by area with 80% efficiency.” The authors should explain how they deduce this conclusion 
logically. Why is the efficiency 80%? 
5) Are the experimental data shown in Figs. 5a-d taken after the nanoantenna devices are 
degraded or before degraded?