In my view, the idea that optical PoW will reduce the energy cost of PoW mining is only temporary, because the Capex aspect of photonic miners is likely to be only a temporary competitive advantage - just like the Capex advantage of building ASIC miners is only a temporary, until each newer and more efficient ASIC is rolled out.
Photonic technology is new and not as readily available - for now.
If it turns out to be a way of minting money, demand will rise, and the competitive process of PoW will surely result in every photonic-miner owner trying to build a bigger, faster farm, just like all the others, and then we're right back to energy.
The ratio of HW cost to energy use is different for each computing platform and depends on algorithm etc. For example, it seems Cuckoo Cycle PoW has somewhat higher capex/opex in ASICs because of the heavy memory use.
For photonics the ratio changes significantly due to fundamental physical properties of analog computation, photons vs. electrons and so on.
Anyhow the process of the Si Photonics (and memristors/other analog approaches to AI) maturing to the same degree as digital ASICs will likely be long and gradual so this isn't a practical issue for next decade.
I had high hopes for Cuckoo Cycle PoW, but it turns it cannot be scaled linearly, or even at all predictably. So unless the consensus is to regularly tweak the algorithms pretty heavily, it runs the risk of reaching a situation in the future where the currently immense memory requirements being a piece of cake.
It will take many years for 1GB of on-chip cache to be common, but when that happens, hopefully commodity computing devices can support somewhat efficient Cuckoo mining...
Photonic technology is new and not as readily available - for now.
If it turns out to be a way of minting money, demand will rise, and the competitive process of PoW will surely result in every photonic-miner owner trying to build a bigger, faster farm, just like all the others, and then we're right back to energy.