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I am looking for the material with the lowest possible refractive index. Googling did not help much, the best I found is this article suggesting that n can be as low as 1.39, but not giving any reference which material this refers to. Other than that, the lowest numbers I found are around 1.5 for fused silica and 1.31 for ice (which is lower than the 1.39, making me question this number).

To be precise, by "solid" I mean any material that can form stable, micron sized lumps. Not meta-materials which can only exist in thin layers or the like. (If the lowest n materials are very exotic, I would also be interested in the lowest n non-exotic ones.)

Is there anything better than fused silica and ice?

To be more specific, I am interested in the Casimir force between two spheres, as described for instance in dx.doi.org/10.1103/PhysRevLett.99.170403. As far as I understand, the leading order contribution comes from large wavelengths. Rephrasing my question: given two spheres of radius 1 micron in distance L (say 3 micron), for which material is the Casimir force between them the smallest?

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    $\begingroup$ which wavelength? Refractive index is highly dependent on wavelength, so even the numbers you gave are for a single wavelength. Check most fluorides: calcium fluoride, sodium fluoride, barium fluoride, etc. this website can help you $\endgroup$ Commented Jun 16, 2020 at 11:08
  • $\begingroup$ I still do not like the question, it premises as deep enough by mentioning meta-materials but then fails to get basic refractive index concepts correctly. What is your goal (i.e. why do you need a low refractive index material)? $\endgroup$ Commented Jun 16, 2020 at 11:14
  • $\begingroup$ Sorry, fair point. My expertise is in fact as far removed from solid state physics and optics as it could possibly be, apologies for not giving more details. What I am interested in is the Casimir force (as in dx.doi.org/10.1103/PhysRevLett.99.170403, specifically the polarizability in eq. (12) of that paper). My understanding of this is still very rudimentary, but as far as I get it, the leading order contribution comes from low frequencies - which as a theoretician I interpret as $\lambda \to \infty$, not sure what that means in practical terms. $\endgroup$
    – André
    Commented Jun 17, 2020 at 20:02
  • $\begingroup$ Ok, my quantum mechanics is long gone, I must say I got nothing from that. So, should this $\lambda\rightarrow\infty$ then be why you want a low refractive index material? But then you need to go <1. In that sense most metals have refractive indices close to 0, see silver for example. What happes is that these materials just display an evanescent wave which is a negative exponential, so the phase velocity for that is almost nothing, and lambda increases drastically. $\endgroup$ Commented Jun 18, 2020 at 0:19
  • $\begingroup$ Metals are a special case and are relatively easy to understand, effectively one can take the limit $\epsilon \to \infty$ and $\mu \to 0$ there, and the precise value of $n$ does not matter. What I need is a dielectric with $n$ as close to 1 as possible. After some more reading, I found the Sellmeier equation, which tells me that $n(\lambda)$ converges to the sum of the Sellmeier $B_i$ coefficients in the limit $\lambda \to \infty$. So essentially, I am looking for the material for which $\sum_i B_i$ takes the smalles value. $\endgroup$
    – André
    Commented Jun 19, 2020 at 13:50

2 Answers 2

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If dielectric films are included, North Carolina State University researchers announced "a film made of aluminum oxide that has a refractive index as low as 1.025 but that is mechanically stiff" on http://news.ncsu.edu/2015/10/chang-refractive-2015/

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I have searched this for other reasons.

My notes ( and brain) are rather scattered. I will do my best to massage the correct neurons.

Crystals. Fused Silica, amorphous as you indicated. But crystal quartz is interesting too. Several crystalline forms? As i recall? Alpha beta gamma? Maybe?

Careful with crystals. Birefringence and polarization. LiF, CaF2 and MgF2 are interesting materials and utilized in high-end optics, apochromat and syperachromat color correction lenses. Aerial and space type stuff. WWII ( some pre war) Harvard optical lab and MIT "crystal growth" lab research. Stockbarger process. And a few other. Zone re- melting. Etc. Schumann circa 1880s 1890s original fluorite for UV transparent optics / astronomy.

Brand name Cytop. A fluoropolymer. Asashi? Ahashi? Glass and chemical Corp?

Brand name perfluoro-x-ane?. Ummm? Fluorinert. By 3M. A liquid. Sorry you want solid. But I might use thus for my thing.

Also Cargille optical liquids. A whole catalog of specially blended calibration liquids. A great resource. Probably downloadable.

Lately I have seen sone interesting glasses on a glass map. Not your traditional "Schott" only map. I am sorting through them today. Hence my finding your post.

Sub 1.5 and maybe sub 1.4 refractive index are supplied by the following companies : Hoya, Schott, Sumita, CDGM, Ohara.

I have only done CDGM today. Much study / collection / collation still needed.

1.4565, 1.47047, 1.48656 ; H-FK71, H-QK1, and H-FK2 respectively.

Hopefully these hints will help you.

Good Luck !!!!

Happy hunting !!!!

P.S.

Sigma Aldrich / Alfa Aesar/ other chemistry catalog???

I found MANY liquid polymers? Possibly plastics? With refractive in the low region. Fluoropolymers mostly. As i recall? Also note PTFE is a candidate.

Consider these sources too.

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  • $\begingroup$ For companies selling fluoride crystal ( Mg / Li / Ca) try crystran UK, Almaz US, ISL ( NY ? Or ISP ? Optics ? I forget). I ended up buying from Russian supplier. I forget name. 10 $ crystal. 90$ shipping. US supply is flipped prices. So you cant win. Edmund optics will supply these things too. $\endgroup$ Commented Jun 20, 2022 at 22:46
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    $\begingroup$ I get that your notes are quite scattered, but I think that this answer could benefit a lot from a more coherent exposition, as opposite to a "stream of consciousness". Could you edit it to make it more easy to follow? $\endgroup$
    – Prallax
    Commented Jun 21, 2022 at 9:46
  • $\begingroup$ 2 answer Prallax, & restore the deleted / moved message that I'd posted ; I offered what I had at the moment, in the spirit of answering the ?. Perhaps I should explain my need & utility, & it will give form 2 the above answer. The human cornea has a refr. ind. hovering around 1.38 or 1.40, & sim. vals. for the human lens. I assume the orig. post-er did not want to go grave rob? But my search has always centered around this range of refr. ind. I forgot to mention the many liquids in this region, but the reference 2 the industrial sci.ic supplier "Cargille Opt. Liq.", ... ... 2B cont. $\endgroup$ Commented Dec 14, 2023 at 2:02
  • $\begingroup$ continued ... despite their prob. proprietary & trade-secret formulae, are prob. simp. hyrdorcarb. liq. in mixture & able 2B guessed, perhaps, by their MSDS sheets. I also offered good optical glass catalogs, whose glass offerings extend below the range of normal 18th & 19th century glasses used for that purpose, i.e. below the 1.5 region. Anything in 1.4 or 1.3 is considered low, IMO. It is quite possible, or probable, that metamaterials & other physics oddities exist in the lab, but I was focused on ofering my expertise as an optics consumer. Deleted / moved mesg : 2B cont ... $\endgroup$ Commented Dec 14, 2023 at 2:09
  • $\begingroup$ For companies selling fluoride crystal ( Mg / Li / Ca) try crystran UK, Almaz US, ISL ( NY ? Or ISP ? Optics ? I forget). I ended up buying from Russian supplier. I forget name. 10 crystal.90 shipping. US supply is flipped prices. So you cant win. Edmund optics will supply these things too. P.S. Carbon Disulfide is a liq. of low index. Ethanol & isoprop. 2. Liquids in general. But I think water is always lower. Cryolite is a natural gem / mineral of low refr. ind., utilized in low-index-high-index "stacks" of thin layers for dichroic mirrors. refractiveindex.info often useful $\endgroup$ Commented Dec 14, 2023 at 2:14

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