Micromechanical mirror performs under pressure... of light

Apr 06, 2012 By John Lawall
Top: The gratings are fabricated with approximately 700 nm between ridges. Each has slightly different spacing and thickness, affecting reflectivity and mechanical performance. Bottom: A single grating measuring 50 micrometers on a side.

(Phys.org) -- A team of scientists from PML's Quantum Measurement Division has designed and tested a novel device that may lead to substantial progress in the new and fast-moving field of optomechanics.

That discipline studies and exploits the ways in which the feeble force of light interacts with very small mechanical objects. Or, as the team leader John Lawall says, "it involves coupling optical to in which the coupling is provided by ."

To be sure, the forces and involved are extremely small. But understanding  and measuring them is important to projects ranging in size from the lasers and mirrors at the giant Laser Interferometer Gravitational Wave Observatory to the minuscule cantilever probes used in atomic force microscopy.

A typical lab setup for this kind of work involves an optical cavity with two mirrors: a fixed mirror on one end, and a movable mirror on the other. (See schematic below.) Laser light injected into the cavity reflects back and forth, pushing on the mirrors as it goes. The resulting displacement in the movable mirror changes the distance between the mirrors, which in turn affects the resonance frequency of the cavity.

Most researchers use a moving mirror made up of 16 to 40 layers of dielectric film with different indices of refraction, culminating in a stack structure a few micrometers thick. That's not exactly huge. But "as you add layers," Lawall says, "the mass per unit area and the thickness just keep going up. And we need really low mass because the optical force is so weak."

In this schematic diagram of the optical cavity, the laser light resonates between the fixed mirror on the left and a single grating on the membrane at right. The gratings are tested individually.

So Lawall, Utku Kemiktarak, Michael Metcalfe, and Mathieu Durand of the Quantum Metrology and Processes Group decided to take a completely different approach.  Metcalfe, a former postdoc in Lawall's group, pointed out that with careful design, gratings can work as mirrors as long as the spacing between the ridges is below the wavelength of light, which in the team's case is about 1560 nm.

With colleagues at NIST's Center for Nanoscale Science and Technology, they produced a grating with a spacing of about 700 nanometers, beginning with a membrane of silicon nitride – a material known to have exceedingly low mechanical losses – and then etching it with reactive ions. (The original membrane has a reflectivity of about 27%; the final product has a reflectivity of 99.6% and has a smaller mass.)

Eventually 81 such gratings, each with slightly different finger widths and spacings, were arranged on a single membrane. In this way the researchers could experimentally determine the reflectivity corresponding to a particular grating design, and choose a grating to couple optimally to a particular mechanical mode of the membrane.

The result is a micromechanical reflector that is more than an order of magnitude less massive than conventional stack reflectors, has a mechanical quality factor (Q = 7.8 X 105) two orders of magnitude higher, and reflects 99.6 % of the incident light. "We are not the first people to use gratings as reflectors," says Kemiktarak, "but we think we are the first to study them closely for their mechanical properties as well as their optical properties."

The grating reflector is mounted at one end of a Fabry-Perot cavity which has a mirror on the other end. Then the entire device is placed in a vacuum chamber (right) and illuminated with laser light (red line) that enters the chamber through a port (inset) in the side. The output beam exits the chamber at a second port (far right) and is routed to a photo-detector. Scientists adjust the frequency of the laser beam so that it matches the cavity resonance condition at a particular time, and thus determine the performance characteristics of each individual grating.

"These devices will likely be of great importance in MEMS devices and optomechanical systems employing radiation pressure," Lawall says.

They may also be of considerable use in pursuing some of the most intriguing phenomena that occur when optomechanical systems approach quantum limits. For example, scientists are interested in preparing fabricated mechanical systems in a state of motion that is equivalent to the ground state (lowest energy level) of a single atom. That will require, among other things, elimination of virtually all the thermal noise in the system – difficult to achieve in mesoscopic conditions.

"In order to get to the point where the system is down to only a few motional quanta, as opposed to 10 million or so, what you want to do is get rid of that thermal excitation," Lawall says. The brute force way to do that is just stick it in a cryostat. But in many cases, the temperatures you need are very low. Instead, you can turn to techniques that aren't terribly different from those employed to laser-cool atoms.

"Basically, you can just take the excess thermal energy corresponding to mechanical motion and transfer it to the optical field via scattered light and therefore reduce the amount of thermal energy in the device itself."

This research was partly supported by the National Science Foundation through the Physics Frontier Center at the Joint Quantum Institute. The research was performed in part at NIST's  Center for Nanoscale Science and Technology.

Explore further: Could 'Jedi Putter' be the force golfers need?

More information: "Mechanically compliant grating reflectors for optomechanics," Utku Kemiktarak, et al. App. Phys. Lett. 100, 061124 (2012).

Related Stories

Laser light used to cool object to quantum ground state

Oct 05, 2011

For the first time, researchers at the California Institute of Technology (Caltech), in collaboration with a team from the University of Vienna, have managed to cool a miniature mechanical object to its lowest ...

Physicists cool semiconductor by laser light

Jan 22, 2012

Researchers at the Niels Bohr Institute have combined two worlds – quantum physics and nano physics, and this has led to the discovery of a new method for laser cooling semiconductor membranes. Semiconductors ...

Recommended for you

Could 'Jedi Putter' be the force golfers need?

Apr 18, 2014

Putting is arguably the most important skill in golf; in fact, it's been described as a game within a game. Now a team of Rice engineering students has devised a training putter that offers golfers audio, ...

Better thermal-imaging lens from waste sulfur

Apr 17, 2014

Sulfur left over from refining fossil fuels can be transformed into cheap, lightweight, plastic lenses for infrared devices, including night-vision goggles, a University of Arizona-led international team ...

User comments : 10

Adjust slider to filter visible comments by rank

Display comments: newest first

Burnerjack
not rated yet Apr 06, 2012
Refering to F=Ma, does this mean photons have mass?
kaasinees
3 / 5 (2) Apr 06, 2012
The notion that photons dont have mass is getting old.
Photons do not have REST mass, meaning their energy and mass is equal.
Photons are not a trans-physical particle. If they were they wouldnt interact with other mass.
Vendicar_Decarian
5 / 5 (1) Apr 06, 2012
No. It means that Newton's laws do not describe all physical phenomenon.

Einstein and others have extended those laws.

"Refering to F=Ma, does this mean photons have mass?" - Burnerjack
Vendicar_Decarian
5 / 5 (1) Apr 06, 2012
The path of photons is changed by gravity and hence they exert a gravitational force of their own. If placed inside a cavity, caged photons will appear to a gravity detector as if they were mass of equivalent energy.

This does not mean that they have mass, but does mean that the term mass loses it's meaning when closely examined.

"The notion that photons dont have mass is getting old." - Kaas
Benni
1 / 5 (1) Apr 07, 2012
Refering to F=Ma, does this mean photons have mass?


It means exactly that.....the confusuion so many have about the mass of a photon is that it can only exist at the speed of light (it can never come to "rest"), this being the case it cannot have "rest mass", so it is assigned "rest mass equivalence" and of course it must retain its' gravity component or it violates E=mc^2.

Two outstanding examples proving photons have "mass" is gravitational lensing & black hole formation, neither could otherwise occur. Black holes are "black" because of two sources of gravity acting on one another & the largest mass so overcomes the mass of the smallest source of gravity that it pulls it onto its surface, hence "black hole". Gravitational lensing is similar except the largest gravity source is not strong enough to pull the weaker gravity photon onto its' surface, therefore causing only causing it to "bend".
Tennex
1 / 5 (1) Apr 07, 2012
The special relativity deals with light spreading in form of light - it doesn't care about some quantum phenomena, photons the less. In aether physics the photons are really different artefacts from light wave, they're solitons of light wave. At the water surface you can have two kinds of solitons: the bright Russel solitons which are moving in lower speed, than the surface ripples and the dark Falaco solitons, which are moving faster. Apparently, the Russel solitons correspond the photons and the Falaco solitons could correspond the neutrinos. Just because every soliton is composed of waves interferring mutually, it cannot move as fast, as the normal harmonic wave. And because it moves slower, it could have certain minute mass. The fans of special relativity object after then: but when the photons have nonzero mass, they couldn't spread to the infinite distances, after then. But we can see the photons of most distant stars without problem..
Tennex
1 / 5 (1) Apr 07, 2012
The solution of this paradox follows from the fact, the photons which we can see in telescopes aren't the original photons emanated with distant stars. The photons undergo so-called the quantum decoherence, after all, in similar way, like the real solitons at the water surface. They're dissolving in vacuum and emerge somewhere else in the path of original photon. In this sense, the photons undergo the similar quantum oscillations, like the neutrinos. And because each individual photon cannot travel to very large distance without decay, the nonzero rest mass of photon doesn't even violate the theorem, every virtual particle should have zero rest mass, or it couldn't mediate interactions at the infinite distances. The whole problem with mass of photon in relativity is, the relativity doesn't deal with quantum phenomena, so it always considers the light as a harmonic wave of infinite scope and zero rest mass.
Tennex
1 / 5 (1) Apr 07, 2012
Because the physicists don't like the aether model, they cannot realize, that the photons are quite normal solitons, which appear at every water surface. The general relativity doesn't make such understanding more easier - on the contrary, because it's slightly biased view from human perspective and it doesn't recognize the quantum phenomena, resulting from interactions of light waves with density fluctuations of vacuum, which are manifesting like the CMBR noise. In relativity the space-time is simply locally smooth and the quantum phenomena and extradimensions have nothing to do with it there. The photons are therefore equivalent to harmonic wave in this theory and harmonic wave has no reference frame defined. From perspective of harmonic wave the time effectively stops in general relativity. You can imagine it like the disappearance of ripples from perspective of boat, which is floating with speed of surface waves at the water surface. All undulations will disappear for such boat.
Tennex
1 / 5 (2) Apr 07, 2012
The dense aether model even leads to some predictions. In my opinion, if we would use the photons of wavelength longer than the wavelenght of CMBR noise, i.e. the microwaves, these photons would behave like the tachyons - they would be dispersed with CMBR noise in the way, which would appear like expanding and popping of bubbles. Such microwaves would exert the attractive force instead of repulsive force to the mirror, but it would push the microwave fluctuation of vacuum instead. In another sense, the source of microwaves between mirrors of different surface area would behave like the reaction-less drive. We already know, that such device can exist - it's EM drive. The fact, this device wasn't even attempted to replicate with mainstream physics just illustrates the pronounced pattern of mainstream behaviour, it avoids to research every phenomena, which doesn't fit the mainstream theories well (whereas the opposite is studied the more).
antialias_physorg
5 / 5 (1) Apr 07, 2012
The dense aether model ...

Got banned again, did we Calippo, rawa, zephyr, ... ?

More news stories

Making graphene in your kitchen

Graphene has been touted as a wonder material—the world's thinnest substance, but super-strong. Now scientists say it is so easy to make you could produce some in your kitchen.