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led the work with Meyhofer. “The LED, with this reverse bias trick, behaves as if it were at a lower tem- perature.
Measuring this cooling, and
proving that anything interesting has occurred, is considerably compli- cated. To get enough infrared light to flow from an object into the LED, the
two would have to be extremely close together, less than a single wave- length of infrared light. This is neces- sary to take advantage of near-field or evanescent coupling effects, which enable more infrared photons to cross from the object to be cooled into the LED. Reddy and Meyhofer had an
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advantage, as they had already been heating and cooling nanoscale devices, arranging them so that they were only a few tens of nanometers apart. At this close proximity, a pho- ton, that would not have escaped the object to be cooled, can pass into the LED, almost as if the gap between them did not exist. The group proved the principle
by building a miniscule calorimeter, which is a device that measures changes in energy, and putting it next to a tiny LED about the size of a grain of rice. They were constantly emitting and receiving thermal pho- tons from each other and elsewhere in their environments. “Any object that is at room tem-
perature is emitting light,” Mayhofer says. “For example, a night vision camera is basically capturing the infrared light that is coming from a warm body.” Once the LED was reverse-
biased, it began acting as a very low temperature object, absorbing pho- tons from the calorimeter. At the same time, the gap prevents heat from traveling back into the calorimeter by conduction, resulting in a cooling effect. The team demonstrated cooling
of 6 watts per square meter. Theo - retically, this effect could product cooling equivalent to 1,000 watts per square meter, or about the power of sunshine on the Earth’s surface.
Engineering a Solution for the Lab
After the construction phase of
the Center of Excellence in NAMSE was completed, a vibration survey was done on the ultra-low vibration lab chambers. The measurements demonstrated that even when a sin- gle vehicle was driving on a nearby street, the vibrations exceeded the NIST-A specifications necessary for the ULVL.
In order for the ULVL to be suc-
cessful it was required to have a vibration criterion (VC) of NIST-A (1 micro-inch, 1 to 20 Hz; 125 micro- inch between 20 and 100 Hz). The VC criteria were developed in the early 1980s by Eric Ungar and Colin Gordon. They were originally devel- oped for use in the semiconductor industry, but have found application in a wide variety of technological applications. The NIST-A criterion was developed for metrology, but has gained popularity within the nan- otechnology community. The
recommendations to
achieve the required specifications included controlling traffic on nearby streets in direct proximity of the new building addition, as well as upgrad- ing the planned pneumatic vibration isolators on optical tables with nega- tive-stiffness isolators, designed and manufactured by Minus K Tech - nology. The University of Michigan
ordered seven customized tabletops and 31 custom Minus K negative- stiffness vibration isolators with pedestals provided for the eight ULVL chambers. Negative-stiffness isolators em -
ploy a unique and completely mechanical concept in low-frequency vibration isolation. They do not require electricity or compressed air. There are no motors, pumps, or chambers, and no maintenance, because there is nothing to wear out. They operate purely in a passive mechanical mode. “In this vibration isolation sys-
tem, vertical-motion isolation is pro- vided by a stiff spring that supports a weight load, combined with a nega- tive-stiffness mechanism,” explains Dr. David Platus, president of Minus K Technology. “The net vertical stiff- ness is made very low, without affect- ing the static load-supporting capa- bility of the spring. Beam-columns connected in series with the vertical motion isolator provide horizontal motion isolation. A beam-column behaves as a spring combined with a negative-stiffness mechanism. The isolator provides 0.5 Hz iso-
lation performance vertically and horizontally. Note that for an isola- tion system with a 0.5 Hz natural frequency, isolation begins at about 0.7 Hz and improves with increase in the vibration frequency. The natural frequency is more commonly used to describe the system performance. Negative-stiffness isolators res-
onate at 0.5 Hz. At this frequency there is almost no energy present. It would be very unusual to find a signif- icant vibration at 0.5 Hz. Vibrations with frequencies above 0.7 Hz (where negative-stiffness isolators begin iso- lating) are rapidly attenuated with increase in frequency. Transmissibility with negative-
stiffness isolators is substantially improved over
air systems.
Transmissibility is a measure of the vibrations that are transmitted through the isolator relative to the input vibrations. The negative-stiffness isolators,
when adjusted to 0.5 Hz, achieve 93 percent isolation efficiency at 2 Hz; 99 percent at 5 Hz; and 99.7 percent at 10 Hz. Negative-stiffness isolators deliver very high performance, as measured by a transmissibility curve.
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