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Topic Name: Nano-sized Electronic Circuit Promises Bright View of Early Universe

Category: STAR (Space, Telecommunications & Radioscience)

Research persons: Michael Gershenson

Location: NASA’s Jet Propulsion Laboratory, Rutgers University, United States

Details

A newly developed nano-sized electronic device is an important step toward helping astronomers see invisible light dating from the creation of the universe. This invisible light makes up 98% of the light emitted since the “big bang,” and may provide insights into the earliest stages of star and galaxy formation almost 14 billion years ago.

The tiny, new circuit, developed by physicsts at Rutgers University, NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and the State University of New York at Buffalo, is 100 times smaller than the thickness of a human hair. It is sensitive to faint traces of light in the far-infrared spectrum (longest of the infrared wavelengths), well beyond the colors humans see.

“In the expanding universe, the earliest stars move away from us at a speed approaching the speed of light,” said Michael Gershenson, professor of physics at Rutgers and one of the lead investigators. “As a result, their light is strongly red-shifted when it reaches us, appearing infrared.”

Because the Earth’s atmosphere strongly absorbs far-infrared light, Earth-based radiotelescopes cannot detect the very faint light emitted by these stars. So scientists are proposing a new generation of space telescopes to gather this light. Yet to take full advantage of space-borne telescopes, detectors that capture the light will have to be far more sensitive than any that exist today.

Detectors of infrared and submillimeter waves, known as bolometers, measure the heat generated when they absorb photons, or units of light. Today’s infrared bolometer technology is mature and has reached the limit of its performance.

“The device we built, which we call a hot-electron nanobolometer, is potentially 100 times more sensitive than existing bolometers,” Gershenson said. “It is also faster to react to the light that hits it.”

The team is led by Gershenson and Boris Karasik of the Jet Propulsion Laboratory (JPL), a NASA center managed by the California Institute of Technology (CalTech). Most of the fabrication and measurement work was done at Rutgers by graduate student Jian Wei, now a post-doctoral associate at the Northwestern University; postdoctoral researcher David Olaya, now with the National Institute of Standards and Technology; and postdoctoral researcher Sergey Pereverzev, now with JPL and CalTech. The theoretical support for this research was provided by Andrei Sergeev of the State University of New York at Buffalo.

Made of titanium and niobium metals, the novel device is about 500 nanometers long and 100 nanometers wide. The physicists built it using thin-film and nanolithography techniques similar to those used in computer chip fabrication. The device operates at very cold temperatures – about 459 degrees below zero Fahrenheit, or one-tenth of one degree above absolute zero on the Kelvin scale.

Photons striking the nanodetector heat electrons in the titanium section, which is thermally isolated from the environment by superconducting niobium leads. By detecting the infinitesimal amount of heat generated in the titanium section, one can measure the light energy absorbed by the detector. The device can detect as little as a single photon of far infrared light.

“With this single detector, we have demonstrated a proof of concept,” said Gershenson. “The final goal is to build and test an array of 100 by 100 photodetectors, which is a very difficult engineering job.” Rutgers took the lead on fabrication and electrical characterization of the single detector, and JPL will take the lead on the optical characterization of the detector and developing detector arrays.

Gershenson expects the detector technology to be useful for exploring the early universe when satellite-based far-infrared telescopes start flying 10 to 20 years from now. “That will make our new technology useful for examining stars and star clusters at the farthest reaches of the universe,” he said.

Note for Infrared Spectroscopy
Infrared spectroscopy (IR spectroscopy) is the subset of spectroscopy that deals with the infrared region of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify compounds or investigate sample composition. Infrared spectroscopy correlation tables are tabulated in the literature.

The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. The far-infrared, approximately 400-10 cm-1 (1000–30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. The mid-infrared, approximately 4000-400 cm-1 (30–1.4 μm) may be used to study the fundamental vibrations and associated rotational-vibrational structure. The higher energy near-IR, approximately 14000-4000 cm-1 (1.4–0.8 μm) can excite overtone or harmonic vibrations. The names and classifications of these subregions are merely conventions. They are neither strict divisions nor based on exact molecular or electromagnetic properties.

Infrared spectroscopy exploits the fact that molecules have specific frequencies at which they rotate or vibrate corresponding to discrete energy levels. These resonant frequencies are determined by the shape of the molecular potential energy surfaces, the masses of the atoms and, by the associated vibronic coupling. In order for a vibrational mode in a molecule to be IR active, it must be associated with changes in the permanent dipole. In particular, in the Born-Oppenheimer and harmonic approximations, i.e. when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the neighborhood of the equilibrium molecular geometry, the resonant frequencies are determined by the normal modes corresponding to the molecular electronic ground state potential energy surface. Nevertheless, the resonant frequencies can be in a first approach related to the strength of the bond, and the mass of the atoms at either end of it. Thus, the frequency of the vibrations can be associated with a particular bond type.

About Radio Telescope
A radio telescope is a form of directional radio antenna used in radio astronomy and in tracking and collecting data from satellites and space probes. In their astronomical role they differ from optical telescopes in that they operate in the radio frequency portion of the electromagnetic spectrum where they can detect and collect data on radio sources. Radio telescopes are typically large parabolic ("dish") antenna used singularly or in an array. Radio observatories are located far from major centers of population in order to avoid electromagnetic interference (EMI) from radio, TV, radar, and other EMI emitting devices. This is similar to the locating of optical telescopes to avoid light pollution, with the difference being that radio observatories will be placed in valleys to further shield them from EMI as opposed to clear air mountain tops for optical observatories.

The range of frequencies in the electromagnetic spectrum that makes up the radio spectrum is very large. This means the variety and types of antennas that are used as radio telescopes vary in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10 MHz - 100 MHz), they are generally directional antenna arrays similar to "TV antennas" or large stationary reflectors with moveable focal points. Since the wave length being observed with these types of antennas are so long, the "reflector" surfaces can be constructed from coarse wire mesh. At shorter wavelengths “dish” style radio telescopes predominate. The angular resolution of a dish style antenna is a function of the diameter of the dish in proportion to the wavelength of the electromagnetic radiation being observed. This dictates the size of the dish a radio telescope needs to have a useful resolution. Radio telescopes operating at wavelengths of 3 meters to 30 cm (100 MHz to 1 GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths above 30 cm (1 GHz) range in size from 3 to 90 meters in diameter.

About Bolometer
A bolometer is a device for measuring the energy of incident electromagnetic radiation. It was invented in 1878 by the American astronomer Samuel Pierpont Langley.

It consists of an "absorber" connected to a heat sink (area of constant temperature) through an insulating link. The result is that any radiation absorbed by the absorber raises its temperature above that of the heat sink—the higher the energy absorbed, the higher the temperature will be. Temperature change can be measured directly or via an attached thermometer (composite design).

While bolometers can be used to measure radiation energy of any frequency, for most wavelength ranges there are other methods of detection that are more sensitive. However, for sub-millimetre wavelengths (from around 200 µm to 1 mm wavelength), the bolometer is the most sensitive detector for any measurement over more than a very narrow wavelength range.

Bolometers are therefore used for astronomy at these wavelengths. However, to achieve the best sensitivity, they must be cooled down to a fraction of a degree above absolute zero (typically from 50 millikelvins to 300 mK).

Bolometers are directly sensitive to the energy left inside the absorber. For this reason they can be used not only for ionizating particles and photons, but also for non-ionizing particles, for any sort of radiation and even to search for unknown forms of mass or energy (like dark matter); this lack of discrimination can also be a shortcoming. They are very slow to respond and slow to reset (i.e., return to thermal equilibrium with the environment). On the other hand, compared to more conventional particle detectors, they are extremely efficient in energy resolution and in sensitivity. They can be used to test very high radio-purity. They are also known as thermal detectors.

The term bolometer is also used in high-energy physics (particle physics) to designate an unconventional particle detector. They use the same principle described above. The bolometers are sensitive not only to light but to every form of energy.

The operating principle is similar to that of a calorimeter in thermodynamics. However, the approximations, ultra low temperature, and the different purpose of the device make the operational use rather different. In the jargon of high energy physics, these devices are not called calorimeters since this term is already used for a different type of detector.

The first bolometer used for infrared observations by Langley had a very basic design: It consisted of two platinum strips, covered with lampblack, one strip was shielded from the radiation and one exposed to it. The strips formed two branches of a wheatstone bridge which was fitted with a sensitive galvanometer and connected to a battery.

Electromagnetic radiation falling on the exposed strip would heat it, and change its resistance, the circuit thus effectively operating as a resistance temperature detector.

This instrument enabled him to feel his way thermally over the whole spectrum, noting all the chief Fraunhofer lines and bands, which were shown by sharp serrations, or more prolonged depressions of the curve which gave the emissions, and discovered the lines and bands of the invisible infra-red portion.

In figure, Physics Prof. Michael Gershenson with laboratory equipment used to fabricate ultra-sensitive, nano-sized infrared light detector.