US Patent-Real-time detection and imaging of terahertz pulse radiation by using photoacoustic conversion

United States Patent 9759689
Inventor:
Guo, Lingjie Jay (Ann Arbor, MI, US) 


http://www.freepatentsonline.com/9759689.html

Methods and devices for high speed detection of terahertz radiation are provided. A photoacoustic transducer receives a pulse of terahertz (THz) radiation. The transducer may comprise a solid, liquid, or semi-solid material. For example, the transducer may be a composite material having a polymer and radiation absorbing particles. The photoacoustic transducer produces an acoustic wave (e.g., an ultrasound wave) in response to receiving the pulse of THz radiation. An acoustic sensor receives the acoustic wave produced by the photoacoustic transducer and thus provides detection of the THz wave.

University of Michigan-Terahertz Detectors Go Handheld

R. Colin Johnson

http://www.eetimes.com/document.asp?doc_id=1322412

PORTLAND, Ore. — Today terahertz detectors are commonplace in airports, where you enter a glass-walled chamber while the detector swings around you, snooping under your clothes for weapons. Now researchers have found a way to downsize the detector portion of those machines into chip-sized devices.
These devices work by converting the elusive terahertz waves into ultrasound, which can be detected by handheld devices. Handheld terahertz detectors could help with medical diagnosis. They could even be attached to telescopes to give astronomers another tool to study planets in other solar systems.
Handheld terahertz detectors "is my hope and are quite doable. In fact, the actual device itself measures only a few millimeters in size and can be made even smaller," University of Michigan professor Jay Guo told EE Times. "With a very compact laser and photodetector, a handheld terahertz detector system is possible. The terahertz light source will be a separate component, which is not part of this work."

Jay Guo's lab produces these tiny detectors for handheld devices

that convert terahertz waves into sound.

(Source: University of Michigan)

Terahertz wave detection has puzzled engineers for more than a decade. According to Guo, his motivation was to convert the waves into something EEs already know how to detect.
"For EEs and all engineers, we knew the solutions to many solved problems already. So when facing this new and difficult problem, we wanted to see if the problem could be reduced or transformed into something for which we already had an answer," he said. "Thinking outside the box is very important for EEs making new breakthroughs."

Guo's research group has produced various lenses to convert terahertz waves into sound.

The terahertz-to-ultrasound detector was built by functionalizing polydimethylsiloxane (PDMS) with carbon nanotubes (CNTs), thus turning the polymer into a sensitive instrument. The carbon nanotubes heat up when irradiated with the terahertz frequency, causing the PDMS to expand and contract, thus producing ultrasound waves.
"The nanotubes are randomly oriented in the PDMS, which is the outcome of how we made such composite, and is preferred as it can absorb the focused THz light from all directions equally effectively," said Guo.
After the waves are converted, the ultrasound detector can produce images by scanning the output from the CNT impregnated PDMS.
"The ultrasound produced by the CNT/PDMS composite detects the amplitude of the terahertz waves. This method works for a very wide range of terahertz frequency due to the broadband absorption property of the nanotubes, all the way to visible and UV light," he said. "Images can be done by raster scanning the terahertz ray with respect to the object, thereby forming the image of the object. In the future, arrays of detectors can be made to make the imaging much faster."
Instead of using a commercial ultrasound detector, the researchers built their own micro-ring resonator, which measures just a few millimeters but can prove the concept by imaging an aluminum cross.
— R. Colin Johnson, Advanced Technology Editor, EE Time

University of Michigan-'T-Ray' Tech Converts Light to Sound for Weapons Detection, Medical Imaging

http://www.pddnet.com/news/2014/05/t-ray-tech-converts-light-sound-weapons-detection-medical-imaging

A device that essentially listens for light waves could help open up the last frontier of the electromagnetic spectrum—the terahertz range.

So-called T-rays, which are light waves too long for human eyes to see, could help airport security guards find chemical and other weapons. They might let doctors image body tissues with less damage to healthy areas. And they could give astronomers new tools to study planets in other solar systems. Those are just a few possible applications.

But because terahertz frequencies fall between the capabilities of the specialized tools presently used to detect light, engineers have yet to efficiently harness them. The U-M researchers demonstrated a unique terahertz detector and imaging system that could bridge this terahertz gap.

"We convert the T-ray light into sound," said Jay Guo, U-M professor of electrical engineering and computer science, mechanical engineering, and macromolecular science and engineering. "Our detector is sensitive, compact and works at room temperature, and we've made it using an unconventional approach."

The sound the detector makes is too high for human ears to hear.

The terahertz gap is a sliver between the microwave and infrared bands of the electromagnetic spectrum—the range of light's wavelengths and frequencies. That spectrum spans from the longest, low-energy radio waves that can carry songs to our receivers to the shortest, high-energy gamma rays that are released when nuclear bombs explode and radioactive atoms decay.

In between are the microwave frequencies that can cook food or transport cell phone signals, the infrared that enables heat vision technologies, the visible wavelengths that light and color our world, and X-rays that give doctors a window under our skin.

The terahertz band is "scientifically rich," according to Guo and colleagues. But today's detectors either are bulky, need to be kept cold to work or can't operate in real time. That limits their usefulness for applications like weapons and chemical detection and medical imaging and diagnosis, Guo says.

Guo and colleagues invented a special transducer that makes the light-to-sound conversion possible. A transducer turns one form of energy into another. In this case it turns terahertz light into ultrasound waves and then transmits them.

The transducer is made of a mixture of a spongy plastic called polydimethylsiloxane, or PDMS, and carbon nanotubes. Here's how it works:

When the terahertz light hits the transducer, the nanotubes absorb it, turning it into heat. They pass that heat on to the PDMS. The heated PDMS expands, creating an outgoing pressure wave. That's the ultrasound wave. It's more than 1,000 times too high for human ears to pick up.

"There are many ways to detect ultrasound," Guo said. "We transformed a difficult problem into a problem that's already been solved."

Though ultrasound detectors exist—including those used in medical imaging—the researchers made their own sensitive one in the form of a microscopic plastic ring known as a microring resonator. The structure measures only a few millimeters in size.

They connected their system to a computer and demonstrated that they could use it to scan and produce an image of aluminum cross.

The response speed of the new detector is a fraction of a millionth of a second, which Guo says can enable real-time terahertz imaging in many areas.

The system is different from other heat-based terahertz detection systems because it responds to the energy of individual terahertz light pulses, rather than a continuous stream of T-rays. Because of this, it isn't sensitive to variations in the outside temperature, Guo says.