Friday, April 3, 2020

Making the invisible visible

Entangled photons can be used to improve imaging and measurement techniques. A team of researchers from the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena has developed a quantum imaging solution that can facilitate highly detailed insights into tissue samples using extreme spectral ranges and less light.

© Fraunhofer IOF
Quantum imaging setup for the microscopic examination of cancer cells.

While optical analysis techniques such as microscopy and spectroscopy are extremely efficient in visible wavelength ranges, they quickly reach their limits in the infrared or terahertz range. That, however, is precisely where valuable information is hidden. For example, bio-substances such as proteins, lipids and other biochemical components can be distinguished based on their characteristic molecular vibrations. These vibrations are stimulated by light in the mid-infrared to terahertz range and are very difficult to detect with conventional measurement techniques. “If these motions could be captured or induced, it would be possible to see exactly how certain proteins, lipids and other substances are distributed in cell samples. For example, some types of cancer have a characteristic concentration or expression of certain proteins. This would mean that the disease could be detected and treated more efficiently. More precise knowledge of the distribution of bio-substances could bring major advances in drug research, as well,” says quantum researcher Dr. Markus Gräfe from Fraunhofer IOF.

Entangled photons – twins yet different

But how can information from these extreme wavelength ranges be made visible? The quantum mechanical effect of photon entanglement is helping the researchers allowing them to harness twin beams of light with different wavelengths. In an interferometric setup, a laser beam is sent through a nonlinear crystal in which it generates two entangled light beams. These two beams can have very different wavelengths depending on the crystal’s properties, but they are still connected to each other due to their entanglement.
“So now, while one photon beam in the invisible infrared range is sent to the object for illumination and interaction, its twin beam in the visible spectrum is captured by a camera. Since the entangled light particles carry the same information, an image is generated even though the light that reaches the camera never interacted with the actual object,” explains Gräfe. The visible twin essentially provides insight into what is happening with the invisible twin.
The same principle can also be used in the ultraviolet spectral range: UV light easily damages cells, so living samples are extremely sensitive to that light. This significantly limits the time available for investigating, for instance, cell processes that last several hours or more. Since less light and smaller doses of radiation penetrate tissue cells during quantum imaging, they can be observed and analyzed at high resolution for longer periods without destroying them.

Small assembly and tiny structures

“We are able to demonstrate that the entire complex process can be carried out in a way that is robust, compact and portable,” says Gräfe. The researchers are currently working to make the system even more compact, shrinking it to the size of a shoebox, and to further enhance its resolution. The next step they hope to achieve is, for example, a quantum scanning microscope. Instead of the image being captured with a wide-field camera, it will be scanned, similar to a laser-scanning microscope. The researchers expect this to yield even higher resolutions of less than one micrometer (1 µm), enabling the examination of structures within individual cells in even greater detail. On average, one cell measures roughly ten micrometers in size. In the long term, they want to see quantum imaging integrated into existing microscopy systems as a basic technology, thus lowering the barriers for industry users.
The demonstrator is one of the results of the Fraunhofer lighthouse project QUILT, which pools the quantum optics expertise of the Fraunhofer Institutes for Applied Optics and Precision Engineering IOF, for Physical Measurement Techniques IPM, for Microelectronic Circuits and Systems IMS, for Industrial Mathematics ITWM, of Optronics, System Technologies and Image Expoitation IOSB and for Laser Technology ILT.

Thursday, April 2, 2020

Abstract-Design of dual-band polarization controllable metamaterial absorber at terahertz frequency

Ben-Xin Wang, Yuanhao He, Nianxi Xu, Xiaoyi Wang, Yanchao Wang, Jianjun Cao

Fig. 1. (a) Side view of dual-band polarization controllable absorber; (b) Top view of…

Dual-band polarization controllable terahertz metamaterial absorber consisting of two horizontal metallic strips and two vertically connected metallic strips is demonstrated. Due to different strip lengths in the two orthogonal directions, two near-perfect absorption peaks are firstly obtained when the incident beam electric field is in the horizontal direction, while two new peaks are next realized when the electric field is selected along the vertical direction. The near-field distributions in two specific directions are provided to investigate the mechanism of polarization controllable dual-band absorption. Our research should have broad application prospects in the selection, control and utilization of polarization-based devices.

Abstract-Exploration of terahertz from time-resolved ultrafast spectroscopy in single-crystal Bi2Se3 topological insulator

Prince SharmaMahesh KumarV.P.S. Awana (CSIR-NPL, India)
In this article, we reconnoiter the differential reflection signal of a Bi2Se3 single crystal flake, using ultrafast transient absorption spectroscopy in the femtosecond time domain and thereby explore the experimental data in terms of terahertz frequency generated in the sample. An exfoliated flake of a well characterized self-flux grown bulk Bi2Se3 single crystal having rhombohedral structure and layered morphology is used in the present study. The kinetic profile of the same being generated through a reflection signal by a pump laser of 650 nm at an average power of 0.5 mW is studied utilizing time-resolved ultrafast technique. The silhouette as a function of probe delay predicting the capability of the terahertz generation is estimated. Here, two methods FFT (fast Fourier transformation) and FFD (filtering high-frequency component followed by fitting data) are performed to estimate the value of terahertz generated in the system. While comparing the two (FFT & FFD) it is found that a large amount of magnitude difference occurs in the prediction of terahertz frequency. Summarily, we not only report the generation of terahertz in Bi2Se3 flake, also but points out that the exact order of magnitude and the capability of the same depends upon the method of analysis. It is important to extract the vibration signal from the background one so that to find the exact order of magnitude and capability of terahertz generation by any quantum material.

Wednesday, April 1, 2020

Abstract-Terahertz three-dimensional monitoring of nanoparticle-assisted laser tissue soldering

Junliang Dong, Holger Breitenborn, Riccardo Piccoli, Lucas V. Besteiro, Pei You, Diego Caraffini, Zhiming M. Wang, Alexander O. Govorov, Rafik Naccache, Fiorenzo Vetrone, Luca Razzari, and Roberto Morandotti

Schematic showing the range of simultaneous photothermal reactions during nanoparticle-assisted laser tissue soldering.

In view of minimally-invasive clinical interventions, laser tissue soldering assisted by plasmonic nanoparticles is emerging as an appealing concept in surgical medicine, holding the promise of surgeries without sutures. Rigorous monitoring of the plasmonically-heated solder and the underlying tissue is crucial for optimizing the soldering bonding strength and minimizing the photothermal damage. To this end, we propose a non-invasive, non-contact, and non-ionizing modality for monitoring nanoparticle-assisted laser-tissue interaction and visualizing the localized photothermal damage, by taking advantage of the unique sensitivity of terahertz radiation to the hydration level of biological tissue. We demonstrate that terahertz radiation can be employed as a versatile tool to reveal the thermally-affected evolution in tissue, and to quantitatively characterize the photothermal damage induced by nanoparticle-assisted laser tissue soldering in three dimensions. Our approach can be easily extended and applied across a broad range of clinical applications involving laser-tissue interaction, such as laser ablation and photothermal therapies.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

MIT researchers use graphene and boron nitride to convert terahertz waves to usable energy

Researchers at MIT are working to develop a graphene-based device that may be able to convert ambient terahertz waves into a direct current. The MIT team explains that any device that sends out a Wi-Fi signal also emits terahertz waves —electromagnetic waves with a frequency somewhere between microwaves and infrared light. These high-frequency radiation waves, known as “T-rays,” are also produced by almost anything that registers a temperature, including our own bodies and the inanimate objects around us.

Terahertz waves are pervasive in our daily lives, and if harnessed, their concentrated power could potentially serve as an alternate energy source. Imagine, for instance, a cellphone add-on that passively soaks up ambient T-rays and uses their energy to charge your phone. However, to date, terahertz waves are wasted energy, as there has been no practical way to capture and convert them into any usable form. This is exactly what the MIT scientists set out to do.
Their design takes advantage of the quantum mechanical, or atomic behavior of graphene. They found that by combining graphene with another material, in this case, boron nitride, the electrons in graphene should skew their motion toward a common direction. Any incoming terahertz waves should “shuttle” graphene’s electrons, like so many tiny air traffic controllers, to flow through the material in a single direction, as a direct current.
“We are surrounded by electromagnetic waves in the terahertz range,” says lead author Hiroki Isobe, a postdoc in MIT’s Materials Research Laboratory. “If we can convert that energy into an energy source we can use for daily life, that would help to address the energy challenges we are facing right now.”
Over the last decade, scientists have looked for ways to harvest and convert ambient energy into usable electrical energy. They have done so mainly through rectifiers, devices that are designed to convert electromagnetic waves from their oscillating (alternating) current to direct current.

Most rectifiers are designed to convert low-frequency waves such as radio waves, using an electrical circuit with diodes to generate an electric field that can steer radio waves through the device as a DC current. These rectifiers only work up to a certain frequency, and have not been able to accommodate the terahertz range.
A few experimental technologies that have been able to convert terahertz waves into DC current do so only at ultracold temperatures — setups that would be difficult to implement in practical applications.
Instead of turning electromagnetic waves into a DC current by applying an external electric field in a device, Isobe wondered whether, at a quantum mechanical level, a material’s own electrons could be induced to flow in one direction, in order to steer incoming terahertz waves into a DC current.
Such a material would have to be very clean, or free of impurities, in order for the electrons in the material to flow through without scattering off irregularities in the material. Graphene, he found, was the ideal starting material.
To direct graphene’s electrons to flow in one direction, he would have to break the material’s inherent symmetry, or what physicists call “inversion.” Normally, graphene’s electrons feel an equal force between them, meaning that any incoming energy would scatter the electrons in all directions, symmetrically. Isobe looked for ways to break graphene’s inversion and induce an asymmetric flow of electrons in response to incoming energy.
Looking through the literature, he found that others had experimented with graphene by placing it atop a layer of boron nitride, a similar honeycomb lattice made of two types of atoms — boron and nitrogen. They found that in this arrangement, the forces between graphene’s electrons were knocked out of balance: Electrons closer to boron felt a certain force while electrons closer to nitrogen experienced a different pull. The overall effect was what physicists call “skew scattering,” in which clouds of electrons skew their motion in one direction.
Isobe developed a systematic theoretical study of all the ways electrons in graphene might scatter in combination with an underlying substrate such as boron nitride, and how this electron scattering would affect any incoming electromagnetic waves, particularly in the terahertz frequency range.
He found that electrons were driven by incoming terahertz waves to skew in one direction, and this skew motion generates a DC current, if graphene were relatively pure. If too many impurities did exist in graphene, they would act as obstacles in the path of electron clouds, causing these clouds to scatter in all directions, rather than moving as one.
“With many impurities, this skewed motion just ends up oscillating, and any incoming terahertz energy is lost through this oscillation,” Isobe explains. “So we want a clean sample to effectively get a skewed motion.”
They also found that the stronger the incoming terahertz energy, the more of that energy a device can convert to DC current. This means that any device that converts T-rays should also include a way to concentrate those waves before they enter the device.
With all this in mind, the researchers drew up a blueprint for a terahertz rectifier that consists of a small square of graphene that sits atop a layer of boron nitride and is sandwiched within an antenna that would collect and concentrate ambient terahertz radiation, boosting its signal enough to convert it into a DC current.
“This would work very much like a solar cell, except for a different frequency range, to passively collect and convert ambient energy,” Fu says.
The team has filed a patent for the new “high-frequency rectification” design, and the researchers are working with experimental physicists at MIT to develop a physical device based on their design, which should be able to work at room temperature, versus the ultracold temperatures required for previous terahertz rectifiers and detectors.
“If a device works at room temperature, we can use it for many portable applications,” Isobe says.
He envisions that, in the near future, terahertz rectifiers may be used, for instance, to wirelessly power implants in a patient’s body, without requiring surgery to change an implant’s batteries. Such devices could also convert ambient Wi-Fi signals to charge up personal electronics such as laptops and cellphones.
“We are taking a quantum material with some asymmetry at the atomic scale, that can now be utilized, which opens up a lot of possibilities,” Fu says.

Tuesday, March 31, 2020

Abstract-Terahertz luminescence and photoconductivity associated with the impurity electron transitions in GaAs/AlGaAs quantum wells

The photoconductivity and photoluminescence spectra of GaAs/AlGaAs quantum wells doped with shallow donors are studied at low lattice temperatures. The optical electron transitions between the first electron subband and donor ground states, as well as between the excited and ground donor states, are revealed in the terahertz photoluminescence and photoconductivity spectra. The temperature evolution of the impurity-related photocurrent in the terahertz spectral range is also studied.