Abstract-The Electrochemical Society Terahertz Spectroscopy: Studying Carrier Dynamics in Semiconductor Nanostructures

Lyubov V. Titova, Sijia Xu, Jean-Marc Baribeau, David J Lockwood,  Frank Hegmann, 

https://iopscience.iop.org/article/10.1149/06914.0051ecst/pdf

Understanding the ultrafast dynamics of photoexcited carriers in semiconductor nanostructures and their dependence on sample morphology is crucial for their incorporation into photonic devices. Time-resolved terahertz (THz) spectroscopy (TRTS) is an all-optical, contact-free technique that directly measures the transient mobile carrier dynamics and terahertz conductivity in materials over picosecond time scales, and is uniquely suited as a probe of conductivity in nanomaterials. Using low temperature MBE-grown silicon films as an example, we show how TRTS can be used to probe microscopic photoconductivity as well as obtain crucial insights into sample morphology. The thin silicon films consist of a mixture of amorphous and crystalline phases, and their relative content changes drastically with growth temperature. Photoexcited carrier dynamics in these films are determined by film crystallinity: in the amorphous phase, carriers are trapped in bandtail states on sub-picosecond time scales, while the carriers excited in crystalline grains remain free for tens of picoseconds. The complex THz conductivity spectra obtained from the TRTS measurements show that the long range conductivity is significantly higher in films grown at higher temperatures that contain a larger fraction of crystalline material with larger crystal grain sizes.

Abstract-Ultrafast terahertz control of extreme tunnel currents through single atoms on a silicon surface

• Vedran Jelic,
• Krzysztof Iwaszczuk,
• Peter H. Nguyen,
• Christopher Rathje,
• Graham J. Hornig,
• Haille M. Sharum,
• James R. Hoffman,
• Mark R. Freeman
• & Frank A. Hegmann

http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4047.html

Ultrafast control of current on the atomic scale is essential for future innovations in nanoelectronics. Extremely localized transient electric fields on the nanoscale can be achieved by coupling picosecond duration terahertz pulses to metallic nanostructures. Here, we demonstrate terahertz scanning tunnelling microscopy (THz-STM) in ultrahigh vacuum as a new platform for exploring ultrafast non-equilibrium tunnelling dynamics with atomic precision. Extreme terahertz-pulse-driven tunnel currents up to 10⁷ times larger than steady-state currents in conventional STM are used to image individual atoms on a silicon surface with 0.3 nm spatial resolution. At terahertz frequencies, the metallic-like Si(111)-(7 × 7) surface is unable to screen the electric field from the bulk, resulting in a terahertz tunnel conductance that is fundamentally different than that of the steady state. Ultrafast terahertz-induced band bending and non-equilibrium charging of surface states opens new conduction pathways to the bulk, enabling extreme transient tunnel currents to flow between the tip and sample.

New window into the nanoworld

This is Vedran Jelic, PhD student at the University of Alberta and lead author on a new paper pioneering microscopy at terahertz frequencies.CREDIT Photo by John Ulan for the University of Alberta

Scientists combine the ultra-fast with the ultra-small to pioneer microscopy at terahertz frequencies

https://www.eurekalert.org/multimedia/pub/133744.php

For the first time ever, scientists have captured images of terahertz electron dynamics of a semiconductor surface on the atomic scale. The successful experiment indicates a bright future for the new and quickly growing sub-field called terahertz scanning tunneling microscopy (THz-STM), pioneered by the University of Alberta in Canada. THz-STM allows researchers to image electron behaviour at extremely fast timescales and explore how that behaviour changes between different atoms.

"We can essentially zoom in to observe very fast processes with atomic precision and over super fast time scales," says Vedran Jelic, PhD student at the University of Alberta and lead author on the new study. "THz-STM provides us with a new window into the nanoworld, allowing us to explore ultrafast processes on the atomic scale. We're talking a picosecond, or a millionth millionth of a second. It's something that's never been done before."

Jelic and his collaborators used their scanning tunneling microscope (STM) to capture images of silicon atoms by raster scanning a very sharp tip across the surface and recording the tip height as it follows the atomic corrugations of the surface. While the original STM can measure and manipulate single atoms--for which its creators earned a Nobel Prize in 1986--it does so using wired electronics and is ultimately limited in speed and thus time resolution.

Modern lasers produce very short light pulses that can measure a whole range of ultra-fast processes, but typically over length scales limited by the wavelength of light at hundreds of nanometers. Much effort has been expended to overcome the challenges of combining ultra-fast lasers with ultra-small microscopy. The University of Alberta scientists addressed these challenges by working in a unique terahertz frequency range of the electromagnetic spectrum that allows wireless implementation. Normally the STM needs an applied voltage in order to operate, but Jelic and his collaborators are able to drive their microscope using pulses of light instead. These pulses occur over really fast timescales, which means the microscope is able to see really fast events.

By incorporating the THz-STM into an ultrahigh vacuum chamber, free from any external contamination or vibration, they are able to accurately position their tip and maintain a perfectly clean surface while imaging ultrafast dynamics of atoms on surfaces. Their next step is to collaborate with fellow material scientists and image a variety of new surfaces on the nanoscale that may one day revolutionize the speed and efficiency of current technology, ranging from solar cells to computer processing.

"Terahertz scanning tunneling microscopy is opening the door to an unexplored regime in physics," concludes Jelic, who is studying in the Ultrafast Nanotools Lab with University of Alberta professor Frank Hegmann, a world expert in ultra-fast terahertz science and nanophysics.

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Their findings, "Ultrafast terahertz control of extreme tunnel currents through single atoms on a silicon surface," appeared in the February 20 issue of Nature Physics.

Abstract-Ultrafast Terahertz Scanning Tunneling Microscopy with Atomic Resolution

Vedran Jelic, Krzysztof Iwaszczuk, Peter Nguyen, Christopher Rathje, Graham Hornig, Haille Sharum, James Hoffman, Mark Freeman, and Frank Hegmann

https://www.osapublishing.org/abstract.cfm?uri=FiO-2016-FF3F.2

We demonstrate that THz-STM can probe single atoms on a silicon surface with simultaneous sub-nm and sub-ps resolution. THz-STM is established as a new technique for exploring high-field nonequilibrium tunneling phenomena with single atom precision.

© 2016 OSA

PDF Article

Patent-Terahertz pulse radiation in treating skin disorders

USPTO Applicaton #: #20150217131 
Inventors: Lyubov Titova, Frank Hegmann, Olga Kovalchuk

http://www.freshpatents.com/-dt20150806ptan20150217131.php

Abstract: The disclosure provides methods for the treatment of skin disorders through the use of minimally invasive terahertz radiation. The method includes exposing skin cells to terahertz radiation in amount sufficient to modulate gene expression in the skin cells. The modulation of gene expression then results in a reduction of the disease state or aspects thereof in the exposed skin cells.

The Patent Description & Claims data below is from USPTO Patent Application 20150217131, Terahertz pulse radiation in treating skin disorders.

RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/936,627, filed Feb. 6, 2014, which is incorporated herein by reference.

Non-melanoma skin cancer (NMSC) and psoriasis are two skin conditions that affect a large number of individuals each year. In the United States alone, there are over 3.5 million cases of NMSC while 1-3% of the population of the United States will develop psoriasis in their lifetime.

NMSC can be primarily divided into two types of skin cancers: squamous cell carcinomas and basal cell carcinomas. Basal cell carcinomas account for 80% of all NMSC cases. This type of cancer develops in cells in the lower epidermis. Such carcinomas occur most frequently on the face, ears, neck, scalp, shoulders, and back. Basal cell carcinoma is usually slow growing and rarely metastasizes, but it can cause significant local destruction and disfigurement if neglected or treated inadequately.

The remaining 20% of NMSC is squamous cell carcinoma. Squamous cell carcinoma develops in the upper most layers of the epidermis. Of the over 700,000 new cases of squamous cell carcinomas diagnosed each year in the United States, over 2,500 will result in death.

Psoriasis is a chronic immune mediated skin disorder effecting 1-3% of the population of the United States. Psoriasis is characterized by hyperplasia of keratinocytes resulting in thickening of the epidermis and the presence of red scaly plaques. The lesions in this chronic disease typically are subject to remissions and exacerbations. Some individuals with psoriasis can also develop psoriatic arthritis and joint pain. Moreover, psoriasis can also be a psychological burden on those afflicted by the disease when the affected skin areas are visible to others.

Current treatment of NMSC generally involves surgical excision of the tumor together with a margin of normal tissue and, when surgery is not feasible or desirable, destruction of the tumor cells by ionizing radiation or other means. Treatment for psoriasis involves topical applications, systemic treatments (e.g., drug injections), and phototherapy involving exposure to UV radiation.

Although each method of treatment can be relatively effective, each has its drawback. For example, removal of a NMSC patch can leave a visible scar. On the other hand, not only is moderate to severe psoriasis resistant to topical treatments, but because of its chronic and recurrent nature, systemic therapy or radiation is often required.

Therefore, there is a need for further methods of treating skin disorders that are rapid, minimally invasive, specific for skin, and easy to use.

Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO3

Spotlight Summary by József A. Fülöp 

https://www.osapublishing.org/spotlight/summary.cfm?URI=oe-23-6-8073

Visualizing a phenomenon is often an important driving force for scientific results and new measurement techniques. It is probably the best way also for learning and understanding. The work of Frank Hegmann’s group on ultrafast imaging of terahertz (THz) waves in lithium niobate (LiNbO3) benefits both technology and understanding.

The researchers at University of Alberta in Edmonton, Canada, applied a technique developed about one and half decades ago to map the generation and propagation of THz Cherenkov waves and what they call transition-like THz radiation in LiNbO3, a nonlinear material commonly used for THz pulse generation. The technological novelty in the work is the use of phase contrast imaging to visualize the full profile of the THz Cherenkov cone in bulk LiNbO3 in a transverse imaging geometry, where an expanded optical probe (imaging) beam travels in a direction perpendicular to the optical pump pulse generating the THz radiation. The THz electric field modulates the phase of the optical probe pulse and the phase modulation is converted to amplitude modulation through Talbot imaging, where the camera is moved out of the image plane of the sample. Talbot imaging is easy to set up and yields qualitative field images.

Ultrafast phase-contrast imaging can find applications in the development and optimization of pulsed optical or THz sources. The technique enables the direct observation and visualization of various nonlinear optical interaction processes. For example, one of the most widely used methods for the generation of intense THz pulses is optical rectification of femtosecond pulses with tilted pulse front. Such sources use a combination of a diffraction grating and imaging optics to generate the pump pulse-front tilt. Imaging can introduce distortions which limit the useful pumped area and therefore the achievable THz pulse energy. Ultrafast phase-contrast imaging can help to measure and minimize such distortions by providing a direct and easy-to-interpret method, superior, for example, to measuring the output beam characteristics of the THz radiation.

Last, but not least, I would like to emphasize the educational value of the work. The scheme is very well suited for an advanced student laboratory course where a suitable femtosecond laser is available. It can help students to get acquainted with important concepts like pulse front tilt or (non-collinear) phase matching.

Abstract-Generation of terahertz radiation by optical excitation of aligned carbon nanotubes

Lyubov Titova , Cary L. Pint , Qi Zhang , Robert H. Hauge , Junichiro Kono , and Frank Hegmann

Nano Lett., Just Accepted Manuscript

DOI: 10.1021/acs.nanolett.5b00494

Publication Date (Web): April 16, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00494

We have generated coherent pulses of terahertz radiation from macroscopic arrays of aligned single-wall carbon nanotubes (SWCNTs) excited by femtosecond optical pulses without externally applied bias. The generated terahertz radiation is polarized along the SWCNT alignment direction. We propose that top-bottom asymmetry in the SWCNT arrays produces a built-in electric field in semiconducting SWCNTs, which enables generation of polarized terahertz radiation by a transient photocurrent surge directed along the nanotube axis.