Lobachevsky University Gets a Patent for an Original Optical-to-Terahertz Converter

https://qswownews.com/lobachevsky-university-gets-a-patent-for-an-original-optical-to-terahertz-converter/

Lobachevsky University has developed an optical -to-terahertz converter that has successfully received a patent.

Terahertz radiation, which occupies a large portion of the electromagnetic spectrum between infrared light and microwaves, has a number of unusual properties. It can penetrate  opaque materials and selectively interact with substances, thus allowing their identification, and it can even accelerate elementary particles.

The prospects for scientific and practical applications of terahertz radiation are quite tempting, and scientists around the world work hard to create effective sources of this type of radiation. The problem is that terahertz radiation cannot be generated either as light, by using laser methods, or as microwaves, by applying electronic methods. Therefore, special methods have to be invented specifically for this type of radiation.

One such method is optical rectification of ultra-short (femtosecond) laser pulses in a lithium niobate crystal. Due to its specific properties, the crystal extracts from the laser pulse its envelope, as if clearing it of optical field oscillations. The resulting envelope is the source of terahertz radiation.

The optical-to-terahertz transducer developed at Lobachevsky University comprises a sandwich structure in the form of a thin (30-50 microns thick) layer of lithium niobate placed between two silicon prisms. The laser pulse propagates in the crystal as in a waveguide and generates terahertz radiation, which is emitted from it in both directions. Upon total internal reflection in the prisms, radiation emerges into free space as two adjacent beams forming in fact one wide beam.

The authors of the invention are Professor Mikhail Bakunov, Head of the General Physics Department at the UNN Faculty of Radiophysics, and Sergei Sychugin,  Junior Researcher of the same department.

“Using the original solution with a symmetrical output of radiation from the crystal in opposite directions and the subsequent formation of a single terahertz beam due to the effect of total reflection in prisms, we have been able to overcome spectrum notching in the radiation generated, which was a fundamental disadvantage of previous converters of this type. Our experiments have also shown high efficiency of this converter and good quality of the terahertz beam”, Mikhail Bakunov comments.

During the experiments with the new sandwich structure, the radiation of a titanium sapphire laser with a pulse energy of 10 µJ was converted to terahertz radiation with a spectral width of more than 3 THz and an efficiency of 0.35%. The results of the study were published in the journal Optics Letters.

Lobachevsky State University of Nizhni Novgorod  is participating in Project 5-100 aimed to maximize the competitive position of a group of leading Russian universities in the global research and education market.

New data on ultrafast electron photoemission from metallic nanostructures obtained

https://phys.org/news/2019-05-ultrafast-electron-photoemission-metallic-nanostructures.html

by Lobachevsky University

The results of a Russian-Japanese experiment explain the mechanism of electron photoemission by metallic nanostructures under ultrafast laser excitation. Metallic nanoparticle ensembles are capable of emitting short bunches of electrons when irradiated by powerful laser pulses of femtosecond (1 fs = 10-15 s) duration. Scientists at Lobachevsky University have long studied the plasmon effect—the excitation by light of collective electron oscillations in nanoparticles and the amplification of the light field associated with these oscillations in the vicinity of the nanoparticle, which plays the main role in this process. It is the plasmon amplification of the field that provides effective photoemission of electrons from a metal.

The prospects for practical application of plasmon nanostructures are associated with their use as ultrafast photocathodes to create pulsed sources of high-brightness coherent X-ray radiation and to produce microscopes with high temporal resolution.

The photoemission of electrons from metallic nanoparticles is accompanied by the emission of terahertz radiation (its range in the scale of electromagnetic waves is between light and microwaves), which makes it possible to use this radiation as a tool for studying photoemission.

"The intensity of terahertz radiation depends non-linearly on the intensity of the laser pulse and demonstrates a high nonlinearity order (from 3 to 6 in various experiments). Although the mechanism of terahertz radiation generation by photoelectrons is not fully understood, it is believed that the high order of nonlinearity is explained by the multi-photon nature of electron emission, that is, by the need to transfer energy from several laser photons to the electron for performing the work to release the electron from the metal," explains Michael Bakunov, Head of the General Physics Department at Lobachevsky University.

To test the hypothesis of a multi-photon photoemission mechanism, scientists from Lobachevsky University together with their Japanese colleagues from Shinshu University, Osaka University and Tokyo Institute of Technology conducted an experiment in which the same metallic nanostructure, an array of gold nanorods ("golden nanoforest") was irradiated with powerful ultrashort light pulses of various wavelengths—from 600 nm to 1500 nm.

The result was surprising. Despite the fact that the energy of quanta differed more than twofold, the order of nonlinearity was approximately the same (4.5-4.8) for wavelengths from 720 to 1500 nm and even greater (6.6) for a wavelength of 600 nm (with the highest quantum energy).

"These results disprove the hypothesis of multi-photon emission of electrons. At the same time, the experimental dependences are in good agreement with the tunnel emission mechanism, whereby electrons are made to escape from the metal by a plasmon enhanced light field," concludes Michael Bakunov.

The results of Russian and Japanese scientists' research were published in one of the leading scientific journals, Scientific Reports