Multielement terahertz radiation generator

FIELD: electricity.

SUBSTANCE: multielement terahertz radiation generator includes test sample, femtosecond laser, multielement emitter where element emitter is made in the form of crystal semiconductor with sputtered metal mask forming sharp laser illumination gradient for crystal semiconductor layer. On the boundary of illuminated and shaded parts of semiconductor layer, a sharp gradient of photoexcited charge carrier concentration is formed parallel to the semiconductor surface. In addition the device includes elliptical mirror forming a focused terahertz radiation beam, while multielement emitter includes a raster of cylindrical microlenses distributing laser radiation between element emitters and illuminating only those semiconductor layer areas involved in terahertz radiation generation. The metal mask is made in the form of flat metal stripes.

EFFECT: increased power of terahertz radiation, possible application of small test samples.

3 cl, 2 dwg

 

The device relates to pulse generators broadband electromagnetic radiation in the terahertz frequency range, based on conversion of femtosecond laser radiation. Such generators are used to generate the terahertz spectrometers to study the properties of substances and materials in the terahertz region of the electromagnetic spectrum. Such devices should have a high conversion efficiency of laser radiation in the terahertz. To solve specific problems using the powerful radiation it is necessary to use materials having a high radiation resistance. The device should have small dimensions to create a portable spectrometers.

Known technical solutions used in the construction of a photoconductive antennas: TERA15-FC produced by the company "Menio Systems, Germany http://www.menlosvstems.com): G10620-11, G10620-12, G10620-13, manufactured by the company "Hamamtsu, Japan (http://ip.hamamatsu.com). Devices are pulse generators broadband electromagnetic radiation in the terahertz frequency range, based on conversion of femtosecond laser radiation, and represent a photoconductive antenna, installed in a single housing with a silicon lens. Photoconductive antennas represent the Oh deposited on the surface of the semiconductor electrodes. Generation of terahertz radiation occurs when the absorption of femtosecond laser radiation in the semiconductor and related to the appearance in it of a pulsed photocurrent due to the drift of photoexcited charge carriers along the surface of a semiconductor in an electric field applied to the electrodes. The maximum beam of terahertz radiation perpendicular to the direction of drift and the semiconductor surface. Installed silicon lens colliery or focuses the THz radiation.

Such devices have increased noise, because of fluctuations in the external electric field is moved in the terahertz signal and degrade the noise performance of the oscillator as a whole. Also photoconductive antennas are characterized by the saturation power of the generated THz radiation with increasing laser intensity. This saturation is due to shielding of the applied electric field excited charge carriers in the semiconductor.

A disadvantage of the known technical solutions is that in photoconductive antennas is the need of the application of the external electric field created by the electrodes on the semiconductor surface. In addition, between the electrodes flows photocurrent proportional to the intensity of the laser radiation. Electric power to ora is determined by the product of the intensity of applied electric field and the strength of the photocurrent, is converted into heat which must be removed from the device. This determines the maximum laser power and the maximum amplitude of the applied voltage to avoid overheating and/or electrical breakdown of the device.

Known technical solution based on a crystalline semiconductor that is used in pulse generators broadband electromagnetic radiation in the terahertz frequency range, based on conversion of femtosecond laser radiation. The technical solution described in the publication: Vitalij L. Malevich a, Ramunas Adomavicius, Arunas Krotkus, "THz emission from semiconductor surfaces", Science Direct, C.R. Physique 9 (2008) 130-141. Generation of terahertz radiation occurs when the absorption of femtosecond laser radiation in the crystalline semiconductor and related to the appearance in it of a pulsed photocurrent due to the diffusion of photoexcited charge carriers (the effect of Dember) and their drift in the internal electric field of the crystalline semiconductor.

A disadvantage of the known technical solutions associated with a low coefficient of conversion of laser radiation into terahertz radiation, namely the output of the generated terahertz radiation from the near-surface layer of the crystalline semiconductor outward. The low conversion efficiency of mo is but to explain as follows. The direction vectors of the drift and diffusion of photoexcited charge carriers perpendicular to the surface of the semiconductor and collinear with the vectors of the electrical field vector of the electric field due to the effect of Dember, respectively. Thus, the directional diagram of the terahertz radiation is perpendicular to these vectors and parallel to the surface of the semiconductor. Due to the large refractive index of the semiconductor is only a small part of terahertz radiation is derived from generator, reflected from its surface on the inner side. A second disadvantage of the known technical solutions associated with the saturation power terahertz radiation with increasing laser intensity. In the local area illuminated by the laser radiation, can be created in a finite number of photoexcited charge carriers involved in the generation of terahertz radiation, and thus a further increase of the laser power does not increase power terahertz.

Known technical solution used in pulse generators broadband electromagnetic radiation in the terahertz frequency range, based on conversion of femtosecond laser radiation (Patent WO 2010/142313 "A passive terahertz radiation source, the IPC H01S 1/02, G02f 2/02, priority from 201012-16), selected as a prototype. The generator of terahertz radiation includes a source of pulsed laser radiation in the form of a laser emitter, comprising at least one elementary emitter, which is a layer of crystalline semiconductor, a portion of the surface which is illuminated by pulse laser radiation, and under the action of this radiation on the border of lighted and unlighted fields parallel to the surface of the crystalline semiconductor is formed a sharp concentration gradient of the photoexcited charge carriers. Generation of terahertz radiation based on the photoelectric effect of Dember, which can be described as follows. The charge carriers diffuse to the area of lower concentration to form a pulsed diffusion current. A significant difference in the mobility of positive and negative charge carriers leads to their spatial separation and the formation of the electric field intensity vector. The change in the diffusion current in the electric field causes the emission of electromagnetic pulse.

The spectral composition and the pulse duration are determined by the parameters of laser radiation and the properties of the crystalline semiconductor and correspond to the terahertz frequency range. Generator deprived drawback associated with the withdrawal of those who egertova radiation from the layer of crystalline semiconductor output, since the directional diagram of the terahertz radiation is perpendicular to the vector of the electric field due to the effect of Dember, and accordingly the surface of the crystalline semiconductor. In accordance with the law of reflection Fresnel conclusion of terahertz radiation perpendicular, i.e. at an angle of 90 degrees to the surface of the crystalline semiconductor is the most effective since it has the lowest loss at the reflection of terahertz radiation from the inner surface of the crystalline semiconductor. Thus, you receive the highest possible power terahertz radiation.

The use of the photoelectric effect of Dember for generating terahertz radiation does not require the use of an external electric field, the fluctuations of which are transferred to the noise terahertz radiation and degrade the signal-to-noise oscillator as a whole. Also because of this not happening saturation power of the generated THz radiation with increasing intensity femtosecond laser radiation associated with the screening of the applied electric field the photoexcited charge carriers in the crystalline semiconductor.

Generator, consisting of a set of elementary emitters (multielement oscillator)has a greater conversion rate femtosec ndogo laser radiation in the terahertz, than the generator, consisting of a single elementary emitter (singleton generator), as multi-element generator deprived drawback associated with the saturation power terahertz radiation with increasing laser power. Power femtosecond laser radiation is distributed in proportion to the number of elementary emitters involved in the generation of terahertz radiation, and in every elementary emitter its value becomes below the threshold for inclusion mechanism of saturation. Total power terahertz radiation multielement oscillator is the sum of the capacities emitted from each of its elementary emitter. Thus, the power of the terahertz radiation can be significantly increased compared with singleton generator by increasing the power femtosecond laser radiation and the absence of this saturation.

In the prototype described in the scheme of the multichannel generator. It is a crystalline semiconductor layer with a spray to the surface of a periodic structure consisting of metal strips. The profile strips, wedge-shaped. When illuminated by laser radiation part of it is reflected from a metal strip, and the remaining part is absorbed in the crystalline semiconductor. The thicker end of the metal strip is fully isolate a crystalline semiconductor light laser radiation and forms a sharp gradient illumination of the crystalline semiconductor. A thinner edge is translucent and forms a smooth gradient. The faster gradient illumination with laser light, the sharper the density gradient is formed of photoexcited charge carriers in the crystalline semiconductor, the higher the amplitude of the pulse diffusion current and power terahertz radiation. Thus, the main power terahertz radiation is formed by the areas around thick edges of a metal strip. The coated strips similar way to form co-directed vectors of gradients of concentration of photoexcited carriers when illuminated by laser radiation of the whole structure. Thus, the amplitude of the terahertz waves emitted from each region near the thick edges of a metal strip, which is essentially elementary emitter stack.

A disadvantage of the known technical solution is inefficient use of femtosecond laser radiation, which covers the entire surface of the elementary emitter, including areas that are not involved in the generation of terahertz radiation: the entire surface of the metal strip and the area near its thin edges. The total area of the regions not involved in the generation of terahertz radiation, more of the total area involved. An additional disadvantage of the data is th technical solution is to generate a divergent beam of terahertz radiation, that requires the use of large sample size for spectroscopic studies and reduces the energy density of terahertz radiation are required, for example, for biological research.

The authors task was to develop a multi-element generator of terahertz radiation based on conversion of femtosecond laser radiation, with more efficient use of laser radiation.

The problem is solved in that the multi-element generator of terahertz radiation, containing the sample, the laser emitting femtosecond laser radiation, multiple emitter comprising at least one elementary emitter, which is a layer of crystalline semiconductor with sprayed metal mask, forming a sharp gradient light layer of crystalline semiconductor femtosecond laser radiation, on the border of the lit and unlit portions of a layer of crystalline semiconductor formed a sharp concentration gradient of the photoexcited charge carriers parallel to the surface of the crystalline semiconductor layer, further comprises an elliptical mirror, made forming a focused beam of terahertz radiation and containing an opening for passing pentose odnogo laser radiation, and multiple emitter is made containing raster cylindrical microlenses, distributing femtosecond laser radiation between elementary emitters and forming on the layer of crystalline semiconductor light only the areas involved in the generation of terahertz radiation, in addition, a metal mask in the form of flat metal strips, and the crystalline semiconductor layer is made in the form of a crystal of InAs at a wavelength of femtosecond laser radiation 775 nm and at a wavelength of femtosecond laser 1550 nm layer of crystalline semiconductor is made in the form of crystal InSb

The technical effect of the claimed device is to increase the conversion of femtosecond laser radiation in the terahertz radiation; the increase in energy density is focused on the sample terahertz radiation, and the ability to use samples of small size and expanding the range of devices for this purpose.

Figure 1 presents a block diagram explaining the operation of the inventive multi-element generator of terahertz radiation, where 1 - laser, 2 - femtosecond laser radiation, 3 - hole, 4 - multiple emitter, 5 - terahertz radiation, 6 - elliptical mirror, 7 - ASCS is adremy sample.

Figure 2 presents a diagram explaining the operation of the multichannel emitter, where 2 - femtosecond laser radiation, 5 - terahertz radiation, 8 - raster cylindrical microlenses, 9 - elementary emitter, 10 - metal mask 11 - crystalline semiconductor 12 - fotovozbuzhdenie charge carriers, 13 - vector of the gradient of the concentration of photoexcited charge carriers.

The inventive multi-element generator of terahertz radiation works in the following way. The laser 1 generates femtosecond laser radiation 2 at the wavelength of the femtosecond laser 1550 nm or 775 nm. Femtosecond laser radiation 2 is directed through the opening 3 in the elliptical mirror 6 on multiple emitter 4, the radiation area which is located in one of the two foci of the elliptical mirror 6 and which converts femtosecond laser radiation 2 in the terahertz radiation 5. Terahertz radiation 5 is converted elliptical mirror 6 in a focused beam of high energy density concentrated in the region of the second focus of the elliptical mirror 6, where the sample 7, therefore, is one of the technical effects of the invention, allowing to use to study samples of small size.

In multielement emitter proishodit conversion of femtosecond laser radiation 2 in the terahertz radiation 5 as follows. Multiple emitter 4 comprises at least one elementary emitter 9. Femtosecond laser radiation 2 is evenly distributed pattern of cylindrical microlenses 8 between elementary emitters 9 and focuses on the crystalline semiconductor layer 11 in the region involved in the generation of terahertz radiation, namely at the edge of the metal mask 10, in the form of flat metal strips deposited on the crystalline semiconductor layer 11. The metal mask 10 should have sufficient thickness so as not to miss femtosecond laser radiation 2. Periods strips of the metal mask 10 and raster cylindrical microlenses 8 should be the same. The width of the stripes of the metal mask 10 and the distance between the bars should not be less than half the width of the intensity distribution of femtosecond laser radiation 2 in focus raster cylindrical microlenses 8. The lateral dimensions of the metal mask 10 and the region of the crystalline semiconductor 11 covered elementary emitters 9, shall be not less than the transverse dimensions of femtosecond laser radiation 2. The thickness of the crystalline semiconductor layer 11 must be at least 4/α (where α is the absorption coefficient of the crystalline semiconductor) to ensure absorption of not less than 98% of the femtosecond laser and the radiation 2, held inside this layer.

The edges of the metal mask 10 is displaced relative to the center of focus of the corresponding microlens raster cylindrical microlenses 8 by a distance not greater than the diameter of the light spot focused femtosecond laser radiation 2, so that the surface of the crystalline semiconductor 11, which may be in the form of a crystal of InAs at a wavelength of femtosecond laser radiation 775 nm or InSb crystal at a wavelength of femtosecond laser 1550 nm, near the end of the metal mask 10 is formed a sharp gradient illumination of femtosecond laser radiation 2. The gradient of the light absorption of femtosecond laser radiation 2 in the crystalline semiconductor 11 leads to the formation of a maximally sharp density gradient of the photoexcited charge carriers 12, and the proportion of intensity of a beam of laser radiation reflected from the metal mask 10, minimum. The vector gradient of the concentration of photoexcited charge carriers 13 collinear with the vector of the pulse current generated in accordance with the effect of Dember and leading to the generation of terahertz radiation 5 which is emitted perpendicular to the gradient vector of the concentration of photoexcited charge carriers 13 and accordingly the surface of kristallicheskogo semiconductor 11 in the direction opposite to the direction of propagation of femtosecond laser radiation 2. Since the vectors of gradients of concentration of photoexcited charge carriers 13 each elementary emitter 9 directed vectors of terahertz radiation 5 is also collinear, and their amplitudes are added together, forming a common beam.

Thus, by reducing losses femtosecond laser radiation 2 in the reflection from the metal mask 10 and increase its absorption in the crystalline semiconductor 11 increases the conversion of femtosecond laser radiation 2 in the terahertz radiation 5 and achieves the technical effect of the claimed invention.

The advantage of the inventive multi-element generator of terahertz radiation is also the possibility of using metal masks with simple profile shape of the bands allows to simplify the manufacture of the generator and to reduce its cost.

1. Multi-element generator of terahertz radiation, containing the sample, the laser emitting femtosecond laser radiation, multiple emitter comprising at least one elementary emitter, which is a layer of crystalline semiconductor with sprayed metal mask, forming a sharp gradient light layer of crystalline floor is Roudnice femtosecond laser radiation, at the border of the lit and unlit portions of a layer of crystalline semiconductor formed a sharp concentration gradient of the photoexcited charge carriers parallel to the surface of the crystalline semiconductor layer, characterized in that it further comprises an elliptical mirror, made forming a focused beam of terahertz radiation and containing an aperture for transmission of femtosecond laser radiation, and multiple emitter is made containing raster cylindrical microlenses, distributing femtosecond laser radiation between elementary emitters and forming on the layer of crystalline semiconductor light only the areas involved in the generation of terahertz radiation, in addition, a metal mask in the form of flat metal strips.

2. Multi-element generator of terahertz radiation according to claim 1, characterized in that the crystalline semiconductor layer is made in the form of a crystal of InAs at the wavelength femtosecond laser radiation 775 nm.

3. Multi-element generator of terahertz radiation according to claim 1, characterized in that the crystalline semiconductor layer is made in the form of InSb crystal at a wavelength of femtosecond laser 1550 nm.



 

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