RU175818U1 - SOURCE OF NARROW-BAND THERAHZER RADIATION PROCESSED IN A LITHIUM NIOBATE CRYSTAL IN THE DIRECTION OF REVERSE THE EXTENSION OF EXCITING ULTRASHORT LASER PULSES - Google Patents

Abstract

The utility model relates to the field of optical instrumentation and relates to a source of narrow-band terahertz radiation. The source includes a linearly polarized pulsed femtosecond laser, an optical terahertz converter, and a divider for separating laser and terahertz radiation. The terahertz optical converter is made in the form of a plate oriented by the surface of one of its sides with the possibility of laser pulses acting on it and made of lithium niobate single crystal with a laser pulse propagation length in this single crystal exceeding the absorption length of terahertz radiation in it. The crystallographic axis [100] of a lithium niobate single crystal lies in the plane of the input surface of the plate and forms an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and the crystallographic axis [001] of this single crystal forms an angle with the input surface of the plate from a range of 50-69 °. The technical result consists in increasing the conversion efficiency, increasing the signal-to-noise ratio and narrowing the spectral width of the radiation line. 3 s.p. f-ly. 5 ill.

Description

The utility model relates to sources of narrow-band terahertz radiation and can be used for highly sensitive equipment for spectroscopy, microscopy, and imaging.

One of the most common techniques for generating narrow-band terahertz radiation, which is most closely related to the claimed terahertz radiation source, is based on the optical rectification of femtosecond laser pulses in electro-optical crystals.

In this technique, an electro-optical crystal is irradiated with femtosecond laser pulses, which, when propagated in the indicated crystal, induce non-linear polarization in it, which emits terahertz radiation (see the work in English by G. Kh. Kitaeva “Terahertz generation by means of optical lasers” - LASER PHYS. LETT. 2008, v. 5, p. 559).

Sources based on this technique consist of a femtosecond laser emitting laser pulses, an optical terahertz converter based on an electro-optical crystal and an element for separating terahertz and laser radiation and have the following characteristics: high repetition rate of terahertz pulses (usually from tens of kHz to hundreds of MHz ), which allows us to consider the dynamics of fast processes, a narrow spectral line of radiation, which allows us to study the spectral properties of substances a given frequency without affecting neighboring frequencies and relatively high energy characteristics of the radiation.

In these sources, the most common crystal of lithium niobate (LiNbO ₃ ), which is part of the ferroelectric group, is used as an optical terahertz converter. The specified crystal has a high threshold of optical destruction ~ 1 TW / cm ² and a large nonlinear coefficient. Thus, the non-linear coefficient of a lithium niobate crystal is 168 pm / V, which is three times higher than the non-linear coefficient of a zinc telluride crystal.

In this group of sources, the optical terahertz converter is made in the form of a periodic structure of thin single-crystal lithium niobate plates, in which the direction of the crystallographic axis [001] periodically changes. So in work in English. lang authors G. Xu et al. “Efficient generation of backward terahertz pulses from multiperiod periodically poled lithium niobate” - OPTICS LETT. 2009, v. 34, p. 995 in a periodically polarized structure of lithium niobate single crystal wafers, terahertz radiation was generated at an operating frequency of 0.37 THz with a spectral line width of 18.1 GHz. The laser pulse polarization was parallel to the crystallographic axis [001] of the single-crystal lithium niobate plates.

The disadvantage of the sources is a small input aperture, a labor-intensive manufacturing process, low strength parameters, and a relatively (proposed in the present description of the utility model) wide spectral emission line.

Due to the fact that the physical mechanism ensuring the operability of the proposed source of narrow-band terahertz radiation is based on optical rectification due to nonlinear mixing of the spectral components of the ordinary and extraordinary waves formed after refraction of the exciting laser beam on the input surface of the plate of a lithium niobate single crystal under conditions of formation of lithium niobate single crystal plate induced non-linear polarization with variable polarity, and goes beyond the principle of action of teraher sources ovogo radiation, these analogs in the group above. Because the comparison of the possibilities of terahertz radiation in them and in the claimed source is not correct due to the basic differences in the methods for generating narrow-band terahertz radiation (for the specific features of the physical mechanism for generating narrow-band radiation using the proposed source, see below), the disclosure of the essence of the proposed terahertz radiation source in utility model formula without prototype.

The technical result from the use of the proposed utility model is the development of a source of narrow-band terahertz radiation generated in a lithium niobate single crystal in the direction opposite to the propagation of exciting ultrashort laser pulses in the range of technologically permissible thicknesses of this single crystal, with an optical-terahertz conversion efficiency (not less than 10 ^-5 ) corresponding to the optimal requirements of the signal-to-noise ratio, with a narrowed spectral width of the emission line and increased the production quality of the specified source due to the proposed structure and composition of this source.

In addition, the proposed source expands the promising arsenal of hardware for current and sought-after narrow-band terahertz radiation.

To achieve the technical result, a narrow-band terahertz radiation source is proposed, comprising a femtosecond laser with linearly polarized pulsed radiation, an optical terahertz converter made in the form of a plate oriented by the surface of one of its sides with the possibility of laser pulses acting on it and made of lithium niobate single crystal with the propagation length of laser pulses in a given single crystal, exceeding the absorption length of terahertz radiation in it by at the operating frequency in order to ensure, in the opposite direction to the propagation of these laser pulses, a terahertz output with increased narrowband as a result of optical rectification of the laser pulses due to nonlinear mixing of the spectral components of the ordinary and extraordinary waves formed after refraction of the exciting laser beam on the input surface of the plate under conditions of formation in single-crystal niobate induced nonlinear polarization with variable polarity, and a divider for separation said laser and terahertz radiation mounted in front of the input surface of said plate.

Moreover, the crystallographic axis [100] of a lithium niobate single crystal lies in the plane of the input surface of the plate and forms an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and the crystallographic axis [001] of this single crystal forms an angle with the input surface of the plate from 50–69 ° .

In particular cases in the proposed source

for generating terahertz radiation with an operating frequency of 0.35 THz and a spectral line width of 3 GHz with an efficiency of 1.4 × 10 ⁵ in the direction opposite to the propagation of exciting linearly polarized femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 500 fs, an optical plate the terahertz converter is oriented normally by the surface of one of its sides to the direction of propagation of the laser pulses acting on it and is made of a lithium niobate single crystal with a linear size that defines m the length of the passage of the indicated laser pulses in it equal to 10 mm, with a crystallographic axis [100] lying in the plane of the input surface of the indicated plate and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and with the crystallographic axis [001] directed at an angle of 69 ° to the input surface of the same plate, and the divider for separating the exciting laser and output terahertz radiation is made in the form of a thin film made of nitrocellulose;

for generating terahertz radiation with an operating frequency of 0.5 THz and a spectral line width of 3.6 GHz with an efficiency of 1.3 × 10 ^-5 in the direction opposite to the propagation of exciting linearly polarized femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 350 fs, the plate of the optical terahertz converter is oriented normally by the surface of one of its sides to the direction of propagation of the laser pulses acting on it and is made of lithium niobate single crystal with a linear size, defining the passage length of said laser pulses in it equal to 7 mm, with a crystallographic axis [100] lying in the plane of the input surface of the indicated plate and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and with the crystallographic axis [001] directed at an angle of 65 ° to the input surface of the same plate, and the divider for separating the exciting laser and output terahertz radiation is made in the form of a thin film made of nitrocellulose;

to generate terahertz radiation with an operating frequency of 1.1 THz and a spectral line width of 16 GHz with an efficiency of 1 × 10 ^-5 in the direction opposite to the propagation of exciting linearly polarized femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 160 fs, an optical terahertz plate the transducer is oriented normally by the surface of one of its sides to the direction of propagation of the laser pulses acting on it and is made of a lithium niobate single crystal with a linear size that sets the propagation length of said laser pulses in it equal to 4 mm, with a crystallographic axis [100] lying in the plane of the input surface of the indicated plate and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and with the crystallographic axis [001] directed under angle of 50 ° to the input surface of the same plate, and the divider for separating the exciting laser and output terahertz radiation is made in the form of a thin film made of nitrocellulose.

Known source of narrow-band terahertz radiation based on the difference frequency generation upon irradiation of a lithium niobate single crystal with two laser pulses of nanosecond duration with close frequencies (see T. Akiba's work in English by “Terahertz wave generation using type II phase matching polarization combination via difference frequency generation with LiNbO ₃ "- JAP. J. APPL. PHYS. 2015, v. 54, p. 062202), which consists of two nanosecond pulsed lasers and an optical terahertz converter based on a lithium niobate single crystal (in which relatively large duration of the laser pulse can not achieve a high optical intensity, which affects the efficiency of optical-terahertz conversion, which is ~ 10 ^-9) is consistent with the proposed novelty narrowband terahertz radiation source, since this known source of narrow-band terahertz radiation uses other sources of laser radiation in combination with a lithium niobate single crystal, which functions as an optical terahertz converter based on the difference frequency generation, different in comparison with the generation of narrow-band terahertz radiation in the proposed source based on optical rectification.

In FIG. 1a schematically shows the proposed narrowband terahertz radiation source; in FIG. 1b is an optical terahertz converter as a part of the source in FIG. 1a; in FIG. 2 is a diagram of a femtosecond laser pulse irradiation of a lithium niobate single crystal during operation of the proposed source; in FIG. 3 - dependence of the operating frequency of the proposed source on the angle formed by the crystallographic axis [001] of the lithium niobate single crystal from which the plate of the terahertz converter is made, with its input surface; in FIG. 4 - normalized spectral power density of terahertz radiation of the proposed source; on figa - the dependence of the absorption length of terahertz radiation in a single crystal of lithium niobate on the operating frequency of the proposed source; in FIG. 5b shows the dependence of the optical terahertz conversion efficiency and the optimal laser pulse duration on the angle formed by the crystallographic axis [001] of the lithium niobate single crystal from which the plate of the terahertz converter is made with its input surface.

The proposed terahertz radiation source contains (see Fig. 1a): a femtosecond laser 1, a divider 2, made of a thin film of nitrocellulose and an optical terahertz converter made in the form of a plate 3 of a lithium niobate single crystal. The surface of the ABCD plate 3 of a single crystal of lithium niobate - an optical terahertz converter is optically polished (see Fig. 1b).

In the examples of the proposed source of narrow-band terahertz radiation for generating terahertz radiation with an operating frequency of 0.35 THz, 0.5 THz and 1.1 THz and a spectral line width of 3 GHz, 3.6 GHz and 16 GHz with an efficiency of 1.4 × 10 ^-5 , 1.3 × 10 ^-5 and 1 × 10 ^-5 in the direction opposite to the propagation of linearly polarized exciting femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 500 fs, 350 fs and 160 fs, plate 3 of terahertz the transducer is oriented normally to its direction by its input surface the propagation of the laser pulses acting on it and is made of a lithium niobate single crystal having a linear size that specifies the passage length of the indicated laser pulses in it equal to 10 mm, 7 mm, and 4 mm with the crystallographic axis [100] of the lithium niobate single crystal lying in the input plane the surface of the specified plate 3, and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and with the crystallographic axis [001] of this single crystal, directed at an angle θ = 69 °, 65 ° and 50 ° to the input surface of the same plate (see FIG. 1a and 2).

The proposed source of terahertz radiation works as follows.

Linearly polarized femtosecond laser pulses from a femtosecond laser 1 pass through a thin-film divider 2 and fall onto the input surface ABCD of the plate 3 — an terahertz optical converter (see FIGS. 1b and 2), and the lithium niobate single crystal is oriented in such a way that the radiation polarization vector the laser pulse forms an angle of 45 ° with the crystallographic axis [100] lying in the plane of the input surface of the specified plate, and the crystallographic axis [001] of the same single crystal is directed at an angle θ, irradiated interval of 50-69 °.

After entering the single crystal of lithium niobate, the laser pulse is divided into a superposition of pulses of ordinary and extraordinary waves (see Fig. 1A and 2). As a result of nonlinear mixing of the spectral components of the waves in a single crystal of lithium niobate, nonlinear polarization is induced, which simultaneously emits terahertz radiation in the direction of propagation of the laser pulses and in the direction opposite to the specified propagation, with a predominance of radiation efficiency in the direction opposite to the propagation of laser pulses. After leaving the face ABCD of the plate 3, the indicated predominant terahertz radiation is reflected from the divider 2 and is displayed in free space for working use. When this laser radiation passes through the divider 2 without significant reflection.

The following physical and computational justification serves to disclose the physical mechanism that ensures the operability of the proposed source of narrow-band terahertz radiation generated in a lithium niobate single crystal in the direction opposite to the propagation of exciting ultrashort laser pulses.

As a femtosecond laser 1, we consider an erbium laser with a central wavelength of 1.05 μm, the pulses of which arrive (at an angle of 90 °) on the input surface of the plate 3 of a lithium niobate single crystal (see Fig. 2). The pulses propagate along the x axis, the polarization vector of which form an angle of 45 ° with the crystallographic axis [100] of the lithium niobate single crystal.

We consider a one-dimensional model with a Gaussian envelope of the electric field strength - the electric field of the laser pulse is considered symmetrical with respect to the y, z axes, and at the entrance to the plate, the electric field strength has the form of the following formula

ω ₀ = 2π / sλ,

where E ₀ is the amplitude of the intensity of the exciting electric field of the laser pulse,

λ is the central wavelength of the exciting laser pulse,

c is the speed of light in vacuum;

τ is the duration of the exciting laser pulse.

When refracting at the input boundary of the plate 3, the laser pulse is divided into a superposition of pulses of ordinary and extraordinary waves.

The choice of the indicated angle of 45 ° with the crystallographic axis [100] of the lithium niobate single crystal was determined in view of the equality of the amplitudes of the pulses of the indicated ordinary and extraordinary waves, which ensures the highest value of nonlinear polarization and, therefore, the associated efficiency of the optical terahertz conversion (which is the first component - a contribution to the efficiency of the desired optical terahertz conversion, not less than 10 ^-5 , corresponding to the optimal requirements of the signal-to-noise ratio and consisting of of the first part and from the second component, due to the proposed interval of the angle θ in connection with the rationale described below).

As a result of nonlinear mixing of the spectral components of these waves, a nonlinear polarization occurs, which can be written as

, Δn=n_o-n_e и L=2cτ/Δn' с

. n_o, n'_о и n_e, n'_е - индексы преломления и групповые индексы обыкновенной и необыкновенной волн при заданном угле θ, ε₀ - диэлектрическая проницаемость вакуума, χ_eƒƒ=χ₁₅cosϑ+χ₂₂sinϑ - эффективный нелинейный коэффициент, χ₁₅ и χ₂₂ - нелинейные коэффициенты кристалла ниобата лития.where ξ = t-n'x / c,

, Δn = n _o -n _e and L = 2cτ / Δn 's

. n _o , n ' _o and n _e , n' _e are the refractive indices and group indices of the ordinary and extraordinary waves for a given angle θ, ε ₀ is the dielectric constant of the vacuum, χ _eƒƒ = χ ₁₅ cosϑ + χ ₂₂ sinϑ is the effective nonlinear coefficient, χ ₁₅ and χ ₂₂ are the nonlinear coefficients of a lithium niobate crystal.

It follows from formula (2) that the nonlinear polarization induced in a single crystal of lithium niobate is alternating with a period of λ / Δn.

To find the spectral power density of terahertz radiation, we solve the wave equation in Fourier space with a source in the form of nonlinear polarization (see expression 2) in three homogeneous regions: half-space x <0, single crystal of lithium niobate and half-space x> d, where d is the thickness of the specified single crystal equal to the propagation length of exciting ultrashort laser pulses in a given single crystal. Then we take the inverse Fourier transform.

The solution implies the existence of two terahertz waves: one propagates in the direction of laser pulses, the other in the opposite direction.

Consider a wave that propagates in the opposite direction. The wave frequency is given by

where n _t is the refractive index at the working terahertz frequency Ω _b / 2π.

Thus, the source operating frequency Ω _b / 2π is set to change the angle θ in view of the dependence of the extraordinary group index and the index of refraction of the laser radiation on n 'and Δn.

In FIG. 3, we plotted the dependence of the working frequency Ω _b / 2π of the narrow-band terahertz radiation source on the angle θ. For example, generation at the operating frequency Ω _b / 2π = 0.5 THz corresponds to an angle θ = 65 °.

The expression for the power spectral density S (Q) as a function of the terahertz frequency Ω

Where

k = Ωn _t / c,

where f is the error function.

In FIG. Figure 4 shows the normalized spectral power density of terahertz radiation calculated according to formula 4 at a pump intensity of 100 GW / cm ² , a thickness of lithium niobate single crystal d = 7 mm, an angle θ = 65 °, and a duration τ = 350 fs. For these parameters, the source of narrow-band terahertz radiation will emit terahertz radiation at a frequency of Ω _b / 2π = 0.5 THz with a spectral line width at half height - 3.6 GHz.

To confirm the narrowing of the emission spectral line of the proposed source, we compare the obtained characteristics of terahertz radiation with a source based on a periodically polarized lithium niobate crystal (see the article in English by G. Xu et al. “Efficient generation of backward terahertz pulses from multiperiod periodically poled lithium niobate ”- OPTICS LETT. 2009, v. 34, p. 995). In this article, the spectral line width is 18.1 GHz at an operating frequency of 0.37 THz, which is six times wider than the width of the 3 GHz spectral line of the proposed source at the same operating frequency.

As a result of numerical calculations, it was found that to ensure the generation of narrow-band terahertz radiation generated in a lithium niobate single crystal in the direction opposite to the propagation of exciting ultrashort laser pulses, the length of the single crystal should be no less than the absorption length L _a at the operating terahertz frequency Ω _b / 2π

To determine the thickness of a lithium niobate single crystal d greater than L _a at various operating frequencies Ω _b / 2π of terahertz radiation, a dependence of the absorption length L _a on the working terahertz frequency Ω _b / 2π is shown, shown in FIG. 5a.

Also, as a result of the calculation, it was found that the optimal duration τ _{opt of} exciting femtosecond laser pulses (corresponding to the maximum efficiency of the optical terahertz conversion) can be selected according to the dependence on the angle θ in FIG. 5b (see dashed curve).

Integrating formula 4 over the terahertz frequency Ω, we find the efficiency of the optical terahertz conversion. In FIG. 5b, the solid curve shows the dependence of the optical terahertz conversion efficiency in the direction opposite to the propagation of laser pulses at the previously indicated optimal pulse duration and the thickness of a lithium niobate single crystal and a pump intensity of 100 GW / cm ² .

Efficiency (at least 10 ^-5 ) that meets the optimal requirements of the signal-to-noise ratio, at which stable operation of the narrow-band radiation source generated in the lithium niobate single crystal in the direction opposite to the propagation of exciting ultrashort laser pulses corresponds to the angle range θ (50 ° -69 °), which in turn corresponds to the operating frequency range Ω _b / 2π from 1.1 to 0.35 THz.

When leaving the specified range in the direction of decreasing the angle θ, the mentioned efficiency sharply decreases - when deviating by 5 °, the efficiency decreases by 20% and when leaving the specified range in the direction of increasing the angle θ, the technologically permissible thickness of the lithium niobate single crystal, equal to 10 mm, is exceeded.

The stated rationale in expanded form will be published in an article in English. lang authors E.A. Mashkovich, S.A. Sychugin and M.I. Bakunov “Generation of narrowband terahertz radiation by an ultrashort laser pulse in a bulk LiNbCb crystal” in Optics Letters magazine, in which this article was received in early March 2017.

Currently, tests are being prepared for experimental samples of the proposed narrow-band terahertz radiation source.

Thus, the above justification, which works for femtosecond lasers and with other central wavelengths, confirms the generation of narrow-band terahertz radiation in a lithium niobate single crystal in the direction opposite to the propagation of exciting ultrashort laser pulses in the range of technologically permissible thicknesses of this single crystal with an opto-optical efficiency conversion (not less than 10 ^-5 ), corresponding to the optimal requirements of the signal-to-noise ratio, with a narrowed spectral width no emission lines of the specified source due to the structure and composition of this source, and the simplicity of the design of the latter minimizes the use of technical means.

Claims (4)

1. A source of narrow-band terahertz radiation containing a femtosecond laser with linearly polarized pulsed radiation, an optical terahertz converter made in the form of a plate oriented by the surface of one of its sides with the possibility of laser pulses acting on it and made of lithium niobate single crystal with laser length pulses in this single crystal, exceeding the absorption length of terahertz radiation in it at the operating frequency to ensure in the opposite direction of propagation the transmission of these laser pulses, a terahertz output with increased narrowband as a result of optical rectification of the laser pulses due to nonlinear mixing of the spectral components of the ordinary and extraordinary waves formed after refraction of the exciting laser beam on the input surface of the plate under conditions of formation of a nonlinear polarization induced in the niobate single crystal with variable polarity , and a divider for separating said laser and terahertz radiation, set n in front of the input surface of the said plate, the crystallographic axis [100] of a lithium niobate single crystal lying in the plane of the input surface of the plate and forms an angle of 45 ° with the direction of the polarization vector of the exciting laser beam, and the crystallographic axis [001] of this single crystal forms an angle with the input surface of the plate from the interval of 50-69 °. 2. The source of narrow-band terahertz radiation according to claim 1, characterized in that for the generation of terahertz radiation with an operating frequency of 0.35 THz and a spectral line width of 3 GHz with an efficiency of 1.4 × 10 ^-5 in the direction opposite to the propagation of exciting linearly polarized femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 500 fs, the plate of the opto-terahertz converter is oriented normally by the surface of one of its sides to the direction of propagation of the laser impulses acting on it It is made of a single crystal of lithium niobate with a linear size that specifies the passage length of the indicated laser pulses in it equal to 10 mm, with a crystallographic axis [100] lying in the plane of the input surface of the plate and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam , and with the crystallographic axis [001] directed at an angle of 69 ° to the input surface of the same plate, and the divider for separating the exciting laser and output terahertz radiation is made in the form of a thin film and made from nitrocellulose. 3. The source of narrow-band terahertz radiation according to claim 1, characterized in that for generating terahertz radiation with an operating frequency of 0.5 THz and a spectral line width of 3.6 GHz with an efficiency of 1.3 × 10 ^-5 in the direction opposite to the propagation of exciters linearly -polarized femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 350 fs, the plate of the optical terahertz converter is oriented by the surface of one of its sides normally to the direction of propagation of the laser impulses acting on it It is made of lithium niobate single crystal with a linear size that specifies the passage length of the indicated laser pulses in it equal to 7 mm, with a crystallographic axis [100] lying in the plane of the input surface of the plate and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam , and with the crystallographic axis [001] directed at an angle of 65 ° to the input surface of the same plate, and the divider for separating the exciting laser and output terahertz radiation is made in the form of a thin film and made from nitrocellulose. 4. The source of narrow-band terahertz radiation according to claim 1, characterized in that for generating terahertz radiation with an operating frequency of 1.1 THz and a spectral line width of 16 GHz with an efficiency of 1 × 10 ^-5 in the direction opposite to the propagation of exciting linearly polarized femtosecond laser pulses with a central wavelength of 1050 nm and a duration of 160 fs, the plate of the optical terahertz converter is oriented normally by the surface of one of its sides to the direction of propagation of the laser pulses acting on it owl and is made of a lithium niobate single crystal with a linear size that specifies the passage length of the indicated laser pulses in it equal to 4 mm, with a crystallographic axis [100] lying in the plane of the input surface of the plate and forming an angle of 45 ° with the direction of the polarization vector of the exciting laser beam , and with the crystallographic axis [001] directed at an angle of 50 ° to the input surface of the same plate, and the divider for separating the exciting laser and output terahertz radiation is made in the form of a thin film, made from nitrocellulose.

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