Novel thin film terahertz radiation source based on AZO material and implementation method thereof

Abstract

The invention provides a terahertz radiation source based on an AZO (aluminum-doped zinc oxide) material and a testing method of the terahertz radiation source, and the terahertz radiation source comprises an AZO thin film and a pump light source. The material is characterized in that the material is composed of a substrate, an aluminum oxide (Al2O3) layer and an AZO layer. A test material needs to be excited by using a femtosecond laser with specific parameters under an oblique incidence condition, and wide-spectrum terahertz radiation with relatively high power can be generated. The invention can be used for an efficient and transparent thin film wide-spectrum terahertz source, and can be widely used for terahertz devices and related instruments and equipment.

Description

TECHNICAL FIELD

[0001] The application relates to a terahertz wave radiation device based on an AZO nanometer film, which can be used to form a terahertz radiation source and other related devices and belongs to the technical field of terahertz science. BACKGROUND

[0002] Terahertz (THz) waves, usually refer to electromagnetic waves with a frequency in the range of 0.1-10 THz (1 THz = 10 12 Hz), are the only waveband in the entire electromagnetic spectrum named after its frequency. Terahertz waves have excellent properties such as low photon energy, non-polar substance penetration and molecular fingerprint spectrum, and have great potential research value and development prospects in the fields of biological medicine, safety monitoring and material identification. The microwave waveband adjacent to the left of the terahertz waveband is in the lower frequency region, and the higher frequency region is from infrared to ultraviolet. Based on the inherent properties of materials, low-frequency electromagnetic waves such as microwaves are more easily obtained by electronic methods; high-frequency electromagnetic waves are more easily obtained by means of photonics. The special wave spectrum position of the terahertz wave gives it unique properties, but its stable and convenient light source research has always lagged far behind that of microwaves and near-infrared radiation, and at the end of last century, it was once called the “terahertz gap (THz gap)”.

[0003] Although progress has been made in recent years in achieving broadband terahertz radiation in air plasmas and liquids, terahertz emission is usually achieved by pumping solid-state non-centrosymmetric nonlinear crystals (such as ZnTe, GaP and LiNbO3), and a near-infrared waveband femtosecond laser is usually used as a pumping source. However, the strength, bandwidth and pumping wavelength of the generated terahertz signal are often limited by the phase matching conditions in the bulk material.

[0004] This problem has stimulated interest in the development of ultra-thin terahertz emitters, which can even be reduced to a few atomic layers in thickness. In particular, optical metamaterials composed of split-ring resonators are considered to be excellent nonlinear terahertz sources due to their magnetic dipole resonance characteristics, and their spectral bandwidth is not limited by the phase matching condition, nor is it affected by the absorption of the remaining radiation band material in traditional nonlinear crystals. However, the complex nanofabrication process and the low laser damage threshold have greatly hindered their widespread application.

[0005] In addition, nanoscale terahertz emitters such as single-layer graphene and tungsten disulfide have also been realized, but their efficiency and strength of radiating terahertz waves are very limited. There are also studies exploring the surface terahertz emission of semiconductors such as InAs, InSb and GaAs, but their terahertz emission can only be achieved in a reflective configuration.

[0006] Recently, materials with a real part of the permittivity approaching zero in certain spectral ranges (i.e. so-called "epsilon-near-zero" (ENZ) materials) have attracted much attention in the field of nonlinear optics, and have been applied in the fields of second and third harmonic generation, all-optical switching, and tunable absorption. The enhancement of nonlinear optical response in sub-wavelength thick materials caused by the ENZ effect significantly relaxes the phase matching condition restrictions for the generation of terahertz waves by traditional nonlinear crystals, thus making it possible to generate observable terahertz radiation by nanometer-thin film optical rectification. The use of ENZ material nanometer-thin films as terahertz radiation sources solves the key problem of the large volume and difficulty of coupling of traditional nonlinear crystals, and provides strong help for the future integration of terahertz source devices.

[0007] In the present application, we demonstrate the generation of terahertz waves from a self-made AZO material thin film by using the ENZ effect. We observed terahertz signals emitted from a 50 nm thick AZO thin film when the pump wavelength was close to the ENZ wavelength of the AZO, and the bandwidth of the terahertz signals was up to about 2.7 THz, which was limited only by the bandwidth of the pump laser and the probe crystal. We confirmed that the terahertz emission originated from the surface optical rectification process of the AZO thin film and was enhanced by the ENZ effect by measuring the pump wavelength and power dependence. SUMMARY

[0008] The purpose of the present application is to provide a thin film material that is simple to prepare and is very suitable for application in a high-efficiency, transmissive excitation thin film broadband terahertz radiation source.

[0009] The purpose of the present application is achieved in that:

[0010] S1: Place a double-sided or single-sided polished substrate into a magnetron sputtering chamber; in the samples tested in the present patent, we used a double-sided polished 1 cm*1 cm quartz substrate to ensure high transmittance of the laser pump during transmissive excitation.

[0011] S2: Heat the substrate in the magnetron sputtering chamber; in the samples tested in the present patent, we used a heating temperature of 600°C. The heating needs to be maintained for about 40 minutes before it is considered complete, because the heat of the substrate itself needs to be balanced for a certain period of time to ensure that the entire substrate reaches the target temperature and the temperature is uniform.

[0012] S3: Sputter Al2O3 on the substrate. By controlling the thickness of the sputtered Al2O3, the doping concentration and the nature of the excess region at the substrate-AZO interface can be adjusted, thereby affecting the intensity of the THz radiation signal. In the samples tested in the present patent, the sputtering thickness of Al2O3 was about 5 nm.

[0013] S4: Depositing AZO thin film on substrate using AZO target material; in the samples tested in this patent, this step uses direct current sputtering, and the sputtering power is 50 W. During sputtering, the sample needs to be uniformly rotated to prevent the sputtered AZO from generating a single-directional crystal, which affects the in-plane isotropic properties thereof.

[0014] S5: Characterizing the AZO thin film by using a laser-pumped terahertz detection system.

[0015] The reason why AZO can efficiently radiate broadband terahertz waves at a subwavelength thickness lies in its special electromagnetic state of "near-zero permittivity" (ENZ). When the pump laser is tuned to the ENZ wavelength of AZO (usually falling within 1800-2200 nm), the real part of the permittivity ε' of the thin film passes through zero and enters the interval of ε'≈0, ε" still remains a small positive value. At this time, the normal electric displacement continuous equation determined by the Maxwell boundary condition

[0016] A huge "field squeezing" effect is generated on the interface: if AZO is regarded as medium 2 and the substrate used is medium 1, when ε2→0, the normal component E2 must be sharply amplified to maintain the flux continuity, forming

[0017] The order of magnitude of the intrinsic field enhancement is improved. This intrinsic field enhancement is independent of the thickness, and even if the thin film is only about 50 nm, it can still provide a peak field of the order of tens of megavolts per meter on the surface, which is provided by traditional bulk crystals, thereby amplifying the originally weak second-order nonlinear susceptibility χ⁽²⁾ to an observable level. Since this field enhancement effect only acts on the normal component of the light field, the light needs to be obliquely incident to excite the sample, and the normal component of the light field polarization is provided by the incident angle. At the same time, the low group velocity in the ENZ region makes the pump pulse "slow down" in the thin film, which equivalently prolongs the light-matter interaction time, further accumulating the nonlinear polarization intensity. For ITO, a highly doped degenerate semiconductor, its free electron dispersion can be described by the Drude model:

[0018] where ω p is the plasma frequency, which is proportional to the carrier concentration N. This explains why the additional growth of an Al2O3 layer at the substrate-AZO thin film interface can change the terahertz wave radiation properties of the samples of the present application, because the presence of Al2O3 changes the doping concentration of AZO at the interface, further increasing its carrier concentration N, and ultimately affecting its permittivity curve.

[0019] In summary, the dynamic field enhancement induced by ENZ effect can efficiently convert the surface photorefractive-induced difference frequency polarization into a wideband THz radiation from 0.1 to 2.7 THz. Since the whole process occurs in a subwavelength scale, the natural condition of phase mismatch Δk ≈ 0 is automatically satisfied, and the bandwidth is no longer limited by the coherence length, but only determined by the pump pulse width and the response of the detection crystal. Therefore, a 50 nm thick AZO film on a quartz substrate can radiate THz waves with an amplitude comparable to that of a 1 mm thick ZnTe crystal under the transmission excitation. The physical origin of the present application is the simultaneous excitation of the electric field enhancement and nonlinear effect in the ENZ state. BRIEF DESCRIPTION OF DRAWINGS

[0020] Fig. 1 is a schematic diagram of the structure of a THz radiation device based on AZO nanofilms. The material itself consists of a substrate, an aluminum oxide (Al203) layer, and an AZO layer.

[0021] Fig. 2 is a schematic diagram of the process of generating THz waves by excitation of an AZO nanofilm sample.

[0022] Fig. 3 is a sample actually processed according to the present application. The structures of the three samples in the figure are all identical, consistent with the structure shown in Fig. 1. As can be seen from the figure, the three samples all maintain a relatively high visible light transmittance. Figure 1

[0023] Fig. 4(a) is the test result of the THz wave signal generated by the sample actually processed according to the present application, and the different curves in the figure represent different pump powers. As can be seen from the figure, the sample can generate stable THz waves under different powers, and the signal shape and phase are very stable. Fig. 4(b) is a spectrum diagram of the THz wave radiation generated by the sample. As can be seen from the figure, the spectrum occupied by the signal is very wide, and there is intensity distribution from 0.1 to 2.7 THz, which is a very high-quality wide-spectrum THz light source.

[0024] Fig. 5(a) shows the measured dielectric constant curve of the sample. It can be seen that the real part of the dielectric constant of the AZO film layer of the sample passes through zero at about 2030 nm, but due to the rise of the imaginary part of the dielectric constant in this wavelength range, the zero point of the comprehensive dielectric constant is calculated to be near 1970 nm. The solid part of Fig. 5(b) shows the measured THz signal intensity of the sample as the pump wavelength changes, and the dashed line in the figure represents the field amplification factor curve brought by ENZ calculated from the curve in Fig. 5(a). It can be seen that the experimental results and the calculated curve basically match, indicating that the THz generation of the sample is mainly controlled by the ENZ field amplification effect.

[0025] ​Figure 6 is a comparison chart of the THz signal intensity generated by the AZO thin film and a 1mm thick zinc telluride (ZnTe) crystal under the same test conditions. As can be seen from the chart, the THz radiation intensity generated by the AZO sample described in the present patent is about 1 / 3 of that of the zinc telluride, and the thickness is only 5*10 -5 times that of the zinc telluride, which is a very high THz wave generation efficiency. DETAILED DESCRIPTION

[0026] The present application will be further described below in conjunction with specific examples.

[0027] S1: First, place the double-sided or single-sided polished substrate into the magnetron sputtering chamber; in the samples tested in the present patent, a 1cm*1cm double-sided polished quartz substrate was used to ensure high transmittance of the laser pump during transmission excitation. The quartz substrate needs to be cleaned with propylene glycol before use to remove possible chemical residue on the surface and avoid affecting the final processing result.

[0028] S2: Heat the substrate in the magnetron sputtering chamber; in the samples tested in the present patent, a heating temperature of 600°C was used. The heating process needs to be maintained for about 40 minutes before it is considered complete.

[0029] S3: First, sputter Al2O3 on the substrate; in the samples tested in the present patent, the sputtering thickness of Al2O3 was about 5 nm. The process gas used in sputtering Al2O3 was argon, which was first pumped to a pressure of 10 -3 Pa below to meet the parameter requirements for processing the Al2O3 thin film. After reaching the target vacuum degree, continuously introduce 25 sccm of argon, while adjusting the pumping flux to keep the chamber pressure dynamically stable at the order of 10 -1 Pa. After about 100s of sputtering, the target thickness is reached.

[0030] S4: Use the AZO target to deposit AZO thin film on the substrate; in the samples tested in the present patent, direct current sputtering was used in this step, with a sputtering power of 50 W. The process gas used in sputtering AZO was argon, which was first pumped to a pressure of 3*10 -4 Pa below to meet the parameter requirements for processing the AZO thin film. After reaching the target vacuum degree, continuously introduce 15 sccm of argon, while adjusting the pumping flux to keep the chamber pressure dynamically stable at the order of 10 -1The sample should be rotated uniformly during sputtering to prevent the sputtered AZO from generating unidirectional crystal, which affects the in-plane isotropic characteristics of the AZO. In the sample tested in the present patent, the rotation speed is set to 45 rad / min. The sample should also be heated during the sputtering process. In the sample tested in the present patent, the sputtering time of the AZO is 7 min, and the AZO film with a thickness of about 50 nm is grown.

[0031] S5: The AZO film is characterized by a laser-pumped terahertz detection system. For the sample, a titanium sapphire regenerative amplifier is used as a front end, and the output center wavelength is 800 nm, the pulse width is 35 fs, and the repetition frequency is 1 kHz. A light parametric amplifier is used as a pump output, the center wavelength is adjustable from 1600 nm to 2500 nm, and the pulse width is 100 fs. A 1-mm-thick zinc telluride (ZnTe) crystal is used to detect the terahertz signal.

[0032] Obviously, the above embodiments of the present application are only examples for clearly illustrating the present application, and are used to help understand the method of the present application and its core idea, and are not intended to limit the embodiments of the present application. For those skilled in the art, on the basis of the above examples, some forms of improvement and modification can be made without departing from the principles and material properties of the present application. Any obvious changes and modifications of the technical solutions of the present application still fall within the protection scope of the present application.

Claims

1. A novel terahertz wave radiation device based on AZO (Aluminum-doped Zinc Oxide) nanofilm, comprising methods for material preparation and testing, characterized in that... Includes the following steps: S1: Place the double-sided or single-sided polished substrate into the magnetron sputtering cavity; S2: Heating the substrate inside the magnetron sputtering cavity; S3: Sputtering Al2O3 onto the substrate; S4: An AZO thin film is deposited on a substrate using an AZO target; S5: Terahertz radiation characterization of AZO thin films using a laser-pumped terahertz detection system.

2. The AZO thin film can generate terahertz waves by exciting its surface second-order nonlinear effect with a femtosecond laser, and can be made into a terahertz radiation source and other related devices. The amplitude and phase of the terahertz waves can be synchronously controlled by adjusting the wavelength, power or incident angle of the pump light.

3. The method for realizing the novel terahertz radiation source according to claim 1, characterized in that, The substrate in S1 can be an insulator that allows terahertz waves to pass through, such as sapphire, quartz, or glass.

4. The method for implementing the novel terahertz radiation source according to claim 1, characterized in that, The heating temperature in S2 can vary within the range of 300℃-600℃.

5. The method for implementing the novel terahertz radiation source according to claim 1, characterized in that, The sputtering thickness of the Al2O3 layer in S3 can vary in the range of 0-10 nm.

6. The method for implementing the novel terahertz radiation source according to claim 1, characterized in that, The near-zero dielectric constant (ENZ) wavelength of the AZO thin film is in the range of 1800–2200 nm, preferably 1900 nm or 2000 nm.

7. The method for realizing the novel terahertz radiation source according to claim 1, characterized in that, The laser-pumped terahertz detection system in S5 uses a femtosecond laser as the excitation source. During excitation, the pump light needs to be obliquely incident on the sample, and the incident angle can be selected within the range of 10°-80°.

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