A terahertz wave detection system based on electromagnetic wave and optical phonon coupling

Abstract

This invention provides a terahertz wave detection system based on electromagnetic wave and optical phonon coupling, relating to the field of terahertz wave detection, including: a laser source, an acousto-optic modulator, a dichroic mirror, a wide-focusing microscope, and [C8H 20 The invention comprises an N4]Cu2Br6 ionic crystal thin film, a plano-convex lens, a single-photon detector, a single-photon counter, and a signal generator. An acousto-optic modulator generates a pulsed laser beam, which is incident on a dichroic mirror. The crystal is raised to an excited state and receives the incident terahertz wave, emitting fluorescent photons. The fluorescence is focused into parallel light by a wide-focusing microscope and passes through the dichroic mirror and plano-convex lens to reach the single-photon detector. The single-photon detector is connected to the single-photon counter, which is connected to the signal generator. Information about the terahertz wave is obtained based on changes in the number of photons. This invention determines the information of the terahertz wave by monitoring changes in fluorescence intensity, representing a new development direction for terahertz signal detectors and possessing significant scientific research and application value.

Description

Technical Field

[0001] This invention relates to the field of terahertz wave detection technology, and more specifically, to a terahertz wave detection system based on the coupling of electromagnetic waves and optical phonons. Background Technology

[0002] Terahertz waves, located between the infrared and microwave bands in the electromagnetic spectrum, represent a transitional region between photonics and electronics. The unique characteristics of this frequency band, such as high penetration, high bandwidth, and low photon energy, make it highly promising for applications in security detection, communication, and bioimaging. However, terahertz waves still face many challenges in generation and detection. From an electronics perspective, the high frequency of terahertz waves leads to high heat and high losses in waveguides; from a photonics perspective, the weak photon energy of terahertz waves, far smaller than the optical bandgap of typical semiconductors, poses technical challenges to the fabrication of terahertz sources and detectors. Currently, developing high-performance terahertz sources and detectors remains the core of terahertz technology.

[0003] Terahertz detection technology can be broadly categorized into two types: terahertz spectral detection and direct detection. Terahertz spectral detection can measure terahertz pulse signals and spectra, while direct detection typically only detects the power and energy of terahertz radiation. Regardless of the specific detection technique or method, they are all based on the physical effects caused by terahertz waves. For example, electro-optic detection based on the Paulcile effect detects terahertz waveforms by detecting changes in the refractive index of electro-optic crystals caused by terahertz waves. Other methods include photoconductive antenna detection based on the accelerated motion of charged particles in an electromagnetic field, and gas plasma detection combined with mixing techniques. Furthermore, detection techniques based on the thermal effects of terahertz waves are relatively mature, such as pyroelectric terahertz detectors. However, this thermal effect usually comprises two indistinguishable parts: the driving effect of terahertz waves on charge carriers and the coupling effect between terahertz waves and phonons. Terahertz detectors purely based on electromagnetic wave-phonon coupling have not yet been developed. Therefore, designing a terahertz detection system based on electromagnetic wave-phonon coupling is a technical problem that needs to be solved. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art or related technologies, and provides a terahertz wave detection system based on electromagnetic wave and optical phonon coupling, which realizes effective detection of terahertz waves based on electromagnetic wave-optical phonon coupling.

[0005] This invention is achieved through the following technical solution: a terahertz wave detection system based on electromagnetic wave and optical phonon coupling, comprising: a laser source, an acousto-optic modulator, a dichroic mirror, a wide-focusing microscope, and [C8H 20The system comprises a [N4]Cu2Br6 ion crystal thin film, a plano-convex lens, a single-photon detector, a single-photon counter, and a signal generator; wherein, a laser source is used to provide pump light; an acousto-optic modulator, while receiving the pump light, generates a pulsed laser according to the control signal of the signal generator, which is incident on a dichroic mirror; a wide-focusing microscope is provided on the reflected light path of the dichroic mirror to project the pump light onto [C8H ... 20 The surface of the N4]Cu2Br6 ionic crystal thin film elevates the crystal to an excited state, during which [C8H 20 The N4]Cu2Br6 ionic crystal thin film is also used to receive incident terahertz waves and emit fluorescent photons. The fluorescence is focused into parallel light by a wide-focusing microscope and passes through a dichroic mirror and a plano-convex lens to reach a single-photon detector. The single-photon detector is connected to a single-photon counter, which is connected to a signal generator. The signal generator is used to generate laser source switching drive signals and counting trigger signals and record the number of photons per unit time. Within the fluorescence lifetime, the information of the terahertz wave is obtained based on the change in the number of photons.

[0006] In this technical solution, [C8H 20 [N4]Cu2Br6 ionic crystals have a large band gap of approximately 4.2 eV and a fluorescence lifetime of up to 113 μs, thus enabling the detection of terahertz signals within a single cycle, and the detection system is relatively inexpensive. [C8H 20 The fluorescence emission of [N4]Cu2Br6 originates from exciton recombination, where excitons are bound to [Cu2Br6]. 4- The structure is surrounded by a large exciton binding energy, belonging to Frank excitons. Therefore, there are almost no free charge carriers in the crystal within its fluorescence lifetime, which suppresses the absorption and interference of charge carriers on terahertz waves.

[0007] The terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention preferably further includes: a terahertz source for generating terahertz wave irradiation [C8H] 20 The N4]Cu2Br6 ionic crystal thin film overlaps with the pump laser spot.

[0008] According to the terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention, preferably, the laser source is a 280nm laser, the pump light emitted by the laser is converted into a pulsed laser after passing through an acousto-optic modulator, and the first-order diffraction spot is taken and incident on the dichroic mirror at an angle of 45°.

[0009] According to the terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention, preferably, the signal generator outputs a square wave with a high level of +5V, a low level of 0V, a frequency of 10kHz, and a duty cycle of 20% as a driving signal to drive the acousto-optic modulator, so that the acousto-optic modulator controls the switching state of the laser source according to the high and low levels, and the laser pulse time interval is 200μs.

[0010] According to the terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention, preferably, the time resolution of the single photon counter is 10 ps; the trigger counting signal is controlled by a signal generator, and the signal mode is the same as the mode driving the acousto-optic modulator; the detection period is greater than the fluorescence lifetime.

[0011] According to the terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention, preferably, the dichroic mirror reflects laser with a wavelength less than 400 nm, allowing fluorescence with a wavelength greater than 450 nm to pass through, and the pump laser of the incident light forms a 45° angle with the dichroic mirror.

[0012] The terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention preferably uses a silicon single-photon detector with an operating range of 400nm to 1000nm and a peak quantum efficiency of 72% at 650nm.

[0013] In the terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention, preferably, the numerical aperture of the wide-focusing microscope is 0.78.

[0014] The terahertz wave detection system based on electromagnetic wave and optical phonon coupling provided by the present invention preferably includes [C8H] 20 The preparation method of [N4]Cu2Br6 crystalline thin film includes: adding 0.05 mmol C8H 20 N4 was dissolved in 2 mL of water, and 0.05 mmol of cuprous bromide was dissolved in 2 mL of hydrobromic acid. The two solutions were rapidly mixed at room temperature and allowed to stand for 1 hour. [C8H] was prepared by spin coating. 20 [N4]Cu2Br6 crystalline thin film: The mixed solution was dropped onto a silicon wafer, the spin-coating speed was set to 4500 rpm, and the spin-coating time was 40 seconds; after spin-coating, it was placed on a heating platform at 100 degrees Celsius to dry for 60 seconds. Preparation of [C8H 20 The entire process of operating the N4]Cu2Br6 crystalline thin film was carried out in a glove box under nitrogen protection.

[0015] The beneficial effects achieved by this invention include at least the following: due to [C8H 20The N4]Cu2Br6 ionic crystal has a large exciton binding energy, which avoids the absorption of terahertz waves by free carriers, and has a long fluorescence lifetime, which can complete terahertz wave detection within one lifetime cycle, reducing time and device cost. Attached Figure Description

[0016] Figure 1 A schematic diagram of the optical path of a terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to an embodiment of the present invention is shown.

[0017] Figure 2 A schematic diagram of an electromagnetic wave-optical phonon coupling mode according to an embodiment of the present invention is shown.

[0018] Figure 3 [C8H] is shown according to an embodiment of the present invention. 20 A schematic diagram of the phonon density of states and Raman spectrum of N4]Cu2Br6 crystal.

[0019] Figure 4 A schematic diagram of the terahertz response time according to an embodiment of the present invention is shown. Detailed Implementation

[0020] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] According to Huang Kun's research in 1950: Equations (1) and (2) reveal the basic principle of electromagnetic wave coupling with phonons:

[0022]

[0023] P = b 21 W+b 22 E (2)

[0024] In the formula It represents a macroscopic quantity that describes the motion of optical waves. Ω represents the reduced mass of ions in the unit cell, μ is the unit cell volume, and Ω is the ion reduced mass. + μ - The displacements are for positive and negative ions. E is the macroscopic electric field intensity, and P represents the macroscopic polarization intensity. Combining this with Maxwell's equations, the electromagnetic wave-optical phonon coupling frequency is obtained:

[0025]

[0026] In the formula, ε(0) and ε(∞) represent the static dielectric constant and the high-frequency dielectric constant, respectively; ω0 represents the transverse optical wave frequency; and c and q represent the speed of light and the wave number, respectively.

[0027] like Figure 2 As shown in the figure: and These are high-frequency and low-frequency electromagnetic waves, respectively; ω TO and ω LO Let represent the frequencies of the transverse and longitudinal optical waves, respectively; and plot the coupling mode of the electromagnetic wave and the optical phonon according to equation (3). From this, we can see that... and The two lines and the transverse optical phonon frequency ω TO And longitudinal optical phonon frequency ω LO In the intersecting region, electromagnetic waves and optical phonons couple. For example, the optical phonon frequency of CH3NH3PbI3 is 1-3 THz. Therefore, the phonon properties of ionic crystals can be studied by detecting the absorption spectrum in the terahertz band. Based on the above principle, the coupling effect between terahertz waves and optical phonons in ionic crystals can be used to detect terahertz waves.

[0028] Since direct observation of phonons is extremely difficult, this invention utilizes the fluorescence properties of crystals to reflect phonon states, enabling indirect measurement of terahertz waves. Because strong electron-phonon coupling exists in most semiconductor ionic crystals, phonon characteristics are crucial determinants of fluorescence intensity, energy, and lifetime. Among these, fluorescence lifetime is closely related to phonon energy. The fluorescence lifetime τ can be represented by equation (4).

[0029]

[0030]

[0031] In the formula τ r τ represents a radiative transition. nr This represents a nonradiative transition. The nonradiative transition rate W can be represented by equation (5). Where α and β are material-related parameters, and ΔE is the transition energy. This refers to phonon energy.

[0032] Based on the above principles, this invention proposes a detection method and its supporting system: the terahertz signal is first applied to the optical phonon to increase the phonon energy, thereby increasing the nonradiative transition probability and changing the fluorescence lifetime, and finally the indirect detection of the terahertz wave is achieved by detecting the change in fluorescence lifetime.

[0033] In addition, to achieve accurate detection of terahertz waves, the following problems need to be addressed: charge carriers in ionic crystals are easily coupled with terahertz waves and cannot be distinguished from phonons; furthermore, signals with short fluorescence lifetimes present technical challenges for detection.

[0034] To realize a terahertz wave detection system based on electromagnetic wave and optical phonon coupling and overcome the above-mentioned technical problems, this invention provides the following... Figure 1The terahertz wave detection system shown is based on electromagnetic wave and optical phonon coupling, including: a laser 1, an acousto-optic modulator 2, a dichroic mirror 3, a wide-focusing microscope 4, and [C8H] 20 5. N4]Cu2Br6 ion crystal thin film; 6. Plano-convex lens; 7. Single-photon detector; 8. Single-photon counter; 9. Signal generator; and 10. Terahertz source. Figure 2 As shown in the figure, the shaded area represents the optical phonon mode frequency range, [C8H 20 The optical phonon energy of N4]Cu2Br6 crystal is in the range of 0.1-7.5 THz, which theoretically enables the detection of pulsed or continuous terahertz signals in the range of 0.1-7.5 THz.

[0035] In this embodiment, a continuous-wave laser with a wavelength of 280 nm is used as the pump source. The pump light passes through an acousto-optic modulator to form a pulsed laser. A continuous high-level signal is applied to the acousto-optic modulator to keep it operational. At this point, the laser exhibits zero-order, first-order, and second-order diffraction spots. The first-order diffraction spot is selected and incident on a dichroic mirror at a 45° angle. The pulse width and frequency of the generated laser pulse are controlled by a signal generator. The signal generator has two output ports, connected to the acousto-optic modulator and a single-photon counter, respectively. A square wave with a high level of +5V, a low level of 0V, a frequency of 10kHz, and a duty cycle of 20% drives the acousto-optic modulator. The acousto-optic modulator controls the laser switch according to the high and low levels, and the time interval between the two output laser pulses is approximately 200 μs. The dichroic mirror reflects lasers with wavelengths less than 400 nm while allowing fluorescence with wavelengths greater than 450 nm to pass through. The pump laser is angled at 45° to the dichroic mirror to maximize reflection and transmittance. Pump laser is incident on [C8H] through a wide-focusing microscope 20 On the surface of the N4]Cu2Br6 ionic crystal thin film, the ionic crystal emits fluorescent photons. The emitted fluorescence is scattered light, which is collected and focused into parallel light by a wide-focusing microscope, and then passes through a dichroic mirror and a plano-convex lens to reach the single-photon detector. The numerical aperture of the wide-focusing microscope is 0.78. The plano-convex lens focuses the collected fluorescence onto the photosensitive area of ​​the single-photon detector. The single-photon detector is a silicon single-photon detector with an operating range of 400-1000 nm and a peak quantum efficiency of 72% at 650 nm. The output signal of the single-photon detector is a pulse signal, with each pulse representing one photon. The pulse signal is input to the single-photon counter. The time resolution of the single-photon counter is set to 10 ps. The trigger counting signal is controlled by a signal generator, and the signal mode is the same as the mode driving the acousto-optic modulator. Due to the high time resolution of the single-photon counter, the test can be completed within one cycle (200 μs).

[0036] According to another embodiment of the present invention, the working principle and detection process of the terahertz wave detection system based on electromagnetic wave and optical phonon coupling of the above embodiments are also disclosed:

[0037] The 280nm continuous laser beam first passes through an acousto-optic modulator, which switches the laser on and off, outputting a pulsed laser-like signal. The drive signal for the acousto-optic modulator is provided by a signal generator. The pump light emitted from the acousto-optic modulator passes through a dichroic mirror, which reflects light with wavelengths below 400nm and transmits light with wavelengths above 450nm. The pump light then illuminates [C8H] through a wide-focusing microscope lens. 20 On a [N4]Cu2Br6 crystal thin film, [C8H] 20 [N4]Cu2Br6 emits fluorescence; the wide-focusing microscope lens focuses the pump light onto the crystal surface and collects the fluorescence, forming collimated light that passes through a dichroic mirror; a single-photon detector detects the fluorescent photons and outputs a single-photon pulse signal, which is acquired by a single-photon counting system to obtain the number of photons per unit time; the single-photon counter has a time resolution of 10 ps, ​​enabling the measurement of fluorescence lifetime within one cycle; the signal generator is configured to output a square wave signal with a frequency of 10 kHz, with each high-level signal having a time interval of approximately 200 μs. The two outputs of the signal generator are connected to an acousto-optic modulator and a single-photon counter, respectively, serving as the laser switch drive signal and the counting trigger signal. At this time, the pump laser raises the ion crystal to an excited state. After a short time delay, the trigger signal input from the signal generator starts the single-photon counter to record the number of photons per unit time of 10 ps, ​​with a recording time of 200 μs per cycle. Within the fluorescence lifetime (approximately 113 μs), the ionic crystal interacts with the terahertz wave through electromagnetic wave-optical phonon coupling, causing a change in the number of photons detected by the counter. Finally, the information of the terahertz wave under test is obtained based on the change in the number of photons.

[0038] Figure 4 The above terahertz detection method yields a terahertz response time diagram. The dashed line represents the data measured under no terahertz field conditions, while the solid line represents the data measured under terahertz field conditions. Arrow (1) in the diagram indicates the start time of the pump laser, and arrow (2) indicates the incident time of the pulsed terahertz wave. Compared to the curve without a terahertz field, the fluorescence intensity rapidly decreases when the terahertz wave is incident on the crystal surface, which can characterize the recognition of the terahertz signal. During the experimental detection of terahertz waves, a pulsed terahertz source with a frequency range of 0.1-4.5 THz was used. Generally, the terahertz source is triggered to emit terahertz waves tens of picoseconds after the triggering pulse laser. The terahertz waves are incident on [C8H 20The surface of the N4]Cu2Br6 ionic crystal thin film coincides with the pump laser spot, and the terahertz spot is generally larger than the laser spot.

[0039] According to yet another embodiment of the present invention, [C8H] is also disclosed. 20 A method for preparing N4]Cu2Br6 crystalline thin films includes: adding 0.05 mmol of C8H... 20 N4 (Rhodotoryl) was dissolved in 2 mL of water; 0.05 mmol of cuprous bromide was dissolved in 2 mL of hydrobromic acid. The two solutions were rapidly mixed at room temperature and allowed to stand for 1 hour. [C8H] was prepared by spin-coating. 20 [N4]Cu2Br6 crystalline thin film: The mixed solution was dropped onto a silicon wafer, which was rotated at 4500 rpm for 40 seconds. It was then placed on a heating platform at 100 degrees Celsius to dry for 60 seconds. The entire process was carried out in a nitrogen-protected glove box.

[0040] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A terahertz wave detection system based on electromagnetic wave and optical phonon coupling, characterized in that, include: Laser source, acousto-optic modulator, dichroic mirror, wide-focusing microscope, [C8H 20 The system comprises a [N4]Cu2Br6 ion crystal thin film, a plano-convex lens, a single-photon detector, a single-photon counter, and a signal generator; wherein, the laser source is used to provide pump light; the acousto-optic modulator, while receiving the pump light, generates a pulsed laser according to the control signal of the signal generator and directs it onto the dichroic mirror; the wide-focusing microscope is provided on the reflected light path of the dichroic mirror to project the pump light onto the [C8H]Cu2Br6 ion crystal thin film. 20 The surface of the N4]Cu2Br6 ionic crystal thin film elevates the crystal to an excited state, during which the [C8H] 20 The N4]Cu2Br6 ionic crystal thin film is also used to receive incident terahertz waves and emit fluorescent photons. The fluorescence is focused into parallel light by a wide-focusing microscope and passes through the dichroic mirror and the plano-convex lens to reach the single-photon detector. The single-photon detector is connected to the single-photon counter, and the single-photon counter is connected to the signal generator. The signal generator is used to generate a laser source switching drive signal and a counting trigger signal and record the number of photons per unit time. Within the fluorescence lifetime, the information of the terahertz wave is obtained based on the change in the number of photons.

2. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 1, characterized in that, Also includes: A terahertz source is used to generate the terahertz wave to irradiate the [C8H] 20 The N4]Cu2Br6 ionic crystal thin film overlaps with the pump laser spot.

3. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 1, characterized in that, The laser source is a 280nm laser. The pump light emitted by the laser is converted into a pulsed laser after passing through an acousto-optic modulator. The first-order diffraction spot is taken and incident on the dichroic mirror at a 45° angle.

4. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 1, characterized in that, The signal generator outputs a square wave with a high level of +5V, a low level of 0V, a frequency of 10kHz, and a duty cycle of 20% as a driving signal to drive the acousto-optic modulator, so that the acousto-optic modulator controls the switching state of the laser source according to the high and low levels, and the laser pulse time interval is 200μs.

5. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 4, characterized in that, The time resolution of the single-photon counter is 10 ps; the trigger counting signal is controlled by the signal generator, and the signal mode is the same as the mode driving the acousto-optic modulator; the detection period is greater than the fluorescence lifetime.

6. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 1, characterized in that, The dichroic mirror reflects laser light with a wavelength less than 400 nm, while allowing fluorescence with a wavelength greater than 450 nm to pass through. The pump laser of the incident light forms a 45° angle with the dichroic mirror.

7. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 1, characterized in that, The single-photon detector is a silicon single-photon detector with an operating range of 400nm to 1000nm and a peak quantum efficiency of 72% at 650nm.

8. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to claim 1, characterized in that, The wide-focusing microscope has a numerical aperture of 0.

78.

9. The terahertz wave detection system based on electromagnetic wave and optical phonon coupling according to any one of claims 1 to 8, characterized in that, The [C8H] 20 The methods for preparing N4]Cu2Br6 crystalline thin films include: 0.05 mmol C8H 20 Dissolve N4 in 2 mL of water, dissolve 0.05 mmol of cuprous bromide in 2 mL of hydrobromic acid, mix the two solutions rapidly at room temperature, and let stand for 1 hour. [C8H] was prepared by spin coating. 20 [N4]Cu2Br6 crystal thin film: The mixed solution was dropped onto a silicon wafer, the silicon wafer rotation speed was set to 4500 rpm, and the spin coating time was 40 seconds; After spin coating, place it on a heating platform at 100 degrees Celsius to dry for 60 seconds.

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