Multi-photon detection system and method based on frequency up-conversion THz wave
By utilizing the spatial resolution capability of multiphoton detectors and temporal and spatial domain coincidence detection technology, the problem of spontaneous parametric upconversion fluorescence interference was solved through a THz upconversion multiphoton detection system based on frequency upconversion, thus achieving effective detection of THz upconversion signals and room temperature photon-level counting.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- SHANDONG UNIV
- Filing Date
- 2026-01-13
- Publication Date
- 2026-04-10
AI Technical Summary
Existing THz detection technologies are unable to effectively detect interference from spontaneous parametric upconversion fluorescence, resulting in the inability of single-unit SPDs to effectively detect THz upconversion signals.
A THz upconversion multiphoton detection system is adopted, which utilizes the spatial resolution capability of multiphoton detectors and temporal and spatial domain coincidence detection technology, combined with spatial optical gate technology, to achieve effective detection of THz upconversion signals.
In the presence of spontaneous parametric upconversion fluorescence, it can effectively detect THz upconversion signals, achieve room temperature photon-level counting detection, and reduce external noise interference.
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Figure CN121830489A_ABST
Abstract
Description
Technical Field
[0001] This application belongs to the field of photon detection technology, specifically relating to a THz wave multiphoton detection system and method based on frequency upconversion. Background Technology
[0002] THz waves refer to electromagnetic waves with frequencies ranging from 0.1 to 10 THz (1 THz = 10¹² Hz) and wavelengths approximately between 0.03 mm and 3 mm. Terahertz waves occupy a transitional position between microwaves and infrared waves in the electromagnetic spectrum. They exhibit long-wave characteristics similar to electronics, while also possessing short-wave characteristics similar to optics, offering broad application prospects. Terahertz waves possess excellent photoelectric properties and occupy a relatively unique position in the electromagnetic spectrum. This uniqueness endows them with many distinctive properties, including but not limited to biosafety, penetrability, and "fingerprint" characteristics. Their applications can cover multiple fields such as imaging, spectrum analysis, and communications, with specific applications including security monitoring, biomedical monitoring, materials quality monitoring, environmental atmospheric monitoring, and radar communications.
[0003] However, the application of terahertz technology relies heavily on highly sensitive THz detection methods. Because typical THz radiation sources usually have low output power, various molecules in the atmosphere strongly absorb THz waves, and there are many interference signals in the environment that hinder THz detection, many applications require highly sensitive detection methods. In THz photon counting, existing techniques such as superconducting nanowire single-photon detectors all require cryogenic cooling.
[0004] THz frequency upconversion detection, as a detection technique for THz waves, holds promise for indirectly detecting THz photons by upconverting the frequency of the target THz wave to a near-infrared signal and then detecting photons in the upconverted signal. In traditional upconversion photon counting detection (e.g., mid-infrared photon detection based on frequency upconversion), to avoid the influence of difference-frequency spontaneous parametric downconversion fluorescence, a single-unit SPD can be used to detect the upconversion signal, typically employing a sum-frequency upconversion scheme for photon counting. However, recent research indicates that spontaneous parametric fluorescence exists in the THz band for both sum-frequency and difference-frequency upconversion, making single-unit SPDs ineffective for detecting the upconversion signal. Summary of the Invention
[0005] To address this technical problem, this invention proposes a THz upconversion multiphoton detection system and method. This technology transforms THz upconversion signal detection from a single time-domain detection to a time- and spatial-domain coincidence detection. Simultaneously, utilizing the spatial detection capability of a multiphoton detector, when facing time-synchronized fluorescence, this technology can be combined with spatial gate technology to perform detection within the expected signal arrival gate, enabling the detection of THz upconversion signals exhibiting spontaneous parametric upconversion fluorescence. Ultimately, it achieves effective detection of THz upconversion signals with spontaneous parametric upconversion fluorescence. The technical solution is as follows: A THz wave multiphoton detection system based on frequency upconversion includes a pump source unit, a beam combiner, a nonlinear crystal, a beam splitter, a spatial photon counting detector, an optical shaping device, and a controller. The pump source unit provides a pump source for emitting an initial laser beam as the pump light during the upconversion process. The beam combiner combines the THz wave to be measured and the pump source beam, which are then collinearly incident on the nonlinear crystal. The nonlinear crystal generates an upconversion signal based on phase matching. The beam splitter spatially separates the upconversion signal and the pump source beam. The optical shaping device spatially shapes the upconversion signal and fluorescence beam. The spatial photon counting detector detects and counts photons in the shaped upconversion signal. The controller includes time gating and spatial gating. Time gating controls the time synchronization between the incident THz wave to be measured, the pump source, and the spatial photon counting detector. Spatial gating controls the size of the spatial gate of the spatial photon counting detector.
[0006] Preferably, the pump source unit includes a laser, and a polarization unit and an oscillation unit are provided in the optical path between the pump source unit and the beam combiner. The pump source enters the nonlinear crystal through the polarization unit and the oscillation unit.
[0007] Preferably, the pump light source passes through a filter and lens L1 and is simultaneously incident on a beam combiner along with the THz signal to be measured. The beam combiner includes a parabolic mirror with a small hole. The THz wave to be measured passes through a filter (germanium filter) and the pump light source is combined on the parabolic mirror.
[0008] Preferably, the optical shaping device includes a reflector M4, a lens L2, and a lens L3; the nonlinear crystal is a DAST crystal or a DSTMS crystal. According to the second-order nonlinear effect, under type 0 phase matching, the upconversion signal and the THz signal to be measured are frequency-matched in the nonlinear crystal to generate an upconversion signal. The upconversion signal is collimated and paralleled by the reflector M4 and the lens L2. Since the emitted near-infrared upconversion signal is mixed with residual upconversion pump and spontaneous parametric upconversion fluorescence, the collimated upconversion signal is separated from the residual pump by the two gratings of the beam splitting unit. After being filtered by the grating and the bandpass filter, it is focused onto the spatial photon counting detector by the lens L3, and the spatial photon counting detector performs photon counting.
[0009] Preferably, the two gratings are near-infrared reflective gratings with a grating density of a = 1200 lines / mm and a blaze wavelength of 1μm.
[0010] Preferably, the relationship between photon count and incident light energy includes: ； Where E represents the single pulse energy in the incident space photon counting detector; or This represents the correction factor, which is related to the detector's detection efficiency and detector parameters; C This represents the count value of the photon counting detector; h Represents Planck's constant; n This represents the photon frequency of the incident photon counting detector.
[0011] A multiphoton detection method for THz waves based on frequency upconversion is proposed. A pump source, acting as an upconversion pump light, and the THz wave to be measured are combined by a beam combiner and then incident into a nonlinear crystal. The beam combiner transmits the pump light and reflects the THz wave to be measured. The two waves generate an upconversion signal in the nonlinear crystal based on second-order nonlinear effects, phase matching conditions, and frequency. A beam splitter spatially splits the upconversion signal light and the pump light. The upconversion signal is then spatially shaped by a beam shaping device. Simultaneously, time gating controls the time delay of the THz wave to be measured, the pump source, and the spatial photon counting detector, ensuring that the THz wave to be measured and the pump source are simultaneously incident into the nonlinear crystal. At the same time, the opening time of the spatial photon counting detector is controlled, allowing the upconversion signal to be incident into the detector for coincidence counting.
[0012] Preferably, the beam splitter can spatially separate the upconversion signal and the upconversion pump at a set angle according to the wavelength difference, and can maintain the invariance of the optical path for a specific wavelength.
[0013] Preferably, the time gating uses the THz wave to be measured as an external trigger, and the time for the THz wave to be measured to be incident on the nonlinear crystal is... The time it takes for the pump light to enter the nonlinear crystal is The time it takes for the upconversion signal to be incident on the space photon counting detector is Time gating allows for the control of time synchronization by adjusting the time delay between different waves and the detector's gate opening time. The relationships between these delays include: Pump light source time delay ; Space photon counting detector door opening time delay .
[0014] Compared with the prior art, the beneficial effects of this application are as follows: This invention spatially repositions the THz upconversion signal and spontaneous parametric upconversion fluorescence, and utilizes the spatial resolution capability of a multiphoton detector to still have the ability to detect unavoidable spontaneous parametric upconversion fluorescence in the upconversion signal. Attached Figure Description
[0015] Figure 1 This is a frequency upconversion THz wave multiphoton detection technology.
[0016] Figure 2 This is a schematic diagram of an embodiment of THz wave multiphoton detection technology based on frequency upconversion.
[0017] 1-Beam combining device, 2-Nonlinear crystal, 3-Beam splitting unit, 4-Optical shaping device, 5-Spatial photon counting detector. Detailed Implementation
[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. It should be understood that the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.
[0019] This invention proposes a THz wave multiphoton detection system based on frequency upconversion. By transforming time-domain detection into time- and spatial-domain composite detection and fully utilizing the spatial photon resolution capability of a near-infrared SPD array, room-temperature photon-level counting detection of THz waves is achieved. The technical device includes: a pump source, a beam combiner, a nonlinear crystal, a beam splitter (including two gratings), an optical shaping device (mirror M4, lens L2, and lens L3), a spatial photon counting detector, and a controller (setting time and spatial gating). The nonlinear crystal upconverts the THz wave frequency to a near-infrared light signal, but simultaneously generates spontaneous parametric upconversion fluorescence noise consistent with the temporal, spatial, and spectral characteristics of the signal, resulting in signal-noise mixing and causing the single-unit SPD to fail. This technology uses an optical shaping device to spatially separate the signal and noise photons, and uses a near-infrared spatial photon counting detector combined with spatiotemporal coincidence measurement to achieve photon counting detection of the signal, thereby realizing room-temperature photon counting detection of THz waves.
[0020] A polarization unit and an oscillation unit are provided in the optical path between the pump light source unit and the beam combiner. The pump light source enters the nonlinear crystal through the polarization unit and the oscillation unit.
[0021] The polarization unit includes a half-wave plate (HWP) and a polarization beam splitter (PBS); the oscillation unit is either a KTP-OPO or a BBO-OPO. Spatial photon counting detectors include, but are not limited to, indium gallium arsenide (InGaAs) MPPC and MCT e-APD arrays.
[0022] The pump light and the THz wave to be measured are combined by a beam combiner and then incident on a nonlinear crystal. The beam combiner transmits the pump light and reflects the THz wave to be measured. The two waves generate an upconversion signal light in the nonlinear crystal according to second-order nonlinear effects, phase matching conditions, and frequency. A beam splitting system spatially separates the upconversion signal light and the pump light. The upconversion signal is then spatially shaped by a beam shaper to achieve a uniform beam size. Simultaneously, utilizing the spatial resolution of a spatial photon counting detector, this technique can be combined with spatial gating to perform detection within the expected gate of the time-synchronized fluorescence. Time gating controls the time delay of the THz wave to be measured, the pump source, and the spatial photon counting detector, ensuring that the THz wave to be measured and the pump source are simultaneously incident on the nonlinear crystal. Simultaneously, the opening time of the spatial photon counting detector is controlled, allowing the upconversion signal to be incident on the detector for coincidence counting, thereby reducing interference from external noise on signal detection. Spatial gating works in conjunction with optical shaping devices to reduce the impact of fluorescence and external noise on signal detection by controlling the size of the spatial optical gate of the spatial photon counting detector.
[0023] The relationship between photon count and incident light energy includes: ； Where E represents the single pulse energy in the incident space photon counting detector; or This represents the correction factor, which is related to the detector's detection efficiency and detector parameters; C This represents the count value of the photon counting detector; h Represents Planck's constant; n This represents the photon frequency of the incident photon counting detector.
[0024] Time gating uses the THz wave under test as an external trigger, and the time it takes for the THz wave to be incident on the nonlinear crystal is... The time it takes for the pump light to enter the nonlinear crystal is The time it takes for the upconversion signal to be incident on the space photon counting detector is Time gating allows for the control of time synchronization by adjusting the time delay between different waves and the detector's gate opening time. The relationships between these delays include: Pump light source time delay ; Space photon counting detector door opening time delay .
[0025] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. It should be noted that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, for ease of description, the accompanying drawings show only the parts relevant to the invention, not the entire structure.
[0026] Example 1: This embodiment provides an architecture for THz wave multiphoton detection technology based on frequency upconversion. An initial laser emits an initial 532nm laser beam with a pulse width of 7ns and a pulse repetition frequency of 100Hz. The 532nm laser beam emitted from the laser passes through a half-wave plate and a polarizing beam splitter, and then enters the KTP-OPO through a 45° reflecting mirror M1 (AR@532 nm, HR@1.2-1.7μm). The KTP crystal has a size of 5*10*27 mm3, with coatings on both sides of AR@532 & 1200-1700 & 750-950nm. The crystal cutting angle is designed so that the angle between the laser beam and the optical axis is 63° when the laser is incident normally. The KTP crystal is fixed on a rotary motor, and different rotation angles can achieve different phase matching to obtain a tunable near-infrared output of 1.25~1.65 μm. The OPO front cavity mirror M2 (AR@532&750-950nm, T=30%@1.2-1.7μm) and the rear cavity mirror M3 (HR@532&1200-1700&750-950nm) are both planar mirrors. The bichromatic anisotropic window consists of a 100mm focal length off-axis parabolic mirror and a 100μm thick germanium sheet. The parabolic mirror has a small hole in the center to facilitate the transmission of the upconversion pump. The THz signal to be detected is collimated and focused by the two parabolic mirrors and then incident on the DAST crystal. The germanium sheet in the middle blocks visible light and near-infrared light from the environment, so that there is no external light noise interference during the upconversion process.
[0027] By adjusting the upconversion pump wavelength output from the KTP-OPO, it passes through a lens L1 (f=150mm) and is simultaneously incident on a 0.75mm thick DAST crystal along with the THz signal to be measured. Based on the second-order nonlinear effect, under type 0 phase matching (all three waves are o-light, polarization direction is horizontal), the upconversion signal and the THz signal to be measured can be summed in the DAST crystal to generate a near-infrared upconversion signal. The upconversion signal is collimated and paralleled by a mirror M4 (HR@1.2-1.7μm) and a lens L2 (f=300mm). Since the emitted near-infrared upconversion signal contains residual upconversion pump and spontaneous parametric upconversion fluorescence, the collimated upconversion signal is separated from the residual pump by a beam splitting system using two gratings. The two gratings are near-infrared reflective etched gratings (Thorlabs, GR25-1210), with a etch density of a=1200 lines / mm and a blaze wavelength of 1μm.
[0028] After passing through the dual grating filter, the near-infrared upconversion signal is focused onto the detector C-RED 1 (Andor) by the lens L3 (f=50mm), which serves as a spatial beam repositioning device, after passing through the corresponding bandpass filter, thus realizing multi-photon detection of the near-infrared upconversion signal.
[0029] Example 2: This embodiment provides an architecture for THz wave multiphoton detection technology based on frequency upconversion. An initial laser emits an initial 532nm laser beam with a pulse width of 7ns and a pulse repetition frequency of 100Hz. The 532nm laser emitted from the laser passes through a half-wave plate and a polarizing beam splitter, and then through a 45° reflecting mirror M1 (AR@532 nm, HR@1.2-1.7μm) into a BBO-OPO. The BBO crystal has a size of 5*10*27 mm3, and its two sides are coated with AR@532, 1200-1700, and 750-950nm. The BBO crystal is fixed on a rotary motor, and different phase matching can be achieved by rotating it at different angles to obtain a tunable near-infrared output of 1.25~1.65 μm. The OPO front cavity mirror M2 (AR@532&750-950nm, T=30%@1.2-1.7μm) and the rear cavity mirror M3 (HR@532&1200-1700&750-950nm) are both planar mirrors. The bichromatic anisotropic window consists of a 100mm focal length off-axis parabolic mirror and a 100μm thick germanium plate. The parabolic mirror has a small hole in the center to facilitate the transmission of the upconversion pump. The THz signal to be detected is collimated and focused by the two parabolic mirrors and then incident on the DSTMS crystal. The germanium plate in the middle blocks visible light and near-infrared light from the environment, so that there is no external light noise interference during the upconversion process.
[0030] By adjusting the upconversion pump wavelength output from the BBO-OPO, it passes through a lens L1 (f=150mm) and is simultaneously incident on a 0.41mm thick DSTMS crystal along with the THz signal to be measured. Based on the second-order nonlinear effect, under type 0 phase matching (all three waves are o-light, polarization direction is horizontal), the upconversion signal and the THz signal to be measured can be summed in the DSTMS crystal to generate a near-infrared upconversion signal. The upconversion signal is collimated and paralleled by a mirror M4 (HR@1.2-1.7μm) and a lens L2 (f=300mm). Since the emitted near-infrared upconversion signal contains residual upconversion pump and spontaneous parametric upconversion fluorescence, the collimated upconversion signal is separated from the residual pump by a beam splitting system using two gratings. The two gratings are near-infrared reflective etched gratings (Thorlabs, GR25-1210), with a etch density of a=1200 lines / mm and a blaze wavelength of 1μm.
[0031] After passing through the dual grating filter, the near-infrared upconversion signal is focused onto the near-infrared MPPC by lens L3 (f=50mm), which serves as a spatial beam repositioning device, thus enabling multi-photon detection of the near-infrared upconversion signal.
[0032] Note that the above are merely preferred embodiments and technical principles of the present invention. Those skilled in the art should understand that the present invention is not limited to the specific embodiments described above, and various modifications, reconfigurations, combinations, and substitutions can be made without departing from the scope of protection of the present invention. Therefore, although the present invention has been described in detail through the foregoing embodiments, the present invention is not limited to these embodiments; more equivalent embodiments may be included without departing from the design concept of the present invention, and their scope of protection is determined by the appended claims.
Claims
1. A frequency upconversion THz wave multiphoton detection system, characterized in that, It includes a pump source unit, a beam combiner, a nonlinear crystal, a beam splitter, a spatial photon counting detector, an optical shaping device, and a controller; the pump source unit provides a pump source for emitting an initial laser beam as the pump light in the upconversion process, and the beam combiner is used to combine the THz wave to be measured and the pump source beam, which are collinearly incident on the nonlinear crystal; The nonlinear crystal generates an upconversion signal based on phase matching; the beam splitter is used to spatially separate the upconversion signal and the pump source; the optical shaping device is used to spatially shape the upconversion signal and fluorescence; the spatial photon counting detector detects and counts photons in the shaped upconversion signal; the controller has time gating and spatial gating; the time gating is used to control the time synchronization between the incident THz wave under test, the pump source, and the spatial photon counting detector; the spatial gating is used to control the size of the spatial gate of the spatial photon counting detector.
2. The THz wave multiphoton detection system based on frequency upconversion according to claim 1, characterized in that, The pump source unit includes a laser. A polarization unit and an oscillation unit are provided in the optical path between the pump source unit and the beam combiner. The pump source enters the nonlinear crystal through the polarization unit and the oscillation unit.
3. The THz wave multiphoton detection system based on frequency upconversion according to claim 1, characterized in that, The pump light source passes through a filter and lens L1, and is simultaneously incident on a beam combiner along with the THz signal to be measured. The beam combiner includes a parabolic mirror with a small hole. The THz wave to be measured passes through the filter and is combined with the pump light source on the parabolic mirror.
4. The THz wave multiphoton detection system based on frequency upconversion according to claim 1, characterized in that, The optical shaping device includes a reflector M4, a lens L2, and a lens L3. The nonlinear crystal is a DAST crystal or a DSTMS crystal. Based on the second-order nonlinear effect, under type 0 phase matching, the upconversion signal and the THz signal to be measured are frequency-matched in the nonlinear crystal to generate an upconversion signal. The upconversion signal is collimated and paralleled by the reflector M4 and the lens L2. The collimated upconversion signal is then separated from the residual pump signal by the two gratings of the beam splitting unit. After being filtered by the gratings and the bandpass filter, the light is focused onto the spatial photon counting detector by the lens L3. The spatial photon counting detector performs photon counting.
5. The THz wave multiphoton detection system based on frequency upconversion according to claim 1, characterized in that, The two gratings are near-infrared reflective gratings with a grating density of 1200 lines / mm and a blaze wavelength of 1μm.
6. The THz wave multiphoton detection system based on frequency upconversion according to claim 1, characterized in that, The relationship between photon count and incident light energy includes: ； Where E represents the single pulse energy in the incident space photon counting detector; η This represents the correction factor, which is related to the detector's detection efficiency and detector parameters; C This represents the count value of the photon counting detector; h Represents Planck's constant; ν This represents the photon frequency of the incident photon counting detector.
7. A method for detecting THz wave multiphotons based on frequency upconversion, characterized in that, The pump source, acting as the upconversion pump light, and the THz wave under test are combined by a beam combiner and then incident into the nonlinear crystal. The beam combiner transmits the pump light and reflects the THz wave under test. The two waves generate an upconversion signal in the nonlinear crystal according to the second-order nonlinear effect and phase matching condition. The beam splitting unit spatially splits the upconversion signal light and the pump light. The upconversion signal is then spatially shaped by a beam shaping device. Simultaneously, time gating controls the time delay of the THz wave under test, the pump source, and the spatial photon counting detector, ensuring that the THz wave under test and the pump source are simultaneously incident into the nonlinear crystal. At the same time, the opening time of the spatial photon counting detector is controlled, allowing the upconversion signal to be incident into the detector for coincidence counting.
8. The THz wave multiphoton detection method based on frequency upconversion according to claim 8, characterized in that, The beam splitter can spatially separate the upconversion signal and the upconversion pump at a set angle based on the wavelength difference, and can maintain the invariance of the optical path for a specific wavelength.
9. The THz wave multiphoton detection method based on frequency upconversion according to claim 8, characterized in that, Time gating uses the THz wave under test as an external trigger, and the time it takes for the THz wave to be incident on the nonlinear crystal is... The time it takes for the pump light to enter the nonlinear crystal is The time it takes for the upconversion signal to be incident on the space photon counting detector is Time gating allows for the control of time synchronization by adjusting the time delay between different waves and the detector's gate opening time. The relationships between these delays include: Pump light source time delay ; Space photon counting detector door opening time delay .