High-sensitivity Terahertz Spectroscopic Detection Based On Quantum Weak Measurement Principle

Due to its low photon energy,high temporal and spatial resolution,terahertz（THz）electromagnetic wave based spectral sensing technology has become an important supplement to existing spectral detection technologies such as infrared（IR）spectroscopy and Raman spectroscopy in the field of analysis and detection.It also has unique advantages in many aspects,such as weak intermolecular interactions such as hydrogen bonding,van der Waals forces,and skeleton vibrations of biological macromolecules are all located in the THz band.Among them,high sensitivity quantitative analysis of substances based on THz spectroscopy technology is an important branch of THz science and applications.Currently,due to the problems such as low power of THz sources,low detection sensitivity of THz detectors,and strong absorption of THz waves by the target samples to be detected,the sensitivity and detection limit of THz spectroscopy for substance detection are still at a relatively low level,far from meeting the actual requirements for trace detection.The detection limit for quantitative detection of biofilms or solid mixtures using conventional terahertz time-domain spectroscopy（THz-TDS）is generally at the mg/m L scale or greater than 10.0%mass fraction.In considering THz detection for solution-based samples,water molecules’strong absorption to THz wave is a detrimental factor,whose detection limit based on attenuated total reflection（ATR）method is generally between 10^-1～10⁰ mg/m L,orμMol/L range.Even with the use of periodic structured metamaterial THz sensors,the THz sensing accuracy of biological solution-based samples can be improved to the order of 10^-2～10^-1 mg/m L,but this still cannot meet the high sensitivity quantitative analysis needs in fields such as pharmaceutical chemistry and biomedicine.Moreover,precise control of the dimension of the artificial periodic structures,the use of complex and precise processing equipment,and sample-related micro/nano manufacturing solutions are also required.Along the direction of the aforementioned investigations,there is a continued quest for detecting extremely low concentrations and/or mass fractions of target molecules with high sensitivity.In recent years,the emergence of the quantum weak value amplification（WVA）technique has provided an opportunity to tackle the problems mentioned above.Since its first proof-of-concept demonstration by Aharonov,Albert,and Vaidman（AAV）in1988,it has attracted intense interest as an effective approach for high-precision metrology and has demonstrated remarkable results as such.To date,weak measurement approaches have been applied to determine a multitude of ultrasmall parameters,such as beam shift of the order of frad,nanometal film thickness,graphene layers,extremely small optical phase delay,magneto-optical constant,and even nonlinear effect of a single photon.Overall,if one prepares the system judiciously,meaning to select a“favorable”pre-selected state and to filter the system-weak-coupled-with-pointer mixed states by post-selection,it is possible to measure extremely small changes of physical quantities,with improvement by a factor of 10³～10⁴ over ordinary measurement in theory.However,WVA with abnormal amplification has not been attempted in the THz field up to now.This work introduces the quantum weak measurement principle into the field of high-sensitivity THz spectrum detection for the first time,and makes the following innovative achievements:（1）First,a theoretical model of THz spectrum detection based on quantum weak measurement principle was established,including slight changes in refractive index of a sample,the slight deflection of the polarization plane of the THz wave caused by an optically active sample,and the small change of the refractive index and the rotation simultaneously.（2）Based on the established THz wave transmission model that combines the principle of quantum weak measurement,simulation analysis was conducted in both transmission and reflection modes.The simulated minute variations of physical parameters mainly included:slight changes in refractive index of a sample（Δn=0.001）,slight changes in optical rotation of a sample（Δα=0.0001°）,and slight changes in refractive index and optical rotation of a sample simultaneously（Δn=0.001,Δα=0.0001°）.It included two situations:"pre-selection was horizontal;post-selection was vertical"and"pre-selection was vertical;post-selection was horizontal".In addition,theoretical simulations were also conducted on THz sensing methods without pre-selection and post-selection processes,verifying the effectiveness of weak value amplification based on the quantum weak measurement principle.The THz detection method based on quantum weak measurement principle had a detection accuracy of1～2 orders of magnitude higher than traditional THz methods for slight refractive index changes in samples（Δn=0.001）;The THz detection method based on quantum weak measurement principle can also effectively detect changes in the optical rotation of a sample（Δα=0.0001°）,while the traditional THz detection method was unresponsive to changes in the optical rotation of a sample.The advantages and disadvantages of two different propagation modes（transmission and reflection）were discussed,in order to provide corresponding theoretical basis and technical guidance for the construction of THz sensing systems based on quantum weak measurement principles in the future.（3）By artificially constructing scenarios of small physical parameter changes,such as simulating the attenuation of THz radiation intensity and the rotation of linearly polarized light polarization plane by rotating a polarizer and a half-wave plate,has been found that traditional THz detection method can detect larger polarizer and half wave plate rotation angles（0°～90°）effectively.However,it is not possible to effectively detect extremely small deflection angles（polarizer:0°～1°,half wave plate:0°～2°）,and THz detection methods based on quantum weak measurement principles can make up for this deficiency.This work demonstrated the effectiveness of the weak value amplification effect in THz high-sensitivity detection in the principle of quantum weak measurement.（4）The method proposed was applied to the highly sensitive quantitative analysis of biological samples.The samples were glucose biofilms with concentrations ranging from 0.01 to 0.10 mg/m L,glucose solutions with concentrations ranging from 0.01 to0.10 mg/m L,and glucose solid tablets with mass fractions ranging from 5.0%to 10.0%,for all of which we obtained better detection results than using traditional THz detection method.Compared with THz detection methods,the new approach proposed has improved detection sensitivity for these three forms of glucose samples by 1～2 orders of magnitude,2 orders of magnitude,and 1.5 times,respectively.At the same time,it was also compared with quantum weak measurement applications based on visible light bands.The detection method combining THz-TDS technology and quantum weak measurement principle had a lower detectable concentration range and smaller detectable concentration difference for glucose solution,which may be related to the sensitivity of THz waves to hydrogen bonds,van der Waals forces,and the changes of molecular conformations.And due to the coherent detection principle of THz-TDS technology,the refractive index of the sample can be directly extracted.In addition,THz can penetrate non transparent media that cannot be penetrated by visible and near-infrared（NIR）,so it also had certain advantages in detecting sample forms.This work paves a new path for achieving high sensitivity detection of THz spectroscopy technology in biomedical,pharmaceutical chemistry,materials science,and other fields.