Bound states in the continuum (BIC) refers to the non-radiative state located in the radiative continuum. BIC provides a novel method for the research and development of functional devices with ultra-high quality factor (Q) in the terahertz band. It has the potential to be used in several applications, including narrow linewidth filtering, terahertz slow light devices, and the enhanced interaction between terahertz waves and matter. In this study, terahertz BIC metasurfaces composed of classical metallic split ring resonators (SRRs) are proposed and numerically studied based on the symmetry protection principle of the structure. The leakage of BIC to the far field can be observed in the spectrum by changing the gap width of SRR to form an observable quasi BIC (QBIC) mode. Moreover, the influence of ohmic loss on the Q of QBIC is systematically studied by applying the Drude model. The proposed BIC and QBIC also have unique responses to the incident angle. The BIC based on SRR metasurface proposed in this study not only provides a new framework with clear mechanism and easy implementation for the development of high-Q terahertz functional devices, but also provides research ideas for subsequent studies on the terahertz BIC metasurface from the aspects of loss and tilted incidence.

The metasurfaces are composed of different superlattices based on classical metallic SRRs. A single unit cell is composed of either 2 or 4 SRRs. For the superlattices with two SRRs in the lattice, two adjacent SRRs with different orientations are arranged vertically [superlattice ① in Fig. 1(b)] or horizontally [superlattice ② in Fig. 1(c)] to form two types of superlattices. For the superlattices composed of four lattices, each SRR orients in a clockwise direction (superlattice ③ in Fig. 4). All metasurfaces have 2-μm-thick high resistivity silicon wafer as substrate. The refractive index of silicon is set as 3.4 and the SRR is set as perfect electric conductor (PEC). The structure is simulated in CST microwave Studio.

First, the BICs in superlattices ①, ②, and ③ are numerically investigated using the eigen-mode solver. Subsequently, the frequency solver is applied to calculate the transmission of the corresponding QBIC metasurfaces by breaking the structural symmetry of the BICs. The field monitor is used to observe the field distribution to clarify the relationship between a BIC and its derivative QBIC. The evolution from BIC to QBIC is effectively presented by changing the gap widths of the SRRs, and the Fano coupling mode is used to calculate the Q of the QBICs. The influence of ohmic loss on the QBICs is investigated by applying the Drude model to the SRRs. Tilted incidence is realized by changing the input and output directions of the ports in the frequency solver, and the unique dependence of the QBICs in superlattices ① and ② is obtained.

Only one BIC exists in superlattices ① and ②. For superlattice ③, which is composed of 4 SRRs, there are two different BICs existing in the metasurface. QBICs with Fano line shape appear in the transmission spectra when the symmetry of the superlattices is broken. The Q of QBIC exhibits an inverse quadratic correlation with the asymmetric parameter. The residual ohmic loss in the SRRs deteriorates the Q of the QBICs, in which the Q of the metasurface calculated using the Drude model drops to half compared to the result with PEC. Regarding the incidence dependence, a tilted incidence with transverse electric (TE) polarization induces a leakage of the BICs in superlattices ① and ②, in which the linewidth of the derivative QBICs is proportional to the oblique angle. However, the tilted incidence with transverse magnetic (TM) polarization will not perturb the BICs in the superlattices.

In this study, we construct symmetry-protected BICs in three superlattices based on SRRs. Subsequently, the bound states at Γ point in these superlattices are investigated via numerical simulation. When the structural symmetry is broken, BICs are converted to the corresponding QBICs, and the Q of the QBICs decreases with the increase in structural asymmetry. The Q of the QBIC is also strongly correlated to the ohmic loss in the SRRs, which was generally neglected in previous studies related to terahertz metasurfaces developed by SRRs. In addition, the superlattices ① and ② have a certain pitch angle dependence. Oblique incidence of TE polarization with an electric vector parallel to the gap can lead to the leakage of BIC. The Q of the formed QBIC decreases with the increase in incident angle, while the TM wave does not have a similar effect. The metasurface designed in this study has a clear mechanism and is conveniently fabricated, which provides a novel direction for the design of high-Q terahertz devices.

Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}

Since its invention, the laser has developed tremendously; stimulated important breakthroughs in numerous related fields such as physics, chemistry, biology, and information science; and played crucial roles in fundamental science and technology application research. The National Science Foundation of China compiles statistics on the funding of Key Programs, Major Programs, Research Programs of National Major Research Instruments, General Programs, and Youth Science Foundation Programs. Based on the statistics compiled between 2017 and 2021, this study analyzes hot words in titles and keywords of previously funded projects to summarize the key developments and challenges of laser science and technology in China and propose topics requiring further research and discussion.

Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}

Bound states in the continuum (BIC) refers to the non-radiative state located in the radiative continuum. BIC provides a novel method for the research and development of functional devices with ultra-high quality factor (Q) in the terahertz band. It has the potential to be used in several applications, including narrow linewidth filtering, terahertz slow light devices, and the enhanced interaction between terahertz waves and matter. In this study, terahertz BIC metasurfaces composed of classical metallic split ring resonators (SRRs) are proposed and numerically studied based on the symmetry protection principle of the structure. The leakage of BIC to the far field can be observed in the spectrum by changing the gap width of SRR to form an observable quasi BIC (QBIC) mode. Moreover, the influence of ohmic loss on the Q of QBIC is systematically studied by applying the Drude model. The proposed BIC and QBIC also have unique responses to the incident angle. The BIC based on SRR metasurface proposed in this study not only provides a new framework with clear mechanism and easy implementation for the development of high-Q terahertz functional devices, but also provides research ideas for subsequent studies on the terahertz BIC metasurface from the aspects of loss and tilted incidence.

The metasurfaces are composed of different superlattices based on classical metallic SRRs. A single unit cell is composed of either 2 or 4 SRRs. For the superlattices with two SRRs in the lattice, two adjacent SRRs with different orientations are arranged vertically [superlattice ① in Fig. 1(b)] or horizontally [superlattice ② in Fig. 1(c)] to form two types of superlattices. For the superlattices composed of four lattices, each SRR orients in a clockwise direction (superlattice ③ in Fig. 4). All metasurfaces have 2-μm-thick high resistivity silicon wafer as substrate. The refractive index of silicon is set as 3.4 and the SRR is set as perfect electric conductor (PEC). The structure is simulated in CST microwave Studio.

First, the BICs in superlattices ①, ②, and ③ are numerically investigated using the eigen-mode solver. Subsequently, the frequency solver is applied to calculate the transmission of the corresponding QBIC metasurfaces by breaking the structural symmetry of the BICs. The field monitor is used to observe the field distribution to clarify the relationship between a BIC and its derivative QBIC. The evolution from BIC to QBIC is effectively presented by changing the gap widths of the SRRs, and the Fano coupling mode is used to calculate the Q of the QBICs. The influence of ohmic loss on the QBICs is investigated by applying the Drude model to the SRRs. Tilted incidence is realized by changing the input and output directions of the ports in the frequency solver, and the unique dependence of the QBICs in superlattices ① and ② is obtained.

Only one BIC exists in superlattices ① and ②. For superlattice ③, which is composed of 4 SRRs, there are two different BICs existing in the metasurface. QBICs with Fano line shape appear in the transmission spectra when the symmetry of the superlattices is broken. The Q of QBIC exhibits an inverse quadratic correlation with the asymmetric parameter. The residual ohmic loss in the SRRs deteriorates the Q of the QBICs, in which the Q of the metasurface calculated using the Drude model drops to half compared to the result with PEC. Regarding the incidence dependence, a tilted incidence with transverse electric (TE) polarization induces a leakage of the BICs in superlattices ① and ②, in which the linewidth of the derivative QBICs is proportional to the oblique angle. However, the tilted incidence with transverse magnetic (TM) polarization will not perturb the BICs in the superlattices.

In this study, we construct symmetry-protected BICs in three superlattices based on SRRs. Subsequently, the bound states at Γ point in these superlattices are investigated via numerical simulation. When the structural symmetry is broken, BICs are converted to the corresponding QBICs, and the Q of the QBICs decreases with the increase in structural asymmetry. The Q of the QBIC is also strongly correlated to the ohmic loss in the SRRs, which was generally neglected in previous studies related to terahertz metasurfaces developed by SRRs. In addition, the superlattices ① and ② have a certain pitch angle dependence. Oblique incidence of TE polarization with an electric vector parallel to the gap can lead to the leakage of BIC. The Q of the formed QBIC decreases with the increase in incident angle, while the TM wave does not have a similar effect. The metasurface designed in this study has a clear mechanism and is conveniently fabricated, which provides a novel direction for the design of high-Q terahertz devices.

Optical parametric oscillators(OPOs)have been proven to be effective,coherent light sources that can expand the wavelengths of commercial lasers(typically limited to narrow emission lines and bands)to a broad range from visible to far-infraredbands. Q-switched lasers with high peak powers have significantly promoted the development and applications of OPOs with the following advantageous characteristics:system compactness(for example,two cavity mirrors and a nonlinear crystal),relatively high conversion efficiency,singly resonant operation,and frequency-agile tunability(for example,angle tuning).A plane-parallel cavity with a large mode volume is well-suited for Q-switched laser pumps.This type of OPO is widely adopted,for example,as a pump/seed source in nonlinear terahertz or mid-infrared(MIR)generation or directly as an MIR source,owing to the wide tuning range and ease of construction.As the earliest configuration in a laser resonator,a plane-parallel cavity is critically stable and sensitive to mirror misalignment.The misalignment of laser cavities,including those of argon ion,CO₂,and Nd∶YAG lasers,has been analyzed previously;however,studies on OPO cavities have rarely been reported.In this study,we performed an experimental investigation on the misalignment characteristics of a plane-parallel cavity-based OPO.

In this study,an OPO based on a plane-parallel cavity structure was developed.A potassium titanyl phosphate(KTP)crystal was utilized as the nonlinear medium(cut at θ=60°,φ=0°,and 10 mm×7 mm×20 mm,anti-reflection(AR)-coated at 532 nm/800-900 nm/1300 1600 nm).A frequency-doubled Nd∶YAG laser(532 nm,10 ns,and 10 Hz)was employed as the pump source.Two flat mirrors(AR-coated at 532 nm/1300-1600 nm and highly reflection-coated at 800 900 nm)formed a singly resonant cavity.The ns-pulsed OPO was operated at a wavelength of 1514 nm via o→e(signal)+o(idler)critical phase matching.The cavity mirrors were precisely controlled using piezoelectric optical mounts for alignment.Each mount was equipped with two piezo actuators,which could provide a two-dimensional(2D)adjustment(axes 1 and 2 for output mirror M1 and axes 3 and 4 for input mirror M2)with an angular resolution of ≤0.7 μrad.

Typical output results(pulse envelopes and beam profile)of the KTP-OPO are presented in Fig.2.The piezoelectric optical mounts with a motion controller module facilitate quantitative analysis of the influence of mirror misalignment on the OPO output.The variation in the output pulse energy with angular tilt δ_xis measured while scanning each cavity mirror along two directions around the well-aligned position(δ_x=0),as presented by the 2D graphs in Fig.3.The subscripts x=1–4 correspond to the four actuators,axes 1–4,respectively.The four curves presented in Fig.3 present envelopes along the principal axis(with the other three δ=0).The full widths at half maximum of the curves(called alignment tolerance)from axes 1 to 4 are 0.171,1.861,0.177,and 1.933 mrad,respectively,which are determined at a pump beamdiameter Φ=4 mm,cavity length L=65 mm,and output pulse energy=6.6 mJ.The discrepancy between the two mirrors along the same direction(axes 1 and 3 and axes 2 and 4)is minimal,which is verified by alternating the two mounts.The tolerances along the horizontal direction(axes 2 and 4, y-principal dielectric axis)are approximately 10 times those along the vertical direction(axes 1 and 3, x-z-principal plane).This can be attributed to the critical phase matching configuration(Fig.4).As presented in Fig.5(a),the alignment tolerance increases with the beam size at a specific pump intensity and cavity length because a larger interaction region(cross-section)can provide more effective round trips for misaligned signal beams.The relationship between the tolerance andoutput energy,shown in Fig.5(b),demonstrates an increasing trend because a higheroutput energycorresponds to a higher single-pass gain(easier to build up).The alignment tolerances of different cavity lengths are compared at fixed input[Fig.6(a)]and output pulse energies[Fig.6(b)].In addition,the other output characteristics vary with the OPO cavity length.A longer cavity length results in a higher threshold and lower energy conversion efficiency(Fig.7).The divergent angles decrease with the cavity length at approximately equal output energies and beam sizes(left y-axis of Fig.8).Better beam quality and worse stability can be obtained with a more extended cavity(right y-axis of Fig.8),and the root-mean-square(RMS)of pulse energy fluctuation increases(3.09%→3.61%→3.96%).

Herein,we quantitatively characterize the mirror misalignment of a plane-parallel cavity-based OPO,which has been widely utilized as a convenient coherent light source with a desired wavelength.A green laser-pumped KTP-OPO equipped with piezoelectric optical mounts is constructed.An almost circular Gaussian beam with a wavelength of 1514 nm is delivered with a slope efficiency of ≥25% and a pulse energy fluctuation(RMS)of ≤4%. The output shrinkage is measured by scanning the cavity mirrors around a well-aligned position.The alignment appears to be significantly more sensitive in the critical direction than in the noncritical direction,which can be explained based on the phase-matching configuration.The alignment tolerance increases with the beam size and input intensity.In addition,the cavity length dependence is analyzed at specific input and output pulse energies.This paper presents a type of ns-pulsed,singly resonant,and critical phase-matched OPO with a wide-angle tuning capability.

The increasing burn mortality rate places an urgent need for accurate diagnosis and treatment of burns. Currently, the third-degree quartile is internationally used to classify the degree of burns based on burn depth, and clinical treatment methods for different degrees of burns are significantly dissimilar. Burn surgeons overestimating the severity of burns can lead to unnecessary surgery, whereas underestimating them leads to treatment delays and worsening of the burn conditions. In addition, studies have shown that burn severity changes dynamically over time, with superficial Ⅱ burn worsening to deep Ⅱ or Ⅲ burns within 48 h of burn occurrence. Therefore, overcoming the defects of subjective judgment using the naked eye and quantitatively monitoring the dynamic changes in the burn degree in real time has become a challenge in the early diagnosis of burns. Burn diagnosis methods based on photonics, such as near-infrared spectroscopy, reflective confocal microscopy, and laser Doppler flowmetry, are developing rapidly. However, their clinical application is limited owing to low accuracy, invasiveness, high detection environment requirements, and high costs. In this study, a noninvasive quantitative method for assessing the burn degree was developed based on spatial frequency-domain imaging (SFDI). Combined with the systematic clustering method and multiparameter dimensionality reduction analysis, the proposed method results in improved classification accuracy of different burn degrees and shortened classification time, thus indicating the potential for early diagnosis of clinical burns.

In this study, the SFDI technique was applied to a rat burn model. First, the backs of Sprague-Dawley (SD) rats were depilated, and a thermostatic iron heated to 100 ℃ was used on the backs of the anesthetized SD rats for 4, 12, and 24 s, respectively, to establish a rat burn model with different burn degrees. Next, the sinusoidally modulated structural patterns were projected onto the surface of each burned area, and the backscattered structural patterns from the tissues were captured using a charge-coupled device (CCD) camera. Subsequently, we used single-snapshot multifrequency demodulation (SSMD) to extract the modulation transfer function (MTF) of light from the burned tissues. Compared with the traditional three-phase shift demodulation method, SSMD only requires a single snapshot to achieve parameter extraction, which significantly suppresses the problem of motion artifacts and improves the signal-to-noise ratio of imaging using filtering technology. Based on the photon diffusion transmission theory, the optical parameters (μ_a and μ′_s) were then recovered using the look-up table method at the 5th, 10th, 30th, 60th, 90th, and 120th minutes after burn. Finally, systematic clustering and multiparameter dimensionality reduction analysis were performed on the optical parameters to quantify and classify different burn degrees.

Different degrees of burns can be effectively distinguished by the relative changes in the two optical parameters at the three wavelengths. The results show that the magnitude of the absorption coefficient positively correlates with the degree of burn. In contrast, the magnitude of the reduced scattering coefficient negatively correlates with the degree of burn. Although the distinction between optical parameters is not significant at the beginning of burns, the optical parameters of the 4 s burn group gradually decrease or gradually recover to the unburned state with observation time. In contrast, the optical parameters of the 12 s and 24 s groups gradually deviate from the normal state (Fig. 6). The burn results are divided into two categories through optimal analysis of systematic clustering. The 4 s group is classified as mild burns, whereas the 12 s and 24 s groups are classified as severe burns. Although the classification accuracy is less than 85% in the first 10 min after burn, it is 100% in the later stages (Table 1). Two new factors (the absorption factor FAC1 and the reduced scattering factor FAC2) reflecting approximately 93% of the original variable information can be generated using the principal component analysis to reduce the dimensionality of the six optical parameters. The results show that the absorption factor, FAC1, distinguishes the degree of burns in a large category (mild burns in the 4 s group and severe burns in the other two groups) and increases the difference between deep Ⅱ degree burn in the 12 s group and Ⅲ burn in the 24 s group. In addition, the assessment of burn severity using principal constituent factors can reduce interference and improve classification accuracy in the early stage after burn (Fig. 9).

The quantitative burn imaging device based on real-time spatial frequency-domain imaging technology has remarkable advantages over existing diagnostic techniques, for example, ease of handling, compact structure, and high precision. Through dynamic monitoring of changes in optical parameters combined with cluster analysis and parameter dimensionality reduction, the degree of burns can be determined through noninvasive assessment, providing a reliable guarantee for the precise treatment of burns. In future studies, we will supplement the pathological verification, characterize additional physiological parameters (such as hemoglobin content, blood oxygen saturation, and melanin concentration) from the optical parameters, and extend this technology to clinical applications so as to significantly reduce the treatment cycle and cost to patients.