The Detection of Explosive Cyclotrimethylenetrinitramine (RDX) Using Optical Microcavity

Optical microcavity, which is based on light-matter interaction, has the advantage of high sensitivity as a sensor. It has been widely used in the security, medicine, and environment felds these years. Detection of atoms and molecules at the nanoscale is one of the practical usages of optical microcavity. In this paper, we focus on a ultra-high-quality factor optical microcavity of whispering gallery mode (WGM) to detect the explosive of cyclotrimethylenetrinitramine (RDX). To demonstrate the high sensitivity of the method, we detect the solution at diferent concentrations. Diferent types of optical microcavities are suitable for the detection of diferent matters. Considering the efects of the results and the simplicity of operation, we use silica microspheres chiefy. Tis is the frst time to detect the explosive using highly sensitive optical microcavity. Te detection of pure explosives will be helpful for establishing the database of the explosives.


Introduction
Analogous to the principle of tuning forks, optical microcavity could confne light to a certain volume to create resonant recirculation.Te principle of optical microcavity is based on the light-matter interaction.Because of the total refection, light is restricted in the optical microcavity that has a symmetrical confguration.Te light in the microcavity couples with the light in the fber.Under certain conditions, the intensity of light will be strengthened.Optical microcavity has its own resonant mode because of its special confguration.When molecules adhere to the surface of a microcavity, the change in system confguration leads to the change of the resonant mode.Generally speaking, these changes in the form of frequencies are easy to observe.Te ideal condition is that the resonator can be indefnite or lossless, so the resonant frequencies are precise.Te quality of optical microcavity is measured by Q factor [1][2][3].Q factor and the microcavity volume are the main parameters of optical microcavity [4][5][6].According to the confnement style, optical microcavities could be divided into three categories: Fabry-Perot microcavity [7][8][9][10], photonic crystal microcavity [11], and whispering gallery mode (WGM) microcavity [12][13][14].Fabry-Perot microcavities [7,8] have two mirrors to feedback light.Te emitter of micropost (quantum dot for example) triggers single photon emission via the Purcell efect.Te high Q and small volume could reach Q � 2000 and V � 5 (λ/n) 3 , where n is the refraction index.Instead of the center of mass motion, the ultracold atom in a strong couple system interacted with the vacuum cavity mode has a ultrahigh Q: F � 4.8 × 10 5 (F is opposed to Q) with V � 1.69 × 10 3 μm 3 [9,10].Photonic crystal microcavities [11] have a hole drilled in the middle of the structure, and a neutral atom located in the hole forms the donor mode geometries with Q � 1.3 × 10 4 .High Q WGM microcavities include microdiscs [15][16][17][18] with Q � 12000 and add/drop flters (semiconductor or polymer) [19,20] with Q � 7000 or Q � 1.3 × 10 5 respectively.Silica or quartz microsphere [21] and microdisk type [5,22] WGMs are ultrahigh Q microcavities with Q � 8 × 10 9 and Q � 10 8 .Lights are refected continuously in spherical and toroidal structures.Because of the high sensitivities and practicabilities of nanoparticle detection, we focus on microsphere WGMs.
WGM optical microcavity is a highly potential technology in forensic science.It has many advantages.First, the price is much cheaper than other traditional instruments such as a gas chromatograph spectrometer.Second, some detection technologies search certain ligand rather than the whole molecule [23,24].Tis will cause the high error detection rate.Optical microcavity detects the molecule, so this technology has more accuracy [25,26].Tird, traditional instruments detect either liquid phase or gas phase (only one phase), and the samplings are all liquid phases (gas phase needs to heat the liquid sampling).Tis is a limitation.Te sample injection of optical microcavity can be liquid phase or gas phase which means there is no need to prepare the gas sample into solution.Te gas sample can be detected by injecting the gas directly into microcavity [27].Fourth, as long as there is a molecule attached to the microcavity, the response to the change in frequencies is obvious [28,29].Optical microcavity has ultrahigh sensitivity which could reach nanoscale [30,31].Fifth, it is easy to operate and really takes a short time to get the result.When the sample is injected into or onto optical microcavity (depends on the confguration of microcavity), the results will be immediate and obvious.Tis is a dynamic real-time processing.All of these advantages show the necessity and potential of this technology application in forensic science.In this paper, we show the viability of this detection technology in forensic science, so we use the WGM silica microsphere cavity to detect RDX as an example.
Tis paper is organized as follows: In Section II, we introduce the method and apparatus used in our experiment.In section III, we give the results of detection of solutions of diferent concentrations and compare them with the results of solution with no determinant.Finally, in section IV, the main results are concluded and further work is discussed.

Method and Apparatus
In the frst step, we fabricate the microsphere cavities with diameter 64 μm using CO 2 laser device of SYNRAD 48-2KAM-AP (see Figure 1).First of all, we get rid of the protective layer of the optical fber (taper fber), clean it with alcohol, and weight an object at the end of the fber vertically.Lately, the fber has been heated by a low-power CO 2 laser.Te heat melts the fber but it doesn't break it.Next, we cut of the weight at the appropriate position of the fber using a high-power CO 2 laser.After that, the rest of the fber is heated to refux.Finally, we get a microsphere cavity.
In the second step, we dip the microsphere cavities into a diluted solution or drip the solution drop onto the microsphere cavities.Te laser propagates along the optical fber and interacts with the molecules suspended in the solution (see Figure 2).Te molecules suspended in the solution change the vibration mode of the microsphere cavity.Tese changes afect the responses on the oscilloscopes.Te laser device is the New Focus Tunable Laser Controller TLB-6700.Te wave range from 1540 nm to 1550 nm.Te power of the laser we used is 2 mW.Te signal generator used in the experiment is Tektronix AFG3022C and the oscilloscope is Tektronix MDO3104.We use a triangular wave with frequency 50 Hz and amplitude 2V(−1V ∼ 1V).Te standard solution is made of the explosive of Cyclotrimethylenetrinitramine (RDX) in acetone of concentration of 1000 ppm, and it is diluted by alcoholic.

The Results and Analysis of Diluted Standard Solution of Different Concentration
Te microsphere cavities of whispering gallery mode (WGM) could be used to detect very few molecules and small microparticles.On the contrary, too many molecules and small microparticles would reduce the Q factor and thus make the results unreasonable.According to this, we dilute the standard solution into three concentrations.Te ratios of the standard solution and the alcoholic solution are 1 : 1, 1 : 2, and 1 : 3.And we compare the 1 : 2 result with acetone solution which is diluted by equivalent alcohol.All experiments with diferent concentrations are repeated three Te coupling method we use is critical coupling, and there is a small gap between the optical fber and the microcavity, the fber is not placed on the periphery of the sphere.
2 Quantum Engineering times, and the results are similar.According to the standard deviations, we chose the fgures of the best results here to demonstrate.

Te Result of Ratio 1 : 1 (Standard Solution: Alcoholic Solution)
. We dip the microsphere cavities in the diluted solution of ratio 1 : 1. Te vibration mode of the microsphere cavity is changed because of the existence of the molecules suspended in the solution (see Figures 3 and 4). Figure 3 shows the original mode of the microsphere cavity.5 and 6).Te result is summarized in Table 2.

Te Result of Ratio 1 : 3 (Standard Solution: Alcoholic Solution)
. We continue diluting the standard solution with alcohol in the ratio 1: 3.In the beginning, the frequency of the vibration mode of the cavity is ω 1 � 1223.13682THz with width Δω 1h � 5.71579 × 10 − 5 THz.Te target molecules are adsorbed to the external surface of the cavity which gives us two smaller frequencies of new modes ω 11 � 1223.13536THz and ω 12 � 1223.13556THz.Teir widths are Δω 11h � 2.79271 × 10 − 4 THz and Δω 12h � 3.09351 × 10 − 4 THz.Te split frequency turns to be ω 12 − ω 11 � 0.20 GHz (see Figures 7 and 8).From the results above, we can conclude, the lower the concentration is the smaller the split frequency becomes (see Table 3).Quantum Engineering

Conclusion and Discussion
Te optical microcavity, which is widely used in medical science [32], biology [33], and environmental science [34], is a new kind of sensor with ultrahigh sensitivity [35][36][37].For example the applications include diagnosis of diseases, detection of protein of blood, the pollution of environment and so on [38][39][40].We want to research the usage of optical microcavity sensor in public security feld because the appearance of more and more terrorist attacks.Optical microcavity has its advantage of ultrahigh sensitivity which could reach nanoscale [30,31].Not only the explosive itself but also things and people that used to touch the explosive can be found using optical microcavity sensor.For example, we wipe someone's hands or clothes or something using flter papers, and then prepare the liquid or gas sample detecting in the optical microcavity sensor.Te result will tell us whether someone or something touched explosive before.Because even only one nanoparticle of explosive adhere to the microcavity, it could cause the split and movement of the mode frequency peak.
In this paper, we use silica or quartz microsphere type whispering gallery mode (WGM) microcavity to show the efect of the detection of typical explosive RDX.After dipping the microsphere cavities into diluted solution, RDX molecules suspended in the solution adsorb onto the microsphere cavities and change the original vibration mode.We give three kinds of concentration as examples.Te ratio of the standard solution and the alcoholic solution are 1 : 1, 1 : 2, 1 : 3.As shown in fgures above, RDX molecules low, split and broaden the frequency peak obviously.When there is no RDX molecule in the standard solution which means pure acetone, the frequency peak only moves to a lower value and no split appears.Te efective refraction index of the microcavity increases due to the solution, the frequency of original vibration mode shift to a lower value.According to the symmetry of the microcavity, waves in the microcavity are divided into clockwise mode and counterclockwise mode.When the molecules enter the microcavity, due to scattering, the modes of the two waves become coupled, and the frequency peak is broadened.Te degeneracy due to the coupling causes the split of the frequency.Te scale of the frequency split is GHz.Te available concentration of diluted solution can be extremely low.Te research of the detect limitations of diferent molecules is still processing.Te noise is unavoidable in experiment.Te main noises in microcavity experiment are laser wavelength drift noise, vibration of the fber taper, quantum shot noise and the thermo refractive noise.Tere are several solutions to reduce the experimental noises, one can lock the vibration mode and tighten the fber to conquer the frst two noises.To reduce the quantum shot noise and thermo refractive noise, reduction of light intensity and derivation of the microcavity heat using optimization of material or confguration that has good thermal property are suggested [41].Besides, these noises can be suppressed using the method called backscattered light [42].Contamination and impurity is the pretreatment of detection.Te purifcation and extraction of chemicals is another topic.We focus on the detection process.Here, we studied RDX only one explosive to show the viability of microcavity technique and the procedure detail.We focus on the viability of technique itself in forensic science (explosive RDX as an example).It is worth noting that in this work, we only analyze the feasibility of this method, so the number of experiments is relatively small.For the later study of particle properties, a large number of experiments are needed, and the experimental data should be statistically signifcant.We can use this method to identify explosives.On the basis of the viability, the comparisons of diferent explosives and diferent confgurations of microcavities are further works in the future.

Data Availability
Te data that support the fndings of this study are available on request from the corresponding author.Quantum Engineering

Figure 1 :Figure 2 :
Figure 1: Te manufacturing process of microsphere microcavity.Schematic of the silica or quartz microsphere type whispering gallery mode (WGM) microcavity.

3. 2 .
Te Result of Ratio 1 : 2 (Standard Solution: Alcoholic Solution).As per the procedure we mentioned above, we dilute the standard solution with alcohol of the ratio 1 : 2 this time.Te frequency of the original mode is ω 1 � 1223.64186THz.Its width is Δω 1h � 5.79740 × 10 − 5 THz.Te target molecules in the diluted solution result in the left move and the split of the original mode.Two frequencies of new modes are ω 11 � 1223.63137THz and ω 12 � 1223.63160THz.Teir widths are Δω 11h � 2.93266 × 10 − 4 THz and Δω 12h � 2.52107 × 10 − 4 THz.Te split frequency becomes ω 12 − ω 11 � 0.23GHz (see Figures � 1 : 2).In the end, we discuss the solution of pure acetone with no RDX molecules and compare it with the result above.Because of the absence of target molecules, the mode of microcavity does not split.Te peak of the mode frequency only moves from ω � 1223.94771THz to another position 1223.94444THz with the broadening of the width from Δω � 1.38211 × 10 − 4 THz to 3.40387 × 10 − 4 THz.Te change of the frequency reads 3.27 GHz (see Figures 9 and 10 ).Whether there exist RDX molecules in the solution or not, the peaks of the diluted solution change to smaller values.Because the solution with no RDX molecules changes the mode volume (the mode distribution) of the microcavity.When the RDX molecules adhere to the microcavity, besides the shift of the frequency peaks, the peaks split and broaden simultaneously.

Figure 3 :
Figure 3: Te frequency distribution of microsphere cavity with no solution.Te blue line shows two peaks of original vibration modes.

Figure 4 :
Figure 4: Te frequency distribution of microsphere cavity with diluted solution.Te ratio of the standard solution and the alcoholic solution is 1 : 1. Te blue line and red line represent the split of two original frequencies.

Figure 5 :
Figure 5: Te frequency of microsphere cavity with no solution.Tere is only one original vibration modes.

Figure 6 :
Figure 6: Te frequency distribution of microsphere cavity with diluted solution.Te ratio of the standard solution and the alcoholic solution is 1 : 2. Te blue line and red line show that one peak splits into two frequencies.

Figure 8 :
Figure 8: Te ratio of the standard solution and the alcoholic solution is 1 : 3. Te blue line and red line show the split frequencies.

Figure 9 :
Figure 9: Te frequency of original vibration mode of microsphere cavity with no solution.

Figure 10 :
Figure 10: Te frequency of pure acetone diluted by alcoholic solution with ratio 1 : 2. Te vibration mode moves to another value and no split occurs.