# Non-Destructive Identification of Drugs in Plastic Packaging Using Attenuated Total Reflection Terahertz Time Domain Spectroscopy

^{1}

^{2}

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## Abstract

**:**

## 1. Introduction

_{0}exp(-αd) where I and I

_{0}are the transmitted and incident intensity of the THz wave, d is the sample thickness and α is the absorption coefficient of the sample [13]. Therefore, such samples can be measured by reducing the thickness in such a way that the transmitted signal can be measured with a sufficiently high signal to noise ratio [14,15]. The other option would be to use the reflection mode terahertz measurement system, where the reflected signal from the sample is compared with the reflected signal from the metallic mirror (Reflectance ≈ 100%) to compute the optical constants. In this process, the relative position of the sample surface with respect to that of the reference mirror strongly affects the relative phase measured; therefore, it is difficult to obtain the accurate optical properties of the sample [16]. In order to overcome such problems, attenuated total reflection terahertz time domain spectroscopy (ATR THz-TDS) can be used. In this method, a sample under investigation is placed on a prism where the THz wave undergoes total internal reflection and the evanescent wave generated at the interface between the prism and sample enables the obtainment of the THz properties of the sample [17,18,19]. There are several advantages of the ATR THz-TDS system over other THz TDS systems, such as its ability to measure thick and highly absorbing sample [20,21]. Moreover, samples in solid and powdered form can also be measured without a special need for sample preparation. Therefore, the ATR THz-TDS system has a large potential in sensing and imaging applications [22,23].

## 2. Materials and Methods

#### 2.1. Experiment

_{1}is the refractive index of Silicon prism, n

_{2}is the refractive index of the sample, and θ

_{i}is the angle of incidence of the incoming THz wave. From Equation (1), it is clear that the depth of penetration mainly depends upon the frequency of the THz wave and the refractive index of the sample, assuming the incident angle (θ

_{i}) and the refractive index of Silicon (n

_{1}) are constant. Therefore, we investigated the penetration depth at a different frequency and refractive index of the sample. Figure 2 shows the dependency of the penetration depth on the sample refractive index (n

_{2}) and frequency. This shows that the depth of penetration increases with the decrease in frequency. Similarly, it also increases with the increase in the refractive index of the sample, as long as the condition n

_{1}> n

_{2}remains satisfied. This indicates that the THz properties of the sample can be measured even though the sample is packaged in a plastic bag, provided that the bag thickness is less than the penetration depth of the evanescent wave.

#### 2.2. Sample Preparation

## 3. Results

_{Sam}(t), whereas the other pulse is known as the reference pulse E

_{Ref}(t). These time domain pulses were transformed to intensity spectra using Fourier transformation, which are written as E

_{Sam}(ω) = |E

_{Sam}(ω)|exp{iφ

_{Sam}(ω)} and E

_{Ref}(ω) = |E

_{Ref}(ω)|exp{iφ

_{Ref}(ω)}; here, φ

_{Sam}(ω) and φ

_{Ref}(ω) are phase spectra of the sample and reference signals, respectively. Figure 4b shows the intensity spectra of the sample and reference signals, respectively.

_{Sam}(ω) − φ

_{Ref}(ω) is the phase difference and R is the amplitude reflectance written as

## 4. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**(

**a**) Schematic diagram of the THz time domain spectrometer. The dotted line shows the position of the Silicon prism in the spectrometer. (

**b**) Schematic diagram of a Silicon prism.

**Figure 2.**The penetration depth dependency on the frequency of THz wave and the refractive index of the sample. Here, a Silicon prism (n = 3.41) is used as a medium for total internal reflection.

**Figure 4.**(

**a**) The THz time domain reference and sample pulses, (

**b**) their respective intensity spectra, (

**c**) refractive index, (

**d**) absorption coefficient of lactose sample.

**Figure 5.**(

**a**) The refractive index and (

**b**) the absorption coefficient of the packaged sample. The absorption peak at 0.53 THz is clearly visible, as shown by the arrowhead. Since the penetration depth of the evanescent wave is smaller than the thickness of the plastic bag at high frequencies, the absorption features of the lactose sample at high frequency cannot be measured reliably. Therefore, the absorption coefficient is shown up to 0.8 THz.

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**MDPI and ACS Style**

Hashimoto, K.; Tripathi, S.R. Non-Destructive Identification of Drugs in Plastic Packaging Using Attenuated Total Reflection Terahertz Time Domain Spectroscopy. *Optics* **2022**, *3*, 99-106.
https://doi.org/10.3390/opt3020012

**AMA Style**

Hashimoto K, Tripathi SR. Non-Destructive Identification of Drugs in Plastic Packaging Using Attenuated Total Reflection Terahertz Time Domain Spectroscopy. *Optics*. 2022; 3(2):99-106.
https://doi.org/10.3390/opt3020012

**Chicago/Turabian Style**

Hashimoto, Kazuma, and Saroj R. Tripathi. 2022. "Non-Destructive Identification of Drugs in Plastic Packaging Using Attenuated Total Reflection Terahertz Time Domain Spectroscopy" *Optics* 3, no. 2: 99-106.
https://doi.org/10.3390/opt3020012