October 1, 2019.
Since in the recent years the threat from terrorism in Thailand have been increasing, especially, in the southern area of Thailand. That threat for example: bombs, explosions hidden in the vehicle, explosive substance etc. harmed the lives of officers operating at that area and also local people. The most important cause of these losses is the lack of detection technology at the distance. Therefore, the research of technique for the long range explosives detection, based on fundamental mechanism which is studied in IR and THz nanostructure lasers is very important. That technique must be high sensitivity and high selectivity techniques to different kinds of explosive substances and also could be detected the explosives at the distance within short time for the safety of the operation officers. Moreover, this technique must be reliable, not expensive which can be massively manufactured for many security organizations in Thailand. The further outcome can be also continued researching the technology for any security purposes of Thailand. Therefore, the security application is the main objective of this research project. This research project can be divided into three parts which relate each other: the creation of laser source emitting in IR range, the stand-off Raman spectroscopy technology and the Raman signal detection by quantum metrology.
From 16 March 2017 until 15 March 2020, the Fundamental Mechanism Study of IR and THz Nanostructure Lasers for National Security Application research project has progresses as following:
2. The Creation of Laser Source Emitting in IR Range
Infrared radiation (IR) is electromagnetic radiation which has the wavelength from 750 nm to 1 mm. It can be divided by the wavelength regions: near-IR has the wavelength from 750 to 1400 nm, mid-IR has the wavelength from 3000 to 8000 nm and far-IR or the terahertz frequency region has the wavelength from 15 µm to 1 mm. All three regions have special characteristics for beneficial uses, for example: near-IR has the absorption bands of many gas molecule: CO2, CO, SO2 and the transmission zones for atmosphere, so that it is possible to use laser and optoelectronics devices emitting in this spectrum region for the purpose of environmental monitoring and the visible zone telecommunication, mid-IR has the band for atmospheric window for the guided missile application, and terahertz range radiation is non-ionizing radiation which can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics and also can penetrate fog and clouds, therefore, it can be used as the probing of the potentially dangerous materials contained within including explosives for security applications. There are several researches of the development of efficiency and performance of mid-IR and terahertz sources (laser and optoelectronics devices) have been published in the recent years. The publications presented about for example: the design and properties of nanostructure strongly supported the intersubband population inversion between two excited states of electrons energy levels in quantum well (QW) in the condition of current or optical pumping. This kind of nanostructure consists of different types of III-V based semiconductors which is called heterostructures. It has great potential for optoelectronic applications as they cover a huge wavelength range from the infrared (IR) to the ultraviolet (UV), there are several researches contributed on the improvement of laser in IR, Quantum Cascade Laser (QCL) is one of the best designs for heterostructure emitting in mid-IR and terahertz ranges. The theoretical study of the possibility of improvement the THz range emission between two energy levels of electrons in nanostructure with quantum wells of heterostructure of GaAs/AlxGa1-xAs with inserted thin AlAs monolayer is undertaken in the first-year of project. This theoretical study was conducted by simulation program Nexnanomat (www.nextnanomat.com). The result of this study was published in Physica B: Condensed Matter Physics as in the Figure 1. Not only theoretical study is presented in the first-year, but also the photoluminescence (PL) experimental setup was settled at the first time (Figure 2).
Fig. 1 Publication in Physica B: Condensed Matter Physics 534 (2018) 169-172.
Fig. 2 Photoluminescence experimental setup of QW at 77 K to 300 K.
Moreover, during the second to the third year of research project, our project created the active collaboration in research. We actively collaborated with Nonequilibrium Electrons Optics Laboratory, Department of Semiconductor Physics and Nanoelectronics, Institute of Physics, Nanoelectronics and Telecommunications, Peter the Great Saint Petersburg Polytechnic University, Saint Petersburg, Russian Federation and Saint Petersburg National Research Academic University of the Russian Academy of Sciences. The purposes of these collaborations are the joint in nanostructure fabrication technology, education training/exchange of students and research staffs. Also, we have observed the technology of Molecular Beam Epitaxy (MBE) machine at Nanoelectronics laboratory, Russian Federation and Saint Petersburg National Research Academic University of the Russian Academy of Sciences as shown on Figure 3.
Fig. 3. Observation of Molecular Beam Epitaxy (MBE) machine at Nanoelectronics laboratory, Russian Federation and Saint Petersburg National Research Academic University of the Russian Academy of Sciences.
At the meantime, the stand-off Raman spectroscopy technology also have been operated together with the creation of laser source emitting in IR range.
3. Detection of Explosive Substance from a Distance via Stand-off Raman Spectroscopy
The stand-off chemical detection system is composed of pulsed laser source to excite the target, light collecting optics to scavenge scattered photons, and a spectrum analysis component working at very low light level. The also consists of computer simulation part and experiment as follows.
Fig. 4 (left) The structure of designed axicon doublet, and (right) laser beam with (a) Bessel-Gaussian profile showing divergence-free beam within 2 km range, and (b) normal Gaussian profile with larger beam divergence.
3.1 COMSOL Multiphysics Simulation for Producing Non-diverging Laser Beam
This section aims to study new beam profile, and design for an effective laser beam delivery system with minimal loss of intensity when travelling for long distance. The main requirement is that the beam should be virtually diffraction-free or very low diffraction in practice. The simulation was employed to study the propagation behavior of electromagnetic wave in a designed optical component, implemented in COMSOL Multiphysics program equipped with Wave Optics: Beam envelope method. The finite element frequency domain method was used to solve Maxwell equations with beam envelop approximation that allows the fine calculation of electromagnetic wave problem in a large computing domain, compared with the wavelength scale. This beam envelope approximation makes possible of this kind of calculation without using ultra high computing resources. This simulation was aimed to design a beam delivery system that maintains a tight beam profile even at long distance for preserving beam intensity. In order to do so, it was seeking for conditions that suppress beam divergence or diffraction in the radial direction even at kilometer scale. It was found that a simple optical structure called axicon doublet, composing of plano-convex axicon and plano-concave, with right refractive indices as show in Figure 4 (left) can be used to generate non-divergent Bessel-Gaussian laser beam profile from a normal Gaussian profile that normally diverges along a travelling distance. A beam passing through an axicon doublet will create new wavevector component with radially inward direction. At optimum design parameters, the radially propagating beam will compensate beam diffraction. The simulation result of Bessel-Gaussian shows that the beam is well collimated for the propagating range of 2 km Figure 4 (right), unlike a normal Gaussian beam that diverges significantly. High intensity laser beam at distance is very important to the excitation of nonlinear optical process on a target. This allows more effective generation of output signal. The experiment on producing non-divergent beam using an axicon doublet is on the progress.
3.2 The Investigation of Random Raman Lasing for More Efficient Detection
In general, a conventional standoff detection technique is based on spontaneous Raman scattering process which affords high molecular specificity but very low Raman conversion efficiency on the order of 10-10-10-8. This is a main hindrance that make the detection of Raman signal nearly impossible from far distance. Therefore, a nonlinear optical process of Raman emission, called Random Raman Lasing (RRL), is introduced in order achieve stronger output signal. RRL requires stimulated Raman scattering (SRS) as a gain mechanism and relies on multiple scattering to supply optical feedback for generating lasing emission. The principle of RRL is displayed in Figure 5. When a pump beam incident onto a target medium with disordered structure, two types of scattering occures: elastic Rayleigh and inelastic spontaneous Raman scatterings. Multiple scattering in the medium structure allows those two beam component to couple nonlinearly through SRS process, upon which a pump phonton is converted to Stokes photon. This gives rise to optical amplification of Raman signal inside the disordered medium.
Fig. 5 Schematic diagram showing Raman Random Lasing process in a disordered structure.
The schematic diagram for generating and detection of Random Raman Lasing spectrum and intensity is illustrated in Figure 6. It consists of a nanosecond pulsed laser, a polarizer to adjusting beam intensity, a quarter waveplate to alter the beam prolarization from linear to circular, and an optical filter to remove unwated wavelengths. The pump beam size was reduced and incident upon a targeted. Here, barium nitrate powder was chosen as a sample as it possesses high Raman gain coefficient, random structure, and the dominant Raman peak similar to those of many explosive substances. The output of Raman scattering is collected and couple to an optical fiber cable and delivered to a spectrometer for spectrum analyzing.
Fig. 6 Schematic diagram for generating and detection of Random Raman Lasing.
From the experiment, Raman spectrum of the target can be obtained clearly as shown in Figure 7. The dominant peak is observed at 1033.9 cm-1, associated with NO3- in the material structure.
Fig. 7 RRL spectrum of barium nitrate target.
It was also found that the intensity of the Raman peak at 1033.9 cm-1 with respect to pump intensity exhibits nonlinear relationship as shown in Figure 8. It can be noticed that the Raman signal increase rapidly for pump intensity beyond a threshold value ~18 MW/cm2. At this condition, the RRL occurs as the optical gain surpasses loss. The maximum Raman conversion efficiency obtained from this experiment is 4.25 x 10-4.
Fig. 8 The relationship between Raman output intensity and pump intensity showing a threshold behavior for Random Raman Lasing.
3.3 The Detection of Raman Scattering from Distance
A light collecting and spectrum analysis system was designed and constructed as shown in Figure 9. It consists of an 8-inch Schmidt Cassegrain telescope, dichroic beam splitter, and lens set. The scattered light is collected, coupled into an optical fiber, and delivered to a spectrometer for spectrum analysis. At this initial stage, an image intensifier system together with a fast gatinging operation synchronous with laser pules to futher reduce noise haven’t yet included.
Fig. 9 The stand-off detection system based on RRL.
In the experimental, a barium nitrate target at 10 m distance was excited by a laser pump to generate RRL. It was found that the output Raman signal is relatively strong and sufficient for detection from such distance which is almost impossible for spontaneous Raman scattering cases. The spectrum of barium nitrate target measured from 10 m distance is shown in Figure 10, where the characteris peaks can be cleary observed although with higher level of noise. However, this experiment is under the development to equip with signal intensifier unit and noise reducing mechanism to further extend the range of Raman detection.
Fig. 10 RRL spectrum barium nitrate target measure from 10 m distance.
4. Quantum Metrology for Enhanced Measurement
Apart from the IR sources we have mentioned, we also aim to improve the efficiency of our measurement system. We expect the significant improved in precision, low