Using Transmissive Photonic Band Edge Shift to Detect Explosives: A Study with 2,4,6-Trinitrotoluene (TNT)

Photonic crystals (PhCs) possess outstanding optical properties that can be exploited for chemical sensing. We utilized a three-dimensional close-packed PhC structure made of functionalized silica nanoparticles. They consist of alternating high and low refractive index regions and have optical properties, such as photonic band structures, that are very sensitive to the change of physical structures. This study used 2,4,6-trinitrotoluene (TNT) to illustrate a detection method based on the transmissive photonic band edge shift (TPBES) due to the binding of TNT with amine anchored on particle surfaces to form Meisenheimer (amine-TNT) complexes. PhCs are exceptionally sensitive to a small change in refractive index caused by surface modification. As a result, they are suitable for sensing specific reactions between an amine and TNT. This method achieved a wide detection range of TNT concentrations from 10$^{-12}$ to 10$^{-4}$ M. 2,4-Dinitrotoluene (DNT) and toluene were used as a control and blank, respectively. Because of gravitational sedimentation, the TNT-functionalized particles were self-assembled in pure ethanol. They were measured by UV–visible transmission spectroscopy. A three-dimensional model to simulate the detection system was built using the particles’ center-to-center distance (a) and effective dielectric constant (ε) as a function of the TNT concentrations. Two sets of simulations were performed: the first set involved a parametric sweep of the center-to-center distance of TNT-functionalized crystals using ε = 2.015. The second set involved a parametric sweep of the dielectric constant with a = 263.1 nm. These perturbations yield a TPBES response that is in agreement with our experimental results

Probing the Topological Surface State in Bi2Se3 Thin Films Using Temperature-Dependent Terahertz Spectroscopy

Strong spin–momentum coupling in topological insulators gives rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3), allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (<30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics results in an optical mobility exceeding 2000 cm2/V·s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ∼0.18 ps and a carrier density of 6 × 1012 cm–2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ∼1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state

Bolometric detection of terahertz quantum cascade laser radiation with graphene-plasmonic antenna arrays

We present a fast room temperature terahertz detector based on graphene loaded plasmonic antenna arrays. The antenna elements, which are arranged in series and are shorted by graphene, are contacting source and drain metallic pads, thus providing both the optical resonant element and the electrodes. The distance between the antenna's arms of approximately 300 nm allows a strong field enhancement in the graphene region, when the incident radiation is resonant with the antennas. The current passing through the source and drain is dependent on the graphene's conductivity, which is modified by the power impinging onto the detector as well as from the biasing back-gate voltage. The incident radiation power is thus translated into a current modification, with the main detection mechanism being attributed to the bolometric effect. The device has been characterized and tested with two bound to continuum terahertz quantum cascade lasers emitting at a single frequency around 2 THz and 2.7 THz yielding a maximum responsivity of ~2 mA W$^{−1}$.RD, HEB and DAR acknowledge financial support from the Engineering and Physical Sciences Research Council (Grant No. EP/J017671/1, Coherent Terahertz Systems). SH acknowledges funding from EPSRC (Grant No. EP/K016636/1, GRAPHTED). KN acknowledges the University of Cambridge Nanoscience Doctoral Training Centre (EPSRC EP/G037221/1) for financial support

High Open-Circuit Voltages in Tin-Rich Low-Bandgap Perovskite-Based Planar Heterojunction Photovoltaics

Low-bandgap CH$_3$NH$_3$(Pb$_x$ Sn$_{1–x}$)I$_3$ (0 ≤ x ≤ 1) hybrid perovskites (e.g., ≈1.5–1.1 eV) demonstrating high surface coverage and superior optoelectronic properties have been fabricated. State-of-the-art photovoltaic (PV) performance is reported with power conversion efficiencies approaching 10% in planar heterojunction architecture with small (100 cm$^2$ V$^{−1}$ s$^{−1}$) intrinsic carrier mobilities.Engineering and Physical Sciences Research Council (Grant ID: EP/M005143/1), India-UK APEX project, Winton Programme (Cambridge) for the Physics of Sustainability, Cambridge Trust, China Scholarship Council, Nava Technology Limited (PhD scholarship), LDRD program, LAN

Performance of tall wheat under different sowing times and methods using various nutrient sources in western Haryana

To study the effect of different sowing times, methods and nutrient sources on the growth and productivity of tall wheat, a field experiment was conducted in a split-plot design with three replications. The analysis of data revealed that tall wheat crop sown in the last week of October on FIRBS recorded significantly higher plant height, dry matter accumulation (at different growth stages), yield attributes, grain yield (2599 kg ha-1) and biological yield (7212 kg ha-1) as compared to conventionally sown wheat crop in 1st and 2nd week of November. Among the different nutrient sources, the recommended doses of chemical fertilizers provided significantly higher values of growth parameters, yield attributes, grain yield (2658 kg ha-1) and biological yield (7426 kg ha-1) compared to organic fertilizers/ biofertilizers. Maximum temperature showed a significant and positive correlation with yield and yield attributes. In light of the results to maximize the productivity of tall wheat, it may be sown in the last week of October on FIRBS supplied with recommended doses of fertilizers on the basis of soil test

Temperature evolution of topological surface states in bismuth selenide thin films studied using terahertz spectroscopy

We have measured the terahertz (THz) conductance of a 23 quintuple layer thick film of bismuth selenide (Bi2Se3) and found signatures for topological surface states (TSSs) below 50 K. We provide evidence for a topological phase transition as a function of lattice temperature by optical means. In this work, we used THz time-domain spectroscopy (THz-TDS) to measure the optical conductance of Bi2Se3, revealing metallic behavior at temperatures below 50 K. We measure the THz conductance of Bi2Se3 as 10 e2/h at 4 K, indicative of a surface dominated response. Furthermore, the THz conductance spectra reveal characteristic features at ~1.9 THz attributed to the optical phonon mode, which is weakly visible at low temperatures but which becomes more prominent with increasing temperature. These results present a first look at the temperature-dependent behavior of TSSs in Bi2Se3 and the capability to selectively identify and address them using THz spectroscopy

The relationship between the three-dimensional structure of porous GaN distributed Bragg reflectors and their birefringence

Porous GaN DBRs offer an opportunity to provide the high reflectance, lattice-matched components required for efficient GaN VCSELs. The birefringence of these structures is therefore of key interest, as it could be used to control the polarization of the emitted light. Here we present a detailed analysis of the optical birefringence for both laterally etched, patterned structures and selfassembled radial porous structures. We correlate this with the 3D structure of the pores, which we measure through the use of FIB milling and serial block-face SEM imaging (SBI). This is a powerful method for imaging the internal nanostructure of the sample and allows the internal pore morphology to be viewed in a reconstruction of any 3D plane. We measure the birefringence of our porous GaN layers as Δ=0.14, with a lower refractive index parallel to the pores (∥) than perpendicular to them (⟂). Using finite element modelling we accurately reproduce the experimentally observed birefringence trends and find that this can be done by modelling GaN as a perfect dielectric. This indicates that the birefringence arises from the limited width across the pores. This also shows that standard modelling approaches can be used to design porous GaN birefringent devices effectively

Probing the topological surface state in Bi₂Se₃ thin films using temperature-dependent terahertz spectroscopy

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (< 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm2/V•s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×1012 cm-2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state

Probing the topological surface state in Bi₂Se₃ thin films using temperature-dependent terahertz spectroscopy

Strong spin-momentum coupling in topological insulators give rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics such as the spin hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3) allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (&lt; 30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics, obtain an optical mobility, exceeding 2000 cm2/V•s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ~0.18 ps and a carrier density of 6×1012 cm-2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ~1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state

Probing the Topological Surface State in Bi2Se3 Thin Films Using Temperature-Dependent Terahertz Spectroscopy

Strong spin–momentum coupling in topological insulators gives rise to topological surface states, protected against disorder scattering by time reversal symmetry. The study of these exotic quantum states not only provides an opportunity to explore fundamental phenomenon in condensed matter physics, such as the spin Hall effect, but also lays the foundation for applications in quantum computing to spintronics. Conventional electrical measurements suffer from substantial bulk interference, making it difficult to clearly identify topological surface state from the bulk. We use terahertz time-domain spectroscopy to study the temperature-dependent optical behavior of a 23-quintuple-thick film of bismuth selenide (Bi2Se3), allowing the deconvolution of the surface state response from the bulk. The signatures of the topological surface state at low temperatures (<30 K) with the optical conductance of Bi2Se3 exhibiting a metallic behavior are observed. Measurement of carrier dynamics results in an optical mobility exceeding 2000 cm2/V·s at 4 K, indicative of a surface-dominated response. A scattering lifetime of ∼0.18 ps and a carrier density of 6 × 1012 cm–2 at 4 K were obtained from the terahertz time-domain spectroscopy measurement. The terahertz conductance spectra reveal characteristic features at ∼1.9 THz, attributed to the optical phonon mode, which becomes less prominent with falling temperature. The electrical transport measurements reveal weak antilocalization behavior in our Bi2Se3 sample, consistent with the presence of a topological surface state