Excitation of dark multipolar plasmonic resonances at terahertz frequencies

Tuning transmission properties of 3D printed metal rod arrays by breaking the structural symmetry

In this work, one metallic photonic crystal waveguide composed of periodic metal rod arrays (MRAs) is experimentally and numerically demonstrated in terahertz frequencies. Such waveguides fabricated by 3D printers exhibit two resonant modes: the fundamental mode and the high-order mode, separating by a broad bandgap. Compared to the fundamental mode, the high-order mode shows higher field confinement and more sensitive to the geometry changes. By breaking the structure parameter, i.e., increasing or decreasing the metal rod interspace, the spectral positions, bandwidths, as well as the transmittances of high-order modes can be optimized. With broken symmetry in MRAs, the third resonant mode having high transmittance has emerged in the transmission spectrum. Results showing that fine-tuning in the alignment of metal rods leads to a great change in the transmission of high-order modes. These findings suggest that the transportation efficiency of THz waves through an MRA is tunable by breaking the structural symmetry.

Critical Factors for In Vivo Measurements of Human Skin by Terahertz Attenuated Total Reflection Spectroscopy

Attenuated total reflection (ATR) geometry is a suitable choice for in vivo measurements of human skin due to the deep penetration of the field into the sample and since it makes it easy to measure the reference spectrum. On the other hand, there are several critical factors that may affect the terahertz (THz) response in these kinds of experiments. Here, we analyse in detail the influence of the following factors: the contact positions between the thumb and the prism, the contact pressure, the contact duration, and the materials of the prism. Furthermore, we use the THz-ATR technology to evaluate different types of handcream and also establish the theoretical model to investigate the reflectivity after interacting with the skin. The results agree well with experimental ones. Our analysis makes it clear the importance of controlling the above factors during measurements to enable reliable THz response and results which, in turn, may be used to monitor water motion in human skin and to predict possible diseases.

Terahertz composite plasmonic slabs based on double-layer metallic gratings

One composite plasmonic slab with a broad bandgap (40%) is experimentally and numerically demonstrated in the terahertz (THz) region. The composite slab consists of double-layer metallic gratings and a dielectric film, which supports two resonant modes. Electric field vectors and charge distributions proved that the low-frequency resonant mode originates from the symmetric plasmonic mode, while the high-frequency resonant mode is induced by the hybrid mode of plasmonic and dielectric modes. Compared with the double-layer metallic grating, the inserted dielectric film significantly enhances the transmission of the transverse magnetic (TM) waves and induces Fano resonances. The near-field coupling between metal gratings and dielectric film can be manipulated by changing the thickness and the refractive index of dielectric films. We further demonstrated that the plasmonic bandgap can be manipulated by tuning the grating width. These results suggest that this composite plasmonic slab is promising in terahertz integrated components development such as a filter, polarizer, or sensor.

Transfer of orbital angular momentum of light to plasmonic excitations in metamaterials

The emergence of the vortex beam with orbital angular momentum (OAM) has provided intriguing possibilities to induce optical transitions beyond the framework of the electric dipole interaction. The uniqueness stems from the OAM transfer from light to material, as demonstrated in electronic transitions in atomic systems. In this study, we report on the OAM transfer to electrons in solid-state systems, which has been elusive to date. Using metamaterials (periodically textured metallic disks), we show that multipolar modes of the surface electromagnetic excitations (so-called spoof localized surface plasmons) are selectively induced by the terahertz vortex beam. Our results reveal selection rules governed by the conservation of the total angular momentum, which is confirmed by numerical simulations. The efficient transfer of light's OAM to elementary excitations in solid-state systems at room temperature opens up new possibilities of OAM manipulation.

Efficient terahertz polarization conversion with hybrid coupling of chiral metamaterial

We propose an ultrathin terahertz waveplate of bi-layer chiral metamaterial for cross-polarization conversion at asymmetric transmission. The chiral metamaterial is constructed with hybrid coupling plasmonic resonators of a concentric ring and a double-split ring. The terahertz metamaterial can efficiently convert the ${y}$y-polarized wave into the ${x}$x-polarized wave with the cross-polarized transmittance over 97% and the polarization conversion ratio of 99% in simulation. The asymmetric transmission parameter, defined by the difference between two opposite propagating transmittances, can be as high as 0.9. The operation frequency and efficiency are geometrically adjustable with the ring size by exploiting the hybrid coupling effect of electric and magnetic resonances. The presented metamaterial enables the functionality of the nonreciprocal terahertz waveplate with high isolation.

Spoof surface plasmonic transmission line with high isolation and low propagation loss

A novel compact spoof surface plasmonic (SSP) transmission line (TL) consisting of a stem-shaped periodic structure is proposed to achieve high field confinement in the higher frequency range (microwave to terahertz). The dispersion characteristic of the proposed stem-shaped SSP unit cell exhibits much lower asymptotic frequency and higher field confinement than the conventional coplanar waveguide (CPW) and the rectangular SSP unit cell structure without increasing the overall transverse dimension, i.e., improved signal propagation performance with reduced cross-sectional area. The numerical study of the multi-conductor line based on the proposed stem-shaped SSP unit cell shows that it has improved propagation and isolation features in comparison to the conventional TL structures. Here, the improved isolation is characterized in terms of the mutual coupling (MC) between the two adjacent lines designed using the stem-shaped SSP unit cell, which is not found to significantly increase with an increase in the coupling length unlikely in the conventional TLs. For the proposed SSP-based multi-conductor line, the MCs for 1.6, 2.25, and 4.15λ coupling lengths with fixed separation of 0.175λ are found to be 22, -19.8, and -17.55dB, respectively.

3D conical helix metamaterial-based isotropic broadband perfect light absorber

We present the design and fabrication of an isotropic broadband perfect light absorber in the near-infrared range using 3D metamaterials with a single resonator in the unit cell. The metamaterial resonator is comprised of a gold conical helix supported on a silicon pillar with back reflector realized on a silicon substrate. Simulations and experiments have demonstrated that the proposed absorber achieves a broad absorption band of more than 3 µm in the 1.5-4.5 µm wavelength range with an average absorbance of more than 90%. The numerical and experimental analyses show that the proposed device can provide both incident angle and polarization independent operations, which further widens the application prospects of our device.

Demonstration of group delay above 40 ps at terahertz plasmon-induced transparency windows

Herein, we demonstrate one of the highest terahertz group delay of 42.4 ps achieved experimentally at 0.23 THz, on a flexible planar metamaterial. The unit cell of metasurface is made up of a textured closed cavity and another experimentally concentric metallic arc. By tuning the central angle of the metallic arc, its intrinsic dipolar mode is in destructive interference with the spoof localized surface plasmon (SLSP) on textured closed cavity, which results in a plasmon-induced transparency phenomenon. The measured transmittances of as-fabricated samples using terahertz-time domain spectroscopy validate numerical results using extended coupled Lorentz oscillator model. It is found that the coupling coefficient and damping ratio of SLSP relies on the radius of the ring structure of textured closed cavity. As a consequence, the slow light maximum values become manoeuverable in strength at certain frequencies of induced transparency windows. To the best of our knowledge, our experimental result is currently the highest value demonstrated so far within metasurface at terahertz band.

Metallic photonic crystal-based sensor for cryogenic environments

We investigate the design, characterization, and application of metallic photonic crystal (MPC) structures, consisting of plasmonic gold nanogratings on top of a photonic waveguide, as transducers for lab-on-chip biosensing in cryogenic environments. The compact design offers a promising approach to sensitive, in situ biosensing platforms for astrobiology applications (e.g., on the "icy moons" of the outer solar system). We fabricated and experimentally characterized three MPC sensor geometries, with variable nanograting width, at temperatures ranging from 300 K to 180 K. Sensors with wider nanogratings were more sensitive to changes in the local dielectric environment. Temperature-dependent experiments revealed an increase in plasmonic resonance intensity of around 13% at 180 K (compared with 300 K), while the coupled plasmonic-photonic resonance was less sensitive to temperature, varying by less than 5%. Simulation results confirm the relative temperature stability of the plasmonic-photonic mode and, combined with its high sensitivity, suggest a novel application of this mode as the sensing transduction mechanism over wide temperature ranges. To our knowledge, this is among the first reports of the design and characterization of a nanoplasmonic sensor specifically for low-temperature sensing operation.

Highly sensitive terahertz fingerprint sensing with high-Q guided resonance in photonic crystal cavity

In order to solve the problem of low sensitivity and poor selectivity in biochemical sensing using terahertz technology, a new sensing scheme based on photonic crystal cavity structure is proposed. It is composed of two identical photonic crystal slabs, each of which consists of a square lattice of silicon-based cylindrical pillars on a silicon substrate. The geometric parameters of the cavity are optimized to obtain a guided resonance peak at 529.2 GHz with a high quality factor of 529. The detected object is located in the middle of cavity where the electric field is strongly localized and confined. The effective detection of lactose with only a few microns thick is taken as an example to demonstrate the sensing performance of this cavity. A distinct decrease in transmittance at resonance peak is observed. The sensitivity using our proposed cavity is 31 times higher than that of using a substrate. Moreover, the selectivity of this photonic crystal cavity for the target is also verified by using fructose as the non-target. These results show that the photonic crystal cavity has potential to be applied for fingerprint detection with high sensitivity as well as selectivity in terahertz sensing.

Tunable hybridization induced transparency for efficient terahertz sensing

Hybridization induced transparency (HIT) resulting from the coupling between the material absorption resonance and the artificial structure (metamaterial) resonance provides an effective means of enhancing the sensitivity in the terahertz spectroscopic technique-based sensing applications. However, the application of this method is limited by the versatility to the samples with different volumes, because the samples usually have a refractive index larger than unity and their presence with different thicknesses will lead to a shift of the structure resonance, mismatching the material absorption. In this work, we demonstrate that by using InSb coupled rod structures, whose electromagnetic response in the terahertz band can be easily controlled by using ambient parameters like the temperature or magnetic field, the HIT effect can be easily tuned so that without the needs to change the rod geometry, one can realize efficient terahertz sensing with different sample thickness.

Inspecting human colon adenocarcinoma cell lines by using terahertz time-domain reflection spectroscopy

Techniques to inspect and analyze human colorectal cancer cell lines by using terahertz time-domain attenuated total reflection spectroscopy (THz TD-ATR) were investigated. The characteristics of THz absorption spectra of two colorectal cancer cell lines DLD-1 and HT-29 in aqueous solutions with different concentrations were studied. Different spectral features were observed compared to normal cell line. Identification results based on different parameters including absorption coefficient, refractive index, real and imaginary parts of complex permittivity, dielectric loss tangent were discussed. This research may be promising for quick and instant inspection of liquid samples by using THz time-domain spectroscopy in medical applications.

Achieving subwavelength field confinement in sub-terahertz regime by periodic metallo-dielectric waveguides

In this paper, we report on a periodic metallo-dielectric structure that supports geometry-induced surface plasmons in the sub-terahertz regime. The proposed structure is made up of a dielectric-coated metallic grating sandwiched by parallel metal plates. Based on the modal analysis of 2D and 3D structures, the impact of a metal cladding and a customized dielectric coating on the dispersion relation and field distribution of the guided surface wave is investigated. It is found that modal field confinement is improved in the presence of a metal cladding without narrowing the operational bandwidth of the waveguide. Moreover, a customized subwavelength-sized dielectric coating based on high-resistivity silicon (HR-Si) can further improve the confinement. As a result, by incorporating both the HR-Si coating and the metal cladding in a conventional metallic grating, subwavelength field confinement is achieved over nearly a 2:1 frequency bandwidth. The achieved performance makes the realization of extremely-low radiation loss sharp bends possible. In particular, the achieved radiation loss is less than 0.5dB for a 90° bend of radius λ₀/4 based on a waveguide cross-sectional dimension of almost λ₀/10 where λ₀ is the free-space wavelength at the maximum frequency of operation. The proposed waveguide is promising for the implementation of sub-terahertz guided-wave devices and circuits thanks to its outstanding field confinement and ruggedized and shielded structure.

Prism coupling of high-Q terahertz whispering-gallery-modes over two octaves from 0.2 THz to 1.1 THz

We report on prism coupling of high-quality (high-Q) terahertz (THz) whispering-gallery modes (WGMs) in spherical high resistivity float zone grown silicon (HRFZ-Si) resonators over two octaves from 0.2 THz to 1.1 THz. The WGMs are excited using a HRFZ-Si prism and show unprecedented quality factors of up to 2.2 × 10⁴. A detailed discussion of the phase-and mode-matching criteria of the prism coupling scheme implemented in the continuous wave THz spectroscopy system is presented. The results provide numerous opportunities for passive ultra-broadband high-Q devices operating in the THz frequency range.

Photonic crystal-based compact hybrid WDM/MDM (De)multiplexer for SOI platforms

A compact hybrid wavelength- (WDM) and mode-division (de)multiplexer (MDM) is proposed, and its performance is evaluated. The design of the device is based on 2D photonic crystals with a square lattice and Si rods. The device can multiplex two eigenmodes, TM₀ and TM₁, and two wavelengths, 1550 and 1300 nm. Two identical multimode interference couplers and an asymmetric directional coupler are used in implementing both the wavelength- and mode-division multiplexing functions, respectively. To avoid back-reflections, tapers are used at waveguide junctions. The structure is compact with dimensions of 29  μm×12  μm, which is suitable for on-chip integration. Simulation results reveal that the insertion losses and crosstalks are less than -1.0927 and -11.9024  dB, respectively, for all four channels.

Selective bright and dark mode excitation in coupled nanoantennas

Coupled nanoantennas as metamaterial unit elements possess peculiar spectral and radiational behaviors. We show that nanoantennas made of two identical plasmonic slot resonators can greatly enhance the quality factors of resonance spectra and control radiation patterns through the selective excitation of bright and dark coupled modes. We confirm experimentally the enhanced quality factor of a bright mode in coupled nanoantennas. Adding phase modulators to the coupled microwave antennas, we demonstrate the "dark mode only" excitation of coupled microwave antennas with an incident plane wave. We also show that the bright-to-dark mode conversion and the related changes in radiation patterns can be controlled by the polarization of incident waves. In particular, we achieve leftward or rightward uni-directional radiation upon the injection of left or right circularly polarized waves.

Spoof Plasmonics: From Metamaterial Concept to Topological Description

Advances in metamaterials have offered the opportunity of engineering electromagnetic properties beyond the limits of natural materials. A typical example is "spoof" surface plasmon polaritons (SPPs), which mimic features of SPPs without penetrating into metal, but only with periodic corrugations on metal surfaces. They hold considerable promise in device applications from microwaves to the far infrared, where real SPP modes do not exist. The original spoof SPP concept is derived from the description of corrugated surfaces by a metamaterial that hosts an effective plasma frequency. Later, studies have attempted to describe spoof SPP modes with the band structure by strictly solving Maxwell's equations, which can possess band gaps from polaritonic anticrossing principle or Bragg interference. More recently, as inspired by the development of topological framework in condensed matter physics, the topological description of spoof SPPs is used to propose topologically protected waveguiding phenomena. Here, the developments of spoof SPPs from both practical and fundamental perspectives are reviewed.

Experimental realization of Mie-resonance terahertz absorber by self-assembly method

Mie-resonance terahertz absorbers by self-assembly method are designed and demonstrated in experiments and simulations. A monolayer of zirconium dioxide (ZrO₂) microspheres fixed on a copper film with designed grids that were manufactured by direct writing with a composite ink system composed of polydimethylsiloxane (PDMS). More importantly, different spacing and array configurations were created economically and efficiently, showing visual performance. Magnetic resonance leads to near-unity absorption at about 0.4 THz in the samples. This work demonstrates efficient terahertz absorbers and highlights a novel direct writing fabrication method that can be extended to produce other optical devices for applications.

Numerical investigation of terahertz polarization-independent multiband ultrahigh refractive index metamaterial by bilayer metallic rectangular ring structure

Multiband high index of refraction can be realized by thin ring-type terahertz metamaterials composed of multilayer coupled unit cells. We have focused on the numerical investigation of this type of a metamaterial. By drastically decreasing the diamagnetic effect with a thin metallic structure in the unit cell and by increasing the effective permittivity through strong capacitive coupling, a bandwidth of 1.5 THz with an index of more than 24 can be achieved using a single-layer thin brick-type metamaterial. The refractive index peak is 35. Then, we design a ring-type metamaterial structure, achieving a refractive index of 91 at about 0.45 THz, which is due to a decrease in the diamagnetic effect with smaller area surrounded by toroidal currents. Based on the coupling effects of double layer ring-type metamaterials or single-layer double ring-type structures, the refractive index peaks reach 43.2 and 18.68 at 0.43 THz and 0.92 THz, respectively. A three-layer ring-type metamaterial structure is proposed to obtain three band high index metamaterials.

Multiband high index of refraction can be realized by thin ring-type terahertz metamaterials composed of multilayer coupled unit cells.

Dynamic modulation of wideband slow light with continuous group index in polymer-filled photonic crystal waveguide

The dynamic modulation of wide bandwidth and low-dispersion slow light with continuous variation of group index n_g is realized in a polymer-filled photonic crystal waveguide (PF-PCW) with optimal structure. By adjusting the unified radius of air holes under a different refractive index of polymer in the first two rows of holes adjacent to the defect, the structure optimization of PF-PCW is first studied, then the fixed optimal structure is obtained. In the optimal photonic crystal waveguide with hole radius r₀=0.328a, a fixed refractive index n₁=1.74 of polymer in the first-row holes, and by adjusting refractive index n₂, the flattened wideband slow light with large normalized delay bandwidth product of group index from 17.15 to 55.65 has been demonstrated. Then, by filling polymer with electro-optic effect into the second-row holes, the dynamic modulation of the optimized slow light in PF-PCW is investigated. The simulation shows that the center operating frequency slightly shifts linearly to a higher one, and the average group index increases exponentially from 33.943 to 75.546 with a normalized delay bandwidth product larger than 0.3089 as the applied voltage increases. The modulation sensitivity of the average group index is about 0.3467/V when applied voltages vary from 0 V to 120 V. These results open the possibility for the dynamic control of slow light according to the practical requirements of flexibility and tunability.

High extinction ratio electromagnetically induced transparency analogue based on the radiation suppression of dark modes

A high extinction ratio (ER) electromagnetically induced transparency (EIT) analogue based on single-layer metamaterial is designed and experimentally demonstrated in this paper. This design involves four mirror-like symmetrically coupled split ring resonators (SRRs) that exhibit a bright-dark-dark-bright mode configuration. The EIT-like effect is realized by coupling between the bright resonators and dark resonators. The high ER feature is achieved from the suppression of radiative losses, due to opposite directions of electric and magnetic dipoles of two dark modes in the unit cell. Classical coupled resonator model is used to theoretically analyze the device transmission performances and to characterize parameter influence of the ER. Both numerical simulation and experiment results demonstrate that the ER of this device can reach more than 21 dB, which is 11 dB higher than that of conventional bright-dark coupling SRR arrangement. Finally, the potential multi-channel sensing utility of this device is demonstrated to show the importance of high ER feature.

Metallic planar lens constructed by double-turn waveguides for sub-diffraction-limit focusing

We present a conceptual demonstration of a metallic planar lens composed of double-turn waveguides for sub-diffraction-limit focusing. The phase delay of a single double-turn waveguide dependent on its structural parameters is investigated by employing the finite-difference time-domain (FDTD) numerical method. The design utilizes the surface plasmon polaritons (SPPs) that propagate along the metal-insulator-metal (MIM) waveguides to achieve the desired spatial phase modulation in the transmitted field. The simulated focal length achieved is in positive agreement with the design and the full-width at half-maximum (FWHM) is 0.446λ, well beyond the diffraction limit. This superfocusing performance can be maintained very well under the slight change of film thickness and slit width, showing the robustness of the design. The maximum aspect ratio of nanoslits constructing the proposed lens is 3.33, which is far less than the previous reports, alleviating the later fabrication. The metallic planar lens as demonstrated will find its applications in such fields as lithography, integrated optics, and super-resolution imaging.

Multipole Modes Excitation of uncoupled dark Plasmons Resonators based on Frequency Selective Surface at X-band Frequency Regime

In this report, we theoretically and experimentally demonstrates that multipole modes could be excited effectively in dark plasmonic resonators without introducing any other bright resonators at microwave range based on a two-dimensional frequency selective surface (FSS) structure. These excited multipole resonances are closely related to the coupling strength between adjacent S-LSPs resonators (the periodicity of the FSS). The modes splitting effects and resonance frequencies of the excited multipole modes are regulated by changing the numbers of grooves and inner disk radius, both of which play significant roles in the excitation of the dark S-LSPs disk resonator at normal incidence. Moreover, the multipole resonances characteristics of dark S-LSPs resonators in the case of oblique incidence are also presented. Observation of such multipole resonances in dark S-LSPs without introducing extra bright resonance at normal/oblique incidence would find more potential applications in microwave and terahertz based sensors, plasmonic resonance devices and metamaterial devices.

Design and fabrication of woodpile photonic structures through phase SLM-based interference lithography for omnidirectional optical filters

In this Letter, we report a large-area and single-step optical fabrication technique based on phase engineering interference lithography that is scalable and reconfigurable for the realization of submicrometer scale periodic face-centered cubic inverse woodpile photonic structures. The realized inverse woodpile structure on positive having four number axial layers with 740 nm spatial and 1046 nm axial periodicities shows 10% reflectance and 90% transmittance at 776 nm wavelength that can further be improved for the addition of axial layers. The realized structure can be transferred to crystalline silicon for realizing a bandpass/rejection near-infrared filter in a reflection/transmission mode. Further, woodpile structures based on low-contrast silicon nitride (Si₃N₄) are designed as selective narrow frequency filters at 1310 and 1550 nm wavelengths for telecommunication applications and omnidirectional red-green-blue filters for display devices by tuning the design parameters.

Terahertz sensing of highly absorptive water-methanol mixtures with multiple resonances in metamaterials

Terahertz sensing of highly absorptive aqueous solutions remains challenging due to strong absorption of water in the terahertz regime. Here, we experimentally demonstrate a cost-effective metamaterial-based sensor integrated with terahertz time-domain spectroscopy for highly absorptive water-methanol mixture sensing. This metamaterial has simple asymmetric wire structures that support multiple resonances including a fundamental Fano resonance and higher order dipolar resonance in the terahertz regime. Both the resonance modes have strong intensity in the transmission spectra which we exploit for detection of the highly absorptive water-methanol mixtures. The experimentally characterized sensitivities of the Fano and dipole resonances for the water-methanol mixtures are found to be 160 and 305 GHz/RIU, respectively. This method provides a robust route for metamaterial-assisted terahertz sensing of highly absorptive chemical and biochemical materials with multiple resonances and high accuracy.

Two-port connecting-layer-based sandwiched grating by a polarization-independent design

In this paper, a two-port connecting-layer-based sandwiched beam splitter grating with polarization-independent property is reported and designed. Such the grating can separate the transmission polarized light into two diffraction orders with equal energies, which can realize the nearly 50/50 output with good uniformity. For the given wavelength of 800 nm and period of 780 nm, a simplified modal method can design a optimal duty cycle and the estimation value of the grating depth can be calculated based on it. In order to obtain the precise grating parameters, a rigorous coupled-wave analysis can be employed to optimize grating parameters by seeking for the precise grating depth and the thickness of connecting layer. Based on the optimized design, a high-efficiency two-port output grating with the wideband performances can be gained. Even more important, diffraction efficiencies are calculated by using two analytical methods, which are proved to be coincided well with each other. Therefore, the grating is significant for practical optical photonic element in engineering.

Near-field terahertz probes with room-temperature nanodetectors for subwavelength resolution imaging

Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.

3D printed hollow core terahertz Bragg waveguides with defect layers for surface sensing applications

We study a 3D-printed hollow core terahertz (THz) Bragg waveguide for resonant surface sensing applications. We demonstrate theoretically and confirm experimentally that by introducing a defect in the first layer of the Bragg reflector, thereby causing anticrossing between the dispersion relations of the core-guided mode and the defect mode, we can create a sharp transmission dip in the waveguide transmission spectrum. By tracking changes in the spectral position of the narrow transmission dip, one can build a sensor, which is highly sensitive to the optical properties of the defect layer. To calibrate our sensor, we use PMMA layers of various thicknesses deposited onto the waveguide core surface. The measured sensitivity to changes in the defect layer thickness is found to be 0.1 GHz/μm. Then, we explore THz resonant surface sensing using α-lactose monohydrate powder as an analyte. We employ a rotating THz Bragg fiber and a semi-automatic powder feeder to explore the limit of the analyte thickness detection using a surface modality. We demonstrate experimentally that powder layer thickness variations as small as 3μm can be reliably detected with our sensor. Finally, we present a comparative study of the time-domain spectroscopy versus continuous wave THz systems supplemented with THz imaging for resonant surface sensing applications.