Metamaterial based CMOS terahertz focal plane array

The distinctive properties of terahertz radiation have driven an increase in interest to develop applications in the imaging field. The non-ionising radiation properties and transparency to common non-conductive materials have led research into developing a number of important applications including security screening, medical imaging, explosive detection and wireless communications. The proliferation of these applications into everyday life has been hindered by the lack of inexpensive, compact and room-temperature terahertz sources and detectors. These issues are addressed in this work by developing an innovative, uncooled, compact, scalable and low-cost terahertz detector able to target single frequency imaging applications such as stand-off imaging and non-invasive package inspection. The development of two types of metamaterial (MM) based terahertz focal plane arrays (FPAs) monolithically integrated in a standard complementary metal-oxide semiconductor (CMOS) technology are presented in this Thesis. The room temperature FPAs are composed of periodic cross-shaped resonant MM absorbers, microbolometer sensors in every pixel and front-end readout electronics fabricated in a 180 nm six metal layer CMOS process from Texas Instruments (TI). The MM absorbers are used due to the lack of natural selective absorbing materials of terahertz radiation. These subwavelength structures are made directly in the metallic and insulating layers available in the CMOS foundry process. When the MM structures are distributed in a periodic fashion, they behave as a frequency-selective material and are able to absorb at the required frequency. The electromagnetic (EM) properties are determined by the MM absorber geometry rather than their composition, thus being completely customisable for different frequencies. Single band and broadband absorbers were designed and implemented in the FPAs to absorb at 2.5 THz where a natural atmospheric transmission window is found, thus reducing the signal loss in the imaging system. The new approach of terahertz imaging presented in this Thesis is based in coupling a MM absorber with a suitable microbolometer sensor. The MM structure absorbs the terahertz wave while the microbolometer sensor detects the localised temperature change, depending on the magnitude of the radiation. Two widely used microbolometer sensors are investigated to compare the sensitivity of the detectors. The two materials are Vanadium Oxide (VOx) and p-n silicon diodes both of which are widely used in infrared (IR) imaging systems. The VOx microbolometers are patterned above the MM absorber and the p-n diode microbolometers are already present in the CMOS process. The design and fabrication of four prototypes of FPAs with VOx microbolometers demonstrate the scalability properties to create high resolution arrays. The first prototype consists of a 5 x 5 array with a pixel size of 30 μm x 30 μm. An 8 x 8 array, a 64 x 64 array with serial readout and a 64 x 64 array with parallel readout are also presented. Additionally, a 64 x 64 array with parallel output readout electronics with p-n diode microbolometers was fabricated. The design, simulation, characterisation and fabrication of single circuit blocks and a complete 64 x 64 readout integrated circuit is thoroughly discussed in this Thesis. The absorption characteristics of the MMs absorbers, single VOx and p-n diode pixels, 5 x 5 VOx FPA and a 64 x 64 array for both microbolometer types demonstrate the concept of CMOS integration of a monolithic MM based terahertz FPA. The imaging performance using both transmission and reflection mode is demonstrated by scanning a metallic object hidden in a manila envelope and using a single pixel of the array as a terahertz detector. This new approach to make a terahertz imager has the advantages of creating a high sensitivity room temperature technology that is capable of scaling and low-cost manufacture

CMOS Terahertz Metamaterial Based 64 × 64 Bolometric Detector Arrays

We present two terahertz detectors composed of microbolometer sensors (vanadium oxide and silicon pn diode) and metamaterial absorbers monolithically integrated into a complementary metal oxide semiconductor (CMOS) process. The metamaterial absorbers were created using the metal-dielectric-metal layers of a commercial CMOS technology resulting in low-cost terahertz detectors. The scalability of this technology was used to form a 64 × 64 pixel terahertz focal plane array

CMOS compatible metamaterial absorbers for hyperspectral medium wave infrared imaging and sensing applications

We experimentally demonstrate a CMOS compatible medium wave infrared metal-insulator-metal (MIM) metamaterial absorber structure where for a single dielectric spacer thickness at least 93% absorption is attained for 10 separate bands centred at 3.08, 3.30, 3.53, 3.78, 4.14, 4.40, 4.72, 4.94, 5.33, 5.60 μm. Previous hyperspectral MIM metamaterial absorber designs required that the thickness of the dielectric spacer layer be adjusted in order to attain selective unity absorption across the band of interest thereby increasing complexity and cost. We show that the absorption characteristics of the hyperspectral metamaterial structures are polarization insensitive and invariant for oblique incident angles up to 25° making them suitable for practical implementation in an imaging system. Finally, we also reveal that under TM illumination and at certain oblique incident angles there is an extremely narrowband Fano resonance (Q < 50) between the MIM absorber mode and the surface plasmon polariton mode that could have applications in hazardous/toxic gas identification and biosensing

Octave-spanning broadband absorption of terahertz light using metasurface fractal-cross absorbers

Synthetic fractals inherently carry spatially encoded frequency information that renders them as an ideal candidate for broadband optical structures. Nowhere is this more true than in the terahertz (THz) band where there is a lack of naturally occurring materials with valuable optical properties. One example are perfect absorbers that are a direct step toward the development of highly sought after detectors and sensing devices. Metasurface absorbers that can be used to substitute for natural materials suffer from poor broadband performance, while those with high absorption and broadband capability typically involve complex fabrication and design and are multilayered. Here, we demonstrate a polarization-insensitive ultrathin (∼λ/6) planar metasurface THz absorber composed of supercells of fractal crosses capable of spanning one optical octave in bandwidth, while still being highly efficient. A sufficiently thick polyimide interlayer produces a unique absorption mechanism based on Salisbury screen and antireflection responses, which lends to the broadband operation. Experimental peak absorption exceeds 93%, while the average absorption is 83% from 2.82 THz to 5.15 THz. This new ultrathin device architecture, achieving an absorption-bandwidth of one optical octave, demonstrates a major advance toward a synthetic metasurface blackbody absorber in the THz ban

Fractal Metasurface Absorbers with Octave-Spanning Bandwidth

Synthetic fractals offer a degree of freedom for varying resonance frequency, and are an ideal candidate for broadband absorbing devices – especially in the terahertz (THz) band where there is a lack of naturally absorbing materials. Metasurface absorbers often suffer from poor broadband performance, whilst strongly-absorbing broadband devices are typically complex multilayer structures [1,2]. Here, we overcome this limitation by developing an ultra-broadband metasurface absorber based on fractal cross resonators [3], capable of experimentally achieving one Optical Octave bandwidth and peak absorption of 93%. We attribute this to a novel absorption mechanism based on both Salisbury screen and anti-reflection responses. Such work is beneficial in realising THz blackbody absorbers, and for bolometric sensing capabilities

Capsule endoscopy compatible fluorescence imager demonstrated using bowel cancer tumours

We demonstrate a proof of concept highly miniaturised fluorescence imager and its application to detecting cancer in resected human colon cancer tissues. Fluorescence imaging modalities have already been successfully implemented in traditional endoscopy. However, the procedure still causes discomfort and requires sedation. Wireless fluorescence capsule endoscopy has the potential to improve diagnostic accuracy with less inconvenience for patients. In this paper we present a 5 mm x 6 mm x 5 mm optical block that is small enough to integrate into a capsule endoscope. The block integrates ultrathin filters for optical isolation and was successfully integrated with a sensitive CMOS SPAD array to detect green fluorescence from Flavin Adenine Dinucleotide (FAD), which is an endogenous fluorophore responsible for autofluorescence in human tissues, and fluorescence from the cancer selective molecular probe ProteoGREENTM-gGlu used to label colorectal cancer cells. In vitro studies were validated using a commercial ModulusTM Microplate reader. The potential use of the device in capsule endoscopy was further validated by imaging healthy and malignant resected human tissues from the colon to detect changes in autofluorescence signal that are crucial for cancer diagnosis

Unity integration of grating slot waveguide and microfluid for terahertz sensing

Refractive index sensing is attracting extensive interest. Limited by the weak light–matter interaction and the broad bandwidth of resonance, the figure of merit (FoM) of terahertz (THz) sensors is much lower than their counterparts in visible and infrared regions. Here, these two issues are addressed by incorporating a microfluidic channel as a slot layer into a grating slot waveguide (GSW), where guided‐mode resonance results in a narrowband resonant peak and the sensitivity increases remarkably due to the greatly concentrated electromagnetic fields in the slot layer. Both reflective and transmissive sensors are developed with the calculated quality (Q) factors two orders of magnitude larger than metamaterial and plasmonic sensors, and the sensitivities one order of magnitude larger than grating waveguide sensors, contributing to a record high FoM of 692. The measured results match well with the simulations considering the fabrication errors, where the degeneration of narrowband transmission peaks in experiments is attributed to the error of the microfluidic channel height and the divergence of the incident beam. The proposed unity‐integrating configuration with simultaneous optimizations of the resonance mechanism, and the spatial overlap between the sensing field and the analytes shows the potential for high sensitivity bio and chemo sensing

Metasurface optics with on-axis polarization control for terahertz sensing applications

Non-destructive testing and imaging of materials with unknown properties are important applications of terahertz technology. Different contrast forming methods are available to obtain diverse information on the imaged region, including intensity, color, phase and polarization. Polarimetric imaging is relatively under-investigated owing to the difficulty in its implementation with complex optical arrangements. In this paper we present the design and monolithic fabrication of an anisotropic metasurface that incorporates several beam forming functions in a single layer with high efficiency thus simplifying instrument construction. The probe beam generated by the metasurface consists of an orthogonally polarized pair of collinear Bessel beams that create a well-defined on-axis change of polarization. The metasurface is integrated into a polarimetric imaging microscope for 3D beam profiling and sensing experiments at 2.52 THz. An analytical model to describe the on-axis polarization variation was found to be in excellent agreement with both finite-difference time-domain (FDTD) simulations and the experimental results. The setup was used to measure the refractive index and diattenuation of homogeneous sample regions in a proof-of-principle application. We anticipate this technology will find future application in non-destructive testing and polarimetric imaging of polymer composites and 3D profilometry of reflective surfaces

Terahertz Metamaterial Absorbers implemented in CMOS Technology for Imaging Applications: Scaling to 64x64 Focal Plane Array Formats

We present the design and fabrication of terahertz (THz) metamaterial (MM) absorbers and their monolithic integration into a commercial CMOS technology along with its respective readout electronics to produce a low-cost, uncooled and high resolution THz camera. We first describe the work done on single band and broadband MM absorbers on custom substrates then progress with a description of the integration of such resonators into a six metal layer 180 nm CMOS process and its coupling with two types of microbolometer sensors: vanadium oxide (VOx) and silicon (Si) pn diode. Additionally, we demonstrate the integration of the THz sensors with readout electronics to form a monolithic THz focal plane array (FPA). Reflection images of a metallic object hidden in a manila envelope are recorded using both the VOx and Si pn diode detectors, demonstrating the suitability of the technology for stand-off detection of concealed objects. Lastly, we present the current work towards scaling this technology into a 64 x 64 FPA

Uncooled CMOS terahertz imager using a metamaterial absorber and pn diode

We demonstrate a low-cost uncooled terahertz (THz) imager fabricated in a standard 180 nm CMOS process. The imager is composed of a broadband THz metamaterial absorber coupled with a diode microbolometer sensor where the pn junction is used as a temperature sensitive device. The metamaterial absorber array is integrated in the top metallic layers of a six metal layer process allowing for complete monolithic integration of the metamaterial absorber and sensor. We demonstrate the capability of the detector for stand-off imaging applications by using it to form transmission and reflection images of a metallic object hidden in a manila envelope