13 research outputs found
Theory and operation of practical formula plans for the investment of university endowment funds
Thesis (M.B.A.)--Boston Universit
Generation of continuous wave terahertz frequency radiation from metal-organic chemical vapour deposition grown Fe-doped InGaAs and InGaAsP
We demonstrate the generation of continuous wave terahertz (THz) frequency radiation from photomixers fabricated on both Fe-doped InGaAs and Fe-doped InGaAsP, grown by metal-organic chemical vapor deposition. The photomixers were excited using a pair of distributed Bragg reflector lasers with emission around 1550ânm, and THz radiation was emitted over a bandwidth of greater than 2.4âTHz. Two InGaAs and four InGaAsP wafers with different Fe doping concentrations were investigated, with the InGaAsmaterial found to outperform the InGaAsP in terms of emitted THz power. The dependencies of the emitted power on the photomixer applied bias, incident laser power, and materialdoping level were also studied
Reclaiming heritage: colourization, culture wars and the politics of nostalgia
This article considers the discursive continuities between a specifically liberal defence of cultural patrimony, evident in the debate over film colourization, and the culture war critique associated with neo-conservatism. It examines how a rhetoric of nostalgia, linked to particular ideas of authenticity,canonicity and tradition,has been mobilized by the right and the left in attempts to stabilize the confguration and perceived transmission of American cultural identity. While different in scale, colourization and multiculturalism were seen to create respective (postmodern) barbarisms against which defenders of culture, heritage and good taste could unite. I argue that in its defence of the âclassicâ work of art, together with principles of aesthetic distinction and the value of cultural inheritance,the anti-colourization lobby helped enrich and legitimize a discourse of tradition that, at the end of the 1980s, was beginning to reverberate powerfully in the conservative challenge to a âcrisisâ within higher education and the humanities. This article attempts to complicate the contemporary politics of nostalgia, showing how a defence of cultural patrimony has distinguished major and minor culture wars, engaging left and right quite differently but with similar presuppositions
The 2023 terahertz science and technology roadmap
Terahertz (THz) radiation encompasses a wide spectral range within the electromagnetic spectrum that extends from microwaves to the far infrared (100 GHzââŒ30 THz). Within its frequency boundaries exist a broad variety of scientific disciplines that have presented, and continue to present, technical challenges to researchers. During the past 50 years, for instance, the demands of the scientific community have substantially evolved and with a need for advanced instrumentation to support radio astronomy, Earth observation, weather forecasting, security imaging, telecommunications, non-destructive device testing and much more. Furthermore, applications have required an emergence of technology from the laboratory environment to production-scale supply and in-the-field deployments ranging from harsh ground-based locations to deep space. In addressing these requirements, the research and development community has advanced related technology and bridged the transition between electronics and photonics that high frequency operation demands. The multidisciplinary nature of THz work was our stimulus for creating the 2017 THz Science and Technology Roadmap (Dhillon et al 2017 J. Phys. D: Appl. Phys. 50 043001). As one might envisage, though, there remains much to explore both scientifically and technically and the field has continued to develop and expand rapidly. It is timely, therefore, to revise our previous roadmap and in this 2023 version we both provide an update on key developments in established technical areas that have important scientific and public benefit, and highlight new and emerging areas that show particular promise. The developments that we describe thus span from fundamental scientific research, such as THz astronomy and the emergent area of THz quantum optics, to highly applied and commercially and societally impactful subjects that include 6G THz communications, medical imaging, and climate monitoring and prediction. Our Roadmap vision draws upon the expertise and perspective of multiple international specialists that together provide an overview of past developments and the likely challenges facing the field of THz science and technology in future decades. The document is written in a form that is accessible to policy makers who wish to gain an overview of the current state of the THz art, and for the non-specialist and curious who wish to understand available technology and challenges. A such, our experts deliver a 'snapshot' introduction to the current status of the field and provide suggestions for exciting future technical development directions. Ultimately, we intend the Roadmap to portray the advantages and benefits of the THz domain and to stimulate further exploration of the field in support of scientific research and commercial realisation
Terahertz photonic integrated circuit for frequency tuning and power modulation - dataset
The quantum cascade laser is a powerful solid-state source of terahertz-frequency radiation. Nevertheless, integrating multiple photonic functions into a monolithic integrated platform in this frequency range is non-trivial due to the scaling of photonic structures for the long terahertz wavelengths and the low frequency tuning coefficients of the quantum cascade lasers. Here, we have designed a simple terahertz-frequency photonic integrated circuit by coupling a racetrack resonator with a ridge laser, exploiting evanescent field coupling, in the longitudinal direction to design a notch filter. The transmission properties of this filter structure are dependent on the phase matching and losses in the coupled racetrack and results in a comb of stopband frequencies. We have optimised the comb separation by carefully selecting the cavity dimensions of the racetrack resonator to suppress longitudinal modes in the ridge laser such that emission is predominant at a single frequency. The emission frequencies and output power from laser are controlled through appropriate control of drive currents to the ridge and the racetrack resonator. The emission frequency is electrically tuned (mode hops) over ~81 GHz exploiting Stark shift of the gain as a function of drive current at the ridge laser, coinciding with an output power variation of ~27% of the peak power (at a heat sink temperature of 50 K). The output power from the ridge also varied by ~30% and the frequency was tuned by a further 10 GHz when the driving conditions at the ridge laser are invariant and the current at the racetrack resonator was varied. To our best knowledge, this is the first report of a frequency engineering, tuning and power modulation of terahertz-frequency quantum cascade lasers using a photonic integrated circuit
Diffuse-Reflectance Spectroscopy Using a Frequency-Switchable Terahertz Quantum Cascade Laser
We demonstrate diffuse-reflectance (DR) spectroscopy of powders using a discretely-tunable terahertz-frequency quantum cascade laser (THz QCL) with a heterogeneous active region. DR signatures were obtained at frequencies of 3.06, 3.21, 3.28, and 3.35 THz, and the relative absorption coefficients were inferred at each frequency using a Kubelka-Munk (KM) scattering model. The spectral lineshapes reproduce the absolute Beer-Lambert (BL) absorption spectra of a range of materials, which were also measured using conventional transmission-mode THz time-domain spectroscopy. It is shown that the DR technique works reliably for materials that include pharmaceutical compounds and foodstuffs, with BL absorption coefficients in the range 2-10 mm(-1). This method is potentially suitable for automated material identification, without any requirement for a priori knowledge of the refractive index or scattering properties of the sampled material
Dataset associated with 'Ultrafast switch-on dynamics of frequency tuneable semiconductor lasers'
This dataset contains experimental data presented in the paper titled 'Ultrafast switch-on dynamics of frequency tuneable semiconductor lasers'. Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits, and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity
Improving the Out-Coupling of a Metal-Metal Terahertz Frequency Quantum Cascade Laser Through Integration of a Hybrid Mode Section into the Waveguide
A hybrid mode section is integrated into the end of the metal-metal waveguide of a terahertz (THz) frequency quantum cascade laser (QCL) by removing sub-wavelength portions of the top metal layer. This allows a hybrid mode to penetrate into the air, which reduces the effective index of the mode and improves the out-coupling performance at the facet. The transmission of the hybrid section is further increased by ensuring its length fulfills the criterion for constructive interference. These simple modifications to a 2.5-THz metal-metal QCL waveguide result in a significant increase in the output emission power. In addition, simulations show that further improvements in out-coupling efficiency can be achieved for lower frequencies with effective refractive indices close to the geometric mean of the indices of the metal-metal waveguide and air