Six years after their birth, terahertz quantum-cascade lasers can now deliver milliwatts or more of continuous-wave coherent radiation throughout the terahertz range — the spectral regime between millimetre and infrared wavelengths, which has long resisted development. This paper reviews the state-of-the-art and future prospects for these lasers, including efforts to increase their operating temperatures, deliver higher output powers and emit longer wavelengths.

Terahertz quantum-cascade lasers

SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

https://iopscience.iop.org/article/10.1088/0960-1317/17/6/R01/pdf

SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography

Science and technologies based on terahertz frequency electromagnetic radiation (100 GHz–30 THz) have developed rapidly over the last 30 years. For most of the 20th Century, terahertz radiation, then referred to as sub-millimeter wave or far-infrared radiation, was mainly utilized by astronomers and some spectroscopists. Following the development of laser based terahertz time-domain spectroscopy in the 1980s and 1990s the field of THz science and technology expanded rapidly, to the extent that it now touches many areas from fundamental science to 'real world' applications. For example THz radiation is being used to optimize materials for new solar cells, and may also be a key technology for the next generation of airport security scanners. While the field was emerging it was possible to keep track of all new developments, however now the field has grown so much that it is increasingly difficult to follow the diverse range of new discoveries and applications that are appearing. At this point in time, when the field of THz science and technology is moving from an emerging to a more established and interdisciplinary field, it is apt to present a roadmap to help identify the breadth and future directions of the field. The aim of this roadmap is to present a snapshot of the present state of THz science and technology in 2017, and provide an opinion on the challenges and opportunities that the future holds. To be able to achieve this aim, we have invited a group of international experts to write 18 sections that cover most of the key areas of THz science and technology. We hope that The 2017 Roadmap on THz science and technology will prove to be a useful resource by providing a wide ranging introduction to the capabilities of THz radiation for those outside or just entering the field as well as providing perspective and breadth for those who are well established. We also feel that this review should serve as a useful guide for government and funding agencies.

The 2016 terahertz science and technology roadmap

3.2. Two-Photon Lithography (TPL) 919 4. Serial Writing with Charged Particles 920 4.1. Electron Beam Lithography 920 4.2. Ion Beam Lithography 920 4.3. Scanning Probe Resist Lithography 921 5. Microand Nanomachining 921 5.1. Focused Ion Beam 921 5.2. Scanning Probe Nanomachining 921 6. Direct Writing and Material Deposition 921 7. Moulding 922 7.1. Mould Fabrication 922 7.2. Demoulding: Mould Treatment 924 7.3. Embossing Thermoplastic Materials: Nanoimprint Lithography (NIL) 924

Fabrication approaches for generating complex micro- and nanopatterns on polymeric surfaces.

Semiconductor lasers based on two-dimensional photonic crystals generally rely on an optically pumped central area, surrounded by un-pumped, and therefore absorbing, regions. This ideal configuration is lost when photonic-crystal lasers are electrically pumped, which is practically more attractive as an external laser source is not required. In this case, in order to avoid lateral spreading of the electrical current, the device active area must be physically defined by appropriate semiconductor processing. This creates an abrupt change in the complex dielectric constant at the device boundaries, especially in the case of lasers operating in the far-infrared, where the large emission wavelengths impose device thicknesses of several micrometres. Here we show that such abrupt boundary conditions can dramatically influence the operation of electrically pumped photonic-crystal lasers. By demonstrating a general technique to implement reflecting or absorbing boundaries, we produce evidence that whispering-gallery-like modes or true photonic-crystal states can be alternatively excited. We illustrate the power of this technique by fabricating photonic-crystal terahertz (THz) semiconductor lasers, where the photonic crystal is implemented via the sole patterning of the device top metallization. Single-mode laser action is obtained in the 2.55-2.88 THz range, and the emission far field exhibits a small angular divergence, thus providing a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal-metal waveguides.

Electrically pumped photonic-crystal terahertz lasers controlled by boundary conditions

The authors demonstrate terahertz microcavity lasers with ultralow current thresholds (Ith≈4mA) and with reduced mode volumes of ≈0.7(λeffective)3, i.e., less than one cubic wavelength. A double metal waveguide with reduced active core thickness (5.82μm) is used to achieve confinement in the vertical direction, without compromising the laser performances. Confinement in the longitudinal direction is obtained using microdisk resonators. The guiding properties of surface plasmons are exploited to guide the mode with the metal contact. This makes the use of a resonator with vertical and smooth sidewalls unnecessary. The emission wavelength is λ≈114μm. The devices lase up to 70K in pulsed mode, and they achieve continuous-wave operation up to 60K.

Terahertz microcavity lasers with subwavelength mode volumes and thresholds in the milliampere range

We report on a dry transfer technique for single wall carbon nanotube devices, which allows to embed them in high finesse microwave cavity. We demonstrate the ground state charge readout and a quality factor of about 3000 down to the single photon regime. This technique allows to make devices such as double quantum dots, which could be instrumental for achieving the strong spin photon coupling. It can easily be extended to generic carbon nanotube based microwave devices.

Stamping single wall nanotubes for circuit quantum electrodynamics

We have designed and fabricated shape-controlled three-dimensional structures combining electron- beam and ultraviolet (UV) lithography. Starting with a tiff file format, a gray scale pattern using a high energy beam sensitive mask is produced with electron-beam direct writing. In only a one step UV exposure, an optical lens and a photonic band gap structure in 16.5 μm thick AZ 4562 positive photoresist, with controlled profile and smooth resist surface, were fabricated. A three-dimensional grid structure was also transferred to a 100 μm thick SU-8 negative photoresist.

Fabrication of three-dimensional microstructures using standard ultraviolet and electron-beam lithography

We report the pulsed, room-temperature operation of λ≈7.5μm quantum cascade lasers (QCLs) in which the optical mode is a surface-plasmon polariton excitation. Previously reported devices based on this concept operate at cryogenic temperatures only. The use of a silver-based electrical contact with reduced optical losses at the QCL emission wavelength allows a reduction of the laser threshold current by a factor of 2 relative to samples with a gold-based contact layer. As a consequence, the devices exhibit room-temperature operation with threshold current densities ∼6.3kA∕cm2. These devices could be used as all-electrical surface-plasmon generators at midinfrared wavelengths.

Room-temperature operation of λ≈7.5μm surface-plasmon quantum cascade lasers

It is demonstrated that the active region thickness of THz quantum cascade lasers can be reduced by a factor of 2 without effects on the threshold current density and maximum operating temperature of the laser. Pulsed and continuous-wave operation, with a low threshold J th = 71 A/cm 2 , are obtained for a 5.86 mum-thick THz QC laser. The emission is peaked at lambdasime115 mum and the waveguide resonator is based on a metal-metal geometry

José Palomo

Papers

Terahertz microcavity lasers with subwavelength mode volumes and thresholds in the milliampere range

Stamping single wall nanotubes for circuit quantum electrodynamics

Fabrication of three-dimensional microstructures using standard ultraviolet and electron-beam lithography

Room-temperature operation of λ≈7.5μm surface-plasmon quantum cascade lasers

Low threshold THz QC lasers with thin core regions