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ULTRAQCL Report Summary

Project ID: 665158
Funded under: H2020-EU.1.2.1.

Periodic Reporting for period 1 - ULTRAQCL (Ultrashort Pulse Generation from Terahertz Quantum Cascade Lasers)

Reporting period: 2015-10-01 to 2016-09-30

Summary of the context and overall objectives of the project

The generation of ultrafast and intense light pulses is an underpinning technology throughout the electromagnetic spectrum enabling the study of fundamental light-matter interactions, as well as industrial exploitation in a plethora of applications across the physical, chemical and biological sciences. A benchmark system for such studies is the modelocked Ti:Sapphire laser, which has grown from being a laboratory curiosity to an essential tool in a broad range of application sectors. Beyond Ti:Sapphire systems, there have been impressive developments in semiconductor based devices for pulse generation in the optical range. These benefit from low system costs and are an enabling technology in new application domains including high speed communications.

However, in the terahertz (THz) frequency range, with its proven applications in imaging, metrology and non-destructive testing, a semiconductor based technology platform for intense and short pulse generation has yet to be realised. Ultrafast excitation of photoconductive switches or nonlinear crystals offer only low powers, low frequency modulation or broadband emission with little control of the spectral bandwidth.

In the ULTRAQCL project we will breakthrough this technological gap, using THz quantum cascade lasers (QCLs) as a foundational semiconductor device for generating intense and short THz pulses. QCLs are the only practical semiconductor system that offer gain at THz frequencies, hence making them suitable for pulse generation, with the ‘bandstructure-by-design’ nature of QCLs allowing the frequency, bandwidth and pulse width to be entirely engineered. We will demonstrate: the first self-starting (passive) mode-locked THz QCL; the first hybrid modelocked THz QCL; the first gain-switched modelocked QCL; and, the first QCL-based THz ultrafast pulse amplifier. The ULTRAQCL project will implement these radical schemes for pulse generation enabling ultrafast QCLs to become a ubiquitous technology for the THz range.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The first 12 month period has established the foundations of the work programme, from the design and simulation of new structures, growth of these heterostructures, device realisation and characterisation. This has resulted in the first realisation of a THz saturable absorber and pulse generation that is considerably shorter than the state-of-the-art. The highlights of the research are summarised below.

Overview and Highlights

1) Growth of QCL and related samples (WP1). This has provided the starting point for the ULTRAQCL project. A number of broadband QCLs have been successfully grown and tested, covering the frequency range of 2.1THz to 5.6 THz for WP3 and WP4. High power QCLs with thick active regions have been realised, as well as high power QCLs working in continuous wave (CW) (applied to WP4). Polaritonic samples have been realised for WP4 and WP5.
2) Realization of planar horn like structures for power extraction from metal-metal waveguides with an aim of realising high output fields (WP1).
3) The investigation of anti-reflection schemes have been extensively investigated using a tilt angle concept. However this proved to be unsuccessful and alternative schemes will be applied.
4) Realization of a state-of-the-art high field THz system for the investigation of saturation and nonlinearities of the realised structures (WP2). The first application of such a system has been demonstrated in the saturation dynamics of saturable absorber samples of WP4. The coherent and incoherent dynamics of these have been shown to be on the scale of 1-2 picoseconds scale, highlighting the ultrafast relaxation that will be relevant for passive modelocking.
5) The first demonstration of modelocking of metal-metal QCLs with broadband emission (WP3). The previous bottleneck of generating shorter pulses has been successfully overcome and active modelocked QCLs with a monolithic dispersion compensation scheme have resulted in sub-5ps pulses, considerably shorter than the state-of-the-art. This breakthrough has been achieved considerably in ahead of the project timescale.
6) The first realization of a THz saturable absorber (WP4), opening up the possibility of passive modelocking via on-chip or external cavity devices.
7) A set of simulation tools have been developed to model pulse generation within QCL-based devices (WP4). This has been applied to the first simulated self-starting modelocked sample. A novel design has been proposed where the absorption can be controlled to demonstrate the onset of self-starting modelocking (WP4).
8) Four papers have been published and one patent has been filed (on monolithic dispersion compensation).
9) Recruitment of postdoctoral and doctoral researchers over the course of the first six month period at each partner site.
10) A workshop session related to pulse generation from QCLs was organised at the international QCL workshop and school IQCLSW-2016
11) External industrial advisors to the project have been appointed to the project to anticipate commercial applications.

These first year highlights provide the basis of further advances in short pulse generation during the course of the ULTRAQCL program, and will be applied to exemplar studies in WP5.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The first year research on ULTRAQCL has permitted a range of objectives to be attained from state-of-the-art growth for broadband and high power QCLs, THz saturation absorption and pulse generation that is considerably shorter than previous demonstrations.

The results highlighted above provide the foundations of the project with a number of important new developments. A demonstration of sub-5ps pulses is a particular highlight and will be used to generate shorter pulses to reach the sub-picosecond regime. The realisation of a saturable absorber through bleaching is an important step for the integration of such a material with QCL-based devices. The realisation of self starting modelocking is now anticipated; samples have been modelled, grown and are awaiting characterisation. These developments will be improved over the next 12 month period and exploited in the application workpackage (WP5) to demonstrate proof-of-principle applications using modelocked QCLs.

The partners have also been pro-active in arranging events and communication activities related to the ULTRAQCL project. For example, open days have been arranged at partner sites where bachelor and masters students have been invited to visit laboratories and to learn about the impact that THz technology is having; THz technology was presented in a TV communication campaign by CNR; a workshop session has been organised at the International Quantum Cascade Laser Workshop and School (IQCLSW) that presented current research trends in the field; ULTRAQCL sponsored IQCLSW with the project logo on the conference material, as well as a poster presentation highlighting the project's objectives; presentation of the project at CNRS workshops on the H2020 research program; and the appointment of industrial advisors for the project to determine the requirements for potential end users. Further communication activities are planned for the next research period with an emphasis on the exploitation of the ULTRAQCL technology being specifically targeted in Year 3.

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