Periodic Report Summary 1 - TERA (Novel Compact Terahertz source based on Dual Wavelength Lasers and Photomixers)
TERA is a Marie Curie Industry-Academia Partnership Pathways Project funded by the FP7 people programme. The aim of this type of project is to develop collaborations between public research organisations and private commercial enterprises, in particular SMEs, with the potential to increase knowledge-sharing and mutual understanding of different cultural settings and skill requirements of both sectors.
TERA exchanges researchers and transfers knowledge between two partners; the University of Dundee (UNIVDUN) and Teravil Ltd (TERAVIL), which is an SME. The project focuses on the exchange of expertise in different aspects of laser physics, material science and THz radiation. The aims are to overcome the scientificand technical barrier to the realisation and adoption of low-cost technologies for the fabrication of compact room-temperature terahertz sources emitting few tens of µW powers at 0.3–1.5 THz and beyond for Biophotonics and Safety and Security applications.
In period 1, TERA has made good progress with all partners actively engaged in the project. The project held its Kick Off meeting at The University of Dundee (UNIVDUN) in March 2012. This meeting established the management framework and a clear working practice across the partners. Combined Supervisory Board and HRTRG Board meetings via Skype have subsequently followed on a regular basis to ensure the progress of the project. Face to face consortium meetings were also held in February 2013 and in October 2013.
A project website (www.eu-tera.eu) has been successfully launched. As well as being the tool for public dissemination of the project results, the website is also being used as the key repository for storing and exchanging project information between partners ensuring that all necessary materials are available to the project fellows. A project flyer has also been produced which has been distributed at various events, including SPIE Photonics West 2013 (February 2013), CLEO Europe (May 2013),SPIE Optics + Photonics 2013 (August 2013) and the 2nd Summer School “Photonics meets Biology” (October 2013).
The secondment and recruitment of the researchers is progressing well at all locations and the infrastructure to facilitate the transfer of knowledge is now well-established.
Achievements in the project to date include:
• Development of dual-wavelength CW and pulsed laser diodes.
• Increase of CW terahertz radiation conversion efficiency and new applications research
Excitation spectrum measurements of the surface THz emission and picosecond photoconductivity in quantum dot (QD) layers have been performed for the first time and provided unique information on optical, electrical and dynamical properties of this material.
Ultrafast photoconductors integrated with wide band antennas were fabricated from all investigated materials. Photomixing efficiency dependence on photoexcited charge carriers trapping time was measured for these devices. Clear correlation of the photomixing efficiency and the recovery time was determined by a double-pulse-excitation, THz-emission experiment results were evidenced, while optical pump – THz probe experiment has only provided indirect information on material suitability for THz mixers.
Further development of efficient laser systems suitable for the optical pumping of THz devices.
• Continuous wave (CW) pump sources based on multi-wavelength gratings, providing fixed THz optical beat signals at various wavelength regions and THz beat frequencies.
• CW pump sources based on continuously tunable multi-grating feedback systems, providing arbitrary control of THz beat frequency signal and average wavelength. These have proved useful in the broad wavelength testing of longer-wavelength pumped THz materials and devices. Enabled the probing and determination of optimal optical pump energies of quantum-dot-based THz photomixer devices.
Development and characterisation of novel photoconductive materials and devices developed at both UNIVDUN and TERAVIL.
• Investigation of novel and existing photoconductive THz devices under optical pumping via established Ti:Sapphire pump laser and time-domain spectroscopy methods.
• Similar investigations using optical parametric oscillators over a broad wavelength range to explore potential THz signal generation and detection over different key excitation energies.
• Identification of performance features of novel quantum-dot (QD) based photoconductive devices for THz applications.
• Determination of device efficiency, limiting factors, signal generation and optical-to-THz conversion mechanisms and realisation of a competitive THz signal source based on the novel QD concepts.
Exploration of the coincident long-wavelength (1.1-1.3 µm) operation of both QD lasers and QD THz antennas. This includes the feasibility study into ultrashort pulse generation using QD laser diodes to be detected using QD THz antennas and used for the purposes of coherent THz signal detection and generation. The purpose of this is to establish feasibility of an all-QD ultra-compact THz spectrometer or testing system, with the eventual view to applications such as lab-on-chip.
Preliminary investigation and demonstrations of QD THz antennas as functional devices in THz spectroscopic applications: cell and chemical sensing trials.
The next stages of the project will include:
• Investigations of cw/pulsed optoelectronic THz range components from bismide materials
THz photoconductors integrated with antennas fabricated on GaAsBi substrates are at the moment the best choise for applications based on pulsed 1 micron wavelength lasers. They suffer, however, as it has been demonstrated in previous stage of the project, from relatively long carrier recombination times limiting the use of bismide based components in cw applications. During the next stage of the project the solution of this problem will be sought in two directions: i) High Bi content annealed GaAsBi layers. Our previous reseach has shown, that annealing at high temperatures leads to a precipitation of Bi into nanometer size clusters that can act as fast capture centers for both electrons and holes, thus reducing photoexcited carrier lifetime in the material. ii) Photoconductors from low-temperature GaInAsBi layers. During annealing a part of Bi atoms go to the clusters and the band gap of the material is increasing. In order to compensate for this and possibly shift the optical absorption edge to1,5 micron wavelength range, quarternary GaInAsBi grown on GaAs substrates will be used for fabricating THz antennas.
• Investigation of novel THz mixer antenna designs
The antenna design having tip-to-tip nano-gap electrode structure will be investigated. It provides THz field enhancement and acts as a nano-antenna to radiate the THz wave generated in the active region of the photomixer. In addition, it can improve impedance matching to the THz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation outcoupling. During the next stage of the project, the square-spiral antenna with tip-to-tip nano-gap electrodes will be made for testing. Further optimization of QD-based laser diode systems for generation of ultrashort optical pulses for use in THz systems. This should involve the use of various pulse compression methods such as chirp compensation, chirped Bragg gratings and fiber Bragg gratings.
Further optimisation of QD-based photoconductive THz antenna devices for improved output power and optical pump tolerances. This should include:
• Improved design and production of antenna contacts with finer features in the case for CW antenna devices
• Optimisation of multi-layer semiconductor structure of THz antennas for both pulsed and CW regime, to refine the internal ‘cavity’ parameters, surface and interface features.
• Optimisation of semiconductor materials and structure of THz antennas for both pulsed and CW regime, regarding nature of implanted QD structures. This includes an alternative deposition method which should improve both ultrafast charge carrier control in the device, as well as the all-important thermal tolerance of the semiconductor superstructure.
Further optimisation of GaBiAs-based photoconductive THz antenna materials for long-wavelength optical pumping and THz signal conversion.
Potential investigations into antenna contacts and device geometries for use with stationary THz imaging methods.
TERA currently has eight Marie Curie Early Stage and Experienced Research Fellows working on the project. Two of these have been recruited and the rest of the Fellows undertake inter-sectoral secondments, the lengths of which range from two to twenty-four months. The Fellows have varying levels of expertise in their fields and their knowledge which they regularly share. All fellows are actively encouraged to attend technical conferences and training events and to publicise the project scientifically and to the general public.
To encourage and strengthen transfer of knowledge, one workshop has been hosted by the consortium as part of the 2nd ‘Photonics Meets Biology’ Summer school in October 2013. The second workshop is currently being arranged to take place in August 2014.
To date the project has exceeded expectations of progress in the first period and more encouraging results are anticipated in the next period. The project has made many scientific achievements and has recruited and seconded highly talented researchers. Both the commercial and non-commercial partners have gained immensely from the skills exchanges which have taken place. The consortium has developed long-term collaborations and relationships, and it is planned for these to continue to grow throughout the rest of the project and beyond.
TERA exchanges researchers and transfers knowledge between two partners; the University of Dundee (UNIVDUN) and Teravil Ltd (TERAVIL), which is an SME. The project focuses on the exchange of expertise in different aspects of laser physics, material science and THz radiation. The aims are to overcome the scientificand technical barrier to the realisation and adoption of low-cost technologies for the fabrication of compact room-temperature terahertz sources emitting few tens of µW powers at 0.3–1.5 THz and beyond for Biophotonics and Safety and Security applications.
In period 1, TERA has made good progress with all partners actively engaged in the project. The project held its Kick Off meeting at The University of Dundee (UNIVDUN) in March 2012. This meeting established the management framework and a clear working practice across the partners. Combined Supervisory Board and HRTRG Board meetings via Skype have subsequently followed on a regular basis to ensure the progress of the project. Face to face consortium meetings were also held in February 2013 and in October 2013.
A project website (www.eu-tera.eu) has been successfully launched. As well as being the tool for public dissemination of the project results, the website is also being used as the key repository for storing and exchanging project information between partners ensuring that all necessary materials are available to the project fellows. A project flyer has also been produced which has been distributed at various events, including SPIE Photonics West 2013 (February 2013), CLEO Europe (May 2013),SPIE Optics + Photonics 2013 (August 2013) and the 2nd Summer School “Photonics meets Biology” (October 2013).
The secondment and recruitment of the researchers is progressing well at all locations and the infrastructure to facilitate the transfer of knowledge is now well-established.
Achievements in the project to date include:
• Development of dual-wavelength CW and pulsed laser diodes.
• Increase of CW terahertz radiation conversion efficiency and new applications research
Excitation spectrum measurements of the surface THz emission and picosecond photoconductivity in quantum dot (QD) layers have been performed for the first time and provided unique information on optical, electrical and dynamical properties of this material.
Ultrafast photoconductors integrated with wide band antennas were fabricated from all investigated materials. Photomixing efficiency dependence on photoexcited charge carriers trapping time was measured for these devices. Clear correlation of the photomixing efficiency and the recovery time was determined by a double-pulse-excitation, THz-emission experiment results were evidenced, while optical pump – THz probe experiment has only provided indirect information on material suitability for THz mixers.
Further development of efficient laser systems suitable for the optical pumping of THz devices.
• Continuous wave (CW) pump sources based on multi-wavelength gratings, providing fixed THz optical beat signals at various wavelength regions and THz beat frequencies.
• CW pump sources based on continuously tunable multi-grating feedback systems, providing arbitrary control of THz beat frequency signal and average wavelength. These have proved useful in the broad wavelength testing of longer-wavelength pumped THz materials and devices. Enabled the probing and determination of optimal optical pump energies of quantum-dot-based THz photomixer devices.
Development and characterisation of novel photoconductive materials and devices developed at both UNIVDUN and TERAVIL.
• Investigation of novel and existing photoconductive THz devices under optical pumping via established Ti:Sapphire pump laser and time-domain spectroscopy methods.
• Similar investigations using optical parametric oscillators over a broad wavelength range to explore potential THz signal generation and detection over different key excitation energies.
• Identification of performance features of novel quantum-dot (QD) based photoconductive devices for THz applications.
• Determination of device efficiency, limiting factors, signal generation and optical-to-THz conversion mechanisms and realisation of a competitive THz signal source based on the novel QD concepts.
Exploration of the coincident long-wavelength (1.1-1.3 µm) operation of both QD lasers and QD THz antennas. This includes the feasibility study into ultrashort pulse generation using QD laser diodes to be detected using QD THz antennas and used for the purposes of coherent THz signal detection and generation. The purpose of this is to establish feasibility of an all-QD ultra-compact THz spectrometer or testing system, with the eventual view to applications such as lab-on-chip.
Preliminary investigation and demonstrations of QD THz antennas as functional devices in THz spectroscopic applications: cell and chemical sensing trials.
The next stages of the project will include:
• Investigations of cw/pulsed optoelectronic THz range components from bismide materials
THz photoconductors integrated with antennas fabricated on GaAsBi substrates are at the moment the best choise for applications based on pulsed 1 micron wavelength lasers. They suffer, however, as it has been demonstrated in previous stage of the project, from relatively long carrier recombination times limiting the use of bismide based components in cw applications. During the next stage of the project the solution of this problem will be sought in two directions: i) High Bi content annealed GaAsBi layers. Our previous reseach has shown, that annealing at high temperatures leads to a precipitation of Bi into nanometer size clusters that can act as fast capture centers for both electrons and holes, thus reducing photoexcited carrier lifetime in the material. ii) Photoconductors from low-temperature GaInAsBi layers. During annealing a part of Bi atoms go to the clusters and the band gap of the material is increasing. In order to compensate for this and possibly shift the optical absorption edge to1,5 micron wavelength range, quarternary GaInAsBi grown on GaAs substrates will be used for fabricating THz antennas.
• Investigation of novel THz mixer antenna designs
The antenna design having tip-to-tip nano-gap electrode structure will be investigated. It provides THz field enhancement and acts as a nano-antenna to radiate the THz wave generated in the active region of the photomixer. In addition, it can improve impedance matching to the THz planar antenna and exhibits a lower RC time constant, allowing more efficient radiation outcoupling. During the next stage of the project, the square-spiral antenna with tip-to-tip nano-gap electrodes will be made for testing. Further optimization of QD-based laser diode systems for generation of ultrashort optical pulses for use in THz systems. This should involve the use of various pulse compression methods such as chirp compensation, chirped Bragg gratings and fiber Bragg gratings.
Further optimisation of QD-based photoconductive THz antenna devices for improved output power and optical pump tolerances. This should include:
• Improved design and production of antenna contacts with finer features in the case for CW antenna devices
• Optimisation of multi-layer semiconductor structure of THz antennas for both pulsed and CW regime, to refine the internal ‘cavity’ parameters, surface and interface features.
• Optimisation of semiconductor materials and structure of THz antennas for both pulsed and CW regime, regarding nature of implanted QD structures. This includes an alternative deposition method which should improve both ultrafast charge carrier control in the device, as well as the all-important thermal tolerance of the semiconductor superstructure.
Further optimisation of GaBiAs-based photoconductive THz antenna materials for long-wavelength optical pumping and THz signal conversion.
Potential investigations into antenna contacts and device geometries for use with stationary THz imaging methods.
TERA currently has eight Marie Curie Early Stage and Experienced Research Fellows working on the project. Two of these have been recruited and the rest of the Fellows undertake inter-sectoral secondments, the lengths of which range from two to twenty-four months. The Fellows have varying levels of expertise in their fields and their knowledge which they regularly share. All fellows are actively encouraged to attend technical conferences and training events and to publicise the project scientifically and to the general public.
To encourage and strengthen transfer of knowledge, one workshop has been hosted by the consortium as part of the 2nd ‘Photonics Meets Biology’ Summer school in October 2013. The second workshop is currently being arranged to take place in August 2014.
To date the project has exceeded expectations of progress in the first period and more encouraging results are anticipated in the next period. The project has made many scientific achievements and has recruited and seconded highly talented researchers. Both the commercial and non-commercial partners have gained immensely from the skills exchanges which have taken place. The consortium has developed long-term collaborations and relationships, and it is planned for these to continue to grow throughout the rest of the project and beyond.