Periodic Reporting for period 2 - EXTREME-IR (Extreme Optical Nonlinearities in 2D materials for Far-Infrared Photonics)
Berichtszeitraum: 2022-09-01 bis 2023-08-31
The generation of light across the mid-infrared (MIR) and terahertz (THz) spectral regions of the electromagnetic spectrum has become an enabling technology, opening up a plethora of sensing applications across the sciences, as well as enabling the study of fundamental light-matter interactions. The key disruptor in this domain is the quantum cascade laser (QCL), which has grown from a laboratory curiosity to become an essential and practical optoelectronic source for a broad range of application sectors. The expansion of applications has, however, highlighted a technology gap lying between the MIR and THz domains, between 25 μm and 60 μm (5 – 12 THz), which is termed the far-infrared (FIR). Compared to neighbouring MIR and THz domains, the FIR lacks solid-state source technologies, despite the many sensing applications that such compact sources would enable. In the EXTREME-IR project we will breakthrough this technological barrier by pioneering a radically new platform exploiting nonlinear optics in 2D materials to realize functionalized, compact and coherent FIR sources.
Here we will use the distinct phonon spectra and extreme nonlinearities in 2D transition metal dichalcogenides (TMDs) and Dirac matter (DM) to create new optoelectronic sources for the FIR. In particular, we will capitalize on the new phenomena of giant room temperature intra-excitonic nonlinearities and efficient high harmonic generation through plasmonics and resonators, combined with state-of-the-art QCLs as optical pump sources, to access and exploit this unexplored electromagnetic region.
The overall objectives of the project are
O1: Demonstrate the first up-converted FIR photonic source using plasmonic confinement in DMs to enhance the optical nonlinearities by several orders of magnitude compared to typical semiconductor materials;
O2: Demonstrate the first down-converted FIR photonic source using TMDs with intra-excitonic transitions to provide giant optical nonlinearities (chi(2) ~10 nm/V, two orders of magnitude greater than the bulk properties);
O3: Integrate DMs with THz QCLs to realize the first up-converted FIR source, both in on-chip and external cavity geometries, taking advantage of the inherently TM polarized emission of QCLs;
O4: Integrate TMDs with MIR QCLs to realise the first down-converted FIR source and improving the efficiency tenfold by overcoming the ‘quantum defect’ that plagues other nonlinear sources;
O5: Demonstrate the application of the hybrid 2D-QCL sources in metrology and near-field spectroscopy, opening a pathway for new sensing technologies in the FIR range.
Here we will use the distinct phonon spectra and extreme nonlinearities in 2D transition metal dichalcogenides (TMDs) and Dirac matter (DM) to create new optoelectronic sources for the FIR. In particular, we will capitalize on the new phenomena of giant room temperature intra-excitonic nonlinearities and efficient high harmonic generation through plasmonics and resonators, combined with state-of-the-art QCLs as optical pump sources, to access and exploit this unexplored electromagnetic region.
The overall objectives of the project are
O1: Demonstrate the first up-converted FIR photonic source using plasmonic confinement in DMs to enhance the optical nonlinearities by several orders of magnitude compared to typical semiconductor materials;
O2: Demonstrate the first down-converted FIR photonic source using TMDs with intra-excitonic transitions to provide giant optical nonlinearities (chi(2) ~10 nm/V, two orders of magnitude greater than the bulk properties);
O3: Integrate DMs with THz QCLs to realize the first up-converted FIR source, both in on-chip and external cavity geometries, taking advantage of the inherently TM polarized emission of QCLs;
O4: Integrate TMDs with MIR QCLs to realise the first down-converted FIR source and improving the efficiency tenfold by overcoming the ‘quantum defect’ that plagues other nonlinear sources;
O5: Demonstrate the application of the hybrid 2D-QCL sources in metrology and near-field spectroscopy, opening a pathway for new sensing technologies in the FIR range.
This 12 month period has taken the foundation established in the 1st year activity, based on material characterisation on WP1 and theoretical developments on WP2, to investigate the nonlinear properties of these 2D materials in WP3 and WP4. Importantly, we show the first nonlinear optical signatures from the two targeted approaches - the up-conversion using DMs (WP3) and the down conversion of optical light using TMDs (WP4).
Overview and Highlights
1) Continual improvements in growth (WP1) with the optimisation of TIs and QCLs that are being fed into WP3 and WP4
2) A range of Electronic devices have been processed realised to control the researched nonlinearties. This extends from gate controlled DM (Graphene and TIs) to TMD devices with applied electric fields, as well as the use of resonators to enhance light-matter interactions. (WP1)
3) Theoretical developments with a strong emphasis on the calculations of 2D bandstructures of TMDs, the exciton spectrum and their linear and nonlinear properties. Approach adapted to all TMDs permitting a tool to design TMDs for down-conversion using applied fields and the material environment; Further the thermodynamic model for HHG has been adapted for excitation at high frequencies (WP2).
4) Optimised design and realization of resonators to enhance the power density for THz light-matter interaction with integrated DMs and TMDs (WP1, WP3 and WP4).
5) Harmonic generation (HG) using high field system demonstrated in DMs (TIs and graphene) (WP3). We have demonstrated HG in a range of materials, from graphene and TIs, as well as gate controlled graphene.
6) HG from a graphene layer pumped by a high power THz QCL (WP3). This work has shown that the generation of light at 9 THz with a QCL pump at 3 THz, using a graphene layer coupled to a THz resonator.
7) Demonstration of THz gain in MoS2 (WP4). Continuing from measurements for the 1st year report showing enhanced THz emission, here we have shown the presence of large band THz gain through careful measurements of THz transmission to exclude an impedance matching effect.
8) Layer controlled down-conversion in the TMDs MoSe2, PtSe2 and Ferromagnetic/PtSe2 heterostructures using optical pumps (WP4). Using optical pumps, down-conversion has been demonstrated in a range of TMDs structures. Further the down-converted light is shown to be carefully controlled by number of the atomic layers.
9) Saturable absorption of TMDs in the MIR (WP4). Using a tuneable OPA, we have demonstrated a saturable absorption in PtSe2 layers and highlight the low power requirements when pumping with MIR light, making it compatible for pumping with QCLs
10) Proof of principle investigations have shown how near-field spectroscopy can be used to study the dispersion in TIs and show the role of surface states. Further a metrological grade spectrometer has been realised for future EXTREME-IR sources (WP5)
11) Several scientific papers on EXTREME-IR advances published in high impact journals (e.g. Nature satellite journals and Wiley Infomat) and further are in progress.
12) Recruitment of postdoctoral and doctoral researchers at all partner’s site.
These second year highlights provide the potential of realising nonlinear 2D devices, pumped by QCLs and optical lasers, in the EXTREME-IR programme. Progress has been made over a variety of domains, covering materials, optoelectronic devices, cavities and resonators, theoretical nonlinearities, as well as the demonstrations of a range of nonlinearities (up-conversion, down-conversion, saturable absorption and THz amplification). This will be applied to further improvements in nonlinear FIR generation and integration, and fed into exemplar applications in WP5, that are currently being developed.
Overview and Highlights
1) Continual improvements in growth (WP1) with the optimisation of TIs and QCLs that are being fed into WP3 and WP4
2) A range of Electronic devices have been processed realised to control the researched nonlinearties. This extends from gate controlled DM (Graphene and TIs) to TMD devices with applied electric fields, as well as the use of resonators to enhance light-matter interactions. (WP1)
3) Theoretical developments with a strong emphasis on the calculations of 2D bandstructures of TMDs, the exciton spectrum and their linear and nonlinear properties. Approach adapted to all TMDs permitting a tool to design TMDs for down-conversion using applied fields and the material environment; Further the thermodynamic model for HHG has been adapted for excitation at high frequencies (WP2).
4) Optimised design and realization of resonators to enhance the power density for THz light-matter interaction with integrated DMs and TMDs (WP1, WP3 and WP4).
5) Harmonic generation (HG) using high field system demonstrated in DMs (TIs and graphene) (WP3). We have demonstrated HG in a range of materials, from graphene and TIs, as well as gate controlled graphene.
6) HG from a graphene layer pumped by a high power THz QCL (WP3). This work has shown that the generation of light at 9 THz with a QCL pump at 3 THz, using a graphene layer coupled to a THz resonator.
7) Demonstration of THz gain in MoS2 (WP4). Continuing from measurements for the 1st year report showing enhanced THz emission, here we have shown the presence of large band THz gain through careful measurements of THz transmission to exclude an impedance matching effect.
8) Layer controlled down-conversion in the TMDs MoSe2, PtSe2 and Ferromagnetic/PtSe2 heterostructures using optical pumps (WP4). Using optical pumps, down-conversion has been demonstrated in a range of TMDs structures. Further the down-converted light is shown to be carefully controlled by number of the atomic layers.
9) Saturable absorption of TMDs in the MIR (WP4). Using a tuneable OPA, we have demonstrated a saturable absorption in PtSe2 layers and highlight the low power requirements when pumping with MIR light, making it compatible for pumping with QCLs
10) Proof of principle investigations have shown how near-field spectroscopy can be used to study the dispersion in TIs and show the role of surface states. Further a metrological grade spectrometer has been realised for future EXTREME-IR sources (WP5)
11) Several scientific papers on EXTREME-IR advances published in high impact journals (e.g. Nature satellite journals and Wiley Infomat) and further are in progress.
12) Recruitment of postdoctoral and doctoral researchers at all partner’s site.
These second year highlights provide the potential of realising nonlinear 2D devices, pumped by QCLs and optical lasers, in the EXTREME-IR programme. Progress has been made over a variety of domains, covering materials, optoelectronic devices, cavities and resonators, theoretical nonlinearities, as well as the demonstrations of a range of nonlinearities (up-conversion, down-conversion, saturable absorption and THz amplification). This will be applied to further improvements in nonlinear FIR generation and integration, and fed into exemplar applications in WP5, that are currently being developed.
Currently, there is no practical, spectrally narrowband, solid state-based technology to access the FIR across the 25 μm to 60 μm (12 – 5 THz) range. Available techniques are difficult to use or show poor performance. EXTREME-IR will overcome the limits of current technology by integrating 2D nonlinear materials in innovative cavities with established, compact and powerful pumping sources. Progress in the first reporting period has shown THz sources with record pumping powers for an efficient nonlinear interaction, the development of 2D nonlinear materials that are starting to be investigated for nonlinear frequency mixing, and a theoretical and simulation base to bring a predictive nature to the nonlinearities in these materials. This provides the base of the project that will permit the generation of FIR radiation in the coming research period.