Scientific community

Concept and project objective(s)

The TREASURE project will demonstrate an integrated terahertz (THz) emitter at room temperature, based on a parametric optical process in an AlGaAs device, combining strong material nonlinearity and high optical confinement. The approach followed is based on a quasi-phasematching scheme in the whispering-gallery cavity of a microcylinder containing self-assembled quantum-dots. Compared to existing THz sources like photoconductive antennas, photo-mixers, quantum cascade lasers and optical parametric generators, the TREASURE source will bring together several crucial advantages: room-temperature operation, electrical pumping, compactness, THz power scalable up to the microwatt range, custom emission wavelength, spectral purity, and the perspectives of coherent detection and two-dimensional array schemes. This will be accomplished with the complementary competences of four world-class research groups, plus an industrial partner, leader in the THz technology.

Let us recall that the THz portion of the electromagnetic spectrum, comprised between microwaves and far infrared, goes about from 500 GHz to 5 THz, with 1 THz corresponding to an energy of 4 meV, a wavelength in vacuum of 300 mm, and 33.3 cm-1 in common spectroscopic terms. [1]

Traditional sources of THz radiation, such as gas lasers or backward-wave oscillators, are bulky and expensive. The elevated cost is also the main drawback of THz sources based on multiplied solid-state oscillators. The same holds true for photoconductive dipole antennas excited by voluminous femtosecond lasers, which are currently the most popular structures for generating and detecting broadband THz pulses. Another technique that has been addressed in the past ten years for the generation and detection of continuous-wave (CW) THz radiation is based on optical heterodyne mixing. [2] This technique is also known as photo-mixing and typically makes use of semi-insulating or low-temperature grown GaAs. In this case a charge oscillation is generated in the conduction band by beating two 800 nm CW diode lasers with a frequency difference in the THz range. However, no significant progress in terms of emitted power has been demonstrated in CW photoconductive generation during the last few years, with maximum outputs limited to the 100 nW range below 1 THz, and quickly decreasing to the nW range at 2 THz. [3] At higher powers, THz quantum cascade lasers (QCLs) can emit more than 50 mW (peak) in the pulsed regime, but they are poorly tuneable and only operate at cryogenic temperatures. [4]

To date, the limitations of all these techniques, as well as the difficult implementation of coherent detection schemes with QCLs, have prevented the large-scale development of THz spectroscopy. Yet the interest for THz spectroscopy, with its two branches of broad-band time domain spectroscopy (TDS) and single-frequency CW spectroscopy, has grown at an impressive rate for the last decade, in both scientific and commercial applications.

In research, TDS-based systems have been mostly employed for low-Q spectroscopy (with Q the quality factor of a resonance) and imaging. To date, conversely, narrow-band THz systems have found many applications in astronomical spectroscopy, where radiation-matter resonances exhibit Q factors in the 102-106 range, and in the study of chemical processes in the atmosphere, where many molecules have strong absorption bands in the THz domain. Beyond research, THz spectroscopy has found important applications in security and military perspective. [1]

Despite the considerable demand in terms of applications, today the “ideal” THz system (source + coherent detector) is still lacking. This defines a very challenging arena, where Europe must keep the pace with respect to the competitors in the US and Japan.

Various routes to THz generation have been explored to date in different materials, based on difference-frequency generation (DFG), optical rectification or parametric generation. THz generation by nonlinear optical techniques in semiconductors dates back to the seventies, with the DFG demonstration in bulk GaAs by non-collinear mixing of two CO2 laser lines. [5] A basic requirement for the generation of significant optical power levels at the new frequency is phase matching, where the phase velocities of the nonlinear polarisation and the generated wave must be equal to ensure a coherent conversion. [6] THz generation from ultrashort near-IR pulses was first demonstrated in bulk nonlinear crystals such as ZnSe and LiNbO3, [7, 8] and more recently in periodically poled LiNbO3 (PPLN). [9]

In the last years, Vodopyanov et al. demonstrated the generation of 0.9 to 3 THz radiation in periodically inverted GaAs, with optical to THz conversion efficiencies of 10-3, using a pulsed parametric amplifier system centred at ~ 3 mm. [10] More recently, following a proposal of Berger and Sirtori, [11] Vodopyanov’s group has also demonstrated THz DFG in a planar GaAs waveguide. [12] In this case, in order to phase-match two near-IR pumps with a DFG THz beam, phase matching relies on the anomalous dispersion created by the phonon absorption band in GaAs.

Unfortunately, all these THz nonlinear sources are passive and require external pulsed pump laser sources, making the overall systems neither compact nor practical outside research laboratories. In addition, it is well known that the strong increase of the conversion efficiency is a key factor to make THz DFG practical and low-cost, and to allow for sufficiently narrow emission linewidths.  A great opportunity in this sense is provided by tightly optically confining semiconductor structures, allowing the integration of laser pump sources and DFG media, and resulting in a compact and rugged device of a few square millimetres in size. In fact, the prospect of integrating the optical pump and the frequency converter greatly simplifies device packaging and alignment, thus providing potential major improvements in yield, cost reduction, stability, lifetime, and overall performance in any potential application. The most relevant existing source of this type has been reported recently by Capasso’s group at Harvard: a THz source based on DFG in a dual-wavelength mid-IR QCL. [13, 14] However, due to heavy doping, its performance is strongly limited by free-carrier absorption (FCA) at THz frequencies, and it can only emit pulses of a few hundreds of nW at 300 K.

On the one hand, GaAs is a privileged material for THz DFG, thanks to its large non-resonant bulk nonlinearity () and low losses in the THz range  (≈1 cm-1). [15] Moreover, GaAs-based devices have distinct advantages over other platforms such as LiNbO3 in terms of thermal and mechanical properties, with the underpinning of highly mature growth and fabrication. These advantages, combined with the high optical confinement granted by the high refractive index, result in high power levels, stability and mode control in GaAs lasers.

On the other hand, the need for tight optical confinement leads naturally to the concept of optical microcavities. For example, following the idea of enhanced nonlinear optics in photonic crystals, [16] some theoretical work on THz generation in photonic-crystal coupled microcavities is presently under way at Harvard and MIT. [17]

In this project we will focus on the much simpler technology of microcylinder resonators (MRs), which can store electromagnetic fields in the form of whispering gallery modes (WGMs). [18] In MRs, the vertical confinement of the electromagnetic field can be provided by dielectric or plasmon-enhanced guiding. In the horizontal plane, where guidance is granted by the bent semiconductor/air interface, a WGM resonates along a circular path and gives rise to an intensity build-up. Under appropriate conditions, the WGM modes can be described with the effective-index method (EIM). This two-dimensional (2D) reduction greatly simplifies the description of the resonator modes, which can be divided in transverse electric (TE) and transverse magnetic (TM) modes, in complete analogy with the modes of optical waveguides. MR WGM modes are inherently leaky due to bending (radiation) losses, which means that a characteristic escape time from the cavity is associated to each of them. The performances of such a MR can be measured in terms of Q factor and free spectral range (FSR). Three integer numbers can characterize a WGM: m (the number of field maxima in the azimuthal direction), n (the number of maxima in the radial direction), and l (accounting for the vertical confinement of the mode). Increasing the MR radius, and thus the cavity effective length, more modes can be accommodated in the structure, which will have a smaller FSR.

In the last decade, MRs have attracted some interest, mainly in the context of semiconductor lasers and add-drop filters at telecom wavelengths. [19, 20] Although intra-cavity nonlinear generation is one of the oldest recipes for efficient parametric generation, [21] the first c(2) interaction between WGMs was only proposed in 2003, within a toroidal PPLN cavity, by Ilchenko et al. [22]. They subsequently demonstrated frequency doubling in the same device, and parametric oscillation in a disc of unpoled LiNbO3. [23, 24] Although the only nonlinear effect observed so far in semiconductor MRs is Kerr phase shift, [25] recent fabrication progress has resulted in low-loss GaAs MRs with Q factors exceeding 105, [26] which lend themselves to efficient frequency conversion.

Recently, CEA (Partner 3) and UW (Partner 2) have shown that GaAs/AlAs MRs can sustain high-Q WGMs in the near IR [27]. Experiments performed on MRs containing quantum dots (QDs) reveal that their photo-luminescence is dominated by one family of horizontally polarised WGMs. While Figure 1a presents WGMs recorded at cryogenic temperature, the project aims at room-temperature operation of MRs, with optimised InGaAs QDs providing stronger carrier confinement and long-wavelength emission. Moreover, Q factors >104 are commonly observed in WGM pillars with diameters as small as 3 µm, enabling simultaneous lasing of several WGMs without mode competition, even for pumping levels well above threshold. Calculated and experimental FSRs show that THz generation through DFG between these WGMs could be possible for MRs with diameters above 20 µm, provided that a proper phase-matching scenario is available (see Figure 1b).

Concerning phase matching in MRs, two major issues affect GaAs: its optical isotropy, which hinders birefringent phase matching, and the lack of low-loss guiding structures with periodic c(2) inversion, which hinders standard quasi-phase matching (QPM). However, two phase-matching schemes can be simultaneously used for THz DFG in MRs fabricated on (100)-GaAs: 1) modal phase matching of THz and near-IR modes, associated to the anomalous dispersion created by the phonon absorption band (see Figure 2a); [11] and 2) the QPM associated to the azimuthal periodic modulation of , i.e. the effective c(2) (see Figure 2b). The latter has been theoretically demonstrated in (100)-GaAs MRs, resulting simply from the symmetry of GaAs bulk c(2), without the technological burden of periodic domain inversion. [28-31] No experimental evidence of this effect has been reported to date, basically due to the small overlap between the nonlinear-optics and semiconductor-technology communities.

Based on the above grounds, the TREASURE THz source will consist of a semiconductor microcavity, combining QD laser, high non-resonant optical bulk nonlinearity, and strong optical confinement. More specifically, we will obtain CW THz emission through DFG from two near-IR WGMs in a high-Q AlGaAs MR. These modes, excited by the emission of QDs embedded in the microcavity, will grant efficient THz generation due to an original mix of modal phase matching and effective QPM. Note that the intrinsic coherence of the c(2) generation also opens the perspective of a reciprocal coherent scheme for THz field detection, with much lower noise equivalent powers than those of power (or “incoherent”) detection schemes.

Figure 1. WGMs in a MR cavity (from Ref. [27]). a) Experimental WGM spectrum measured at cryogenic temperature on a 4 µm diameter MR: the spectrum is dominated by one family of TE modes. b) The calculated FSR, estimated within the EIM approximation, reproduces well experimental data.

Figure 2. Phase matching in GaAs MRs. a) Proximity of GaAs refractive indices in the near IR and the THz. b) angular dependence, due to the fact that the only nonzero element of the c(2) tensor is .


Figure 3. TREASURE device on (100)-GaAs substrate: a) 3D view; b) Optimised injection scheme.

The final TREASURE device is sketched in Figure 3a. It is based on a MR etched in a GaAs/AlGaAs double heterostructure that vertically confines the near IR WGMs like a standard dielectric waveguide. The cylindrical structure is capped on both sides by a metallic mirror to ensure a double-plasmon, vertical confinement of the THz WGM. In this nested-waveguide scheme, which is detailed in Figure 4, the AlGaAs spacers provide a useful degree of freedom to optimise the spatial overlap of the three interacting modes. The metallic mirrors are also used as contacts for electrical injection into the doped AlGaAs layers. In order to selectively inject the current at the edges of the cylinder where the near-IR WGMs are located, a highly resistive region will be defined by ion implantation in its central part. InAs QD arrays located within the near-IR waveguide are used as active medium. Unlike quantum wells, the gain curve of QD ensembles is mostly broadened due to QD size fluctuations (inhomogeneous broadening). This is the key factor for obtaining simultaneous lasing of two near-IR WGMs without mode competition. It is important to note that the homogeneous broadening of QDs at 300 K, of about 10 meV, [32, 33] will allow emission at frequencies down to 2.4 THz. Conversely, an upper limit of 6 THz is set by GaAs rest-strahlen band.

CW lasing at 300K of microdisk WGMs sustained by the gain of optically pumped InAs/GaAs QDs has been reported few years ago [34]. Electrical pumping is still to be demonstrated for such microlasers. Furthermore, a thermally induced redshift of the lasing line is observed when the pump power is increased above threshold. The lasing properties of air-suspended microdisks are indeed mainly limited by the poor heat sinking related to this geometry [35]. Besides this mode shift, the heating of the active medium also entails carrier escape from the QDs, higher FCA losses and enhanced non-radiative recombination at the disk surfaces.

Figure 4. Vertical mode profiles: near-IR mode excited by the QDs (solid line), and the DFG THz field (dotted line).

Compared to the standard air-suspended microdisk geometry, the MR structure comes with two crucial advantages:

  • The MR geometry provides an excellent heat sinking, which is essential to get lasing at 300K, and a high-power emission to maximise the DFG efficiency. At variance with microdisks, where heating degrades the lasing properties (mode shifts, power saturation), preliminary results conducted by CEA (Partner 3) at low temperatures (5-150K) on MR WGM lasers do not show any red shift, for pumping powers five times larger than the threshold. Unlike microdisks, lasing modes in MRs also display a clear decrease of their mode linewidth above threshold [27], which is very attractive for high-efficiency THz generation by DFG.
  • Although electrical pumping has been achieved for microdisk lasers (with quantum wells as gain medium), [36] their lasing performances are strongly limited by the fact that the current is injected through the disk pedestal near the centre of the microdisk. In the MR geometry, conversely, the current can be selectively injected at the circumference of the active layer, which sustains the WGMs (see e.g. Figure 3b). From a technological point of view, the lasing of WGMs in MRs containing QDs has been demonstrated until now only at low temperature and under optical pumping [27, 37]. An optimisation of the QD active medium, and a development of the selective electrical injection at the edges of the MRs still need to be developed, in order to reach 300K lasing, minimise the threshold current, and optimise the lasing properties of the WGM modes under high-injection conditions.
  • Figure 5. a) THz emission from the edge of the MR; b) Multispectral THz source

    The DFG process in the TREASURE device involves two high-Q, TE-polarised near-IR WGMs and a low-Q, TM-polarised THz WGM that leaks out all around the MR edge (see Fig. 5a). Due to the  combination of  strong diffraction, reflection from the bottom mirror and circular symmetry  of the MR, the THz field will radiate almost vertically, although the THz WGM in the MR is TM (see WP2 and WP5). [38] To give an example, preliminary calculations (in the case of 6 mm MR height, radius R=40.6 mm, 0.325 mm GaAs thickness, and pure AlAs claddings) predict 1 mW THz emission at n3=4.8 THz (i.e. l3=63.4 mm) for l1=c/n1=0.923 mm and l2=c/n2=0.936 mm. This power is radiated for intra-cavity powers at l1 and l2 in the 10-20 mW range, which is safe with respect to two-photon absorption (TPA). Since n3 is expected to scale gently with the radius, we can envisage 2D arrangements of THz emitters with different radii, potentially useful to obtain a multi-spectral emitter (see Fig. 5b).

    Interestingly, an electrically pumped, coherent THz detector can be developed on a similar, but reciprocal device principle as the one outlined above. Here an incoming THz wave at n3 is mixed with a near-IR WGM at n2, and thus the new near-IR frequency n1=n2+n3 is generated, given suitable phase matching conditions, with an intensity proportional to the THz intensity.

    In conclusion, the TREASURE source constitutes a novel and original goal:

    • From an application-oriented point of view, because: 1) existing CW THz sources are complex and expensive; 2) it paves the way to the fabrication of integrated sources and coherent THz detectors, based on the same technological platform.
    • From a scientific point of view, because it is placed at the fascinating intersection between nonlinear optics, QD physics, microcavity photonics, and THz engineering.

    Recently, UPD (Partner 1) and CEA (Partner 3) have jointly published [38] and patented [39] the preliminary design of the phase-matched TREASURE source. These preliminary results build a strong theoretical and experimental basis for the TREASURE project.

    Project Summary :

    The Project is divided into 6 WorkPackages (WPs):

    WP1: Project coordination

    WP2: Design

    WP3: Micro- and Nano-Fabrication, WGM lasing performances

    WP4: Nonlinear optical characterisation

    WP5: Source optimisation and evaluation of other application prospects

    WP6: Dissemination and exploitation

    ******************************

    Figure 6. SCH, standard GRINSCH and ‘flat GRINSCH’ band profiles (for the sake of simplicity, only the conduction band is presented). The proposed ‘flat GRINSCH’ for the TREASURE source will likely permit to reduce the thickness of doped layers and FCA for THz radiation, while preserving a good optical confinement of near IR WGMs and carrier injection.

    Figure 7. Fabrication steps of the TREASURE device.

    Figure 8. Preliminary test of THz DFG in passive AlGaAs microstructures, with input fields injected into the waveguide by end-fire coupling (a), or into the MR by evanescent coupling (b).

    Figure 9. Simulated coupling between a TM WGM in a simplified Au/GaAs/Au MR (R = 20 µm, h = 2 µm) and TE radiation at 3 THz.

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Hired people

A PhD student is partially funded by the TREASURE project:
- Silvia Mariani

Also funded by TREASURE project is the Project Manager at Paris Diderot University:
- Anne-Sophie Refloc'h

A post doc is being funded by the TREASURE project at Julius-Maximilians Universitaet Wuerzburg:
- Fabian Langer

Conferences

The TREASURE consortium organises:
a workshop session dedicated to electrically-injected nonlinear THz emitters at 300K, within the 7th Terahertz days and GDR-I workshop to be held in Cargèse, Corsica, France, 25-27 March 2013

The TREASURE consortium attended:

7th Optoelectronics & Photonics School, 16-22 March 2013, Trento, Italy

NLO 50 International Symposium, 8-10 October 2012, Barcelona, Spain, "AlGaAs micropillars for THz DFG"

IEEE International Semiconductor Laser Conference San Diego, USA, 7-10 October 2012, "Room Temperature, Continuous Wave Lasing in Microcylinder and Microring Quantum Dot Laser Diodes"

Nonlinear Optics, 17-21 June 2012, Colorado Springs (Colorado), United States, "Optical Characterization of Nonlinear THz Emitters"

3rd EOS Topical Meeting on Terahertz Science & Technology (TST 2012), 17 -20 June, 2012 Prague, Czech Republic (Poster Presentation)

Industrial Exhibits

The TREASURE consortium attended:

International Quantum Cascade Lasers School & Workshop 2012, September 2-6, 2012, Baden bei Wien, Austria.

Future Security, Security Research Conference, September 4 – 6, 2012, Bonn, Germany.

Pittcon 2012, Orlando, FL from 11 March to 15 March 2012. Major new products and technologies in the field of analytical and laboratory instruments.

Laser Applications to Chemical Security and Environmental (LACSEA), 29 January - 1 February 2012, Rancho Bernardo Inn, San Diego, California, United States

SPIE Defense Security & Sensing, "International Defense, Security & Sensing Exhibition", April 23-27 2012, Baltimore, United States

Swiss NanoConvention 2012, “the prime showcase for nanotechnology in Switzerland”, Lausanne May 23rd 2012, Lausanne, Switzerland

FLAIR 2011 - Field Laser Applications in Industry and Research / September 13-17, 2011, Conference Center, Murnau, Germany

10e Rencontres HélioSpir, September 24, 2011, Montpellier, France

LASER World of PHOTONICS, May 23-26, 2011 Munich, Germany

"Alliance" organization was visited in order to prepare a technology offer on the "Enterprise Europe Network"