Multi-Terahertz Sideband Generation on an Optical Telecom Carrier

Jan 12, 2018 - In this paper we demonstrate large wavelength shifts on a telecom carrier at room temperature, achieved by multi-THz sideband generatio...
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Multi-Terahertz sideband generation on an optical telecom carrier with a Quantum Cascade Laser Sarah Houver, Armand Lebreton, Alireza Mottaghizadeh, Maria Amanti, Carlo Sirtori, Gregoire Beaudoin, Isabelle Sagnes, Olivier Parillaud, Raffaele Colombelli, Juliette Mangeney, Robson Ferreira, Jérôme Tignon, and Sukhdeep Dhillon ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01124 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 13, 2018

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Multi-Terahertz sideband generation on an optical telecom carrier with a Quantum Cascade Laser

Sarah Houver1*, Armand Lebreton1, Alireza Mottaghizadeh2, Maria Ines Amanti2, Carlo Sirtori2, Gregoire Beaudoin3, Isabelle Sagnes3, Olivier Parillaud4, Raffaele Colombelli3, Juliette Mangeney,1 Robson Ferreira,1 Jerome Tignon1 and Sukhdeep Singh Dhillon1*

1

Laboratoire Pierre Aigrain, Département de physique de l’ENS, École normale supérieure, PSL Research University, Université Paris Diderot, Sorbonne Paris Cité, Sorbonne Universités, UPMC Univ. Paris 06, CNRS, 75005 Paris, France 2

Laboratoire Matériaux et Phénomènes Quantiques, Université Paris Diderot - Sorbonne Paris Cité, France

3

Centre de Nanosciences et de Nanotechnologies, CNRS, Univ. Paris-Sud, Université Paris-Saclay, France 4

III-V Lab, 1 avenue Augustin Fresnel, Palaiseau France.

* [email protected]; [email protected]

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Quantum Cascade Lasers (QCLs) are the principal semiconductor light sources for the midinfrared (MIR) and terahertz (THz) spectral regions. However, up-converting their emission to the technologically mature telecom spectral region (1.3µm to 1.6µm) remains an important goal for applications. This would permit new QCL functionalities to be realized, such as stabilization and injection locking of the QCL emission to precise and low cost telecom frequency combs for spectroscopy. In this work, we demonstrate the up-conversion of the QCL emission to the telecom band at room temperature, using the QCLs’ inherent resonant optical nonlinearities that reduces the constraints of phase matching. This is based on the generation of multi-THz sidebands on a telecom carrier, where a low power telecom beam at 1.55 µm is injected into the cavity of an InP-based MIR QCL (9 µm, 33 THz), giving rise to the nonlinear sum frequency at 1.3 µm. The results are supported by a theoretical model that highlights the giant enhancement of the nonlinear interaction through the resonant telecom excitation and the reduced role of phase matching. As well as potentially bringing important spectroscopic capabilities to QCLs, this compact and low power all-optical connection between the low-loss transmission windows of 1.3 µm and 1.55 µm would potentially permit QCLs to be applied as ultrafast wavelength shifters in fiber telecommunication networks.

Keywords: Optical communications, Quantum Cascade Lasers, Sideband generation, Up-conversion, Nonlinear Optics

Quantum cascade lasers (QCLs) 1-2 are compact, powerful and practical semiconductor devices based on intersubband transitions within III-V quantum well based materials. Over the last two decades, they have been developed to realize laser action over the entire mid-infrared (MIR) and terahertz (THz) spectral regions with high output powers. These developments have opened-up the possibility of intra-cavity nonlinear optics within these spectral ranges, taking advantage of the high internal power densities within the QCL cavity. For example, important advances have been realized in the generation of THz radiation at room temperature using two MIR QCL active regions within the 2

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same cavity

3-4

. The intracavity power is combined with giant intersubband resonant nonlinearities,

orders of magnitude greater than bulk nonlinearities, permitting high nonlinear conversion efficiencies5. Another development has been the combination of interband and intersubband resonant nonlinearities for THz sideband generation on optical beams. Based on pioneering demonstrations with Free Electron Lasers6-7, resonant sideband generation was first realized in GaAs-based MIR QCLs8. Here a near-infrared pump, ENIR (~1.5 eV), is injected into a QCL cavity operating at EQCL, resonant with the interband transitions, resulting in sidebands on the optical carrier i.e. ENIR ± EQCL. High efficiencies were then shown using THz QCLs9-10, resulting in THz sidebands separated (or ‘shifted’) by a few nanometers in wavelength from the optical pump. Recently larger wavelength shifts (~70 nm, multi-THz) were shown at high temperatures in a collinear geometry within a GaAs based MIR QCL

11

. However, as these QCLs are based on GaAs quantum wells (i.e. interband

transitions around 800 nm (1.5eV)), sidebands are generated in the NIR, and thus not adapted to the telecom range of interest (∼1.3 µm to 1.7 µm). THz sideband generation has been demonstrated in the telecom range using the bulk properties of QCL constituent III-V materials

12

. However, all these

results were limited to cryogenically operated THz QCLs. This bulk nonlinear process cannot be applied to MIR QCLs owing to a low refractive index that inhibits phase matching. (Other demonstrations of sideband generation has been shown around 800 nm, including high harmonic generation, using free electron lasers 13-14).

In this paper we demonstrate large wavelength shifts on a telecom carrier at room temperature, achieved by multi-THz sideband generation within an adapted InP-based MIR QCL (λQCL= 9 µm,

νQCL= 33 THz, EQCL = 140 meV). These QCLs are based on InGaAs/AlInAs quantum wells, and thus the resonant interband transitions fall within the telecom range. Owing to this scheme, wavelength shifts could be demonstrated over the entire telecom band, from 1300 nm to 1700 nm, through nonlinear sum and difference frequency generation (SFG and DFG). This is obtained within the QCL cavity which is simultaneously employed as the source, waveguide and nonlinear material. The generated multi-THz sidebands are separated from the telecom pump by 140 meV (i.e. the QCL 3

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photon) corresponding to a wavelength change of ∼250 nm. The all-optical transition from 1.3 µm to 1.55 µm and vice-versa, connecting the two low loss telecom windows is then easily achieved

15-16

.

Further, we also compare these results with sideband generation with InP-based THz QCLs (ν~4 THz). This shows that the telecom pump can be shifted by 17 meV corresponding to smaller shifts of 30-35 nm in the telecom range. Although limited to low temperatures, a comparison can be made between THz and MIR QCLs and their effects on the sideband generation efficiency. This is performed through a theoretical formalism for the nonlinear susceptibility that correctly predicts the behaviour of MIR and THz QCLs, permitting a predictive tool for future optimisation of the process. As well as applications to telecommunications and high frequency modulation of optical carriers

17-18

, this

nonlinear interaction between the telecom and QCL spectral ranges could be applied for QCL stabilisation and injection locking to telecom optical combs 19-20.

Methods The experimental realization of the sideband generation scheme is based on a collinear geometry for all the interacting waves 9 i.e. in the same plane parallel to the surface of the QCL (figure 1a). Experimentally, the telecom pump, Etelecom, is coupled into one end of the QCL cavity (the facet) operating at EQCL and the generated sidebands exit the opposite facet (Etelecom ± EQCL) (figure 1a). This type of guided transmission geometry for both the QCL emission and the input interband excitation provides an interaction length L ∼ 1/αp where αp are the pump losses. This has an important implication on the telecom pump energy required for the DFG (Etelecom - EQCL) and SFG (Etelecom + EQCL) sidebands. To illustrate this, a schematic of the QCL bandstructure including conduction and valence bands is shown in figure b) and c). For the DFG sideband to be generated and detected, the telecom pump needs to be greater in energy than the effective bandgap of the material (i.e. the lowest lying electron-hole transitions) to excite the resonant nonlinearities (figure 1b). The pump is absorbed after a few microns and the DFG sideband, generated below the effective bandgap, is transmitted and exits the opposite facet. Similarly to observe SFG in this geometry, the telecom pump is below the effective bandgap and transmitted through the QCL cavity. This results in SFG being resonant with the 4

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effective gap (figure 1c), with the non-absorbed part being generated over the last few microns of the cavity. For both SFG and DFG sidebands, when the pump energy is increased, the resonance conditions improve leading to stronger nonlinearities and increasing sideband intensity. On the other hand, for higher pump energy, the DFG (or the pump itself for SFG) attains the bandgap energy and also begins to be absorbed. Thus for both DFG and SFG, the wavelength range that can be explored for sideband generation is dictated by the interband absorption.

The MIR QCLs were based on lattice matched In0.53Ga0.47As/In0.52Al0.48As quantum wells, grown by Metalorganic Chemical Vapour Deposition (MOCVD) on an InP substrate. The active region (AR) is based on a four-well system with a double-LO-phonon depopulation design

21

. This

design has been previously used to demonstrate high temperature laser operation at 9 µm (33 THz, 140 meV) with hundreds of milliwatts of output power. The AR comprises a total of 44 periods resulting in a thickness of 2.3 µm. The injection wells have been n-doped at 1.6×1017 cm-3. This was used for a compromise between high power emission and low threshold currents 22. The waveguide is discussed below. MIR QCLs were processed using photolithography with the QCL ridge realized using inductively coupled plasma (ICP) etching for small ridge widths (8µm). The processed wafer was cleaved into 3 mm long cavities and indium soldered to copper mounts. The THz QCLs used here were also based on In0.53Ga0.47As/In0.52Al0.48As quantum wells, designed to emit at ~4 THz (17 meV). It employs a three-well active region and was grown by molecular beam epitaxy (MBE). The layer sequence of the structure, in nanometers and starting from the injection barrier, is as follows: 1.9/13.7/0.9/15.5/1.9/25. In0.52Al0.48As barrier layers are in bold, In0.53Ga0.47As well layers are in roman. The central part of the 25 nm well was n-doped (5.8×1016 cm-3) over 9 nm. The AR comprises a total of 185 periods resulting in total thickness of 10.9 µm. The THz QCLs were processed into metal-metal waveguides with 150 µm wide ridges, cleaved into 1 mm long cavities and indium soldered to copper mounts.

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In order to realize an efficient nonlinear interaction, it is important to consider the modal confinement of the QCL emission and the external telecom excitation. The THz QCL possesses a metal-metal waveguide that confines both the THz and telecom radiation

10

. However, specific

considerations are required for the MIR QCLs. Figure 2a shows the optical guided modes in the dielectric waveguides typically used for these devices where both sides of the AR are clad by doped InGaAs layers (which increase the refractive index contrast, C1). The MIR mode, the telecom mode and the telecom refractive index are shown in green, red and blue, respectively. The 3rd order telecom mode is shown as it has the largest overlap with the AR compared to the 1st and 2nd order modes. Although both MIR and telecom modes are guided in the AR owing to the low InP index, the telecom mode has an overlap with the InGaAs cladding layers owing to the larger telecom refractive index than that in the AR. This results in the losses on the telecom mode to be increased, and have been experimentally evaluated to ~ 13 cm-1 through transmission measurements 23. As the InGaAs bandgap will be smaller in energy than the first interband transitions of the quantum wells, the pump and the generated sideband will be significantly absorbed in the InGaAs cladding layers. Further, these layers do not contribute to the second order nonlinear process due the lack of an asymmetry in the potential profile 24. To overcome these limitations, a new waveguide was realized without the requirements of InGaAs cladding layers around the AR (figure 2b). Further, the width of InP cladding layers around the AR was increased to 4 µm from 2 µm in the standard waveguide, resulting in a reduction in the losses of the MIR mode from 6 cm-1 to 3 cm-1. The MIR mode overlap with the AR is 63 % with both waveguides. The telecom mode is well confined within the AR, as shown on figure 2b, and there is therefore a strong spatial overlap between the two modes and the active region.

The samples were placed in a continuous flow cryostat, where the temperature was set to 77 K for THz QCLs and varied from 200 K to room temperature for MIR QCLs. The external telecom excitation was provided by continuous wave tunable diode laser (λ = 1330 nm – 1680 nm by Yenista) with output powers of 2-5 mW focused on to the QCLs. (These low pump powers result in a small excited population near the input facet which does not alter the threshold or output powers of the 6

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QCL). The polarization was chosen to be identical to that of the QCL emission, i.e. transverse magnetic (TM), implying that only transitions between light holes and electrons are excited, owing to selection rules 25. The telecom beam was coupled into and out of the QCL cavity using short-focus aspheric lenses. The generated sideband was then detected using a grating spectrometer coupled to an InGaAs array detector (New Imaging Technology). The THz and MIR QCLs were operated in pulsed mode with typically 1 µs pulses at a repetition frequency of 50 kHz, with the telecom laser synchronized to the QCL modulation using an acousto-optic modulator.

Results and Discussion Figure 3 shows the typical spectra of the SFG sidebands between a telecom pump (red) at Etelecom = 0.797 eV (λtelecom = 1555 nm), and the MIR (orange curve) and THz (blue curve) QCLs. The SFG sideband from the MIR QCL is observed at 0.937 eV, i.e. 1325 nm, separated from the pump by the MIR photon (140 meV). (The MIR QCL was held at 250K). SFG from the THz QCL is observed at 0.814 eV, i.e. 1525 nm, separated from the pump by the THz photon (17 meV). Each spectrum (pump, SFG for MIR QCL and SFG for THz QCL) has been taken independently and the efficiency of the process is discussed below. These are the first demonstrations of sideband generation in InP-based THz and MIR QCLs with a resonant excitation in the telecom domain. The DFG sideband can also be observed for the opposite process (i.e. conversion from 1.325µm to 1.555µm) and is shown in the supplementary material.

To illustrate the resonant behavior of the process, the SFG sideband intensity was investigated as a function of the telecom pump energy. Figure 4a shows SFG sideband between the telecom pump and the THz QCL set at 77 K for pump wavelengths from 1550 nm to 1590 nm. The expected evolution is observed: the sideband intensity increases with pump photon energy, until the latter reaches in turn the first interband transition energies (~0.797 eV). Both pump and sideband are then absorbed and the SFG intensity drops rapidly. Similarly, figure 4b shows the SFG spectra with the MIR QCLs at a temperature of 250 K. The telecom pump was set below the bandgap and was varied over a tuning 7

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range from 1520 nm to 1680 nm. SFG is observed over this entire range, generated from 1300 nm to 1415 nm. The wavelength range is experimentally limited by the pump laser wavelength range but, in principle, could be extended beyond 1680 nm. The SFG efficiency decreases progressively for wavelengths shorter than 1620 nm owing to interband absorption of the pump and sideband. Given the relatively high operation temperature (250 K), the interband transitions are broader and the absorption takes place over a range of more than 60 nm, considerably larger than that for THz QCLs. This absorption broadening is consistent with transmission measurements (see supplementary material).

The application of both THz and MIR QCLs here also permits an in-depth analysis of the nonlinear process. Here we compare, and simulate, the second order nonlinear susceptibility, χ(2), obtained for both QCL types with close experimental conditions. From the SFG and pump power measurements (PSFG and Ptelecom, respectively), the SFG efficiency,  = 





, can be deduced

26

. For a MIR

(THz) QCL held at 250 K (77K), a maximum SFG efficiency of 1×10-4 % (2×10-3 %) was measured, under pump excitation at telecom = 1.66 µm. (1.55 µm). From the efficiency measurements, a value for χ (2) for the SFG nonlinear process in MIR and THz QCLs can be extracted using 26:



∆,   () * 2 . + (ℎ *1 4. 56  8     ()  = 4     !"!#$%   &!"!#$% ' ∆,    * 2 . + *1 4.

where Ptelecom (ntelecom), PSFG (nSFG), PQCL (nQCL) are the power (the refractive index) of the telecom pump, the SFG and the QCL respectively. S is the interaction area, telecom is the telecom pump wavelength and L is the QCL cavity length. αSFG represents the losses on the SFG sideband, at energies greater than the bandgap energy. (Losses on the telecom pump and the QCL are taken to be zero). ∆k is the phase mismatch. The intracavity QCL power was evaluated to 85 mW and 20 mW, using calibrated detectors and taking into account a reflectivity coefficient of 0.3 and 0.9

27

for the

MIR and THz QCLs, respectively. Given the ridge dimensions, S can be evaluated to 100 µm2 (MIR) 8

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and 500 µm2 (THz) QCLs. The refractive indices are ∼ 3.5, 3 and 3.2 in the telecom, MIR and THz ranges, respectively. The losses αSFG are calculated using the imaginary part of the linear susceptibility, Im((1)), and are evaluated for both THz and MIR QCLs at the corresponding pump energies. The phase mismatch is determined from the bulk refractive index. For the THz QCL, the phase mismatch ∆k ∼ 700 cm-1 and is negligible compared to the sum losses αTHz ∼ 3000 cm-1. For the MIR QCL, the phase mismatch ∆k ∼ 4000 cm-1 is on the same order as the sum losses αMIR ∼ 5000 cm1

. Using these defined parameters leads to  (2)MIR = 9×10-10 m/V and (2)THz = 6×10-9 m/V, highlighting

an order of magnitude between the two QCLs.

These experimental values for (2)MIR and (2)THz can be compared with theoretical calculations of the nonlinear susceptibility in both MIR and THz QCL bandstructures, which can be expressed for the SFG process as 28:

()

 =

1 :%; :;< :%; + >? − > − )A =>? − > !"!#$% − )A =>;< + >? − > !"!#$% − )A  8 ? %;