Subscriber access provided by - Access paid by the | UCSB Libraries
Communication
Diode-pumped organo-lead halide perovskite lasing in a metal-clad distributed feedback resonator Yufei Jia, Ross A. Kerner, Alex J. Grede, Alyssa N. Brigeman, Barry P. Rand, and Noel C. Giebink Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b01946 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 23, 2016
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
Diode-pumped organo-lead halide perovskite lasing in a metal-clad distributed feedback resonator Yufei Jia1, Ross A. Kerner2, Alex J. Grede1, Alyssa N. Brigeman,1 Barry P. Rand2, Noel C. Giebink1† 1
Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802,USA
2
Department of Electrical Engineering and Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ 08544,USA †
email:
[email protected] Abstract Organic-inorganic lead halide perovskite semiconductors have recently reignited the prospect of a tunable, solution-processed diode laser, which has the potential to impact a wide range of optoelectronic applications. Here, we demonstrate a metal-clad, second order distributed feedback methylammonium lead iodide perovskite laser that marks a significant step toward this goal. Optically pumping this device with an InGaN diode laser at low temperature, we achieve lasing above a threshold pump intensity of 5 kW/cm2 for durations up to ~25 ns at repetition rates exceeding 2 MHz. We show that the lasing duration is not limited by thermal runaway and propose instead that lasing ceases under continuous pumping due to a photoinduced structural change in the perovskite that reduces the gain on a sub-microsecond timescale. Our results indicate that the architecture demonstrated here could provide the foundation for electricallypumped lasing with a threshold current density Jth < 5 kA/cm2 under sub-20 ns pulsed drive.
Keywords: lead halide perovskite, laser, distributed feedback, diode laser
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The goal of a solution-processed, tunable diode laser remains a long-standing challenge for the organic and thin film electronics community.1-3 Hybrid organic-inorganic lead halide perovskite materials have recently renewed hope for achieving this goal with the demonstration of low threshold optically-pumped lasing4-8 and efficient light emitting diodes.9-11 Beyond favorable gain characteristics,12-14 perceived advantages of these perovskite materials over organic semiconductor thin films include a higher charge carrier mobility15, 16 and refractive index17, 18 that respectively facilitate increased injection current and better mode confinement. Here, we demonstrate a metal-clad, second order distributed feedback (DFB) perovskite laser constructed on a silicon substrate that achieves threshold at optical pump intensities of 5 kW/cm2 for durations ≤25 ns at repetition rates exceeding 2 MHz. This result represents three important milestones on the path to a perovskite laser diode. These include (1) the use of a high thermal conductivity substrate for efficient heat rejection, (2) establishing low threshold in an electrically-accessible architecture, and (3) doing so under comparatively long (nanosecond timescale) pulse duration that is compatible with the time constants (e.g. RC) involved in electrical injection. Our findings underscore the importance of short pulsed current drive to minimize the threshold of a future hybrid perovskite laser diode and suggest that a threshold current density < 5 kA/cm2 may be feasible. Figure 1a shows a scanning electron micrograph cross-section of the laser architecture, which is based on a Si substrate grating (period Λ = 403 ± 1 nm) coated with 50 nm of Au to maximize thermal conductivity and prevent lasing mode overlap with the absorbing Si. A 15 nm thick conformal Al2O3 interlayer is deposited on top of the Au to minimize surface recombination and the structure is completed by planarizing with a ~160 nm thick layer of methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) perovskite. In this context, a key
1
ACS Paragon Plus Environment
Page 2 of 25
Page 3 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
challenge19 was the use of a one-step spin coating process employing a solvent exchange that yields smooth, fine-grained films with minimal scattering loss; details are provided in the Methods section. Due to uncertainty in the layer optical constants, another technical challenge arises in realizing the precise combination of grating period and perovskite thickness required to support a mode that meets the Bragg condition within the narrow MAPbI3 gain spectrum (a free space wavelength span of ~20 nm within the range 775 < < 800 nm, depending on temperature12, 13
). Following initial rigorous coupled wave analysis (RCWA) calculations20 to predict the
perovskite thickness needed to support the fundamental transverse electric (TE0) mode, reflectivity measurements were used to adjust the deposited MAPbI3 thickness and fine tune the modal effective index ( ) to satisfy the second order DFB condition, = ⁄Λ, at the gain spectrum target frequency, = 2 ⁄ .
Specifically, incident light that is grating-coupled into the forward and backwardpropagating waveguide modes leads to reflectivity minima (see Fig. 1b) at wavelengths above and below given by: = ± ! sin% + , 1 −
(1)
where % is the incident angle and = ( ⁄( is the linear dispersion of the modal effective
index in the Taylor expansion, )* ≈ , + ) − *. Plotting the reflectivity minima extracted from Fig. 1b versus the sine of the incident angle therefore enables a linear fit to determine from the intercept as shown in Fig. 1c. Note that the shorter wavelength minima cannot be resolved at high angle since absorption above the band edge becomes significant.
2
ACS Paragon Plus Environment
Nano Letters
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Once the appropriate cold cavity characteristics were verified, samples were loaded into an evacuated cryostat and pumped optically at room temperature using both second and third harmonic pulsed Nd:YAG microchip lasers (approximately 0.5 ns pulse width, with 100 Hz and 16.4 kHz repetition rates, respectively) focused to a ~30 x 400 µm2 stripe. A clear output power threshold is observed for both cases in Fig. 2a and is also evident in direct camera images of the pump spot shown in the inset. The different threshold fluence recorded for the different pump wavelengths (-,.// = 91 ± 2 µJ/cm2 and -,/.1 = 40 ± 5 µJ/cm2 for 2 = 355 nm and
2 = 532 nm, respectively) is due in part to the trivial difference in photon energy, with the balance attributed to a narrower spatial gain distribution in the waveguide (and thus reduced modal gain) under 2 = 355 nm excitation owing to the factor of ~3 increase in MAPbI3 absorption coefficient at this wavelength. The transition above threshold is accompanied by collapse of the emission spectrum to a narrow laser line (linewidth Δ = 1.1 nm FWHM) shown in Fig. 2b. The laser line is strongly TE polarized (i.e. polarization parallel to the grating grooves) as shown in Fig. 2c and emits in a double-lobed far-field beam profile according to the photograph in Fig. 2d. This emission profile is characteristic of second order DFB lasing dominated by gain/loss coupling21-23 and is consistent with a large imaginary contribution in the complex coupling coefficient (5 = 59.7 − 38.86 cm-1 estimated from rigorous Floquet mode analysis23) expected from strong modal
interaction with the metal-clad grating. Taken together, the data in Fig. 2 unambiguously demonstrate lasing. Figure 3 examines the temperature dependence of the lasing behavior. The red-shift of the laser line with decreasing temperature observed in Fig. 3a follows a corresponding reduction in the MAPbI3 bandgap. This is accompanied by a dramatic threshold reduction in Fig. 3b,
3
ACS Paragon Plus Environment
Page 4 of 25
Page 5 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
reaching a minimum -,/.1 = 3.0 ± 0.4 µJ/cm2 at temperature T = 160 K, which corresponds to
a threshold excitation density 7 ~8x10:; cm-3. The decrease in threshold is consistent with that observed previously for MAPbI3 amplified spontaneous emission24 and likely results in part from a combination of increased radiative rate and decreased Auger loss.25 Below 160 K, the perovskite undergoes a tetragonal to orthorhombic phase transition that leads to a sudden increase in band gap,17 blue-shifting the gain spectrum14 outside of the DFB resonance window and extinguishing laser emission. We note that it is not uncommon to observe additional laser lines in some spectra, as exemplified by the T = 200 K and T = 160 K cases in Fig. 3b. In the course of many measurements, there is no clear pattern to the number or position of these lines other than that they cluster tightly within ~1 nm of the cold cavity resonance. This apparent randomness leads us to believe that, although the modes are defined longitudinally by the grating, their transverse confinement may be affected by weak residual perovskite surface or grain boundary scattering that leads to small deviations in resonant frequency. To better understand the pump requirements for a future perovskite diode laser operating in the non-impulsive pump regime (i.e. when the pump duration is comparable to or longer than the photoluminescence lifetime), Fig. 4 explores the lasing behavior under illumination by a high power InGaN laser diode at T = 160 K. Figure 4a establishes lasing above a pump threshold (instantaneous) intensity 190 K. As an alternative possibility, we note that the inorganic sub-lattice distortion caused by rotational freedom of the MA cation has also been suggested to cause a (dynamic) shift between direct and indirect band gap configurations,49 which could similarly reduce the net gain. The inset spectra in Fig. 4b argue against this possibility, however, as the lasing modes redistribute in intensity with increasing pulse duration (consistent with a change in the gain spectrum) but do not shift in wavelength (expected for a change in band gap). Ultimately, given that both of these possibilities as well as others50 stem from the presence of the MA cation, inorganic lead halide perovskites such as CsPbBr3 may provide a practical route to avoid the issue of lasing death all together.51 As a fundamentally different cause, it is also important to note that photoexcitation of MAPbI3 at blue wavelengths (as in the present case) has been associated with the formation of a charge transfer complex, resulting in a transient I2-like
7
ACS Paragon Plus Environment
Page 8 of 25
Page 9 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Nano Letters
species that normally regenerates the MAPbI3 ground state on a sub-nanosecond timescale52 but could potentially accumulate over time and adversely affect the lasing response under the intense photoexcitation conditions employed here. Further study of the MAPbI3 lasing death mechanism is clearly warranted. In summary, we have demonstrated metal-clad MAPbI3 distributed feedback lasers that operate at a pump intensity threshold of 5 kW/cm2 for durations up to ~25 ns under InGaN diode laser excitation at low temperature. Owing to effective thermal management by the silicon substrate, we conclude that heating does not limit the lasing duration in this structure and hypothesize instead that a photoinduced increase in dielectric constant shifts the excited state equilibrium from excitons to free carriers, reducing the gain below threshold over a ~25 ns time span. Although more work is needed to understand the mechanism of lasing death and assess its consequences in more detail, the outlook for electrically-pumped MAPbI3 lasing appears bright. If the present architecture can be converted into an efficient, low capacitance LED that does not substantially increase the cavity optical loss and maintains charge balanced recombination under short (
