Toward continuous-wave operation of organic semiconductor lasers

Abstract

The demonstration of continuous-wave lasing from organic semiconductor films is highly desirable for practical applications in the areas of spectroscopy, data communication, and sensing, but it still remains a challenging objective. We report low-threshold surface-emitting organic distributed feedback lasers operating in the quasi-continuous-wave regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This outstanding performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength combined with a mixed-order distributed feedback grating to achieve a low lasing threshold. A simple encapsulation technique greatly reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium otherwise taking place under intense continuous-wave photoexcitation. Overall, this study provides evidence that the development of a continuous-wave organic semiconductor laser technology is possible via the engineering of the gain medium and the device architecture.

Keywords: Continuous-wave; distributed feedback; organic semiconductor lasers.

Fig. 1. Fabrication method of the organic semiconductor DFB lasers.

Schematic representation of the method used to fabricate the organic DFB lasers. The different successive steps involve the fabrication of the DFB resonator structure by electron beam (e-beam) lithography, thermal evaporation of the organic semiconductor thin film, and spin coating of CYTOP polymer film followed by device sealing with a high–thermal conductivity (TC) sapphire lid.

Fig. 2. Structure of the mixed-order DFB resonators.

(A) Schematic representation of the mixed-order DFB grating structure used in this study. SEM images with (B) ×2500 and (C) ×100,000 magnification and (D) SEM image and (E) cross-sectional SEM image of the device after the deposition of a 200-nm-thick BSBCz:CBP blend film.

Fig. 3. Lasing properties of organic DFB lasers in the qCW regime.

Streak camera images showing laser oscillations from a representative BSBCz:CBP encapsulated mixed-order DFB device at repetition rates from 0.01 to 80 MHz over a period of (A) 500 μs or (B) 200 ns (80 MHz only). Excitation intensity was fixed at ~0.5 μJ cm⁻², which is higher than the lasing threshold (E_th). (C) Temporal evolution of laser output intensity at various repetition rates (f) in an encapsulated BSBCz:CBP mixed-order DFB laser. (D) Lasing threshold in several types of DFB devices as a function of repetition rate. Lines serve as visual guidelines.

Fig. 4. Lasing properties of organic DFB lasers in the long-pulse regime.

(A) Streak camera images showing laser emission integrated over 100 pulses from an encapsulated mixed-order DFB device using a BSBCz:CBP (20:80 wt %) film as gain medium and optically pumped by pulses of 30 ms and 2.0 kW cm⁻² (top) or 800 μs and 200 W cm⁻² (bottom). (B) Photograph of DFB device operating in the long-pulse regime (excitation, 30 ms). (C) Lasing threshold (E_th) in various DFB devices as a function of excitation duration. Dotted lines serve as visual guidelines. (D) Change in laser output intensity from organic DFB lasers as a function of the number of incident pulses (100 μs and 200 W cm⁻²).