December 15, 2009 / Vol. 34, No. 24 / OPTICS LETTERS 3923

Blue-light-emitting ZnSe random laser

Toru Takahashi, Toshihiro Nakamura,* and Sadao AdachiGraduate School of Engineering, Gunma University, Kiryu-shi, Gunma 376-8515, Japan

*Corresponding author: nakamura@el.gunma-u.ac.jp

Received September 16, 2009; revised November 16, 2009; accepted November 17, 2009;posted November 20, 2009 (Doc. ID 117180); published December 15, 2009

We observed blue-light-emitting random laser action with coherent feedback in ZnSe particles. Below thethreshold excitation, we observed a broad spontaneous emission band peaking at �470 nm. Above thethreshold, several discrete lasing lines appear at the center of the spontaneous emission band ��475 nm�.The linewidth of each lasing emission is less than 0.4 nm, which is about 40 times narrower than that of thespontaneous emission band. Although the lasing intensity of each line varies from pulse to pulse, the peakwavelengths do not shift significantly. The lasing emission was also found to radiate in all directions.© 2009 Optical Society of America

OCIS codes: 140.5960, 290.0290, 160.6000.

A random laser can be used as a new stimulated lightsource without any precise external laser cavities.Recently, such random lasers have been extensivelystudied by a number of groups because of theirunique characteristics, such as low threshold, spatialemission, and high coherency. The random lasing ac-tion is caused by the multiple scattering of light in again medium consisting of highly concentrated andrandomly shaped particles or column scatterers [1].Several types of materials including dye-dopedglasses [2], solid-state laser materials [3,4], andsemiconductors [5–12] are used as materials in ran-dom lasers. In particular, random lasers using ZnOpowder, first reported by Cao et al. [5,6], have attrac-tive potential for the application in various light-emitting devices.

Semiconductor random lasers using ZnO [5–10],GaN [11], and GaAs [12] particles or columns canemit light in the near-UV or IR region. Thus, the de-velopment of a new random laser emitting in the vis-ible region is awaited with interest. ZnSe has an ex-citonic bandgap of �2.7 eV [13] and has been used asan active layer in blue–green/blue diode lasers oper-ated at room temperature [14,15]. This material istherefore thought to be a promising material for re-alizing random lasers in the visible region.

In this Letter, we report the lasing action and char-acteristics of a blue-light-emitting ZnSe random laseroperated at room temperature. Below the thresholdexcitation, we observe only a broad spontaneousemission band with a peak at �470 nm; however,above the threshold excitation, several discrete las-ing lines appear at the center of the spontaneousemission band ��475 nm�.

The ZnSe crystals used were synthesized by the re-crystallization traveling-heater method and were notintentionally doped [16]. The yellowish ZnSe crystalswere ground in an agate mortar and dispersed inmethanol. To obtain a powdered ZnSe film, the solu-tion was added dropwise onto a Si substrate and thendried. The thickness of this film was about 10 �m.The sizes of ZnSe powder particles were evaluatedusing a JSM-6330F (JEOL) scanning electron micro-scope (SEM). The lasing action was observed by

the excitation with frequency-tripled light pulses

0146-9592/09/243923-3/$15.00 ©

(355 nm) from a Nd:yttrium aluminum garnet laserwith 5 ns duration. The excited light was incident atan angle of 45° on the sample surface. The size of theexcited light spot was 0.25 mm2 at the sample sur-face. The light emitted from the sample was collectedin the directions of 30°, 60°, and 90° from the samplesurface. Photoexcited emission spectra were thenmeasured using a single monochromator equippedwith a CCD (PIXIS:100B, Princeton Instruments).

Figure 1 shows an SEM image of a powdered ZnSefilm. From this image, it is clear that particles in theZnSe powder have highly random shapes. The size ofeach ZnSe particle is several hundred nanometers,which is comparable to the wavelength of the emittedlight. In such a case, light is strongly scattered andclosed loop paths can be formed, enabling the ampli-fication of light when the optical gain exceeds the ab-sorption loss under a high excitation intensity.

Figure 2 shows photoexcited emission spectra ob-tained from the powdered ZnSe film. The emittedlight was collected at 90° from the sample surface. Ata low excitation intensity, a weak broad emissionband with a peak at 470 nm is observed with anFWHM of about 15 nm [Fig. 2(a)]. The broad emis-sion band is ascribed to spontaneous emission owingto the radiative recombination of excitons in ZnSe,

Fig. 1. SEM image of powdered ZnSe film deposited on Si

substrate.

2009 Optical Society of America

3924 OPTICS LETTERS / Vol. 34, No. 24 / December 15, 2009

which has a 2.7 eV (460 nm) excitonic gap at roomtemperature. As the excitation intensity increases,narrow emission peaks appear at the center of thespontaneous emission band ��475 nm� [Fig. 2(b)].With further increasing the excitation intensity, dis-tinct emission lines can be observed in the emissionspectrum [Fig. 2(c)]. The widths of these emissionlines are less than 0.4 nm, which is about 40 timesnarrower than that of the spontaneous emissionband. Because our powdered ZnSe film consists ofhighly randomly shaped small particles (see Fig. 1),it is expected that significant light scattering repeat-edly occurs in the sample. Therefore, it is consideredthat a random laser action occurred in our ZnSesamples, similar to that observed in ZnO powders[5,6,9]. Note, however, that no lasing action has beenobserved from bulk ZnSe platelets or larger-particleZnSe powders (a few micrometers in diameter), evenat high excitation intensities.

As clearly shown in Fig. 2, the lasing emissionlines appear at a slightly longer wavelength than thespontaneous emission peak. The same phenomenonwas observed in a ZnO random laser [7,8]. In bulkZnO and ZnSe platelets, stimulated emission due tothe recombination of electron–hole plasma orexciton–exciton scattering was observed [10,17,18].The emission peaks in such cases exhibited a redshiftrelative to the spontaneous emission peak, similar tothat obtained in the present study. We also foundthat each lasing line shifts slightly toward thelonger-wavelength side with an increasing excitationintensity [Figs. 2(b) and 2(c)]. This tendency has also

Fig. 2. Emission spectra of powdered ZnSe film. The exci-tation intensities are (a) 540, (b) 612, and (c) 720 kW/cm2.The emitted light was collected at 90° from the samplesurface.

been observed in a ZnO random laser [7]. The red-

shift is possibly due to an exciton–exciton collisionmechanism [18] or to the heating of the sample by ex-citation laser illumination [8].

The integrated emission intensity of the ZnSe ran-dom laser as a function of the excitation power isshown in Fig. 3. Below the threshold excitation power[points (a)], the spontaneous emission intensitygradually increases with increasing excitation power.At the threshold [point (b)], the integrated intensitystarts to increase superlinearly. Such a typical lasingbehavior has been reported in various random lasers[5–8]. This result indicates that coherent feedbackoccurred in the present powdered ZnSe sample. Thelasing threshold estimated from Fig. 3 is�600 kW/cm2. This value is nearly equal to that ob-served in various ZnO random lasers [5–8].

Figure 4 shows the pulse-to-pulse fluctuation oflasing spectra obtained by the excitation at720 kW/cm2. Three sharp peaks are always observedat wavelengths of 473.9, 474.7, and 475.6 nm. Thesestable lasing wavelengths indicate that the presentlasing mode can simply be explained by the geometri-cal randomness in the gain medium [19]. On theother hand, the peak intensities of these lasing linesare temporally unstable. This is because the lasingintensity is strongly dependent on the excitation in-tensity (our excitation source exhibits a slight pulse-to-pulse fluctuation). Similar instability has alsobeen observed in a GaAs random laser by Noginov etal. [12]. This type of instability suggests that the ran-dom lasing action is very sensitive to the excitationintensity, i.e., each laser mode may compete depend-ing on available gain/loss mechanisms [20].

Figure 5 shows lasing spectra at three differentangles of radiation. Our present laser is found to ra-diate in all directions unlike conventional Fabry–Perot lasers. The emission intensity is also found notto exhibit a strong dependence on the radiation direc-tion. However, the lasing spectra observed in three

Fig. 3. Integrated emission intensity as a function of exci-tation power. The intensities were integrated over the 450–490 nm wavelength region. Each data point was obtained

by averaging several spectra.

December 15, 2009 / Vol. 34, No. 24 / OPTICS LETTERS 3925

different directions are clearly different from eachother.

In summary, we developed a blue-light-emittingrandom laser using a powdered ZnSe film. Several

Fig. 4. Pulse-to-pulse spectra obtained at excitation inten-sity of 720 kW/cm2 and at fixed excitation spot.

Fig. 5. Emission spectra observed at three different direc-

tions [(a) 30°, (b) 60°, and (c) 90°] from the sample surface.

lasing peaks caused by coherent feedback were ob-served above the threshold for light excitation��600 kW/cm2�. This value was almost the same asthat reported for ZnO random lasers. The linewidthsof these lasing peaks were less than 0.4 nm. Althoughthe lasing intensity fluctuated from pulse to pulse,the lasing wavelengths did not perceptibly fluctuate.Furthermore, the laser emission was observed inall directions. Recently, Ma et al. [21] realized anelectrically pumped UV random laser using ac-axis-oriented ZnO polycrystalline film. Our presentresults for a blue-light-emitting random laser using apowdered ZnSe film may promote the development offuture applications of electrically excited visible-lightrandom lasers in various optical fields.

This work was supported by a Grant-in-Aid for Sci-entific Research (C) (20560290) from the Ministry ofEducation, Culture, Sports, Science and Technology,Japan.

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